PALEOCEANOGRAPHY,
VOL. 4, NO. 1, PAGES19-55, FEBRUARY1989
SURFACE
WATER RESPONSE
OF
EQUATORIAL
ATLANTIC
OCEAN
THE
TO ORBITAL
FORCING
1,2
William
F.
Andrew Mc Intyre,
Ruddiman,
1 Karen Karlin, 1 and Alan
C. Mix 3
Abstract.
The response
of the
mixed layer
of the equatorial
Atlantic
to climate
change,
for
times
greater
than 10 kyr,
is
predominantly
forced
by the
precessional
component
of
periodicities
insolation.
hemisphere
A
zonal
transect
of
three
cores
analyzed
at 1-kyr
intervals
documents this
response
for 0-250
ka
The western
equatorial
Atlantic
is characterized
by minimal
variation
in
surface
water
character,
indicating
temporal
stability
of the mixed layer,
except
during
intervals
of maximum
glaciation.
In contrast,
the
eastern
region
shows marked temporal
variations
in
temperature
assemblages,
estimated
sea
surface
and foraminiferal
with
dominant
1Lamont-Doherty
University,
Palisades,
New
York.
2Queens College of the City
University
of New York,
State
University,
Copyright
1989
by the American
Paper
number
0883-8305/89/88PA-038
95510.00
Union.
surface
ice
temperature
volume.
forced
variations
in
southern
These
of orbitally
(1)
and monsoon-controlled
and (2) advection
of
trade
wind
divergence
heat from high
latitudes.
INTRODUCTION
The
climate
of
the
Earth
undergoes
rhythmic
change.
The last
780 ka were dominated
by
periodicities
centered
on 100 kyr,
41 kyr,
and 23 kyr
[Imbrie
et al.,
.
Earth-Sun
orbital
variations
are considered
the primary
forcing
function
[Hays et al.,
1976;
Imbrie
et al.,
1986].
Climate
response
to
insolation
forcing
is modulated
by
mechanisms.
Imbrie
and
Imbrie
[1980]
that
continental
ice volume
insolation
by approximately
kyr.
Ruddiman and Mcintyre
1984]
88PA03895.
on 23 kyr.
are the product
terrestrial
Corvallis•
Geophysical
sea
continental
signals
Flushing.
3College of Oceanography
Oregon
and
1984]
Geological
Observatory
Columbia
centered
At precessional
periods,
the eastern
equatorial
Atlantic
responds
in
phase with
southern
hemisphere
sea
surface
temperature
and
significantly
leads northern
described
response
of
Atlantic
to
variations
ice
sheets.
mechanisms
the
orbital
in
lags
6 to 9
[1981,
that
high-latitude
link
North
forced
northern
Prell
showed
and
hemisphere
Kutzbach
20
Mcintyre
[1987]
defined
monsoon
in
the
the
role
Indian
paper examines the
equatorial
Atlantic
forcing.
Riehl
of
Surface
momentum
and
most
produces this
the
heat
The energy
gain
function
of
insolation
orbital
The
of
1964]
in
which
then
which
water
transport
in
and
both
zonal
the
the
sense
[Hastenrath,
1980].
Gordon [1985,
1986] has hypothesized
interocean
advection
major
and
salt
this
and
of
thermocline
component
in
budget.
idea
is
salt
the
advection
mixed
modulated
by orbital
the
of
basis
as
ocean's
these
to
of
layer
a
heat
A corollary
that
in
water
the
heat
is
forcing.
On
observations
and
assumptions,
our goals are to (1)
document the history
of the surface
waters
of the equatorial
Atlantic
from approximately
250 ka to the
present;
(2) determine
the
relationship
between
response,
insolation
glaciation);
descriptive
ice
SURFACE
Interhemispheric
this
surface
Atlantic
waters
today
transfer
characterizes
[Lamb,
by
the
1981;
Hastenrath,
1980;
Stommel,
1980;
Oort
and Vonder
Haar,
1976].
Asymmetry
of the continent
and ocean
configures
structure
the
such
tropospheric
that
the
1977]
heat
north
melds
Thus
is
Current
.
Brazil
with
The
Current,
the
southern
advected
Guiana
hemisphere
northward;
both
the
The strong
seasonal
variation
in
the forcing
winds,
the trades
(tropical
easterlies),
produces
a
fluctuating
equatorial
system
[Philander,
1979;
Reverdin,
1985;
Servain
and Legler,
1986].
In
boreal
summer (June-September)
,
strong
southern
trades
invade the
northern
hemisphere
and the ITCZ is
furthest
from the equator.
Along
the equator,
divergence,
SEC speed,
and thermocline
slope
are all
at
their
maximum,
SST in the eastern
equatorial
Atlantic
is at its
minimum,
and the
sea surface
upward to the west
[Katz and
Garzoli,
1982,
1984;
Servain
1986].
the
western
The water
heat
heat
has
southern
hemisphere
trade
winds extend
into
the northern
hemisphere
for much of
the year
[Riehl,
1979] .
Sea surface
temperature
(SST) is cool due to
upwelling/divergence
at and south of
and
1983;
1976]
and
reservoir
reservoir
slopes
transiting
of
Atlantic/Caribbean.
the
This
a
small
annual
variation
in SST but a large
annual
variation
in heat
storage
[Merle,
1980].
Variation
of heat
storage
in
the mixed
layer
of the western
equatorial
Atlantic
reflects
the
export
of heat northward
in western
boundary
currents
and eastward
in
countercurrent
OCEANOGRAPHY
heat
Current.
forms
response.
EQUATORIAL
heat
Equatorial
the SEC warms [Molinari,
Bunker
and Worthington,
volume
and (3) erect
models to explain
south
the
Legler,
oceanographic
forcing
(due to
orbital
variations),
and
(variations
in continental
cloud
radiative
Atlantic
thermal
equator
and the
Intertropical
Convergence
Zone
(ITCZ)
lie
in the northern
hemisphere.
Hadley cell and tropical
mixed layer
is a major control
of the global
heat budget on all time scales.
The
tropical
oceans are corridors
of
meridional
diminishes
[Hastenrath,
for
annually
interaction
in the
Forcing
continental
margin of Brazil
diverts
part of the SEC into the northern
hemisphere
[Croll,
1864; Bjerknes,
and through time.
The tropical
mixed layer of the ocean constitutes
the major heat reservoir
of the
world.
This
enhances
that
geometry,
both
and
(SEC)
momentumand heat is a
controls
equator.
cover
This
response of the
to orbital
of
atmosphere."
Water Response to Orbital
the
the
Ocean.
[1979;
p.1]
stated,
"The
are the source of all
tropics
the
et al.'
In boreal
March),
the
s .
winter
southern
(Decembertrades
are
weak, and part
of the equatorial
surface
water
piled
up in the west
flows
back
[Richardson
Divergence,
SEC speed
its
and
as
countercurrents
and
Reverdin,
thermocline
are minimal,
annual
maximum.
its
attendant
1987]
.
slope,
and
and SST is at
Thus
features
the
SEC
have
two
quite
different
seasonal
aspects.
Statistical-dynamic
models
developed
for and applied
to
equatorial
oceanography
[Cane, 1979;
Mcintyre
et
al.'
Surface
Water
Response
to
Orbital
Forcing
0"
4
21
IO'N
ß
.
.
V25-59
ß
Fig.
1.
range of
Map of
annual
contour
core positions
(triangles).
temperature
variation
at
interval
is
time
the
The
1øC.
the northern
boundary
of the SEC,
while
the eastern
Atlantic
core,
RC24-16,
is positioned
slightly
Cane and Sarachik,
1976,
1977,
1979;
Moore and Philander,
1977;
Philander
and Pacanowski,
1980,
1981,
1986a,b]
support
observational
data with
respect
to the seasonal
response
of
the equatorial
Atlantic.
The
westward-flowing
SEC is trade
wind
forced
and responds
essentially
in
phase with
trade
wind variations.
This convergence
of observation
and
modeling
suggests
two important
questions'
(1) is the annual
cycle
a suitable
analogue
for the slower
physics
of an orbitally
forced
response,
and (2) are there
significant
variations
in
interhemispheric
heat transport
on
orbital
Contours
delineate
50 m [Gorshkov,
1979].
south
of
the
main
axis
of
the
SEC
(Figure
1; Table
1) .
The
relationship
of these
cores
to
annual
SST variation
is clarified
by
comparison
with
the 50-m annual
temperature
variation
(Figure
1) .
The central
and eastern
regions
(V30-40,
RC24-16)
show marked
annual
temperature
variations,
while
the
western
region
(V25-59)
does not.
All
cores
are above
the present
lysocline,
defined
as the depth at
which
undersaturation
carbonate
al.,
1980]
scales?
enhanced
of
the
ion occurs
[Takahashi
.
Documented
intervals
dissolution
in
et
of
the
Today the mixed layer
of the
equatorial
Atlantic
is partitioned
into
a western
region
where
Atlantic,
for example,
isotopic
stage
4 [Crowley,
1983],
are
represented
in these
cores by only
small
increases
in fragmentation
of
foraminiferal
tests.
The sample
variations
interval
DATA
AND
METHOD
in
heat
storage
region
with
We
have
SST
are
chosen
three
sited
water
Atlantic
the
core,
Table
Core
core,
to
V25-59,
Atlantic's
The
central
V30-40,
1.
lies
mean
is
the
mean
time
interval
in
warm
The
revised
A •5-g
beneath
sample
from
piece
5ø2.3'S
1ø22.4 'N
0ø12.0 ' S
each
preparation
Imbrie
and
is obtained
r Depths r and Usable
Longitudes
Depth
m
RC24-16
V25-59
V30-40
sedimentation
per
at
core
In
a
of
kyr.
Core Coordinates
Latitudes
was
rate
of each core
(centimeters
kiloyears)
derived
from initial
carbonate
and biostratigraphy.
the event,
this
provided
data
response
to
both regions.
westernmost
beneath
reservoir.
and
an eastern
response.
cores
document mixed-layer
climate
forcing
in
The
minimal
maximal
and
the opposite
10ø11.5'W
33ø28.9'W
23ø09.0 'W
3559
3824
3706
method is
Kipp
[1971].
from a 1-
Lenqths
Length
cm
1066
841
755
1
22
Mcintyre
cm-wide
slice
normal
to
dried
for
for
weighed.
of 4 g of
of
•m)
bath
Surface
2 hours,
deionized
is
and
is
added to
then
agitated
refer
for
1 hour
at
140
fraction
allowed
to
a shaker
rpm.
by wet
sieve.
The
collected
and
is
settle
until
supernatant
supernatant
is
is
washed
deionized
the
clear.
The
siphoned off,
and the
<63-•m sample is dried
(<40 C) and
archived.
The >63-•m fraction
is
with
o
water
and
dried
(<40øC).
The procedure
of
shaking,
wet sieving,
and drying
repeated
twice.
It has been
determined
•han
that
single
this
method,
sequence with
shaking,
is the
to disaggregate
is
rather
3 hours of
more effective
way
while minimizing
fragmentation.
The cleaned
>63-•m fraction
is
weighed,
and the percent
of >63-•m
fraction
by weight
is calculated.
Next it is randomly
split
into two
halves'
analysis,
The
successively
latter
half
split
and
•m until
planktonic
at
obtained.
consists
Generally,
of 300 to
at
150
least
300 whole
foraminifera
are
a faunal
400 >150-•m
specimens;
determined
a 300 count was
to be the statistically
acceptable
lower
al.,
1973] .
in the splits
limit
count
not
in
et
used
an
planktonic
species
(for
example,
both
or
Globigerinoides
tuber,
white,
and G.
ruber,
pink)
are recorded.
Specimens
not belonging
to one of
these
41
listed
29
have
resolving
taxa
as
or
unidentifiable
"other."
been
Of
shown
to
the
be
foraminiferal
by Q mode factor
analysis
Forcing
Because
the
in
warm
and
of
terms
equations,
of
abbreviated
Tc
for
the
cold
age.
STRATIGRAPHY
The development
of the SPECMAP
time
scale
[Imbrie
et al.,
1984;
Pisias
et al.,
1984,
Prell
et al.,
1986;
Martinson
et al.,
1987]
permits
the construction
of
chronologies
with
errors
less than
+2.5
kyr for records
since
700 ka.
The method utilizes
oxygen isotope
signals
from foraminifera
in which
specific
isotope
signatures,
called
isotope
events,
have been defined
[Pisias
et al.,
1984,
Table
2].
Any
core that
contains
well-preserved
foraminiferal
tests
capable
of
yielding
a continuous
oxygen isotope
signal
is amenable
to application
of
the
SPECMAP isotope
taxonomy.
The isotope
signals
were obtained
the
benthonic
foraminifera
wuellerstorfi
are
41
taxa,
useful
in
assemblages
[Kipp,
1976;
Molfino
et al.,
1982].
SST
estimates
are calculated
using
a new
transfer
function
FA20
[Molfino
et
al.,
1982].
FA20 is based on an
expanded
set of core top samples
in
V25-59,
from the planktonic
foraminifera
Neogloboquadrina
dutertrei
in RC2416, and from Globigerinoides
$acculifer
in
V30-40.
core
was
one
of
those
the
SPECMAP
time
scale.
of
identifications
archive.
Forty-one
morphotypes
Ocean.
estimates
cold
Reexamination
[Imbrie
The portions
are retained
the
Cibicides
is
sieved
for
from
one for oxygen isotope
the other
for faunal
counts.
Orbital
equation.
Estimated
SST values
for
Tw and Tc for all
samples
are listed
in Appendix
A for both depth
and
Samples are partitioned
sieving
through
a 63-•m
<63-•m
to
Tw
sample,
in
to
Atlantic
warm and
(0.45
the
the
Response
the equatorial
region
is
characterized
by annual
migration
the oceanic
thermal
equator,
we
then
filtered
Water
from
(oriented
A disaggregating
solution
sodium metaphosphate
per
water
which
sediment
al.'
the core's
long axis),
12-24hrs
at <40øC,
desiccated
liter
of
et
al.
[1984]
oldest
the
portion
shown
of
latter
to
erect
isotope
used
has
The
used
by Imbrie
that
V30-40,
in
et
the
isotope
events
8.2 through
8.3 were
incorrectly
identified.
When the
SPECMAP time
scale
was applied,
this
produced
a marked change in
sedimentation
rates.
We have
revised
event
identification
such
that
the
anomalous
sedimentation
rates
are
eliminated
and
a rate
commensurage
with
that
younger
than
event
7.5 prevails
to the core
bottom
(Appendix
B) . All
isotopic
data
are
listed
in
the
SPECMAP
Archive
1 [1989].
Tape copies
of
this
archive
can be obtained
by
writing
to the Data Support Section,
National
Center
for Atmospheric
Research,
Boulder,
Colorado.
Mcintyre
et
5.0
al.'
Surface
Water
Response
V25-59
V30-40
018
018
4.0
2.0
3.0
1.0
0.0
0
to
Orbital
Forcing
23
RC24-16
018
-1.0
-2.0
2.0
o.o
1.0
-1.o
-2.0
I
1oo
200
Depth
300
400
-6.2
-6.4
500
600
700
800
Fig. 2.
Oxygen isotope (180) signals plotted
in the depth domain.
The isotope
substages
used to establish
correlation
time scale
are indicated
by the appropriate
numbers
margin
of each core,
and the depth and age for each
Appendix
B.
To
achieve
possible,
the
statistical
criteria
correlation
best
we have
methods
for
goodness
coefficient
be
correlation
applied
and
defined
of
fit.
r
The
between
the isotope
signals
must be >0.90
(meaning that
>80% of the compared
signals
are equivalent).
Where
similar
periodicities
are present,
cross-spectral
analysis
is used to
determine
the coherency
at these
periods
[Bloomfield,
1976; Jenkins
and Watts,
1968] .
Coherency
is the
tendency
for oscillations
over a
given
frequency
band in one signal
to be linearly
related
to comparable
oscillations
in another
signal,
when
the effects
of phase difference
are
removed by alignment
of the two
signals.
The coefficient
of
coherency
k measures
the degree
to
which this
tendency
is present.
If
k is statistically
significant,
then
the phasing
at this
period
is
meaningful.
The former
value
must
ß
>0.80
and
with
the SPECMAP
in the right
are listed
in
the
latter
must
be
in
phase for the signals
to correlate.
V30-40
is our primary
stratigraphic
control
for the other
cores in this
study.
Its isotope
signal
clearly
depicts
all
the
isotope
stages
(Figure
2).
To
determine
if our stratigraphic
revision
was
correct,
we first
transferred
the signal
domain using
the dates
event
identified
in
isotope
analyzed
signal
with
stack.
The
the
into
the
for each
V30-40.
This
was cross-spectrally
the SPECMAP isotope
correlation
SPECMAP
time
stack
of
is
V30-40
to
excellent,
r=0.97.
Intercore
comparison
of different
parameters
necessitates
that
the
chronologies
of V25-59,
RC24-16,
and
V30-40
be the same.
Chronologies
were
established
RC24-16
possible,
events.
by
for
V25-59
and
identifying,
where
the dated
SPECMAP isotope
The
age
of
these
events
was
Mcintyre
Table
2.
Statistics
Spectral
Core
of
the
Surface Water Response to Orbital
Cross-
core,
Oxygen
with
Siqnals
Versus
Coherence
k
r
SPECMAP
Phase
0.92
0.92
0.97
0.84
0.95
0.99
-1.5
ø
6.3 ø
-1.0
ø
Core
Versus
RC24-16
V25-59
V30-40
0.92
0.91
0.85
0.95
0.2 ø
5.6 ø
correlation
coefficient
r
is
computed
for the entire
signal.
The
coherence
k and
phase
values,
in
degrees,
are
only
computed
for
the
23-kyr
period.
The three
equatorial
cores
are
shown
isotope
shown
40.
versus
stack
[1984]
statistics
.
the
from
the
SPECMAP
Imbrie
Intercore
of RC24-16
versus
(3ø32.8'S,
35o13 8'W),
ka record,
also
this
et
al.
correlation
and V25-59
SPECMAP
core
are
V30-
and
RC24-16
V30-40
met our
fit,
were
cross-correlated
and,
if
criteria
for
intercore
the
goodness
correlation
If
considered
established.
"tune"
the
the orbital
resultant
chronologies
frequencies.
The
relevant
statistics
stratigraphic
in Table 2;
Appendix
We did
variations.
temperatures
temperature
water
for
given
are in
B.
difference
SEA
SURFACE
TEMPERATURE
V25-59
is
characterized
r=0.034
(Table
warm SST with low-amplitude
variations
except
for a cool
excursion
late
from ~143 to
foraminiferal
cool interval
in V30-40
in isotope
stage
130 ka (Figure
3) .
The
population
within
this
is equivalent
to that
during
stage
6.
A nearby
the
warmest
stable
a
and
not
Tc
correlative,
3b) .
of cores V30-40
in the region
by seasonal
divergence,
following
characteristics
3a and b) . First,
Tc and,
lesser
extent,
Tw have
signals
Second,
low-
that
are well
high-frequency
are prominent
in isotope
in both Tw and Tc, being
in V30-40
intracore
are
than
in RC24-16.
the Tw and Tc
basically
dissimilar.
SST intercore
differences,
defined
by pairwise
subtraction
of
the mean temperature
value
of each
core,
are deemed significant
if they
the
estimate
These
standard
of
the
standard
1982,
error
FA20
Tc
are
+1.2øC
[Molfino
1] .
for
all
standard
between
of
equation.
errors
for
Table
The
for
et
Tw
three
cores
lie
error.
The
the
V25-59
Tc
mean
and
the
Tc of
the
other
two
cores
exceeds
the
standard
error.
Intercore
SST differences
are
primarily
6,
Tw
are
The SST signals
and RC24-16,
sited
differences
within
the
difference
by
a
between
they
Tw and +1.3øC
The SST signals
in V25-59
differ
markedly
from those
of V30-40
and
RC24-16.
with
signals,
al.,
ESTIMATED
South
range
of all
the cores
These SST signals
presence
of a warm
reservoir
exceed
all
statistics
are
all chronologies
to
thermal
character
through
time.
While
there
appears
to be little
signals
to
entire
Africa
V2.5-59
has
and smallest
(Table
3a) .
document
the
greater
Third,
not
from
cooler
the
we exclude the stage 6 cold
variations
stages 2-3
of
170
ka
dominated
interval,
the SST signal
in stages
1-5 in V25-59
is characterized
by
high-frequency
SST variations
whose
amplitude
is equal
to
(Tw) or
exceeds
(Tc) that
of low-frequency
frequency
defined.
was
in
America.
to
statistics
interval
(J. Imbrie,
personal
1988).
Thus,
once
zone
dominated
with
6
the
last
waters
have the
(Figures
then used to transpose
the isotope
signal
into the time domain by
linear
interpolation.
These time
domain signals
were then correlated
with the SPECMAP isotope
signal.
Last,
the isotope
signals
of V25-59
cool
isotope
stage
communication,
equatorial
RC24-16
V25-59
V30-40
The
V25-56
within
surface
Stack
Forcing
a 0-170
contains
Correlation
Coefficient
Core
of
Analysis
Isotope
et al..
due to
the
estimated
Tc
values.
Intracore
differences
and Tc are
Lnvolving
latitude
striking.
SST estimates
cores
yielded
between
Past
efforts
in highTw and Tc
Tw
Mcintyre
et
al.-
Surface
Water
Response
V25-59 (øC)
22
to
Orbital
Forcing
V$0-40 (øC)
29
15
22
25
RC24-16 (øC)
29
15
29
22
20
40
60
AGE
80
100
120
140
Tc
Tw
160-
220
240'
Tc
260
Tc
Tw
Tw
Fig.
3.
Estimated
sea surface
temperature,
SST, for the warm, Tw,
and cold,
Tc, equations.
Note the marked intracore
signal
difference
in the
listed
signals
were
two eastern
in Appendix
basin
A.
whose amplitudes
similar.
For
(41ø00'N,
32o55.5'W)
and Mcintyre
[1981]
cores,
V30-40
and periods
example,
V30-97
from Ruddiman
has r=0.97
for
Tc versus
Tw.
In the equatorial
cores,
the correlation
between the
Tw and Tc signals
is significantly
lower
[Table
3b].
This difference
was previously
Hays [1976]
noted by Gardner and
and Prell
et al.
[1976].
The younger portions
of the SST
signals
in both RC24-16 and V30-40
are quantitatively
different
from
the older
portions.
Prior
to the
isotopic
6-5 boundary,
the signal
has little
high-frequency
variation.
After
the stage 6-5 boundary,
highfrequency
variability
is present,
particularly
in isotope
2.
These high-frequency
stages 3 and
variations
and
RC24-16.
The
SST data
are
in SST were checked
by resampling
and recounting
50% of the data
levels
valid.
involved
In
and
summary,
both
intercore
responses
equatorial
western
found
estimated
and
an
be
SST defines
intracore
which
subdivide
Atlantic
into
and
to
unstable
thermal
the
a stable
eastern
region.
The latter
is characterized
by stronger
intracore
signal
dissimilarity.
Spectral
analysis
defines
statistically
significant
periodicities.
The method of
spectral
estimation
used calculates
the
Fourier
autocovariance
and Tukey,
The climate
transform
function
of
the
[Blackman
1958; Bloomfield,
1976].
response to orbital
26
Mcintyre
Table
Cores
Equation
3a.
et al.The
Surface Water Response to Orbital
Basic
Statistics
Number
of
Estimated
Temperature,
of
Forcing
SST
Standard
oC
Minimum
Maximum
Range
Mean
Deviation
24.98
19.32
22.94
16.41
23.63
15.73
28.32
25.56
28.40
24.58
28.73
25.91
3.34
6.24
5.46
8.17
5.10
10.18
26.89
23.62
1.106
25.45
1. 058
Samples
V25-59
Tw
Tc
165
165
V30-40
Tw
252
Tc
252
RC24-16
Tw
251
Tc
251
Tc
and
Tw refer
to
values
estimated
by the
20.92
1.697
26.23
0.765
20.61
2. 137
cold
and warm equations,
respectively.
forcing
in
the
concentrated
equatorial
in
narrow
cores
is
the
bands
centered
on the three
primary
orbital
periods.
The 23-kyr
period
(precession)
dominates
the highamplitude
Tc signals
while
the 41kyr period
(obliquity)
is present
in
the low-amplitude
Tw signals.
In
the
ensuing
discussions
we will
refer to these bands by the primary
periods'
23 kyr, 41 kyr, and 100
kyr.
Precession
contains
both
23-
and 19-kyr
kyr signal
at 23 kyr,
components,
but the 19is weak relative
to that
over the time interval
of
our
and
cores,
heterodyne
the
tones
it difficult
concentrate
existence
around
of
19 kyr makes
to examine.
on the 23 kyr
minimum
statistical
The V25-59
SST signal
is
punctuated
late
in isotope
stage
by
a cold
interval.
6
To determine
if
this
low temperature
excursion
materially
altered
the spectra,
truncated
Thus we
precession
for
reliability.
The spectrum of Tc
lacks
statistically
significant
periods,
though there
is power at 23
kyr,
41 kyr,
and at very low
frequencies.
Both Tw and Tc spectra
contain
broad peaks centered
on a
period
of 6 kyr. The sampling
interval
in this core is 1 kyr and
the chronologic
error,
<+2.5 kyr;
thus the significance
of the 6-kyr
period
is questionable.
the
ka and
V25-59
spectrally
at
analyzed
the
shortened
The spectrum of V25-59 Tw (Figure
4) contains
a broadband peak
centered
on a period of 41 kyr which
resulted
in a slight
enhancement
of
the Tc 6-kyr
period,
the appearance
of shoulders
in the spectral
curve
at 23 kyr,
and 12.5 kyr,
and, as
expected,
the disappearance
of power
at very long periods.
Tw spectra
accounts
for
of the Tw signal.
34%
of
the
total
However,
the record length encompasses less
than four 41 kyr periods;
five
is
for
the
Table
at
3b.
Zero
Correlation
Lag Between
Coefficient
r
SST Signals
Equation
V25-59
Tw
Correlation
Coefficient
r
0.233
Versus
V30-40
V25-59
V30-40
RC24-16
Tc
Tw versus
Tw versus
Tw versus
Tc
Tc
Tc
truncated
record
41-kyr
period.
The periods
of
variations
seen
This
the
in
retained
rhythmic
the
time
SST
series
plots
of V30-40 and RC24-16
(Figure
3) yield
well-defined
spectral
peaks
(Figure
4).
Tw in V30-40
has
,
Core
the
series.
we
128
cycle.
variance
time
record
0. 627
0.034
0.256
0.212
significant periods of 41 kyr and 16
kyr which account for 14% and 11% of
the
total
variance,
respectively.
The V30-40
Tc spectra
has
significant
periods
of 23 kyr and
100 kyr.
The 23-kyr
period
accounts
for an impressive
46% of the total
variance,
while
the 100-kyr
period
accounts
other
for
16%.
significant
There
periods
are
in
no
this
Mcintyre
et al.'
Surface
Tw
Water Response to Orbital
Tc
v25-59
2
:
10--• n=146
ø
t=lka
I/
I\
B.W.=O.028
d.f.=6.48
Forcing
during
(Figure
the
5)
RC24-16
V25-59
(8.3øC)
(5.2øC,
glacial.
Seasonality
has a maximum range
,
0 i , , , , , , , ! , ,
V30-40
I--6]41:14%
44
:
m=80
RC24-16
and
ASSEMBLAGES
B.W.=O.016
o
C.1.=31.07
Z
0
0
RC24-
5 41:27%
6
52-• 25:41% n=236
/
H
m=75
/ 100:24%
/,• II
AND
,
, ,
0.1
,.,,
,., ,.,
........
recorded,
17
define
48
produced
,
,
, ,
0.2
,
0
0.0
0.1
0.2
FREQUENCY
(cycles/ko)
Fig.
4.
Variance
spectra
of SST.
Dominant periods
in kiloyears
of
significant
peaks are indicated.
Abbreviations
are.
n, number of
points;
int.,
time
interval
between
data points;
m, lags
(the number of
points
lagged
in the Fourier
analysis);
BW, bandwidth;
df,
degrees
of freedom;
CI, confidence
interval
for
the
lower
limit.
spectrum.
RC24-16
Tw and Tc spectra
contain
most of the same significant
periods
as V30-40,
but the percent
variance
accounted
for
is
different'
27% by 41 kyr in the Tw signal
and
41% by 23 kyr and 24% by 100 kyr in
the Tc signal.
Seasonality
is a measure
of the
annual
range of SST variability.
The Climate'
Long-Range
Investigation,
Mapping,
and
Prediction
of the last
V30-40.
SST
account
four
of
by
data
(CLIMAP)
glacial
reconstruction
maximum [CLIMAP
Project
Members, 1981] mapped
oceanographic
seasonality
and
demonstrated
that
the equatorial
Atlantic
had higher
seasonality
for
the
(Table
4).
the
of
the
assemblages
analysis
The
explain
of
most
and these
six
factor
assemblages
,
in
SST variance
percent
,
40%
Foraminifera
raw
0.0
Tc
The response
of the equatorial
Atlantic
to climate
forcing,
as
monitored
by estimated
SST, can be
analyzed
by determining
which
foraminiferal
components
control
the
SST signals.
Eight
species
of
100:16%
0
in
d.f.=6.42
(J3
Z
<•
fluctuations
f=lka
>-
i-.
by
Seasonality
is primarily
value.
The spectra
of seasonality
contain
significant
23-kyr
power,
explaining
26% of the total
variance
in
3:46% n=257
z
in
and a minimum in
or 3.1øC if the cold
stage
6 is excluded).
in V30-40 and RC24-16
controlled
0
2?
only
of
the
other
two
a small
variance.
The
four
dominant assemblages
are the
tropical,
transitional,
divergence,
and subpolar
as described
by Kipp
[1976] and Molfino
et al.,
[1982].
We use
Kipp's
criteria
but
have
changed one name, substituting
the
term "divergence..
for "gyre."
This
was done because the biogeography
of
the three
dominant
species
in the
assemblage
is
associated
oceanic
upwelling
regions
in
low
The first
the
with
and divergence
latitudes.
stage
mechanism
in understanding
responsible
for
the
equatorial
mixed-layer
response
documented by SST and assemblages
involves
establishing
the
relationship
between
biota
and
oceanography.
Each assemblage
is
composed of ecologically
related
foraminiferal
species.
Factor
analysis,
which
assemblages,
defines
does not
oceanographfic
control
the
we must
rely
these
specify
parameters
groupings.
on
studies
the
that
To do that,
of
the
living
biota.
sophisticated
Development of
sampling methods
[Wiebe et al.,
a generalized
1976] has established
ecology for the
species
the
living
equatorial
thermocline
in the mixed layer
Atlantic.
partitions
The
the euphotic
of
Mcintyre et al.'
28
Surface Water Response to Orbital
V25-59 (øC)
•
5
I
•
V30-40 (øC)
9
I
1
•
5
I
•
Forcing
RC24-16 (øC)
5
9
I
9
20
40
60
AGE
80
100
120
140
160
180_
200_
220-
240-
260
Fig.
5.
from
Tw at
1-kyr
basin
core
V25-59.
zone
into
Seasonality,
an upper
(warm)
lower
(cool)
environment.
of this
stratification,
logical
terms
to treat
of
both
temperature.
species
live
water
above
in degrees
intervals.
the
depth
the
and
the
waters
thermocline.
thermocline.
in
the
equatorial
to
the
nutrient
associated
or
Divergence
species
region
and
with
variation
of the
variation
involves
Tc
western
in
vertical
1986;
.
the
displacement
and slope change due to
seasonal
wind forcing
of surface
water
species
within
computed by subtracting
minimal
The
annual
thermocline
in
Tropical
assemblage
in the warmest surface
adapted
conditions
Because
is
it
Celsius,
the
1980;
Reynolds
and Thunell,
Thunell
and Reynolds,
1984]
and a
ecology
Transitional
and subpolar
prefer
cooler
temperatures
below
Note
are
food
the
thermocline.
The divergence
assemblage
is defined
by a number of
species
whose optima are associated
with
the thermocline
in tropical
regions
[Be and Tolderlund,
1971;
Fairbanks,
et al.,
1982;
Fairbanks
and Wiebe,
1980;
Fairbanks,
et al.,
(see
equatorial
conceptual
explains
variation
advection
ratios.
is
associated
across
variation
produce
the
section
on
surface
oceanography).
cartoon
(Figure
6)
how the annual
thermocline
and changes
in heat
could modify
assemblage
Since
maximum productivity
the
with
nutrient
thermocline,
in thermocline
alternation
in
dominance.
When
the
flux
the
seasonal
depth can
assemblage
thermocline
ascends,
assemblages
adapted
to
lower
temperatures
will
flourish.
The
reverse
thermocline
occurs
descends.
when
the
A
Mcintyre
et
Table
4.
al.'
Surface
The
Four
Water
Response
Foraminiferal
Assemblages
That
Control
Estimated
SST and the
Dominant
Species
That
Define
These Assemblages
in the
Equatorial
Cores
AS SEMBLAGE
FORAMIN
IFERA
to
Orbital
Forcing
assemblage
(Figures
7a,b,c)
that
monitors
this
response
and the
estimated
SST (Figure
3) should
resemble
each other.
Spectral
analysis
highlights
the changing
response
of each assemblage
along
this
equatorial
traverse
and
indicates
which assemblage
is
responsible
Gl obi gerinoi
Tropical
des
ruber
(white)
sacculifer
(no
sac)
Globigerina
falconensis
Globorotalia
inflata
Neogl oboquadrina
Divergence
dutertrei
Globorotalia
Pulleniatina
menardii
obliquiloculata
Globigerinoides
sacculifer
(no
that
for
the
characterize
spectral
each
core's
periods
SST
signal
(Figure
8).
In the eastern
core RC24-16
(Figures
7c and 8), the
tropical
and transitional
assemblages
each contain
the 23-kyr
period
and are coherent
with
each
other
and in phase,
but with
the
opposite
sign.
The divergence
assemblage
shows minimal
variability
and lacks
significant
spectral
power
at the primary
orbital
periods.
The
subpolar
assemblage
contains
significant
spectral
peaks,
but the
power is very
low.
The signal
Globigerinoides
Transitional
29
sac)
Globigerinoides
sacculifer
(with
Globorotalia
sac)
turnida
Globigerina
bulloides
Globigerinita
glutinata
Neogl oboqua dri n a
pachyderma
( dext ral
Neogl oboqua dri n a
pa ch yderma /
Subpolar
)
Tr0pical > Divergence > Transiti0naI > Subp0iar
dutertreii
16
22
28
intergrade
Terminology,
assemblages,
component
species
are
from
[1976],
Molfino
et al.
[1982],
this
paper.
and
Kipp
and
+DIVERGENCE
Tropical = Divergence > Transitional > Subpolar
16
this
process
an analogue
for
the past?
During
times
of
climatically
induced
increase
in
wind-forced
divergence,
does the
divergence
assemblage
predominate
(Figure
6b)?
Alternatively,
or in
concert,
a change in heat advection
into
the equatorial
region
would
effect
the biota;
for example,
a
22
28
Is
decrease
in
heat
would
favor
an
increase
in assemblages
adapted
to
cooler
temperatures
(Figure
6c).
Both
effects
decrease
from
east
to
west along the equator
today.
If the seasonal
forcing
of
thermocline
geometry and divergence
intensity
model,
is
then
the
the
correct
time
descriptive
series
of
the
+DIVERGENCE
- HEAT ADVECTION
Tropical '"Transitional _>divergence > Subpolar
Fig.
6.
Relative
dominance
of the
foraminiferal
assemblages
in the
equatorial
Atlantic
for three
possible
oceanographic
configurations.
These cartoons
are
(a) modern,
(b) increased
divergence
and thermocline
shallowing
due to
increased
trade
wind forcing,
and
(c)
with
increased
increased
cool
water
divergence.
advection
30
Mcintyre
et
al.-
Surface
Water
Response to Orbital
V25-59
TROPICAL
TRANSITIONAL
1.0
0.4
0.0
•1
DIVERGENCE
0.5
Forcing
0.1
SUBPOLAR
0.6
0.0
0.4
20-
40
60
AGE
k.a.
80
• oo
120
• 40
Fig.
7.
V30-40
range
Foraminiferal
assemblage
and (c) RC24-16.
of 0 to 1.
character
of
these
The
latter
assemblages
does
dependent
solely
record
is
not support
a model
on divergence
the lack of
assemblages,
RC24-16
not
this
for
the
reason
for
period
in the other
when the V30-40
and
records
were
truncated
to
an
equivalent
age range
(0-128
ka),
they still
produced
significant
23kyr spectral
power.
This analysis
demonstrates
that
the
character
of
the
time
signal
of each assemblage
often
markedly,
intercore
the 23-kyr
period
is not
series
series
changes,
and that
unique
to
for
represent
three
two
variability
for signal
genesis.
All
four
assemblages
in V30-40
(Figure
7b) contain
significant
23-kyr
spectral
power (Figure
8).
In V2559, only the divergence
assemblage
contains
significant
spectral
power
at 23 kyr
(Figures
7a and 8).
The
short
time
X axes
(a)
Factor
cores
V25-59,
(b)
loadings,
contain
with
a
well-defined
23-
kyr periods,
but the assemblages
responsible
and the percent
variance
accounted
for by each assemblage
change intercore.
Cross-spectral
analyses
of
assemblages
with
estimated
SST in
each core statistically
determine
which assemblages
exert
primary
control
of
estimated
SST.
In
statistical
terms,
a comparison
of
the SST and assemblage
signals
should
yield
a high r and,
for
shared periodicities,
high k and an
in-phase
relationship
(Table
5).
In
RC24-16
the
tropical
In
transitional
and
assemblages
V30-40,
all
control
assemblages
SST.
exert
control,
but the divergence
assemblage
has the lowest
correlation
with
Tc.
In V25-59,
accounted
for by the 23-kyr
period
(Figure
8) in the transitional
assemblage
decreases
from 36% in
relationships
are not well
defined.
Only the divergence
assemblage
contains
a significant
spectral
peak,
23 kyr,
but it has low
coherency
with both Tc and Tw
signals.
The explanation
for this
RC24-16
lack
any one assemblage.
west,
the
fraction
to
28%
in
From east
of variance
V30-40
V25-59,
while
in the
assemblage
the 23-kyr
increases
in
V30-40
to
to
8%
in
from
6% in
RC24-16
to
46% in V25-59.
of
stable
divergence
period
to
All
23%
western
stability
late
in
definition
thermal
involves
character
the
of
the
equatorial
Atlantic.
When
is destroyed,
as occurred
isotope
stage
6, the
Mcintyre
et
al.-
Surface
Water
Response
to
Orbital
Forcing
v3o-4o
TROPICAL
0.2
TRANSITIONAL
1.0
0.0
b
DIVERGENCE
0.7
I
o
I
0.0
SUBPOLAR
0.7
,
,
31
,
0.0
I
20
40
60
80
lOO
120
AGE
k.a. •4o
16o
180
2oo
220
24o
260
Fig.
foraminiferal
momentarily
character
in
the
7.
assemblages
assume the assemblage
and enhanced
response
seen
eastern
basin
cores.
in
divergence
conjecture
one
the
in the west.
that
there
is
climate/ocean
modulates
vary
the
mixed
east
the
atmospheric
chemistry,
carbon dioxide,
is only
documented
for part
of this
time
interval
[Barnola
et al.,
1987],
include
changes
to
constant
which
We
more
inconstant
[Eddy,
1983],
but for
which
no documentation
or acceptable
theory
exists
to derive
a history
mechanism
precessional
ecology
of
history
of
specifically
we
refrain
from adding this
control
to
our study.
Nor is it possible
to
In summary,
the assemblages
responsible
for the 23-kyr
period
change from tropical
and
transitional
(continued)
than
that
forcing
to
equatorial
commensurate
Plausible
layer.
in
we
with
causes
the
know
our
of
solar
to
be
records.
the
observed
variations
in the mixed layer
of the
signals
could
be (1) insolation
in
situ,
(2) monsoon modulation
of
insolation
forcing,
(3) trade
wind
modulation
of insolation
forcing,
and (4) changes
in heat
content
of
water
advected
from high
latitudes.
These will
be the topics
we discuss,
in sequence,
in the ensuing
subsections.
First,
insolation
in
situ
is shown to be unsatisfactory
as an explanation
for the documented
responses.
Second,
the operation,
in concert,
of climate
forced
equatorial
Atlantic.
the
variations
SIGNAL
CAUSE
A significant
assemblage
part
of
variation
is
in narrow bands coherent
centered
periods,
translated
SST and
with,
and
on, the primary
orbital
dominated by precession.
The mystery resides
induced
the
concentrated
variations
into
heat
in how orbitally
in
insolation
and
are
biotal
Because
in
monsoon/trade
wind
32
Mcintyre
et
al.:
Surface
Water
Response to Orbital
RC24-16
TROPICAL
C
TRANSITIONAL
1.0
0.0
0.0
0.8
Forcing
DIVERGENCE
0,1
SUBPOLAR
0.7
0.0
0.5
0
20
40
60
80
100
AGE 12o
140 -
160
180
200
_
220
240
Fig.
intensity
and advection
is
as an acceptable
mechanism
explain
the signals.
Insolation
in
7.
presented
to
(continued)
and
(5)
correlation
situ
the
Precession
is
the
dominant
centered
in most
suggests
on the
23-kyr
period
insolation
in
r=0.53
k=0.92
Tc lags
with
i.e.,
interval
level.
beneath
autumn (August-October)
This suggests that the
much
of
its
heat
within
10øS latitude.
these
cores
analyzed
signals
latitude
were
obtains
the
zone
The Tc signals
cross-spectrally
with five
integrated
band.
SEC
0 ø-
from
insolation
over this
These
insolation
signals
are the (1) annual,
(2)
boreal
summer half
year
(AprilSeptember),
(3) boreal
summer
quarter
year (May-July),
(4) boreal
winter
half
year
(October-March),
phasing
Tc or the phase
it exceeds the
confid•_•ce
The
for
by
•1.9 kyr.
All other insolation
signals
are either
negatively
acceptance
SEC.
and,
and phase
insolation
To determine
if the 23 kyr period
is a direct
response
to local
insolation,
we have used cores RC2416 and V30-40,
which are sited
the
year
This negative
that
correlated
too large,
situ.
quarter
Tc,
period,
-31 ¸ .
indicates
signals
(SST, assemblages)
a direct
response
to
winter
with
23-kyr
equals
insolation
forcing
in low latitudes.
The presence
of strong
spectral
peaks
boreal
(November-January).
The boreal
summer (austral
winter)
half-year
insolation
has the highest
positive
The phasing
insolation
July)
is
of
at
the
boreal
-48 ¸,
while
boreal
June through
When
we
refer
signal
in the
summer
that
refers
to
boreal
summer
this
a
for
(midmonth
September).
summer
discussion,
compilation.
signal
boreal
documented
on
We compiled
signal
for
boreal
ensuing
(May-
is 43¸.
appropriate
summer
midmonth
to
95%
quarterly
for
insolation
to yield
the
SST response
is centered
meteorological
summer.
a quarter-year
insolation
calendric
is
when
This
cross-
it
Mcintyre
et al.'
Surface Water Response to Orbital
TROPICAL
TRANSITIONAL
DIVERGENCE
0.10
0.16
Forcing
33
SUBPOLAR
0.0323:46%
0.04
V25-59
0.02
0.08
0 ß05 ß
0.00
0.02
0.01
0.00
0.00
0.00
V30-40
:38%
00:37%
23:29%
100:30%
100:20%
23:23%
0.3
O.
12
0.0,
,
,
0.09
, -, 0.00__,
0.44 100:31%
0.4
, •
, 0.00
23:36%
0.09
,
,
0.00
0.2
0.0.5
0.0
0.1
RC24-16
0.02
0.01
0.1
ß
100:15%
0.2
0.0
,
23:25%
0:20%
0.00' 0.0
, ,•
i
0.03-
0.10 75:29%
:29%
0.22
,
V
0.00
[ '• 0.00
,
0.2
0.0
0.1
0.2
I
0.0
,
0.1
•
0.2
FREQUENCY
(cycles/ka)
Fig.
with
8.
the
Spectra for each assemblage plotted
in Figures
dominant period(s)
indicated
and their
percent
accounted
spectrally
for
within
analyzed
signal
23-kyr
yields
period,
equals
-0.4
¸.
each
to
the
An
The
alternate
is to
isolate
spectrum.
V30-40
r=0.623
and, for the
k=0.921 and phase
same statistics
for RC24-16 are r=0.606,
and phase equals
3.6 ¸ .
check
from
both
filtering
techniques,
content
controlled
from
the
September
mean
the
by
mixed
in
situ
for
layer,
and compare
of
insolation
June
at
core
heat
insolation
in
summer.
V25-59,
which
does
not
from
contain
any significant
power at the 23-kyr
period,
or by the thermodynamics of
the
system.
RC24-16
power
10øS
in-
is
This simple answer is not
supported
either
by the results
by
The
the
periodic
signals,
window [Blackman and Tukey, 1958]
and compared to the same component
filtered
SST, a proxy
of
boreal
them.
The 23-kyr
component in V3040 was filtered
using a Hanning
through
They are in phase.
correlation
a discrete
component
latitude.
simplest
answer based on these
phase relationships
is that
equatorial
k=0.924,
on
7a, b, and c
variance
equator
at
V25-59,
Tw signals
the
the
41-kyr
annual
V30-40,
contain
period.
insolation
and
spectral
At
the
range
3•
Mcintyre
Table
5.
Correlation
et al.-
and
Surface
Water Response to Orbital
Forcing
Statistics
Versus
Cross-Spectral
SST Signals
of
Assemblages
,
Core
Assemblage
Equation
Correlation
Coherence
Coefficient
Phase
k
r
V25-59
V30-40
RC24-16
All
means
tropical
Tw
0. 425
0. 774
141 ø
transitional
Tc
Tw
Tc
0. 666
0. 024
0.876
0. 480
0. 714
0.839
43 ø
-77 ø
-170 ø
divergence
Tw
0. 484
0. 402
Tc
0.178
0.101
subpolar
Tw
0.139
0.334
Tc
0.775
0.698
tropical
Tc
0. 827
0. 979
transitional
Tc
0. 772
0. 959
170 ø
divergence
subpolar
tropical
Tc
Tc
Tc
0. 413
0. 782
0. 844
0. 960
0. 930
0. 900
167 ø
17 6ø
7ø
transitional
Tc
0.952
0.975
-179
divergence
subpolar
Tc
Tc
0. 377
0. 679
0. 441
0. 650
-17 5 ø
16 ø
k and phase values
that
the assemblage
refer
lags
to the
SST.
annual
contains
period.
these
Tw
insolation
significant
example,
the 41-kyr
phase
insolation
signal
equals
61ø.
insolation
warming
oceanic
mixed layer
temperature
in
situ.
of
the
diffusion
through
would
range
SST
phasing
gives
an in-
the
documented
of
>9øC and
control
through
seasonal
with
the
mixed-
time.
variation
boreal
and lags
that
in
the
the
mixed-
modern
correlation
summer
insolation.
In boreal
summer, SST
is lowered
not by a change in
received
insolation
but by the
shallowing
of the thermocline.
This
is a process
that
is controlled
by
changes
in the velocity
of the trade
winds forced
by insolation.
The
cooler
water
that
upwells
during
divergence
is brought
to the equator
by advection
from high latitudes.
the
and the annual range to
[Schneider
and Thompson,
presumed
forcing
signal
of boreal
summer insolation
and the response
of SST, the Tc signal,
indicates
changes
ø
phase
but regular
leads
insolation
suggest
mechanisms
ø
-7 ø
nonlinear
Thus
layer
of
-170
seasonality
other
The
of
1988]
a
layer
divergence
is
analogue
to explain
to
1979;
Kutzbach
and Otto-Bliesner,
1982;
Kutzbach
and Gallimore,
A detailed
comparison
of the
in
the small
of Tc to
0¸
the
reduce
is
value.
changes
The weak spectral
due
to
insolation
The
thermal
inertia
surface
ocean
and
the
<0.25øC
<0.1øC
phase
the 41-kyr
period
in
in all
three
cores
is
not
there
ø
-89 ø
A negative
relationship
of signal
which,
when averaged,
only the obliquity,
41-kyr,
Cross-spectral
analysis
of
signals
with
annual
at 0 ¸ yields
no
relationship;
for
for V30-40,
r=0.06
and,
for
period,
k=0.05
and the
power
found
at
the Tw signals
seasonal
at
period.
that
is small,
for,
although
the Sun at
zenith
is placed
over each
hemisphere
for
half
the year,
the
zenith
angle
over the equator
varies
only slightly.
However,
this
oscillation
cancels
precession;
thus
the
23-kyr
78 ø
-158
We explore
these
mechanisms
in
the
alternative
next
three
subsections.
.
Monsoon
The
RC24-16
Tc
signals
may record
in
V30-40
changes
and
in
Mcintyre
et
al.'
Surface
Water
Response
thermocline
dynamics and divergence,
modulated
by trade
wind velocity.
The modern annual
cycle
is the
analogue.
One manner of altering
trade
wind velocity
in the
equatorial
Atlantic,
over time
intervals
longer
cycle,
is
intensity.
via changes in monsoon
Monsoons decrease
the
zonal
trade
than
winds
in
(austral
winter)
convergence
over
land
the
Table
is
From
of
gradient
minus
20os
value
for
greater
this
mid
the
the
positive
more
lags
intense
to
the
boreal
summer
20øN.
These
the
monsoon.
by only
two
7.4 ¸,
RC24-
and
these
two
both
that
by minima.
in
boreal
to
the
maximum
with
and
and
for
time
the
meridional
and
component
of
[COHMAP Members,
produce
a marked
monsoon.
[1982],
[1983],
Street-Perrott
[1985]
demonstrated
that
the maximum
monsoon climate
and attendant
wet
periods
of Africa,
centered
on 9 ka,
monsoon
effect,
then
time
perihelion
is
aligned
boreal
summer,
there
should
be
a seasonality
minimum in SST.
The
time series
of V30-40
encompasses
0260 ka; within
this
interval,
this
alignment
of perihelion
with boreal
summer occurred
11 times,
and there
are 10 well-defined
seasonality
minima
(Table
6).
The chronologic
in the core is +2.5 kyr.
but two of the data pairs
within
this
envelope.
discrepancies,
>0.9
summer by a
North
African
Otto-Bliesner
and Roberts
Kutzbach
One
trade-wind
strengthened
Kutzbach
and
Street-Perrott
the
kyr,
data
and modeling
convergence
over
increased
resolution
velocity
outside
+2.5
decreased
the zonal
trade
wind velocity
1988].
This
should
All
of
fall
lag slightly
behind maximum
insolation
at 11 ka (perihelion
aligned
with June).
For this
at regular
way to minimize
seasonality
is
diminish
divergence
by reducing
values
envelope,
16, and these signals are punctuated
component
-
29.5
63.5
81.5
101
122 .5
151.5
174 .5
194
224
-
cores.
every
function
V30-40
Only
resolution
the
~0.5
change
a
zonal
,
diminution
in equatorial
divergence.
If this
scenario
is an analogue
for
SST seasonality
monitors
the
in annual
SST range,
primarily
intervals
ka
RC24-16
59.5
82
102
122
148.5
173.5
194
223
239
Africa
of
kyr.
This
is less
than the
computational
error;
thus the two
signals
are in phase.
There
is further
evidence
linking
the SST signal
with monsoon control.
in
Minima,
11
interval
indicate
signals
of the 23-kyr
are strongly
coherent,
and the V30-40
Tc filter
TC
Basin
32
60
82
104
126
149
175
196
220
241
The
gradient
at
of
ka
with
Two Cores
insolation
and
insolation
the
Seasonality
insolation
by comparing
the 23-kyr
component
obtained
by filtering
the signals
filtered
period
k=0.98,
Eastern
When
Summer
Maxima)
in
V30-40
a function
!
Tc
the
Times
Boreal
Monsoon
Minima
11
insolation
gradient
by 26 ¸
~1 6
kyr.
Further
insight
can be gained
insolation
African
of
in
,
Cross-spectral
analysis
of V30-40
Tc
to this
insolation
gradient
yields
r=0.463.
Both signals
have strong
23 kyr periods
which are coherent,
k=0.91,
but the Tc signal
lags the
V30-40
Occurs
June,
20øN and 20øS (20øN
month
Comparison
summer
June-September).
the
north,
is
between
35
Perihelion
of received
insolation
during
boreal
summer, both annually
and on orbital
time
scales
[Riehl,
1979;
Kutzbach
and Street-Perrott,
1985;
Kutzbach
and Otto-Bliesner,
1982] .
One
measure
6.
(North
by increased
the North African
intensity
Forcing
Seasonality
mass.
Monsoon
Orbital
Perihelion
annual
boreal
to
and
significant
is
the
Despite
correlation
fall
these
is
statistically
at
the
RC24-16
seasonality
similar
correlation
95th
percentile.
minima
(Table
show a
6).
The correlation
of seasonality
minima in equatorial
Atlantic
SST
with perihelion
aligned
with boreal
summer is reinforced
by evidence
that,
at these
times,
productivity
36
Mcintyre
et
al.'
Surface
time
of perihelion
in
with
Equatorial
surface
oceanography
is
directly
controlled
by the trade
winds.
The strength
of these winds
is a function
of the seasonally
low-latitude
oceanic
and
continental
anticyclones
and the
monsoonal
cyclones
over Africa.
The
surface
water
response
is in
Sverdrup
balance
for times
greater
than
120 days [Philander,
1979;
Reverdin,
1985] .
While
the
geography
of continent
and ocean is
effectively
invariant
over the late
Quaternary,
both insolation
and
albedo
varied
and thus changed
atmospheric
forcing
of equatorial
oceanography.
This is proven
by the
SST signals
already
described.
But
are these
signals
indicative
of
changes
in trade
wind orientation
or
trade
wind intensity,
or both?
Signal
variations
in V30-40
and
RC24-16
are due to changes
in the
abundance
of species
adapted
to cool
waters,
reflected
in the Tc and
seasonality
signals.
Since
insolation
in situ
does not produce
the SST time
series
by direct
absorbance,
the simplest
explanation
is variation
in oceanographic
divergence.
This presumes
that
annual
response
to trade
wind
control
is a model applicable
to
orbitally
forced
variations.
The
stronger
the Hadley
circulation,
the
more intense
the zonal velocity
of
the
trade
winds.
This
leads
to
greater
Eckman drift
and increased
equatorial
divergence.
For times
maximum Hadley
development,
there
a
boreal
[1988]
Winds
intensified
decrease
in
the
meridional
wind
show
maximum
at
If
the
function
for
the
CLIMAP
models
the
last
glacial
[1981]
have
maximum by
.
Global
been
used
atmosphere
of
the
climate
to
last
simulate
glacial
simulation
their
ka.
Members
for
This
SST
variations
of
and
are
a
wind-modulated
divergence,
then the tropical
and
divergence
assemblages
should be the
best
indicators.
Their
response
should
be
antithetic,
with
the
divergence
assemblage
characterizing
times of high seasonality
and cold
Tc
all
values.
cores.
This
is
not
In
RC24-16
the
the
case
in
divergence
assemblage,
which lacks
significant
23-kyr
power,
is not
correlative
with
seasonality,
Tc,
and the tropical
assemblage
signals,
all
of which do have significant
23kyr periods
(Figure
7).
In V30-40
the divergence
assemblage
contains
the 23 kyr period
and varies
antithetically
with the tropical
assemblage.
V25-59
should
be too
far
west
to
discussion
divergence
one
in
23-kyr
similar
be
this
core
7).
It can
transitional
assemblages
will
all
included
in
of divergence,
assemblage
is
with
a well-defined
be argued
that
the
and subpolar
also
respond
to
divergence
because
lower
water
temperature.
three
of these
assemblages
respond to increased
increased
abundance,
in
a
yet the
the only
period
and its signal
is
to that
of V30-40
(Figure
summary,
documented
COHMAP
in
documented
increased
of
is
summer.
18
seasonality
This
been
Forcing
circumstantial
evidence
supports
trade
wind modulation
of divergence.
should
has
Orbital
9 ka that
the meridional
component
was stronger
while
the zonal
component
was weaker
relative
to
both today
and the last
glacial
vector,
with
a concomitant
increase
in aridity
in Africa
[Riehl,
1979].
scenario
to
show trade
wind zonality
as strong
or stronger
than today,
a time when
perihelion
is aligned
with boreal
winter.
A corollary
to this
exists
in the modeling
of the youngest
interval
when perihelion
was aligned
boreal
summer the eastern
equatorial
Atlantic
lacked
a well-developed
divergence
signature.
In essence,
seasonal
divergence
collapses
when
boreal
summer and perihelion
are
aligned.
Trade
Response
maximum
[Gates,
1976;
Kutzbach
and
Guetter,
1986;
Manabe and Hahn,
1977;
Manabe and Broccoli,
1985;
Schneider
and Thompson,
1979] .
All
was lower,
indicating
a diminution
in seasonal
divergence.
Mix et al.,
[1986]
demonstrated
that
for the
youngest
Water
be
and
the
are
23-kyr
If
divergence
by
their
signals
They are not
correlative.
RC24-16
this
in
V30-40.
component
In
in
the
divergence
assemblage
becomes
increasingly
significant
from
to west,
relative
to other
east
assemblages
while
within
each
core,
Mcintyre et al.'
Surface Water Response to Orbital
37
Forcing
Berger,
1978; Flohn,
1978].
This
configuration
is characterized
by an
equatorward
expansion
of Antarctic
sea ice and the polar
anticyclone.
oceanographic
divergence,
as
documented by modern oceanography,
has the opposite
trend.
Windmodulated divergence
is a partial
explanation
for the documented
This,
in
turn,
causes
an equatorward
response.
translation
westerlies
Advection
Drift.
As the prevailing
wind belts
move north,
so will
the northern
boundary
of the subpolar
water
mass.
If it impinges
on the tip
of South
Africa,
cold
surface
water
will
be
shunted
into
the Benguela
Current,
The
documented
and biota
variations
may reflect
advected
heat.
in
SST
modulation
The
surface
by
waters
of the equatorial
Atlantic
are part
of the mass transport
path
from
south to north
which begins
with
cool
surface
waters
of the Benguela
Current,
crosses
the equatorial
zone
in the SEC, is deflected
northward
in
the
north
Brazil
and
Guiana
currents,
contributes
to the Gulf
Stream,
and finally
drifts
and
anastomoses
into
numerous paths
in
the high
latitudes
of the North
Atlantic.
Thus the equator
is the
border
across
which hemispheric
exchange
of heat
occurs.
Because
small
the
ocean,
South
any
Atlantic
marked
is
annual
a
or
long-term
variation
in surface
oceanography
must be the result,
in
part,
of external
forcing.
There is
a
net
flow
of
South
into
the
evaporation
within
the
runoff
is
Therefore
surface
water
North
exceeds
basin,
low
[Hoflich,
the
South
flow
from
the
precipitation
and
continental
Atlantic
the
has
a
While there
the extent
of
North
Atlantic
[Merle,
1980;
Philander
and
Pacanowski,
1986a],
it
is clear
that
the surface
water
budget
is balanced
by inflow
Indian
The
the
coldest
highest
correlate
and
from
Pacific
and
oceans.
austral
winter
concurrent,
winter
Tc
values
seasonalities
with
times
i.e.,
is
most
precessional
and
the
in our cores
when aphelion
solstice
when
intense
component
are
austral
because
of
the
insolation
is lowest.
Since
the signal
of the
precessional
component has the same
geometry
for the same month at all
latitudes,
boreal
summer also
has an
insolation
low.
This configuration,
minimum
summer
considered
glaciation
insolation
and
for
austral
to
be
both
winter,
conducive
[Milankovitch,
boreal
is
to
1941;
warm
water
will
be barred
Atlantic.
The
in
heat
waters
content
along
corridor
from
should
Wind
Agulhas
entry
into
the South
resulting
diminution
of
the
the
West
the
mass
be
surface
transport
recorded
in
the
equatorial
mixed-layer
SST.
The transitional
and subpolar
assemblages
have their
optima at
~40øS [Kipp,
1976; Molfinoet
al.,
1982].
If the equatorial
SST and
seasonality
signals
contain
an
advective
component,
it should
be
recorded
by these
assemblages.
In
RC24-16,
the transitional
assemblage
contains
a strong
23 kyr-period
and
varies
antithetically,
r=-0.90,
with
the tropical
assemblage.
It is in
phase with
seasonality,
r=0.94
and,
for the 23-kyr
period,
k=0.99
and
the
1984].
negative
water budget.
is some question
about
return
from
Atlantic,
while
of
the
mid-latitude
which
drive
the
phase
equals
0.0 ¸.
In V30-40,
both the transitional
and subpolar
assemblages
have well-defined
23 kyr
periods
and similar
relationships
to
the tropical
assemblage
and
seasonality.
This may indicate
variation
in advected
heat,
with
minima being
recorded
by the
dominance
of these
assemblages.
CLIMAP published
maps of modern
and last
glacial
maximum zooplankton
biogeography
[CLIMAP Project
Members,
1981,
Maps la and b];
those
for the South Atlantic
depict
foraminiferal
assemblages.
At the
last
glacial
maximum,
the
transitional
and subpolar
assemblages
characterize
the eastern
boundary
current
(Benguela),
and the
transitional
assemblage
dominates
the eastern
portion
of the
equatorial
Atlantic.
N.G. Kipp
(personal
communication,
1988)
has
subsequently
improved
foraminiferal
biogeography
for the South Atlantic
during
the last
glacial
maximum and
the result
is shown in Figure
9.
At
38
Mcintyre
60*
FI
I
I
$0'
I
!
I
I
I
al.'
Surface
O*
I
•.-'..."•2.'..:.
et
I
'--
I
I
I
I
I
Water
South
I
I
I
I
,
Response
Atlantic
considered
!iii!>•%.:.:.:.:..•"%::•::.'.
O*
thermocline
water
input to the
component of
Today
the
South Atlantic
this
global
circulation
warm
Indian
Ocean
water
Evans,
1986].
Gordon
[1986]
emphasizes
that
this
is unique for
an eastern
boundary
current,
in that
the Benguela
Current
begins
with a
significant
heat addition
at high
and
mesoscale
latitudes.
[1986b]
ß
'.
circulation
1986].
45*
i'.
•"1,5'
;50*
35øS could
This
affect
from
mass
[Olsen
and
and Pacanowski
flux
across
the
heat
transport
north.
input
thermocline
I I
Africa
in
eddies
Philander
that
heat
note
further
II I I I I I I I I I I I I I I I I
is
a global
passing
westward
around
the Agulhas
retroflection,
O*
•'.•.:i!iii!
....
of
$0'
flow
"
Forcing
water
component
1985,
involves
Orbital
thermocline
one
[Gordon,
1,5'
to
of
Indian
water
is
Ocean
limited
to
a
narrow passage between Africa
and
the southern
hemisphere
subtropical
convergence,
whose mean latitude
is
I• POLAR
•'•lSUBPOLAR
• SUBTROPICAL
41os [Lutjeharms,
1985] .
The
MARGIN
i• GYRE
:":'•TRANSITIONAL
position
of this
convergence
is
controlled
by the West Wind Drift,
a
Fig.
9.
Maps of the dominant
product
of southern
hemisphere
biogeographic
zones of foraminiferal
westerly
forcing.
The width
of this
assemblages
(top)
for the modern
passage
varies
between
approximately
world
and (bottom)
the last
glacial
60*
50'
O*
maximum after
CLIMAP Project
Members
[1981]
and Molfino
et al.
[1982] .
Note
that
the
assemblage
transitional
dominated
the
equatorial
Atlantic
glacial
maximum.
eastern
during
the
last
the last
glacial
maximum the
equatorward
translation
of cooler
waters,
optimal
for the transitional
assemblage,
supports
our contention
that
variation
in
advection
of
heat
is one viable
explanation
for the
assemblage
signals
in the equatorial
cores.
These patterns
cannot
be
explained
by a general
oceanic
cooling,
because
the zonal
gradient
of estimated
SST along 30os is 8oc
for
the
modern
versus
11øC
for
the
glacial.
This steeper
glacial
gradient
is due to lower SST values
in the east
[CLIMAP Project
Members,
1981,
Maps 3a and b],
commensurate
with
the
water
The
water
euphotic
divergence
water
mechanism
advection
from
of
in
that
increased
glacial
rises
cold
times.
into
the
zone,
when equatorial
occurs,
is thermocline
the
South
Atlantic.
3¸
and
7¸
of
latitude
as
southern
hemisphere
convergence
migrates
impetus
of seasonal
interannual
climatic
The
wider
the
the
advection
the
subtropical
under the
and small
variations.
passage,
the
into
Atlantic.
the
greater
CLIMAP Project
Members
[1981]
showed that
during
the last
glacial
maximum both the southern
hemisphere
subtropical
convergence
and the
southern
hemisphere
subantarctic
front
were positioned
at lower
latitudes
south
evidence
diminution
into
the
of
suggests
of
Atlantic
flow
Africa.
The
a marked
from
Ocean
the
Indian
[Prell
et
al.,
1980].
We have reexamined
RC11-86
(35ø47'S,
18ø27'E),
positioned
southwest
of Cape
Agulhas,
which has SST values
core
3.8oc
colder
at the last
glacial
maximum
than today
[CLIMAP Project
Members,
1981].
The key to deciphering
the
response
of this
region
to climate
change lies
in a comparison
of the
modern versus
glacial
foraminiferal
assemblages.
In RCll-86,
analysis
of core top sediments
representing
modern
conditions
shows
that
the
Mcintyre
et
al.'
Surface
Water
Response
to
Orbital
Forcing
39
v.•o-ao T• (øc)
-2.0
0.0
2..0
-2.0
0.0
2.0
-2.0
0.0
2.0
-2.0
0.0
2.0
......
SP[C•AP
STACK
V•0-97
RC11-120
RC11-120
Fig.
10 Time series
plots
of 23-kyr
filter
signals
of V30-40 Tc
(solid
curves)
versus the northern
hemisphere
signals
of the SPECMAP
oxygen isotope stack and V30-97 Tc and the southen hemisphere signals
of Cycladophora
symbols ) .
transitional
72% and the
foraminiferal
last glacial
indicative
warm
surface
the
dominates
assemblage
32%
foraminifera
total
the
accounts
for
RCll-
86.
Today, this
degree of cold
assemblage
dominance
is present
between the southern
hemisphere
subtropical
convergence
and the
southern
hemisphere
subantarctic
front
in
the
West
Wind
Drift.
If
we
use modern biogeography
as an
analogue
for the glacial
surface
water
scenario,
then this
subtropical
convergence
was
displaced
equatorward
and the West
Wind Drift
collided
with
South
Africa.
This
would
have blocked
Agulhas
advection
into
the
South
Atlantic
colder
the
subpolar
reduction
consider
assemblage
reflects,
in
that
surface
Current,
heat
the
signal
in part,
waters
into
a
in RC24-16
variation
We
higher
southern
latitudes.
PHASING
We have presented
evidence
that
SST and assemblage
signals
are the
product
of variation
in equatorial
divergence
and heat advection
from
higher
southern
latitudes.
The
relationship
of the equatorial
response
to a specific
hemisphere's
insolation
forcing
can be tested
using
the precession
component,
which dominates
these equatorial
time
series.
We
cannot
make
a
similar
study using the obliquity
component,
because
it is poorly
defined
in our equatorial
signals.
We have
10)
compared
and phase
This
the
is
length
(0-257
record
heat
(Figure
comparison
filtering
in
from
with
23-kyr
period
with 23-kyr
signals
from both hemispheres
and present
the results
in both the time
(Figure
producing
content.
transitional
advection
(curves
in the V30-40 Tc signal
periods
in high-latitude
and would have injected
Benguela
RCll-120
are
50% and
in
core
RESPONSE
water
subpolar
at
transitional
of
Tw from
fauna.
During the
maximum, assemblages
instead,
assemblage
and
assemblage
accounts
for
tropical
for 20% of the
of
absent;
davisiana
operation,
11)
domains.
constrained
by
ka) .
The
which
extracts
a discrete
rhythm, uses a weighted
moving average.
The V30-40 23-kyr
40
Mcintyre
et
al.'
Surface
Water
filtered
that
of
and the
0o
-27 ø
-50 ø
Response
to
Orbital
Forcing
signal
consistently
leads
the SPECMAP isotope
signal
V30-97
Tw (Figure
10).
The
phases for this
comparison
presented
in a phase clock
11)
and
show
that
V30-40
are
(Figure
leads
oxygen isotopes
by •2.7
kyr and V3097 SST by ~6.3 kyr.
This phasing
precludes
a major forcing
role
for
the high-latitude
northern
hemisphere
in the equatorial
_96 ø
Atlantic.
-164 ø
Fig.
11.
The phasing
between
the
23-kyr
filtered
signals
(figure
10)
and
minimum
winter,
at
insolation
perihelion
the
in
in
0 ¸ position)
austral
December
plotted
(line
on a
phase clock.
Time is clockwise;
thus
all
signals
lag that
of insolation
by the values,
in degrees,
printed
next
to each signal.
filtered
signal
therefore
begins
at
66.5
ka and ends at 193 ka (126 kyr
long),
which encompasses
greater
than five
precessional
cycles.
While
the records
encompass
different
lengths
of time,
we have
extracted
their
23-kyr
filter
signals
from the same record
length
and age interval
and used the same
statistical
parameters
(lag,
window,
etc.)
as
for
V30-40.
Northern
hemisphere
control
is
assessed
using
two signals.
We have
computed
the 23-kyr
filtered
signal
from the SPECMAP stack
of oxygen
isotopes
[Imbrie
et al.,
1984] which
predominantly
records
volumetric
change in northern
hemisphere
continental
of
ice.
northern
contained
The
second
hemisphere
in
the
measure
climate
estimated
SST
latter
artifactual
latitudes
values
have
variation
in
Antarctic
hemisphere
sea ice,
albedo
the
southern
equivalent
of
hemisphere
continental
northern
areal
extent
of
ice.
Fig.
12.
(Opposite)
Paleoisotherm
reconstructions,
in degrees
celsius,
of the equatorial
and South Atlantic
which schematically
delineate
the
response
to posited
mechanisms.
In
all
figures
the solid
arrows
are
surface
winds,
and the open arrows
surface
currents.
Figure
12a
represents
June perihelion
at 9 ka,
characterized
by high austral
winter
insolation.
The
maximum,
is
and
minimal.
monsoon
is
equatorial
The
SST
at
its
divergence
contours
are
derived
from Mix
[1985],
CLIMAP
Project
Members,
[1981,
1984],
and
this
paper.
The southward
displacement
of the southern
hemisphere
subtropical
convergence
(open dashed line)
permits
the
Agulhas
to export
warm surface
waters
into
the Atlantic.
Figure
12c
depicts
the seasonality
for
time,
and the solid
circles
this
are the
is
additional
in
reconstruction.
Figure
12b
represents
December perihelion
at
136 ka, characterized
by low austral
winter
insolation.
Divergence
is
strong
and the equatorward
displacement
of the southern
hemisphere
subtropical
convergence
core V30-97
(41ø00'N,
32ø55.5'W),
which contains
significant
spectral
power at the 23-kyr
period
[Ruddiman
and Mcintyre,
1981,
1984] .
V30-97
Tw, rather
than Tc, is used because
the
Southern
hemisphere
control
is
assessed
using
signals
similar
to
those
of the northern
hemisphere.
Unfortunately,
there
are no time
series
which directly
measure
the
an
lower
limit
at high
[Ruddiman
and Mcintyre,
has
cores
lowered
heat
used
in
advection
this
in
the
1984].
Benguela
Current.
seasonality
for
Figure
this
time.
The phase relationship
of V30-40
Tc to the northern
hemisphere
is
critical
to defining
oceanographic
response.
The V30-40
Tc 23-kyr
circles
additional
used to modify
the CLIMAP [1982]
last
glacial
maximum reconstruction
to make these
maps of 136 ka.
are
the
12d shows
The solid
cores
Mcintyre
et
al.'
60*
Surface
45*
Water
Response
30*
15'
to
Orbital
O*
Forcing
15'
30*
0ø
15ø
30 ø
45 ø
0o
15ø
30 ø
45 ø
0o
15ø
0o
15'
60ø
45ø
;50ø
15ø
O*
15ø
.50ø
Mcintyre
We
have
the
substituted
davi$iana
which
Antarctic
C.
120
used
variations
is
the
from
Hays,
signal
core
RCll-
This
core
construction
SPECMAP isotope
al.,
same
to
79ø52'E).
in
in
to
part,
1980;
Morley
and
The 23-kyr
filtered
davi$iana
stack
1986]
and
chronology
Surface
Cycladophora
considered
in
ice
(43ø31'S,
was
is
least
sea
[Morley,
1983].
of
species
at
al.'
fluctuations
radiolarian
respond,
et
of
the
[Imbrieet
therefore
has
as V30-40.
the
The phase relationship
with the
southern
hemisphere
shows that
the
V30-40
Tc 23-kyr
filter
slightly
lags that
of C. davi$iana
for the
interval
193-112
ka while
slightly
leading
at 86-66
ka (Figure
10).
There
are three
possible
causes
for
the variable
phasing
of V30-40
to C.
davisiana'
(1) chronology
error,
(2) change in precessional
forcing,
and (3) nonlinear
response
of C.
davisiana.
Chronology
error
can be
ignored,
as the V30-40
and RCll-120
isotopes
were used to construct
the
SPECMAP isotope
stack
from which the
chronology
was derived.
Within
the
time
spanned by these
filters
there
is
some
minor
variation
precessional
correlative
in
period,
with
but
the
C.
it
filter
signal.
We are left
presumption
of a nonlinear
of
C.
davisiana
to
climatic/oceanographic
cannot be directly
the
available
is
not
davisiana
with the
response
Antarctic
forcing
determined
which
from
evidence.
As in the northern
hemisphere
test,
we have chosen a high-latitude
estimated
Tw signal,
in this
case
RCll-120,
and filtered
it for the
23-kyr
component.
The Tc 23-kyr
filtered
signal
of V30-40
lags the
RCll-120
Tw 23-kyr
filter
which,
given the resolution
by 1.7 ø,
of our
chronology,
means that
V30-40
and
RCll-120
SST are in phase
(Figures
10 and 11).
In summary,
V30-40
SST
is in phase with or lags slightly
Water
Response to Orbital
Forcing
which modulate
the response
of
equatorial
Atlantic
to orbital
forcing
(principally
the
precessional
component).
Two,
rely
on southern
hemisphere
insolation,
are
(1) trade
wind
zonality
which
controls
Ekman
pumping
(divergence)
and (2)
advection
from high southern
latitudes
which
budget.
northern
controls
The third,
hemisphere
the
the
which
heat
which relies
insolation,
on
is
monsoon convergence
over
North
Africa,
which
controls
trade
wind
meridionality.
Two end-member
scenarios
explain
the process
(Figure
12) .
When
perihelion
is
aligned
summer,
summer
maximum
over
North
same
low
to
time
at
and
(Figure
(austral
middle
boreal
is
Africa
monsoon dominates
the
with
insolation
12a).
At
winter)
latitudes
a
the
of
in
the
southern
hemisphere,
the insolation
gradient
is steep,
insolation
is
high,
and the southern
hemisphere
subtropical
convergence
is furthest
south permitting
warm Indian
Ocean
water
entry
into
the Atlantic
(Figure
12a) .
Northward
advection
of heat
is enhanced
in the Benguela
Current.
The
equator,
is
divergence,
result,
at
the
a time of minimal
productivity,
and
seasonality,
with
the
warmest
equatorial
SST.
The isotherm
(austral
winter)
and seasonality
maps (Figures
12a and 12c) are
reconstructions
for 9 ka using
core
data
from Mix et al.,
[1986],
COHMAP
members
[1988],
and this
study.
The
temperature
range
the equator
Seasonality
(Figure
is very
12c)
is
only
4øC along
12a).
low (Figure
.
When aphelion
austral
is
winter
aligned
(insolation
with
minimum),
the southern
hemisphere
control
dominates
(Figure
12b) .
During
austral
middle
winter
latitudes
at
low
latitudes
in
the
southern
to
the
southern
hemisphere's
response
to orbital
forcing
and leads
the
northern
hemisphere's
response
to
orbital
forcing
by a large
amount.
hemisphere,
the
insolation
gradient
is
the
insolation
is
EQUATORIAL
Atlantic
RESPONSE
MODEL
low,
and the subtropical
convergence
is
furthest
north,
minimizing
heat
input
from the Indian
Ocean into
the
of
We have devised
a descriptive
model involving
three
mechanisms
shallow,
heat
Ocean.
in
Northward
the
reduced.
At
zonal
velocity
the
Benguela
same
of the
advection
Current
time,
trade
the
winds
is
is
Mcintyre
et
strongest
al.-
Surface
because
the
Water
North
Response
African
monsoon is weakest,
since
perihelion
is aligned
with boreal
winter.
The
result,
at the equator,
is a time of
maximum divergence,
productivity,
and seasonality,
with the coolest
SST.
This
can
be
seen
in
the
SST
isotherm
map for austral
winter
during
isotopic
stage 6 (Figure
12b).
This
reconstruction
utilizes
the estimated
SST from this
study
and CLIPLAP Project
Members,
[1984]
to modify
the CLIPLAP last
glacial
maximum isotherms
in the equatorial
Atlantic
(added core positions
shown
in
Figure
the
12d);
isotherms
outside
are
the
of
same
this
zone
as
published
by CLIPLAP Project
Members,
[1981] .
The increased
divergence
and consequent
thermocline
shallowing
are well defined
by the
seasonality
isotherm
pattern
for
isotopic
stage 6, in which the zone
of
maximum
seasonality
is
centered
on 10øW (Figure
12d),
the center
of
maximum divergence
[Philander
and
Pacanowski,
1986a]
.
to
Orbital
4.
Variation
periods.
2. Southern
hemisphere
climate
change exerts
a greater
influence
on
the equatorial
Atlantic
at orbital
frequencies
than does that
of the
northern
hemisphere.
SST and
foraminiferal
assemblage
signals
are
in phase with,
or slightly
lag,
variations
in southern
SST but lead high-latitude
hemisphere
SST and ice
The
mechanisms
hemisphere
northern
volume.
which
modulate
orbital
forcing
to produce
the
equatorial
SST signals
are ascribed
to variations
in trade
wind velocity
and heat advection
from high
southern
latitudes.
When aphelion
is aligned
with austral
winter,
trade
wind zonality,
equatorial
divergence,
and heat advection
are
at a maximum.
When perihelion
is
aligned
with boreal
summer, the
North
thus
are
African
monsoon
divergence
at
a
in
circulation
South
minimum.
dominates;
and heat
advection
heat
between
Atlantic
advection
the
Indian
oceans.
and
Perihelion
in boreal
summer equates
with
maximum, while
aphelion
in austral
winter
equates
with minimum,
heat
advection.
5. The data
and descriptive
models in this
paper
indicate
that
the short-period
variation
of the
annual
cycle
is only a partial
analogue
for
the response
of the
equatorial
Atlantic
at longer
orbital
periods.
It
is an analogue
for
the response
produced
by changes
in trade
wind velocity
but not the
response
due to variations
in
advected
heat.
6. The
variations
advection
orbital
question
of whether
in interhemispheric
in
time
data.
1. The SST response
of the mixed
layer
of the equatorial
Atlantic
to
orbital
forcing
is partitioned
into
a western
region
of temporal
stability
and an eastern
region
of
marked variation
at precessional
43
northward
into
the equatorial
Atlantic
is controlled
by orbitally
forced
changes in atmosphere/ocean
dynamics
of the southern
hemisphere,
which modulates,
primarily
at the
precessional
period,
the thermocline
the
Atlantic
scales
cannot
unequivocally
CONCLUSIONS
3.
Forcing
We
response
advection
Atlantic
lowest
answered
heat
occur
be
with
demonstrate
that
our
the
records
changes
in
into
the equatorial
at orbital
periods.
heat
advection
is
on
SEC
heat
The
correlative
with
aphelion
in austral
winter,
times
of maximum glaciation.
The
equatorial
signal
of this
effect
is
mirrored
in
South
Atlantic
along
the Benguela
and Imbrie,
1987;
press,
1989]
and
Aghulas
retroflection
have
half
an
orbitally
answer:
forced
APPENDIX
there
of
into
in
the
We
are
variations
northward
advection
South
Atlantic
and
equatorial
cores
axis
[Mcintyre
Imbrie
et al.,
from a core
at
(RCll-86).
in
heat
the
in
the
Atlantic.
A.
ESTIMATED
SEA
SURFACE
TEMPERATURES
Estimated
sea
temperatures,
cold,
Tc,
SST,
equations
their
respective
estimated
ages
Estimates
were
surface
for
are
warm,
listed
Tw,
and
with
depths and
(Table
A1) .
computed from the
FA20 equation
which
is archived
in
the Specmap Archive
1 [1989] .
Mcintyre
Table
A1.
Estimated
et
al.'
Surface
Water
Sea Surface
Response
Table
to
A1.
Orbital
Forcing
(continued)
Temperatures
"
Depth,
cm
Age, ka
Tw,øC
Depth,
cm
Age, ka
!
V25-59
V25-59
0.0
1.50
2.5
5.0
2.26
7.5
10.0
12.5
3.01
3.77
15.0
4.52
5.28
6.04
!7.5
6.79
20.0
22.5
7.55
25.0
25.0
27.5
9.06
9.06
30.O
32.5
32.5
35.O
37.5
8.30
9.52
9.98
10.45
10.45
10.91
11.86
4O.O
12.94
42.5
14.02
45.0
15.02
47.5
16.01
50.0
17.01
52.5
18.00
55.0
60.0
62.5
19.00
19.60
20.20
20.80
67.5
22.00
7O.O
22.60
72.5
72.5
75.0
102.5
105.0
107.5
23.20
23.20
23.80
24.40
25.00
25.60
26.20
26.80
27.40
28.00
28.93
29.85
30.78
31.70
32.63
33.56
57.5
77.5
80.0
82.5
85.0
87.5
90.0
92.5
95.0
97.5
100.0
110.0
34.48
112.5
35.41
115.0
117.5
36.33
37.26
120.0
38.19
120.0
122.5
38.19
39.11
125.0
40.04
Tc,øC
Tw,øC
Tc,øC
(continued)
127.5
40.96
28.2
23.5
27.5
27.1
27.4
27.5
27.3
27.6
25.3
25.3
25.5
25.6
25.2
24.9
130.0
132.5
135.0
137.5
140.0
142.5
41.89
42.81
43.74
44.67
45.59
46.52
27.0
27.4
27.2
27.5
27.1
26.5
23.3
27.4
26.9
26.9
26.6
26.9
27.0
26.4
26.9
25.8
26.3
26.5
26.6
26.6
26.9
26.3
27.1
27.6
26.8
27.2
27.4
26.9
27.8
27.8
26.4
25.1
24.7
24.8
24.6
24.3
24.5
24.3
24.5
23.8
24.4
23.6
23.9
23.6
23.5
23.4
24.7
24.1
23.0
24.2
23.6
24.4
23.5
24.0
23.8
145.0
147.5
150.0
152.5
155.0
157.5
160.0
162.5
165.0
167.5
170.0
172.5
175.0
177.5
180.0
182.5
185.0
187.5
190.0
192.5
195.0
197.5
200.0
200.0
47.44
48.37
49.30
50.22
51.15
52.07
53.00
54.09
55.18
56.27
57.36
58.45
59.55
60.64
61.73
62.82
63.91
65.00
65.31
65.63
65.94
66.26
66.57
66.57
27.3
27.5
27.6
27.3
27.6
27.6
26.9
26.8
27.1
26.8
26.4
27.3
27.0
26.3
26.7
27.5
27.1
27.6
27.4
27.0
28.3
27.7
27.6
27.4
27.0
27.0
26.5
26.9
26.2
26.8
27.0
27.2
27.9
24.0
24.0
23.3
23.2
23.2
24.2
23.2
24.3
24.3
202.5
205.0
207.5
210.0
212.5
215.0
217.5
220.0
222.5
66.89
67.20
68.15
69.10
70.05
71.00
72.29
73.57
74.86
26.0
27.3
26.4
27.0
26.6
26.6
26.5
26.7
27.2
27.1
27.0
27.6
23.7
23.5
23.9
225.0
227.5
230.0
76.14
77.43
78.71
26.7
27.0
26.2
26.8
27.4
26.0
27.1
26.7
27.6
27.2
26.9
23.6
23.8
23.2
23.5
23.7
22.5
23.1
22.8
232.5
235.0
237.5
240.0
242.5
245.0
247.5
250.0
80.00
80.88
81.75
82.63
83.50
84.38
85.25
86.13
26.5
26.9
27.1
26.5
26.3
26.7
27.1
27.2
27.4
26.8
26.9
26.8
22.9
23.4
23.6
22.9
252.5
255.0
257.5
260.0
87.00
87.92
88.83
89.75
26.2
26.7
26.9
26.6
23.9
23.5
23.6
23.8
23.0
23.7
22.4
23.9
23.0
23.1
23.6
23.8
23.6
23.9
23.9
23.7
24.4
24.0
23.8
24.4
24.3
25.1
25.5
24.2
24.7
24.5
23.9
24.2
24.1
23.4
22.7
24.3
23.3
23.9
23.9
23.5
23.8
24.1
24.3
24.6
23.8
23.9
24.5
24.6
24.2
24.4
24.7
24.3
24.5
24.1
23.9
24.2
24.3
Mcintyre
et al.'
Table
Surface
A1.
Water Response to Orbital
Depth, cm
Table
(continued)
,
Forcing
A1.
45
(continued)
,
Age, ka
V25-59
Tw, øC
Tc,øC
Depth,
cm
V25-59
(continued)
262.5
265.0
267.5
270.0
272.5
90.67
91.58
92.50
93.42
94.33
26.4
26.8
26.2
26.8
27.0
23.7
23.9
23.7
275.0
277.5
280.0
280.0
282.5
285.0
95.25
95.79
96.32
96.32
96.86
97.39
26.6
26.9
26.9
26.9
26.6
26.8
24.3
287.5
290.0
292.5
295.0
297.5
300.0
302.5
305.0
307.5
310.0
312.5
315.0
317.5
320.0
322.5
97.93
98.46
99.00
100.14
101.29
102.43
103.57
104.71
105.86
107.00
109.00
111.00
113.00
114.13
115.25
26.2
26.4
26.5
27.0
26.8
26.7
27.0
27.2
27.0
27.9
27.3
27.3
27.0
27.5
27.1
325.0
327.5
330.0
116.38
117.50
118.63
27.1
26.7
26.4
332.5
119.75
27.1
335.0
337.5
340.0
342.5
345 . 0
347.5
350.0
352.5
355.0
357.5
360.0
362.5
365.0
367.5
370.0
372.5
375.0
377.5
380.0
382.5
385.0
387.5
390.0
392.5
120.88
122.00
123.20
124.40
125 . 60
126.80
128.00
128.39
128.78
129.17
129.56
129.94
130.33
130.72
131.11
131.50
132.40
133.29
134.19
135.09
135.98
136.88
137.78
138.67
26.8
27.5
26.2
26.2
26 . 5
25.9
26.3
26.1
25.5
26.0
26.6
26.5
25.0
25.1
25.9
25.5
25.7
26.7
27.3
26.2
26.4
27.2
26.1
27.6
395.0
397.5
400.0
Tw, øC
Age,ka
Tc,øC
(continued)
139.57
142 .79
146.00
27.8
27.2
28.3
22.1
20.7
23.0
23.5
24.2
24.1
23.8
24.3
24.1
24.0
23.8
24.1
24.1
23.7
24.6
24.1
V30-40
0
3
6
9
12
15
18
21
24
27
25.4
22.6
10.29
24.7
22.4
1.50
27.0
2.63
27.1
23.9
24.1
3.75
4.88
6.00
27.0
24.1
26.8
24.6
26.7
23.4
23.7
6.86
26.7
7.71
26.0
24.0
8.57
26.1
23.6
9.43
30
11.14
24.5
23.4
23.5
33
36
12.00
24.0
23.4
23.5
23.5
23.2
23.2
22.4
23.3
24.2
23.7
24.2
24.2
39
42
45
48
51
54
57
60
63
66
69
24.1
24.7
24.4
23.8
24.0
72
75
78
81
84
23.6
23.8
23.4
23.2
23.0
22.9
23.0
87
90
93
96
99
102
105
21.9
108
21.9
21.0
19.4
20.4
20.4
111
114
117
120
123
21.5
20.0
20.1
19.7
19.3
21.0
126
129
132
135
138
141
24.4
22.6
21.9
21.8
13.36
24.9
22.8
14.05
23.7
21.5
14.73
23.2
21.3
12.
68
15.41
25.0
20.7
16.09
24.6
20.0
16.78
25.2
19.5
17.46
27.1
19.3
18.13
18.78
25.0
18.4
27.2
19.0
19.44
24.0
18.2
20.09
20.75
21.40
24.0
25.2
18.4
26.3
24.2
23.5
25.1
23.1
23.3
20.8
21.87
22.35
22.82
23.29
23.76
24.62
25.85
27.09
28.32
29.55
30.79
32.02
33.26
34.49
35.72
36.96
38.19
39.43
40.66
41.89
43.13
44.36
26.9
24.0
23.6
24.1
22.9
28.4
27.4
25.0
23.8
28.0
24.1
27.3
25.1
24.6
27.2
23.9
24.0
19.4
20.8
20.4
21.0
19.6
20.2
22.7
20.2
19.8
19.8
20.3
22.9
21.4
19.3
19.0
21.1
19.7
22.2
19.5
18.6
21.3
18.8
18.1
•6
Mcintyre
Table
Depth, cm
A1.
Age, ka
et al.'
Surface
Water Response to Orbital
(continued)
Tw, øC
Table
Tc, øC
A1.
Depth, cm Age, ka
Forcing
(continued)
Tw, øC
Tc, øC
,
V30-40
144
45.60
147
46.83
48.06
150
153
49.30
156
165
50.53
51.77
53.00
53.86
168
54.71
171
55.57
174
56.43
57.29
159
162
177
180
183
186
189
192
195
58.14
201
59.00
60.50
62.00
63.50
65.00
66.38
67.77
204
207
70.54
198
69.15
210
71.54
213
72.34
216
219
222
73.96
74.76
73.15
225
75.57
228
231
76.37
234
237
240
243
246
249
252
255
77.18
77.99
78.79
79.60
80.54
81.62
82.69
83.77
276
279
282
84.85
85.92
87.00
88.00
89.00
90.00
91.00
92.00
93.00
94.00
285
288
291
294
297
300
96.00
97.00
98.00
99.00
99.94
258
261
264
267
270
273
95.00
(continued)
V30-40
(continued)
27.3
24.7
27.2
25.2
21.8
19.1
20.9
19.7
303
306
309
312
100.88
101.82
102.76
103.69
25.5
24.9
25.4
25.3
22.8
23.2
22.7
22.6
28.1
25.4
27.5
26.2
27.1
24.7
25.3
25.1
23.4
24.2
23.4
26.1
24.1
26.8
25.2
26.4
27.0
23.6
23.7
24.9
24.8
20.9
20.1
21.2
20.6
22.4
21.5
20.5
21.7
22.1
21.7
21.6
22.8
21.3
19.8
20.5
21.1
21.0
18.0
19.0
19.0
20.7
315
318
321
324
327
330
333
336
339
342
345
348
351
354
357
360
363
366
369
372
375
104.63
105.57
106.51
107.45
108.39
109.33
110.27
111.20
112.14
113.08
114.02
114.96
115.90
116.84
117.78
118.71
119.65
120.59
121.53
122.55
123.64
26.1
25.4
26.1
25.0
25.8
24.2
27.3
26.1
26.5
25.3
25.4
25.3
25.9
25.0
25.4
26.3
25.8
24.7
25.7
25.6
26.2
23.2
23.0
23.4
22.6
22.3
20.4
21.7
20.2
21.0
20.1
20.7
20.9
20.9
20.9
21.6
21.0
22.6
22.2
23.4
22.8
23.5
25.6
20.9
378
124.73
24.0
21.4
26.5
24.3
25.1
25.8
25.0
26.0
25.9
27.4
21.8
21.5
21.3
21.1
22.0
23.5
22.8
24.4
381
384
387
390
393
396
399
402
125.82
126.91
128.00
129.75
131.50
133.25
135.00
135.76
25.0
24.5
24.7
24.9
25.9
26.3
26.5
26.3
20.6
20.4
19.6
19.9
17.9
18.1
18.7
18.7
25.3
26.7
25.8
26.4
25.0
22.5
24.6
21.9
22.2
20.9
20.4
20.1
20.9
405
408
411
414
417
136.52
137.29
138.05
138.81
139.57
25.8
26.7
25.8
26.7
26.9
17.3
18.2
17.4
18.5
19.5
420
423
426
140.33
141.10
141.86
20.7
19.4
22.4
19.3
22.0
21.6
20.6
21.0
21.8
22.1
23.2
429
432
435
438
441
444
447
450
453
456
459
142.62
143.38
144.14
144.90
145.67
146.43
147.19
147.95
148.71
149.48
150.24
26.5
26.2
27.2
26.2
26.8
26.1
27.8
25.6
26.2
26.0
25.8
26.0
25.8
25.5
18.4
19.0
19.2
19.5
21.0
20.1
23.0
21.0
20.2
21.3
21.9
22.2
21.9
20.7
26.5
24.7
25.0
27.9
24.8
26.5
24.7
26.5
25.3
25.5
24.8
25.3
25.2
25.7
Mcintyre
et al.Table
Depth,
A1.
cm Age,ka
V30-40
Surface Water Response to Orbital
(continued)
Tw,øC
Table
Tc,øC
Depth,
cm
A1.
462
465
468
471
474
477
480
483
486
489
151.00
152.00
153.00
154 .00
155.00
156.00
157 .00
158.00
159.00
160.00
27.2
26.6
27.0
27.1
26.0
24 .2
25.5
25.8
26.1
25.8
22.2
22.0
20.7
18.8
19.7
16.8
19.1
17.2
18.2
19.1
621
624
627
630
633
636
639
642
645
648
492
495
498
501
161.00
162 .00
163.00
164 .00
25.2
26.3
25.3
25.0
504
507
510
513
516
519
522
525
528
531
534
165.00
166.00
167 .00
168.00
169.00
170.00
171.00
171.83
172 . 67
173.50
174.33
537
540
175.17
176.00
25.2
26.0
24.7
25.7
26.3
25.4
26.1
26.9
26.1
26.5
26.9
26.8
18.0
20.0
19.1
21.0
19.5
20.1
20.2
21.5
21.7
21.3
22.1
21.9
23 ß3
23.1
22.6
23.2
21.3
651
654
657
660
663
666
669
672
675
678
681
684
687
690
693
696
699
543
177 .40
26.0
21.8
702
546
549
552
555
558
561
564
567
570
178.80
180.20
181 . 60
183.00
183.75
184 .50
185.25
186.00
186 . 62
25.2
25.4
26.4
25.6
26.0
26.4
26.1
25.5
26.1
20.3
20.4
19.3
17.2
18.7
18.9
17.5
18.7
18 ß9
705
708
711
714
717
720
723
726
729
573
576
579
582
585
588
591
594
597
600
603
606
609
612
187 .23
187 .85
188.46
189.08
189.69
190.31
190.92
191.54
192 .15
192 .77
193.38
194 .00
195.57
197 .14
25.7
26.0
27.4
26.5
26.9
26.2
26.0
26.5
26.2
26.2
26.3
26.3
26.2
25.7
19.2
20.0
20.4
21.2
21.9
23.1
23.2
23.0
22.9
23.3
23.2
23.8
22.5
22.2
732
735
738
741
744
747
750
753
615
618
198.71
200.29
23.9
24.8
20.1
20.1
(continued)
Tw, øC
Age,ka
V30-40
(continued)
25.0
Forcing
Tc, øC
(continued)
201.86
203.43
205.00
208.50
24.9
24.8
25.1
24.5
21.5
20.0
19.7
20.0
212 . 00
213.33
214 . 67
216.00
217.50
219.00
220.50
222.00
223.50
225.00
226.50
228.00
228.77
229.54
230.31
231.08
231.85
232 . 62
233.38
234.15
234 .92
235.69
236.46
237.23
24 . 0
25.0
25.7
26.0
26.5
26.3
25.9
26.2
25.5
25.9
25.5
25.0
25.3
24.8
23.5
25.4
25.5
25 . 3
25.6
25.1
24.7
25.6
25.7
25.9
19.6
19.8
22.4
21.5
22.4
22.9
22.8
22.8
22.7
23.0
22.2
21.2
19.1
19.1
16.4
17.7
20.3
19 . 2
18.3
21.5
20.9
23.0
23.3
23.0
238.00
239.19
240.38
241.56
242.75
243.94
245.13
246.31
247.50
248.69
249.88
251.06
252.25
253.44
254.63
255.81
257.00
25.9
24.8
25.1
23.2
24.2
24.7
25.5
24.3
25.1
25.5
23.9
24.6
24.6
23.8
24.1
24.0
25.3
23.4
22.6
21.9
20.8
20.5
19.8
19.0
18.8
19.1
19.8
19.0
19.9
20.4
21.1
20.6
21.7
22.7
28.1
26.3
25.8
25.9
24.1
23.7
RC24-16
0
3
6
1.50
2.20
2.90
48
Mcintyre
Table
Depth, cm
A1.
Age, ka
RC24-16
9
12
15
18
21
24
3.60
4.30
5.00
5.70
6.40
7.10
27
7.80
30
33
36
39
42
45
48
8.50
9.20
9.90
10.60
11.30
12.00
12.85
51
54
57
60
63
66
69
72
75
13.69
14.54
15.39
16.24
17.08
17.93
18.78
19.36
19.94
78
20.52
81
84
21.10
21.68
87
22.26
90
93
22.84
23.42
96
99
102
105
108
111
114
117
120
123
126
129
132
135
138
141
144
147
150
153
156
159
162
165
24.00
25.20
26.40
27.59
28.79
29.99
31.19
32.38
33.58
34.78
35.98
37.17
38.37
39.57
40.77
41.96
43.16
44.36
45.26
46.15
47.05
47.95
48.85
49.74
et
al.'
Surface
Water
(continued)
Tw,øC
Response
Table
Tc, øC
Depth, cm
Orbital
A1.
(continued)
Age, ka
Tw, oC
RC24-16
(continued)
to
Forcing
Tc , oC
(continued)
26.2
25.9
26.4
25.9
26.0
26.0
26.3
26.8
26.0
26.6
26.2
25.7
25.9
26.4
26.9
26.4
24.1
23.9
23.7
23.5
23.9
23.7
24.2
22.8
23.7
23.6
23.5
23.1
21.9
22.9
21.0
20.5
168
171
174
177
180
183
186
189
192
195
198
201
204
207
210
213
50.64
51.54
52.44
53.33
54.23
55.13
56.03
56.92
57.82
58.72
59.62
60.51
61.41
62.31
63.21
64.10
25.8
25.6
27.6
26.8
26.7
27.5
26.6
27.1
26.7
26.7
27.5
27.4
27.4
26.5
26.9
26.0
20.3
18.6
22.1
19.0
20.8
20.7
20.5
21.0
21.5
20.7
20.7
21.2
20.9
20.7
21.5
21.1
26.5
26.7
26.2
26.2
26.3
25.8
26.1
26.0
27.4
26.9
26.4
20.6
19.7
18.8
17.3
18.4
17.8
17.7
18.7
18.7
20.2
19.5
216
219
222
225
228
231
234
237
240
243
246
65.00
66.11
67.22
68.32
69.43
70.54
71.41
72.28
73.14
74.01
74.88
26.4
27.3
27.0
27.2
26.0
25.9
26.1
26.5
26.5
26.1
26.3
21.4
20.4
19.7
18.7
17.4
16.2
17.5
17.9
18.6
18.8
19.3
27.3
26.4
27.2
27.3
27.9
27.1
26.1
26.4
26.5
26.0
26.9
25.5
26.3
26.6
25.2
26.0
26.4
25.6
26.6
25.9
25.7
26.2
26.5
26.9
26.0
25.3
19.7
20.6
20.9
19.7
19.9
20.6
18.7
20.7
18.8
19.0
19.5
19.6
18.5
17.9
19.6
18.3
18.4
18.5
17.5
17.1
17.7
17.6
18.8
18.6
17.7
19.2
249
252
255
258
261
264
267
270
273
276
279
282
285
288
291
294
297
300
303
306
309
312
315
318
321
324
25.8
26.5
26.0
25.7
26.9
25.9
26.3
26.4
27.2
25.4
25.6
25.3
25.9
26.6
25.6
25.9
26.0
25.5
25.7
25.7
26.2
26.0
27.0
25.4
27.7
27.3
19.7
18.2
20.1
20.5
20.8
21.6
23.0
23.3
23.4
21.3
19.5
19.5
20.5
19.6
18.9
18.7
19.0
18.7
20.0
21.0
21.1
21.4
23.2
22.6
24.0
23.1
75.75
76.61
77.48
78.35
79.22
80.09
80.95
81.82
82.69
83.94
85.20
86.45
87.71
88.96
90.22
91.47
92.73
93.98
95.24
96.49
97.75
99.00
99.82
100.64
101.46
102.29
Mcintyre
et al.'
Table
Surface
Water Response to Orbital
A1.
(continued)
Depth, cm Age,ka
Tw,øC
327
330
333
336
339
342
345
348
351
354
357
360
363
366
369
372
375
378
381
384
387
390
393
396
399
402
405
408
411
414
417
420
423
426
429
432
435
438
441
444
447
450
453
456
459
462
465
468
471
474
477
480
483
Forcing
Table
Tc, øC
A1.
Depth, cm Age,ka
RC24-16
RC24-16
(continued)
103.11
103.93
104 .75
105.57
106.39
107.21
108.04
108.86
109.68
110.50
111.32
112 .14
112.96
113.79
27.4
27.8
27.4
26.5
25.6
25.9
26.1
26.2
25.5
27.2
26.8
26.3
25.7
26.3
24.1
24.1
23.0
22.1
22.7
22.6
22.0
22.0
19.6
19.3
17.7
19.2
18.1
19.5
486
489
492
495
498
501
504
507
510
513
114.61
115.43
116.25
117 .07
117.89
118.71
119.54
120.36
121 . 18
122.00
122 . 87
123.73
124.60
125.47
126.33
25.7
26.7
27.0
26.7
26.4
26.3
26.5
27.6
26.6
26.4
26 ß 6
26.2
25.6
24.8
24.8
19.0
21.5
22.1
22.9
22.9
21.5
23.8
24.1
24 . 1
24.3
24 . 3
23.7
22.5
22.1
21.9
528
127.20
128.07
25.0
25.5
22.2
21.6
128.93
129.80
130.67
131.53
132 .40
133.27
134.13
135.00
135.50
136.00
136.50
138.11
139.72
141.33
142 .94
144 .56
146.17
147 .78
149.39
151.00
151.74
152 .47
25.3
24.6
25.2
25.1
25.8
26.0
26.6
26.5
25.8
26.5
25.0
25.8
26.5
25.9
26.1
26.3
26.8
26.8
26.2
25.9
27.2
27.1
21.1
20.7
20.6
18.9
17.7
18.6
18.1
18.2
18.1
18.2
17.3
17.0
17.0
17.9
17.5
18.8
19.9
20.4
20.8
20.5
21.7
22.2
516
519
522
525
531
534
537
54O
543
546
549
552
555
558
561
564
567
570
573
576
579
582
585
588
591
594
597
6OO
603
606
609
612
615
618
621
624
627
630
633
636
639
642
•9
(continued)
Tw,øC
Tc,øC
(continued)
153.21
153.95
26.8
26.5
19.4
19.6
154.68
155.42
156.16
156.89
157.63
158.37
159.11
159.84
26.2
26.4
26.6
26.3
26.0
27.0
26.3
26.1
20.2
18.2
19.1
18.8
19.8
19.1
19.5
19.3
160.58
161.32
162.05
162.79
163.53
164.26
165.00
166.13
167 .25
168.38
169.50
170.63
171.75
172 .88
174 .00
175.13
176.25
177.38
178.50
26.0
25.9
26.0
24.8
26.1
25.8
26.2
25.6
25.8
26.0
26.4
26.4
27.3
26.7
26.2
26.0
25.9
27.0
26.6
18.2
18.9
18.1
18.6
18.0
17.6
19.1
19.9
19.6
20.6
20.7
20.5
19.9
20.2
21.8
20.6
20.2
20.5
20.6
179.63
180.75
181.88
183.00
183.58
184 .16
184.74
185.32
185.89
186.47
187 .05
187.63
188.21
188.79
189.37
189.95
190.53
191.11
191.68
192.26
192 .84
193.42
194 .00
195.24
26.0
25.4
25.2
25.4
26.0
25.7
26.4
26.3
26.0
26.1
26.3
27.5
26.8
27.0
27.6
25.2
26.8
28.7
28.3
27.1
26.5
27.3
27.0
27.0
20.1
20.7
20.2
17.1
18.2
17.5
19.4
17.0
15.7
18.6
18.5
18.4
20.9
20.8
23.5
20.7
23.3
25.5
24.6
23.9
23.4
25.1
24.3
23.7
50
Mcintyre
Table
et
A1.
(continued)
Age, ka
Tw,øC
al.'
Surface
Water
cm
to
Orbital
defining
SPECMAP isotopic
then adjusted
for best
fit
methods
Depth,
Response
described
stratigraphy
Tc,øC
and
in
in
section
the
references
Forcing
events
and
by the
the
of
this
paper
listed
in
this
,
RC24-16
paragraph.
(continued)
645
648
651
654
657
660
663
666
669
672
196.48
197.72
198.96
200.20
201.44
202.68
203.92
205.16
206.40
207 ß 64
25.6
26.7
25.1
25.7
24.5
25.6
25.0
23.6
24.5
23.9
22.3
22.8
20.9
21.6
19.4
21.2
19.5
20.4
20.1
21.4
675
678
681
684
687
690
693
696
699
702
705
708
711
714
717
720
723
726
729
208.88
210.12
211.36
212 . 60
213.84
215.08
216.32
217.56
218.80
220.04
221.28
222.52
223.76
225.00
225.93
226.86
227.79
228.71
229.64
25.2
24.4
24.8
24 . 7
25.9
24.9
25.4
24.8
26.2
25.6
25.9
26.0
26.3
26.3
26.3
25.6
25.2
26.4
25.3
20.6
21.5
22.6
23 . 3
24.3
23.2
23.7
23.2
24.4
23.8
23.9
24.2
23.2
24.5
21.4
19.5
19.2
19.6
19.3
732
735
738
741
744
747
750
230.57
231.50
232 .43
233.36
234.29
235.21
236.14
24.9
25.8
26.5
25.4
26.4
27.5
27.3
20.4
19.2
18.8
19.2
22.1
23.3
22.6
APPENDIX
B.
CORE
Table
Isotopic
Substage
et
the
at.,
1987].
SPECMAP
chronology
et
al.
Core
stack
V30-40
cores
was established
[1984]
.
We have
is
one
of
whose
by
Imbrie
modified
their
age model for this
core for
events
prior
to 7.5
(see text
for
explanation).
Age models for V25-59
and RC24-16
were first
erected
by
Chronology
Depth in Core (cm)
V25-59
V30-40
RC24-16
1.5
6.5
11.0
12.0
14.0
17.8
18.87
19.0
21.4
24.0
3.1
28.0
-.-
44.4
3.3
53.0
4.0
4.2
-.5 0
5.1
59.0
65.0
67.2
70.5
71.0
80.0
-.5 2
82.7
87.0
0.0
16.5
35.5
-.42.5
ß
-.55.0
-.-.92.5
-.160.0
-.187
5
205.0
-.215.0
232.5
-.252.5
-.-
95.3
275.0
5 3
5.4
99.0
107.0
292.5
310.0
297
-.-
317.5
337.5
350.0
372.5
-.395.0
400.0
415.0
427.0
445.0
457.0
-.370.5
387.0
-.399.0
-.-
6 3
6.4
-.-.-.-
113.0
122.0
128.0
131.5
135.0
139.6
146.0
151.0
156.0
163.5
167.7
6.5
-.6 6
7.0
7.1
7.2
-.7.3
-.7.4
7.5
-.-
171.0
176.0
183.0
186.0
194.0
205.0
212.0
216.0
225.0
228.0
238.0
257.0
-.-.-.-.-.-.-.-.-.-.-.-.-
--
The chronology
for
each core
(Table
B1) was established
using the
SPECMAP oxygen
isotope
taxonomy
[Imbrie
et al.,
1984;
Pisias
et at.,
1984;
Prell
et at.,
1986;
Martinson
Age
ka
Core
-.1.1
-.2 0
-.2 22
-.2.2
2.24
3.0
5.5
6.0
-.6.2
CHRONOLOGY
B1.
, --
0.0
12.0
-.33
0
-.58
5
-.-.75
0
91.5
-.-.162.0
183.0
195.0
-.-
0.0
-.-.45.0
-.-.69.0
-.-.96.0
-.147
0
-.-.216
0
--
o-
231
208
241.5
.
0
-.273.0
-.-
0
312
-.261
-.-
-.-
-.-
462
522.0
540.0
555.0
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A. C. Mix,
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(Received
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19, 1988.)
55