DEEP CIRCULATION CHANGES IN
THE SOUTH ATLANTIC OCEAN:
RESPONSE TO INITIATION
OF NORTHERN HEMISPHERIC GLACIATION
A Thesis
Presented to
The Faculty of the Department of Geology
San Jose State University
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
By
Rene L. Turnau
August 1988
ACKNOWLEDGEMENTS
Samples and isotopic data were obtained from Dr. Paul
Loubere, Northern Illinois University (Site 548), Dr. Paul
Ciesielski, University of Florida (Site 514), Dr. Bob
Thunell, University of South carolina (Site 516A) and Dr.
Dave Hodell, University of Florida (Sites 516A, 517 and
518) .
Funds in support of this research and graduate study
were obtained from a grant from the Achievement Rewards for
College Scientists (ARCS) Foundation through San Jose State
University, the Packard Foundation, and NSF Grants
DPP-8316992 and DPP-8613823.
My sincere thanks go to Dr. Michael Ledbetter, of Moss
Landing Marine Laboratories, for his support, introducing
me to the field of Paleoceanography, and allowing me to
explore new concepts and ideas through research.
Finally, thank you to the entire Moss Landing Marine
Laboratories community for providing an outstanding
environment in which to conduct research and study marine
sciences.
iii
TABLE OF CONTENTS
Page
ABSTRACT ••••.
.viii
INTRODUCTION.
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Modern Ocean Circulation. • • • • • • • • • • • • • • • • • • • • • • • • • • • 3
Water Masses ...••...
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Antarctic Intermediate Water . . . . . . . . . . . . . . J
Mediterranean Outflow Water . . . . . . . . . . . . . . . 4
Upper Circumpolar Water ..•.•.............. 5
North Atlantic Deep Water . . . . . . . . . . . . . . . . . 6
Lower Circumpolar Water .•................. 6
Weddell Sea Deep Water .•.................. 7
Water Mass Interactions ..••.•.................. 8
Late Pliocene Climate •.•...........•............... 11
Core Locations •.
e
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••••••••••••••••••••••
DSDP Site 514.
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oooaooooooeoe•o•••••l7
DSDP Sites 516A, 517, and 518 . . . . . . . . . . . . . . . . . 17
DSDP Site 548 •••••.•••••••. ., •.
METHODS • •
RESULTS o
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DISCUSSION •.
Grain-size Statistics.
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Climate/Deep-water Interactions ..•..•.............. 36
3 o 2 to 2 • 9 Ma. •' •••••
2 • 9 to 2 • 6 Ma
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2 • 6 to 2 • 5 Ma ••••••••
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Ill
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to 2 • 3 Ma • • • • • • • • • • " • • • • • • • • • • • • • • • •
2•5
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Interpretation of Site 548 ....•••..•...•........... 51
CONCLUSIONS o
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REFERENCES CITED .......
APPENDIX A •••
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LIST OF ILLUSTRATIONS
Figure
1.
Page
Water-mass Interactions in the Atlantic
Ocean . •••••••....•.
a
.............
'* • • • • ~ ~ •• "' ., • • • • • • • •
9
2.
Location and Bathymetry of DSDP Sites 548 1
516, 517, 518, and 514 .....•••.•................. 15
3.
Positions of DSDP Sites 516A, 517, and 518
Relative to the Deep Waters of the South
Atlantic Ocean ..... " ........... " ....... ............ 18
4.
Position of DSDP Site 514 Relative to the
Modern-Day Polar Front and Deep-Water
Masses .
o
•••••••••••••••••••••••••••••
G
•••••••••••
19
5.
Comparison of Temporal Fluctuations in the
Grain-size statistics versus the Geochemical
Data for DSDP Site 548 .......•................... 24
6.
Comparison of Temporal Fluctuations in the
Grain-size Statistics versus the Geochemical
Data for DSDP Site 516A •......................... 25
7.
Comparison of Temporal Fluctuations in the
Grain-size Statistics versus the Geochemical
Data for DSDP Site 517 ......•.................... 26
a·.
Comparison of Temporal Fluctuations in the
Grain-size Statistics versus the Geochemical
Data for DSDP Site 518 . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.
Comparison of Temporal Fluctuations in the
Grain-size Statistics versus the Radiolarian
Faunal Assemblage Data for DSDP Site 514 ......... 28
10.
Contour Plot of Grain-size and Oxygen-isotope
Data . . . . . . . . . . . . . . . . . . . . . . . • . . . . .. . . . . . . . . .
11.
o
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Silt Particle-size Distribution for Sediment
Deposited in AABW, NADW, and the Transitional
Zone in the Rio Grande Rise Region ............... 34
Table
1.
Page
Core Locations and their Present Water
Masses . • . • .. . . . . • . . . . . . . •
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16
ABSTRACT
Silt particle-size statistics (coarsest one-percentile
and mean) of samples from DSDP Sites 514, 516A, 517, 518,
and 548 have been used to infer relative changes in
paleospeeds of deep-water masses in the Atlantic Ocean
during the Late Pliocene initiation of Northern Hemispheric
glaciation.
The integration of these grain-size statistics
with geochemical and faunal assemblage studies by other
authors enhances the understanding of how climatic forcing
regulates deep water-mass interactions.
Based on the between-site comparison of coarsest
one-percentile values, the Late Pliocene can be divided
into four intervals of differing deep-water activity.
These intervals occurred at 3.2 to 2.9 Ma, 2.9 to 2.6 Ma,
2.6 to 2.5 Ma, and 2.5 to 2.3 Ma, with pulses of inferred
paleospeeds occurring at 3.15 to 3.10, 2.85, 2.71, 2.6, and
2.4 Ma.
Except for a brief increase in paleospeeds
-3.1
Ma, the first time-interval is characterized by low
inferred paleospeeds corresp~nding to depleted
o18 o
values.
o13 c
and
The event at 3.1 Ma is inferred to be a
result of increased Antarctic circumpolar Current (ACC)
viii.
circulation in response to a brief, but non-permanent,
Antarctic glaciation and subsequent cooling of Antarctic
surface and bottom waters.
The interval from 2.9 to 2.6 Ma is dominated by
climatic cooling and an influx of deep waters originating
in the North Atlantic.
The introduction of significant
volumes of Mediterranean Outflow Water (MOW), as
demonstrated by increased inferred paleospeeds at Site 548,
and an expanding glaciation beginning at 2.9 Ma, resulted
in increased paleospeeds and a deepening of North Atlantic
Deep Water (NADW) in the South Atlantic.
A pulse of
increased inferred paleospeeds at 2.7 Ma resulted in the
temporary deepening of NADW to the depth of Site 518
(presently 3944 m) .
The 2.6 to 2.5 Ma interval represents the retreat of
deep water, originating in the North Atlantic, from the
depth of Site 518 to the depth of Site 517 (presently
2963 m).
Increased inferred paleospeeds at Site 518
correspond with depleted
o13 c
values and a northward
shift in the polar front, thus suggesting the
reintroduction of Circumpolar Water to depth of site 518
and the establishment of a deep-water stratification
similar to that present today.
A continuing trend towards
ix
this stratification continues into the latest times
interval (2.5-2.3 Ma) as inferred deep-water paleospeeds
increased in response to the formation of extensive
Northern Hemispheric ice sheets.
X
INTRODUCTION
The formation of deep waters requires a mechanism to
initiate thermohaline flow by increasing the density of
surface waters.
Because climate directly affects the
physical and chemical properties of surface water masses,
it may be an important regulator of thermohaline flow
(Duplessy and Shackleton, 1985).
If this is true, a change
in deep-water paleocirculation may have resulted from the
initiation of Northern Hemispheric glaciation from 3.2 to
2.4 Ma. (Shackleton et al., 1984).
The objective of this
study is to examine paleocirculation of the deep Atlantic
Ocean as related to Late Pliocene paleoclimatic events
associated with the initiation of Northern Hemispheric
glaciation.
Several paleoceanographic tools have been used to
delineate the response of water masses to paleoclimatic
forcing.
Oxygen isotopes, eolian deposits, ice-rafted
debris and faunal assemblages have been used for
paleoclimate analysis; carbon isotopes, carbonate
preservation, faunal assemblages, hiatus frequencies, and
silt-mean grain size have been used in water-mass studies.
This study utilizes temporal variations in grain size of
the non-biogenic silt fraction to infer fluctuations in the
2
speed of intermediate- and deep-water masses for the
time-interval from 3.2 to 2.4 Ma.
The data from this study
are compared to other published results that describe Late
Pliocene climate to illustrate the response of deep-water
masses to paleoclimatic events associated with Northern
Hemispheric glaciation.
Silt-mean grain size has been used to infer
fluctuations in production rates of deep- and bottom-water
masses (Ledbetter et al., 1978, Ledbetter, 1981, 1984).
The silt-size distribution at any site is inferred to
result from the fractionation within the current of fine,
suspended sediment that has been supplied upstream.
Periods of intense current activity allow the coarser
silt-fraction to be deposited for two reasons:
first, the
coarser particles can be transported farther downstream to
the site, and second, the fines remain suspended in the
turbulent flow (McCave and Swift, 1976).
As current speed
subsides, coarser silt particles are deposited nearer the
source.
As a result, only medium and fine silt and clay
particles are held in suspension long enough to reach the
site of analysis.
Therefore, the finer silt fraction will
be indicated by a finer particle-size distribution.
3
Ocean circulation
Water Masses
An understanding of the processes responsible for the
formation and distribution of the modern South Atlantic
deep-water masses is necessary to infer the response of
paleocirculation to climate forcing.
Generally, the water
masses present in the South Atlantic Ocean originate in the
North Atlantic Ocean, the Pacific Ocean in the Drake
Passage, and the Weddell Sea.
The water masses have
acquired their diagnostic chemical and physical properties
in these different areas and lie at depths corresponding to
their densities; these depths vary laterally according to
the geostrophic structure of the density field (Reid et
al., 1977).
Although these water masses are mixed and
modified by diffusive and advective processes, they may be
recognized by their original properties over distances of
thousands of kilometers.
Antarctic Intermediate Water.
Antarctic
Intermediate Water (AAIW) originates from the relatively
fresh surface water that encircles the Antarctic
continent.
Because of its surface-water origin, the water
mass is high in dissolved oxygen and is relatively fresh
(Johnson, 1982).
AAIW sinks at the Antarctic Convergence
and flows northward at a depth of approximately 900 to 1000
4
meters to approximately 40°S within the Falkland current;
from there AAIW turns eastward as part of the subtropical
anticyclonic gyre.
AAIW flow continues around the eastern
part of the gyre and reaches the coast of South America
again north of 30°S.
Between 30° and 40°S, AAIW
moves southward as part of the gyre.
The tongue of AAIW
can be traced as far north as -25°N where it loses its
characteristic properties as it mixes with highly saline
Mediterranean water and other intermediate water masses.
Mediterranean Outflow Water.
Mediterranean Outflow
Water (MOW) is a high-salinity (-35.5°/oo), hightemperature (-11°C) water mass that occurs between 600
and 2500 m water depth in the North Atlantic (Reid, 1979).
MOW extends west, via the Straits of Gibraltar, from the
Mediterranean Sea across the North Atlantic Basin and
combines with the Gulf Stream circulation opposite the
carolinas.
More importantly for this study, the MOW
extends north to the Norwegian Sea, where it contributes
the salt necessary for the creation of North Atlantic Deep
Water (Reid, 1979).
The formation of MOW depends on several climatic
factors in the Mediterranean Basin.
include:
These factors
a warm climate in the eastern basin, high
regional seasonality, an east-west gradient in surface
5
ocean temperatures, and a mechanism to increase the density
of the surface water in the western basin until it is dense
enough to sink, and mix vertically, with eastern
Mediterranean basins (Sankey, 1973).
Upper Circumpolar Water.
Circumpolar Water (CPW)
is a product of the mixing of North Atlantic Deep Water,
Antarctic Bottom Water, and the intermediate water masses
present near the Antarctic continent.
CPW becomes
entrained in the Antarctic Circumpolar Current (ACC) and,
as a result, is out of contact with the atmosphere for
prolonged periods (Reid et al., 1977; Johnson, 1983};
therefore, a principal characteristic of this water mass is
its low oxygen content.
After flowing through the Drake
Passage, the water mass is deflected northward to about
40°S within the Falkland Current and encounters NADW,
which is of comparable density.
CPW is then split into
upper (UCPW) and lower (LCPW) branches.
After CPW is
split, the two branches turn southward within the offshore
return flow.
UCPW lies at depths greater than 1500 m
within the South Atlantic anticyclonic gyre and near the
sea-surface at Antarctica, where it has a minimum
temperature and salinity and maximum of oxygen as it
approaches the cooled surface layers (Reid et al., 1977).
6
North Atlantic Deep Water.
North Atlantic Deep
Water (NADW) forms as MOW enters the Norwegian-Greenland
Sea through the Iceland-Scotland Ridge, where heat is lost
to the atmosphere and the resulting cooled and saline water
mass sinks (Gordon, 1975).
This dense water mass flows
across the Greenland-Iceland-Farce Ridge and combines with
intermediate water masses south of the ridge to form NADW.
Extreme characteristics of the NADW are evident in the
western boundary current as far as 40°S.
Beyond 40°S
the waters move southward but further offshore.
Lower NADW
(LNADW) and parts of the Upper NADW (UNADW) eventually turn
eastward and parts are entrained within Circumpolar Water
(CPW) in the ACC.
LNADW turns eastward north of 45°S
within the ACC, and its density range south of 45°S is
similar to that of CPW.
The salinity maximum of lowest
NADW, however, is evident all the way to the Weddell Sea.
Near 60°S the UNADW isopycnal lies near the sea surface
(-200 m) and the effect of vertical exchange with the cold,
fresher, oxygen-rich surface waters is evident (Reid et
al., 1977).
Lower Circumpolar Water.
Lower Circumpolar water
(LCPW) has the same origin as UCPW (northward deflection of
the eastward flowing CPW east of the Drake Passage), but is
7
of higher density, and is present at depths of
approximately 3700 to 4100 m in the Rio Grande Rise area.
Just north of this area, the oxygen minimum used to define
LCPW disappears.
From this point northward, NADW
predominates at this depth (Reid et al., 1977).
Weddell Sea Deep Water.
Weddell Sea Deep Water
(WSDW), a component of Antarctic Bottom Water (AABW),
originates along the western shelf of the Weddell Sea or
farther east along the shelf margin.
The formation of AABW
is associated with sea-ice formation and the interaction of
upwelled CPW with the bottom of the floating-ice shelves
(Gordon, 1975).
As the ice forms, particularly during the
winter months, most salt is restricted from the ice so that
the salinity and density of the water increases to create
thermohaline flow.
WSDW characteristics are intermediate
in value between CPW and newly formed bottom water and may
be accounted for by vertical diffusion between these two
layers within the Weddell cyclonic gyre (Reid et al.,
1977).
The WSDW extends northward into the Argentine
Basin, via
a gap
in the Falkland Fracture Zone, and becomes
entrained in the western boundary undercurrent (LePichon et
al., 1971; Georgi, 1981; Ledbetter, 1986).
WSDW extends
from the Argentine Basin through the Vema Channel and flows
into the Brazil Basin.
The water mass continues northward
8
but loses its distinct properties north of approximately
40°N, in the North American Basin (Gordon, 1975).
Water Mass Interactions
The intermediate and deep waters of the South Atlantic
interact in such a way that the properties and advective
processes of one water mass may be directly influenced by
those of another (Johnson, 1982).
Johnson {1982) described
three types of water-mass interactions ("teleconnective
linkages") that are represented in modern oceans (fig. 1).
The examples cited are particularly appropriate to this
study for projecting possible climate and water-mass
interactions of the past.
The first type of interaction is characteristic of
modern MOW and NADW.
"Reinforcement" occurs when the
mixing of high-salinity MOW water with the cold {-1.5°C)
surface waters of the Norwegian Sea creates a deep
thermohaline flow that enters the Atlantic by cascading
over the sills of the Greenland-Iceland-Farce Ridge.
This
water mass eventually mixes with water in the Labrador Sea
to form NADW (Johnson, 1982).
This reinforcement may be
necessary to form the modern NADW.
Climatic and tectonic
changes in the past, however, restricted MOW from
interacting with Norwegian Sea surface water.
the advective properties of NADW may have been
significantly different.
As a result,
A. RE·INFORCINC TflECONNECTION
·C. WEDGING
_,.
3
B. lATERAl BLOCKING/RE-DIRECTION
--i..-. NAOW .:
-
.·.:
.
~
·•
km
.
4
1-
2
5
IO"N
lim
o•
to•s
. 20"5
30"5
from Johnson ( 1982)
4
CPW It NADWI
5
Figure 1.
Water-mass interactions in the Atlantic Ocean (Johnson, 1982).
10
The second type of water-mass interaction is
"blocking/redirection."
In the South Atlantic this occurs
as the northward-deflected CPW intercepts the motion of
southward-flowing NADW.
Because the two water masses are
of comparable density, the NADW splits the CPW into upper
and lower branches (Reid et al., 1977).
NADW continues to
flow southward past the point of blocking, and it is
entrained within the eastward circumpolar flow.
The
relative production rates, and intensification, of the
water masses determines their spatial arrangement with
respect to each other and the topography of the South
Atlantic.
The third teleconnection is referred to as "wedging"
and is exhibited by the NADW/WSDW interaction.
In the
tropical and sub-tropical western Atlantic, southward
flowing NADW overrides denser, northward-flowing WSDW.
Between the cores of the two water masses lies a broad zone
of turbulent mixing.
Today, the top of this turbulent zone
slopes upward from a depth of 4000 m at 40°N to 3200 m at
30°S (Johnson, 1982).
The depth interval and the slope
of this mixing zone are dependent upon the relative rates
of production of each water mass.
Thus, past changes in
the relative production of these water masses may have
resulted in a wedge geometry different from what occurs
today.
11
Late Pliocene Climate
A useful tool for investigating paleoclimate is
oxygen-isotopic stratigraphy.
This tool is based on the
theory that changing climatic conditions result in varying
fractionation, or differential concentration, of oxygen
isotopes in seawater. The lighter isotope, 16 o, exhibits
a higher vapor pressure than the more rare, 18 o isotope.
Therefore, during times of increased evaporation, as occurs
during glacial periods, higher concentrations of the
lighter isotope are fractionated in vapor, and seawater
becomes enriched in the heavier isotope.
Most importantly,
ice sheets lock up freshwater, preventing normal
recirculation of this water back into the oceans.
Because
freshwater is enriched in the lighter isotope/ a drop in
s~a
level associated with glacial events corresponds to
higher concentrations of 18 o in the remaining seawater.
Marine invertebrate organisms record these isotopic changes
in seawater upon secretion of their calcium carbonate
skeleton. The changes are measured as 18 o; 16 o ratios
by a mass spectrometer.
Several studies of Late Pliocene climate suggest that
climatic cooling began at approximately 3.2 Ma and resulted
in the first permanent Northern Hemispheric ice-volume
growth about 2.4 Ma.
Shackleton and Opdyke (1977) used
oxygen-isotope fluctuations in the benthic foraminifera
12
Globocassidulina subglobosa to infer Northern Hemispheric
glacial events at 3.2 and 2.4 Ma.
An isotopic enrichment
of +0.4°joo, just prior to the Mammoth Subchron (3.2 Ma),
was interpreted as being equivalent to a 40 m drop in sea
level due to stored ice.
In addition, the event was
approximately simultaneous with the inception of ice-rafted
debris observed in sediments at DSDP Site 116 (Berggren,
1972).
The isotopic event at 2.4 Ma (+l.0°joo)
represented a change in the character of isotopic
excursions to a pattern more characteristic of those
representing the Pleistocene glacial-interglacial cycles.
Since the study by Shackleton and Opdyke (1977), many
authors have suggested that the 3.2 Ma glacial event was a
brief pulse at best and that the first significant ice
build-up did not occur until 2.5 to 2.4 Ma.
For example,
Shackleton and others (1984) compared oxygen-isotope
results from DSDP Site 552A, on the west flank of Rockall
Bank, to the onset of ice-rafting.
episode coincided with positive
A brief ice-rafting
o18 o
values at 2.5 Ma;
prior to 2.5 Ma, however, ice-rafted debris was present in
insignificant amounts.
They attributed the absence of
ice-rafted debris to the lack of extensive calving of
icebergs in the North Atlantic prior to 2.5 Ma.
In
addition, Backman (1979) reevaluated the biostratigraphy of
DSDP Site 116 and placed the beginning of ice-rafting at
2.5 Ma.
13
Prell (1985) examined the oxygen-isotopic evidence for
major glaciation at 3.2 Ma by assuming that a signif'icant
increase in the volume of continental ice sheets should be
recorded by enrichment of oxygen-isotopic values of both
planktonic and benthic foraminifera.
Except for a brief
co-variance from 3.2 to 3.15 Ma (suggesting a brief,
non-permanent ice-growth) , planktonic
o18 o records from
3.6 to 2.8 Ma did not exhibit the step-like enrichment that
the benthic record did.
He attributed this lack of
co-variance to represent new or increased production of
cold bottom waters in the high latitudes due to surface
cooling rather than an increase in ice-volume.
Similar
views are expressed by Hodell et al. (1985) in their
analysis of DSDP Sites 516A, 517, and 518; Thunell and
Williams (1983) in DSDP Sites 125 and 132; Backman (1979)
in DSDP Sites 111 and 116; and Loubere and Moss (1986) in
DSDP Site 548.
Ciesielski and Grinstead (1986) interpreted
the 3.2 Ma 18 o enrichment as at least partially related
to cooling of surface waters caused by northward expansion
of polar waters, production of colder bottom waters, and
expansion of the West Antarctic ice sheet.
They added that
the enrichment did not involve an increase in AABW
production.
Other studies by Stein (1986) and Rea and
Janacek (1982) show that increased accumulation of eolian
particles was directly correlated with increased
14
desertification and enhanced atmospheric circulation
associated with increased pole to equator temperature
gradients during the onset of climatic cooling near 3.2 Ma.
Thus, these studies suggest that the onset of climatic
deterioration, associated with a Northern Hemispheric
cooling event, commenced at 3.2 Ma with a brief, temporary
Northern Hemispheric ice-growth, and permanent cooling of
bottom waters.
Climatic conditions from 3.2 to 2.6 Ma were
unstable but relatively warm.
At approximately 2.6 Ma
there was a progressive cooling leading to a more permanent
glaciation at 2.4 Ma.
Core Locations
DSDP Sites 514, 516A, 517, 518, and 548 were selected
for this study because their modern positions in the
Atlantic Ocean provide the potential for recording
fluctuations of Atlantic deep waters in the past (fig. 2,
Table 1).
Additionally, each site contains complete
depositional records of pelagic sedimentation for the Late
Pliocene; thus they are ideal for monitoring the response
of deep waters throughout the Late Pliocene climatic event
and onset of Northern Hemispheric glaciation.
Because
deposition at each site is affected mainly by pelagic
sedimentation, grain-size fluctuations in the non-biogenic
silt fraction are inferred to be the result of water-mass
velocity changes.
15
Figure 2. Location and bathymetry of DSDP Sites 548,
516, 517, 518, and 514 (modified from Barker et
., 1983,
Ludwig
al., 1983, and deGraciansky et
., 1985).
16
TABLE 1:
CORE LOCATIONS AND THEIR PRESENT WATER MASSES
DSDP SITE
LOCATION
DEPTH
WATER MASS
516A
RGR
30°16'S
35°17'W
1313
AAIW-UCPW
517
vc
30°56'S
38°02.5'W
2963
NADW
518
vc
29°58'S
38°08.1'W
3944
LCPW-AABW
514
MAR
46°02.8'S
26°51.3'W
4318
AABW
548
GS
48°54.9'N
12°09.8'W
1251
MOW
RGR= Rio Grande Rise; VC= Vema Channel;
Atlantic Ridge; GS= Goban Spur.
MAR= Middle
AAIW= Antarctic Intermediate Water; UCPW= Upper
Circumpolar Water; NADW= North Atlantic Deep Water; LCPW=
Lower Circumpolar Water; AABW= Antarctic Bottom Water;
MOW= Mediterranean Outflow Water.
17
DSDP Site 514
DSDP Site 514 is situated on the lower flank of the
Middle Atlantic Ridge (MAR) in the Southeast Argentine
Basin, -250 miles north of the present-day position of the
polar front (fig. 2).
At a depth of 4318 m, the site is
influenced today by a sluggish southward return flow of
Antarctic Bottom Water (Ledbetter, 1986).
Due to the
presence of this site in the sluggish return flow, only
major fluctuations in AABW paleospeeds will be recorded by
the silt-fraction.
However, the proximity of site 514 to
the polar front (fig. 3) has distinguished it as site
well-suited for paleoceanographic studies of the
Pliocene-Pleistocene variations in the position of the
Antarctic convergence (Ciesielski and Grinstead, 1986).
DSDP Sites 516A, 517, and 518
Sites 516A and 517 are located on the Rio Grande Rise
in the Southwest Atlantic Ocean (fig. 2).
At a water depth
of 1313 m, Hole 516A lies within the transition zone
present between AAIW and UCPW (fig. 4).
Site 517 (2963 m)
lies within the core of NADW today (fig. 4).
Site 518 lies within the Vema Channel (fig. 2), a major
conduit between the Brazil and Argentine Basins.
At a
water depth of 3944 m, Site 518 is situated near the base
of the transition between AABW and NADW on the eastern
18
40°5
46°5
SUBTROPIC.lll.
CONVERGENCE
SITE
514
50°5 .
64°5
ANTARCTIC
CONVERGENCE
ANTARCTIC
DIVERGENCE
~~lh~*--~---~·~~~~~'~~
--
1000
]
1500
-
j:
-
a.
c
IIJ
2000
.........
/
2500
_.,.
{
3000
)
.)
(
)
4000
Figure 3.
Position of DSDP Site 514 relative to the
modern day polar front and deep-water masses (modified from
Ciesielski and Grinstead, 1986) .
19
0 2 ( JJ.M /Kg)
220
00
260
DRILL
SITES
_ AAIW
-+516
UCPW
E
.;g
2
--Salinity (%o)
:I:
1--
i
a..
w
0
-Oxygen (uM/Kg)
3
I
./
,..
,.
/
·-·Temperature (° C)
.I
NADW
./
/
4
.(
!S
•517
LCPW
•518
AABW
0
2
34.2
34.6
!
4 Tl'"C)
Sl"l.... l
Figure 4. Positions of DSDP sites 516A, 517, and 518
relative to the deep waters of the south Atlantic Ocean
(modified from Barker et al., 1983).
20
margin of the channel (fig. 4).
site 518 is therefore
well-situated to record Late Pliocene fluctuations of the
AABW/NADW transition zone.
DSDP Site 548
Site 548 is located southwest of Ireland in 1251 m of
water (fig. 2).
Its modern position in MOW provides the
potential to record Late Neogene history of MOW with
respect to changing climatic conditions.
METHODS
Samples from DSDP Sites 516A, 517 (3.21-2.85 m.y.
samples only), 518, and 548 were wet-sieved using a 63
screen.
analysis.
(<45
~m)
The fine fraction (<63
~m)
~m
was saved for
Fine-fraction samples from DSDP Sites 514
and 517 (2.91-2.13 m.y. samples) were provided
from previous investigations.
Fine fractions were triple-decanted to leave only the
silt particles.
The samples were oven dried at 40°C and
dry split to -0.4 gram sub-samples; the remainder was
vialed and saved.
The sub-samples were treated with 10%
Hcl to remove carbonate.
After washing, the carbonate-free
sub-samples were boiled for 30 minutes in 1.0 M Na 2 co 3
to remove biogenic silica.
Sub-samples were then rinsed
twice to remove excess Na 2 co 3 solution.
Selected
samples were examined under a microscope to insure that all
the biogenic material had been removed.
In a few cases not
all the biogenic silica was removed so it was necessary to
re-boil the batch in the same concentration of Na 2 co 3 ,
but for half the original time.
It was useful to stir the
sample carefully so that the siliceous particles were
better exposed to the solution.
It is necessary to remove
the biogenic constituents because the size of the carbonate
and siliceous particles reflects factors other than
bottom-water velocity (Blaeser and Ledbetter, 1982).
22
Once all the biogenic components were removed, -100 ml
of distilled water was added to the sub-sample.
This
sub-sample was analyzed on an Elzone (Particle Data Inc.)
electronic sizing instrument using a 120
~m
orifice.
This
instrument has a computer interface similar to that
described by Meurdter et al. (1981).
For a complete
discussion of particle-size methods consult Blaeser and
Ledbetter (1982).
The samples were analyzed until two
consecutive runs were within 0.05 phi of each other.
The
average of the two consecutive runs was computed and used
as the mean-size value for that sample.
The coarsest
one-percentile value was obtained from a statistical run of
the last of the two samples that were within 0.05 phi of
each other.
RESULTS
The mean particle size and coarsest one-percentile of
the silt fraction for samples in each core were plotted
versus age (figs. 5, 6, 7, 8, 9; Appendix A).
It should be
noted that the paleomagnetic time scale is included for
reference only, due to ambiguity in the magnetostratigraphy
for Sites 516A, 517, and 518.
This ambiguity is a result
of the physical nature of the major lithologic unit, which
consists of nannofossil-foraminiferal ooze (Barker et al.,
1983).
The age framework for sites 516A, 517, and 518 is
derived from Hodell et al.
(1985) and is based upon
interpolation between planktonic foraminifera datum levels,
which are based upon intercalibration of the polarity time
scale of McDougall et al.
Biostratigraphic correlation of
these sites was refined by matching percent carbonate and
isotopic curves for planktonic foraminifera between sites
(Hodell et al., 1985).
Ages for samples at Site 514 were determined by
extrapolation from sedimentation rates between well-defined
paleomagnetic boundaries (Ciesielski and Grinstead, 1986).
Stratigraphy for Site 548 is ambiguous due to the weak
paleomagnetic intensity and the lack of stratigraphically
useful taxa.
The stratigraphy was determined using a
combination of biostratigraphy, isotope stratigraphy, and
eco-stratigraphy (deGraciansky et al., 1985).
Between-site
SITE 548
1251 m
SILT MEAN, 0
COARSEST 1%,0
benthic
'2.5 Ma
150
E
:i
I-
160
ll..
w
c
170
Figure 5. Comparison of mean and coarsest one-percentile of the silt fraction
to oxygen isotope data (from Loubere, 1986) for DSDP Site 548. A vertical line is
drawn through the mean of the data for each parameter.
Periods of relatively
enriched oxygen isotopes and higher than average grain-size statistics are darlcened.
>>= z !::
~ 0
ui
CJ
<
a: a:
:X: <
..J
0
11..
.....:
<
2.4
0
~
SITE 516A
1313 m
SILT MEAN ,0
6.110
11.40
COARSEST 1%,0
5.00
4.110
4.20
~
U
2.80
18
Dpoa
'lloo
2.40
benthic
u~ 18.Opoe L
0.60
0.00 1.00
planl<lonlc
Q t3CPDB 'lf.o
0.16
benthic
2.60
2.00
pl11nklonkl
I-
I a.o
ffi
m
..J
a
Figure 6. Grain-size statistics versus geochemical data for DSDP Site 516A.
oxygen and carbon isotope data are from Hodell et al., 1985. A vertical line is
drawn through the mean of the data calculated for each parameter. Periods of
relatively enriched oxygen isotopes, depleted carbon isotopes, and higher than
average grain-size statistics are darkened.
N
lJl
>=
::f
w
(!J
SITE 517
>z !::
0
a: a:
<(
<(
::1:
0
2.4
:IE
..J
0
ll.
SILT MEAN ,0
6.80
8.60
COARSEST 1%,0
!UO
4.90
s::-•e
U
3.00
2963 m
~ 10
U OPDBL
OPDB%.
2.50
0.50
0.00
i.OO
0.75
0.50
2.50
~
<(
, benllllc
plank Ionic
Figure 7. Grain-size statistics versus geochemical data for DSDP Site 517. ,
Oxygen and carbon isotope data are from Rodell et al., 1985. A vertical line is
drawn through the mean of the data calculated for each parameter. Periods of
relatively enriched carbon isotopes, depleted carbon isotopes, and higher than
average grain-size statistics are darkened.
SITE 518
3944 m
SILT MEAN, 0
COARSEST 1%,\l)
S:O.te
0
B'eoPDB -..
OI'DB'A.
0.25
0.00 -0.;!5 -0.50
s:-13
o
tl.
Cpoe'A.
1.10 0.110 0.50 0.20
2.00
1.60
CARBONATE
10
80
benthic
Figure 8. Grain-size statistics versus geochemical data for DSDP Site 518.
Oxygen and carbon isotope data are from Hodell et al., 1985 and carbonate
preservation data are from Hodell et al., 1983. A vertical line is drawn through
the mean of the data calculated for each parameter. Periods of relatively enriched
oxygen isotopes, depleted carbon isotopes, increased carbonate preservation, and
higher than average grain-size statistics are darkened.
N
-J
SITE 514
4318 m
COARSEST 1 'II.
,0
FACTOR 2 LOADING
FACTOR 4 LOADING
(ANT ARCTIC)
(NORTHERN DUIIANTAilCTIC)
POLAR FRONT POSITION
RELATIVE TO SITE 614
&OUIDG Of I 14
fACtOR 0
P.f.
PACTOR 8
NOiHH Of 114
fACtOR ll
0.19
Figure 9. Grain-size statistics versus faunal assemblage information
(Ciesielski and Grinstead, 1986). A vertical line is drawn through the mean of the
data calculated for the grain-size parameters.
Periods of higher than average
grain-size statistics are darkened.
N
00
29
correlation of paleoceanographic events should be made with
this lack of magnetic resolution in mind.
To allow for down-core interpretation of relative
changes in grain size, the mean of the grain-size data was
computed for each site and a line was drawn through the
plot at the appropriate value.
Oxygen-isotope data
obtained from benthic foraminifera at Site 548 are shown on
figure 5.
Carbon- and oxygen-isotope data obtained from
benthic and planktonic foraminifera at Sites 516A, 517, and
518 are included and shown adjacent to the appropriate
plots (figs. 6, 7, 8).
In addition, percent carbonate data
are presented for Site 518 (fig. 8), and radiolarian faunal
assemblages are presented for Site 514 (fig. 9).
The relationship between plots of the mean and coarsest
one-percentile of the silt fraction is unique for each
site.
The covariance between plots of the two grain-size
parameters for each site was determined by linear
regression.
These values are: 0.71 (Site 514), 0.73 (Site
516A), and 0.81 (Site 548).
Plots for sites 517 and 518
show very little covariance (0.14 and 0.04 respectively).
The unique relationship of the mean and coarsest
one-percentile plots for each site may be a result of the
type(s) of water masses affecting each site through time
and is discussed later.
The data from the coarsest one-percentile plots are
arranged on a time scale and contoured between sites as
30
illustrated on the left contour plot in figure 10.
Comparison of the coarsest one-percentile values between
sites generally indicates decreasing grain size with
increasing depth.
Coarsest one-percentile values at Site 516A range from
5.16 to 4.12 phi, and display a general coarsening trend
up-core (fig. 10, Appendix A); values at Site 517 range
from 5.26 to 4.71 phi and also display a general coarsening
up-core.
Coarsest one-percentile values at Site 518 range
from 5.29 to 4.63 phi; however, 70% of the silt fraction is
between 5.1 and 5.29 phi.
Coarsest one-percentile values
at this site also coarsen up-core.
The transition from
fine- to coarse-silt particle size appears most distinct at
site 518, and occurs at -2.78 Ma.
Coarsest one-percentile
values at Site 514 range from 5.66 to 4.72 phi; only three
values (at 2.98, 2.78, and 2.20 Ma) are coarser than 5.0
phi.
As with the previous sites, coarsest one-percentile
values coarsen up-core.
A transition from fine- to
coarse-silt particle size is not distinct at Site 514.
Conversely, the grain-size increase generally appears to
occur gradually as each peak reaches a coarser value than
the preceeding one (fig. 9).
Although Site 548 is not
included in the contour diagram, coarsest one-percentile
values coarsen up-core from 4.32 to a high of 4.10 phi;
these values indicate that the silt range is consistently
coarser than at any of the other sites.
COARSEST 1% ,CI
SILT MEAN ,0
6.45
6.40
<
::!;
<
>-
2.4
11 < 4.9 0
0 4.9-5.1
~
<
::!;
[J
§
2.5
> 3.2%o
3.0-3.2%o
0
6.1-6.3
2.8-3.0% 0
Iilli < 2 . ao'/00
E:::;J
>5.3l/>
2.6
2.7
C/l
C/l
::J
2.8
<
Cl
2.9
3.0
3.1
514
518
517
516A
518
517
4318m
3944m
2963m
1313m
3944m
2963m
516A
1313m
Figure 10. Oxygen isotope values (Leonard et al., 1983; Hodell et al., 1985)
versus grain-size data illustrated in contour diagrams to allow for comparison of
climatic events to deep-water activity. The silt-mean plot (Bark, 1986) represents
AABW activity.
32
The contour diagram {fig. 10) shows five events during
which coarser sediment was deposited throughout the·time
period from 3.2 to 2.3 Ma.
The coarsening event at -2.62
to 2.52 Ma appears to differ from the rest in that the
sediment is markedly coarser and the event appears most
distinct at Site 518.
The other coarsening events at -3.15
to 3.10, 2.85, 2.71, and 2.40 Ma appear at all depths
represented by the sites.
An overall coarsening of the
silt fraction deposited by deep-water currents occurs at
2.89 Ma and continues until 2.30 Ma.
A contour diagram of benthic oxygen-isotope data
derived from samples of the same time period from Sites
516A, 517, and 518 (Leonard et al., 1983; Hodell et al.,
1983, 1985) is juxtaposed with the coarsest one-percentile
contour diagram to illustrate possible climate/deep-water
interactions (fig 10).
Periods of oxygen-isotopic
enrichment at Site 517 occur at 3.15 to 3.10, 3.05, 2.68 to
2.57, and 2.45 to 2.38 Ma.
Similar isotopic enrichment
events occur at site 518 but are not as prominent for the
3.15 to 3.10 and 3.05 Ma events.
In addition, an isotopic
enrichment (+3.09°joo} occurs at 2.95 Ma at Site 518 that
is not seen at Site 517 (+2.54°joo).
Site 516A shows a
gradual relative oxygen-isotopic enrichment up-core, with
highest values present between 2.95 to 2.8 Ma (2.9°joo}.
DISCUSSION
Grain-size statistics
The unique relationship between the mean and coarsest
one-percentile plots of the silt-size distribution for each
core reflects the different hydrodynamic properties of the
specific water masses affecting sedimentation at each
site.
This is reflected today at sites near the Rio Grande
Rise-Vema Channel region: Blaeser and Ledbetter (1982)
determined that textural characteristics of sediment vary
between sites influenced by AABW, transition zone, and NADW
water masses (fig. 11).
Blaeser and Ledbetter (1982) found
that sediment influenced by AABW has a coarse-fraction
component in the silt-size distribution that affects all
textural statistics.
As a result, the silt fraction of
sediment deposited by AABW has the largest average grain
size and the coarsest one-percentile values, and the
poorest sorting of the three water masses.
The comparison
of mean vs. coarsest one-percentile plots of the silt
fraction from sediment deposited by AABW, then, should show
simultaneous fluctuations.
Sediment textural statistics
from the AABW/NADW transition zone can change rapidly
depending on the gradient or transition between the two
water masses.
When transition-zone sediment is influenced
by the encroachment (shallowing) of AABW, a coarse silt
Representative Frequency Distributions of the Vema Channel
16~------------------------------------------------------------------~
I
<"II'
'
I
I I,\..
'\
12
~
Transition
..
>-
• '
r·
0
@
•
fl •
II~
• I
Zone,f~
'
u
c 8
Q)
::J
tT
Q)
"-
lL
4
7.6
7.2
6.8
6.4
6.0
Particle Size,
5.6
5.2
4.8
4.4
4.0
cp
Figure 11. Textural characteristics of sediment deposited from AABW, NADW, and
the transition zone in the Rio Grande Rise-Vema Channel region of the South Atlantic
Ocean (after Blaeser and Ledbetter, 1982).
35
sub-population is introduced to the water mass.
This
coarse silt sub-population is evident in the coarsest
one-percentile statistics, but not necessarily the mean
statistics, of the silt fraction.
Because the transition
zone sediment also has a large component of very fine silt
(a result of the particle deposition in the zone of least
motion between northward-moving AABW and southward-moving
NADW), the injection of this fine component may prevent the
mean value from showing an increase simultaneous with the
coarsest one-percentile statistics.
If, however, the site
is influenced by a transitional water mass more affected by
NADW (deepening), the silt population will not include the
coarse sub-population that is characteristic only of
sediment deposited by AABW.
This will be revealed in the
coarsest one-percentile statistics, but again, not
necessarily by the mean size.
The coarsest one-percentile
and mean size of the silt populations should be finest at
the "level of least motion" within the transition zone due
to its relatively low velocity and inability to support a
medium to coarse silt population (Blaeser and Ledbetter,
1982).
Sediment deposited under the influence of the core
of NADW is the best sorted of the above examples and is
characterized by a moderate mean grain size.
The well
sorted nature of this sediment should result in a mean silt
value that fluctuates simultaneously with the coarsest
36
one-percentile value.
Due to the lower relative speed of
NADW compared to AABW, finer silt particles are deposited
and grain-size statistics should not reach values as high
as those of AABW.
It is probable that sedimentation at deep-water sites
was controlled and dominated, historically, by different
water masses.
Because mean statistics do not necessarily
differentiate between water masses, the coarsest
one-percentile of the silt fraction best shows vertical
fluctuations of the deep-water masses.
In addition, the
use of coarsest one-percentile statistics eliminates the
factor of entrapment of fine silt by clay floes, which
would result in a misrepresentative (finer) mean silt
value.
The mean statistics, however, may be used in
conjunction with the coarsest one-percentile statistics to
determine if down-core grain-size changes in the silt
population reflect fluctuations in speed within one water
mass or changes in the relative position of water mass(es)
affecting the site.
Climate/Deep-water Interactions
When grain-size data are compared with oxygen- and
carbon-isotope data, percent-carbonate data (Hodell et al.,
1985), and faunal-assemblage data (Ciesielski and
Grinstead, 1986; Hodell et al., 1985), a possible origin
37
for the deep waters can be inferred for the events
illustrated on the contour diagram of coarsest
one-percentile values (fig. 10).
In addition, the contour
diagram shows the relative intensity of pulses in
deep-water paleospeeds, as well as the stratification of
the deep-water column at certain times during the 3.2 to
2.3 Ma interval.
Where the grain size at each site is
approximately the same for a specific time period, one
water mass may have dominated the deep-water column.
Where
grain sizes differ between sites, the water column was
apparently well-stratified and may have resembled that of
today (fig. 4).
Because the deep waters of the past may
not have had physical and chemical properties identical to
those of today, acronyms representing the present water
masses are used only to imply the possible source of the
water mass and its resulting water-mass properties.
,~
The time period 3.2 to 2.3 Ma may be divided into four
time intervals based on deep-water activity inferred from
grain-size statistics.
Silt particle sizes deposited
between 3.2 to 2.9 Ma were relatively fine at all sites,
suggesting that the water column was dominated by
relatively quiescent paleospeeds, except for a pulse of
increased paleospeed at 3.15 to 3.10 Ma.
From 2.9 to 2.6
Ma, silt particle sizes, and deep-water paleospeeds,
increased at all sites; from 2.6 to 2.45 Ma, deep-water
paleospeeds decreased slightly except at Site 518, where
38
paleospeeds increased dramatically between 2.6 and 2.5 Ma;
from 2. 45 to 2. 3 Ma paleospeeds at Sites 514, 518, a·nd 517
increased and then decreased again by 2.3 Ma.
3.2 to 2.9 Ma
The low inferred paleospeeds from 3.2 to 2.9 Ma are
coincident with relatively variable, depleted benthic
o18 o values at Sites 516A and 517 (figs. 6, 7, a, 10).
Pulses of 18 o enrichment occur at 3.12 and 3.05 Ma at
o18 o values at 516A
of o18 o at Site 518
Site 517, whereas
depleted.
Values
remain relatively
remain slightly
enriched during the time period but do not fluctuate to the
extremes as those values at Site 517.
The lower relative percent carbonate values and
depleted values of
o13 c
for Site 518 (fig. 8) show the
corrossivity and/or "age" of the water mass influencing
sedimentation at this site.
because depleted
o13 c
This conclusion is made
values generally indicate an
increased supply of carbon from metabolic processes of
marine plankton.
As "older" and deeper water masses have
accumulated more nutrients, they are generally more
depleted in 13 c than "newer" and surface waters.
Therefore, a change in the cycling of nutrients generated
by a decrease (or increase) in the circulation rates is
reflected in the 13 c of the water and is recorded in the
calcium carbonate tests of microfossils.
39
Low mean and coarsest one-percentile statistics of the
silt fraction also suggest a sluggish water mass.
Thus,
together these factors suggest that a slowly moving
deep-water mass, with a source from the Antarctic and
hydrodynamic properties similar to the modern transitional
water mass, was present at Site 518.
The fine grain-size
values also indicate a deepened AABW such that there was
not an injection of the coarse-silt subpopulation at the
depth of Site 518.
Site 517 shows simultaneous decreases in both mean and
coarsest one-percentile values of the silt fraction during
this time interval.
The o13 c values fluctuate but remain
relatively depleted from 3.18 to 2.98 Ma and become
enriched from 2.98 to 2.90 Ma.
This enrichment corresponds
to a relative increase in the mean size and relatively
unchanged coarsest one-percentile values.
The overall
depleted o13 c values indicate a probable Antarctic source
for the deep waters at this site, but the fluctuations
therein imply that a shallower water mass with lower
nutrient values periodically deepened to this site.
Enrichment of o1 3 c values at 2.98 Ma implies a deepening
of deep waters, with a probable North Atlantic origin, at
this time.
Site 516A is characterized by fluctuating, but
relatively low, mean and coarsest one-percentile values
40
from 3.2 to 2.9 Ma.
with depleted
The low inferred paleospeeds coincide
a13 c values until about 3.04 Ma when o13c
values became enriched and correlate with a slight peak in
coarsest one-percentile values.
At approximately 3.0 Ma,
however, grain-size values return to those similar to the
beginning of the time interval while o13 c values remain
enriched.
As with Site 517, Site 516A was probably
influenced by a deep-water mass with an Antarctic source
throughout most of this time interval.
The peak in
coarsest one-percentile statistics at 3.05 to 3.02 Ma may
have involved a permanent encroachment of faster-moving,
oxygen-enriched AAIW to depths of Site 516A.
Following a hiatus from 3.86 to 3.18 Ma (Ciesielski and
Grinstead, 1986), indicating enhanced AABW speeds, the silt
fraction at Site 514 progressed from relatively coarse at
3.2 ·to 3.0 Ma, to fine at 3.0 to 2.9 Ma.
of "mixed-zone" and
11
The association
upwelling 11 radiolarian factors
(fig. 9) during this time period (3.2 to 2.9 Ma) suggests
that the position of the polar front was near or directly
above Site 514 (Ciesielski and Grinstead, 1986) .
This
indicates that, although Antarctic climate was colder than
today, the colder climate did not result in a significant
increase in AABW paleospeeds as would be expected.
Instead, a decrease in coarsest one-percentile values was
observed following the hiatus, suggesting a subsiding AABW
41
speed.
These observations are confirmed by a composite
plot of mean values of the silt fraction, shown in figure
10, compiled from four deep Islas Orcadas piston cores
presently located within AABW (Bork, 1986).
The relatively low inferred paleospeeds and depleted
o13 c
values at all sites, and evidence of increased
carbonate dissolution rates at Site 518 (fig. 8), indicate
that deep waters originating in the Antarctic were present
at all sites during the time period from 3.2 to 2.9 Ma.
The expansion of this deep-water mass implies a reduction
in the volume of both NADW and AABW in the South Atlantic.
This concurs with the findings of Ledbetter and Ciesielski
(1986)· in their observation of an increased trend for early
Gauss shallow hiatuses (CPW) and a decreasing trend for
deep hiatuses (AABW) for the same time period in the South
Atlantic sector of the Southern Ocean.
These results are inconsistent with the interpretations
of Hodell et al. (1985).
Hodell et al. (1985) suggested an
increased influence of AABW near Site 518 based on
increased occurrences of Nuttalides umbonifera, a benthic
foraminifer present in modern 13 c-depleted AABW.
However, it is possible that slowly moving CPW, depleted in
13 c, had chemical properties similar to those of modern
AABW, thus allowing the habitation of this species of
foraminifera within CPW.
42
An increase in LCPW volume at the expense of AABW and
NADW would suggest that significant glaciation and global
cooling had not yet begun to allow large volumes of North
and South Atlantic bottom waters to form and infiltrate the
South Atlantic.
Furthermore, the pulse of increased
inferred paleospeeds at 3.1 Ma (fig. 10) does not
correspond with a change (enrichment) of o13 c values nor
an increase in carbonate preservation at Site 518 as would
be expected with an increased
bottom water.
in~luence
of new (Antarctic)
I attribute the increased paleospeeds to a
short pulse of increased Antarctic Circumpolar Current
(ACC) circulation, resulting in simultaneous increased
paleospeeds of LCPW.
Higher paleospeeds at the shallower
sites (517, 516A), are inferred to have been a result of
less attenuation of Coriolis deflection at shallower
depths.
If AABW had shallowed to depths near Site 518,
grain size would be coarsest at this site during the
temporary increase in paleospeeds at 3.1 Ma.
The widespread global enrichment of 18 o at 3.1 Ma is
therefore interpreted as not associated with an increase in
bottom-water production, but partially related to cooling
of surface and bottom waters associated with a cooling
climate.
The cooler climate increased pole to equator
temperature gradients, enhanced wind patterns (Rea and
Janacek, 1982; Stein, 1986), and intensified the ACC
circulation.
43
2.9 to 2.6 Ma
A significant change in the pattern of South Atlantic
deep ocean circulation began at 2.89 Ma with inferred
pulses of increased paleospeeds occurring at 2.85, 2.72,
and 2.62 Ma (fig. 10).
Oxygen-isotopic enrichment occurs
at 2.85 Ma and 2.7 Ma to 2.6 Ma and is synchronous with
periods of increased inferred paleospeeds.
Thus, these
pulses record the first flux of an oxygen-enriched,
nutrient-depleted, North Atlantic deep water into the South
Atlantic during the Late Pliocene (fig. 10).
NADW
dominated the deep water column and expanded to depths near
Site 518 (3944 m), with its core at or near depths of Site
517 (2963 m).
Largest coarsest one-percentile values at depths of
Site 518, approximately 2.7 Ma, allows the inference that a
pulse of increased paleospeed occurred at this time.
This
inferred increase of paleospeed coincides with enriched
613 c and high percent carbonate values from samples at
the same site, suggesting that the core of NADW deepened
temporarily to this depth.
Consistent with this is the
slight depletion in 613 c values at Site 517 to values
less than at Site 518 (Hodell et al., 1985).
This
indicates that the core of NADW had moved from Site 517 to
depths of Site 518 and was replaced, temporarily, by deep
waters derived from the Antarctic.
44
Between these pulses, inferred paleospeeds remained
higher than those of the previous time period.
the character of the benthic
o18 o
A change in
values also occurs at
this time to more consistent and similar between-site
values for sites 516A, 517, and 518 (fig. 10).
Although the record is discontinued for Site 516A after
2.78 Ma, it is evident that a dramatic increase in both
coarsest one-percentile and mean values of the silt
fraction coincides with an enrichment in
o13 c
values.
This would indicate a deepening of AAIW to this depth
(1313 m).
A short 13 c depletion event at 2.8 Ma may
correspond with UCPW shoaling, but conditions similar to
those at the beginning of this time period were
re-established at 2.78 Ma.
Relatively low coarsest one-percentile and mean values
of the silt fraction at Site 514 characterize this time
period and correspond with low mean values from the AABW
composite plot.
As mentioned previously, decreased
sensitivity of this site to fluctuations in AABW, due to
its position in a sluggish return flow, eliminates this
site from extensive interpretation during periods of low
paleospeeds.
Factors involved in the expansive penetration of NADW
into the South Atlantic were the combination of the absence
45
of significant production of CDW (Ciesielski and Grinstead,
1986) and AABW (Ledbetter and Ciesielski, 1986; Bork, 1986)
along with the significant Northern Hemispheric cooling
from 3.0 to 2.5 Ma.
The pulses of increased deep-water
paleospeed and the deepening of NADW from 2.7 to 2.65 Ma
occurred simultaneously with the oxygen-isotopic
enrichments (fig. 10).
This indicates possible climatic
forcing upon the production of NADW and its penetration
into the South Atlantic.
Further evidence of climatic
forcing upon deep-water activity is cited by Stein and
Bleuil (1986).
They found that increased winnowing of the
fine fraction and sorting effects in the sand fraction of
deep-water Site 141 (4148 m) coincided with almost
contemporaneous hiatuses in the NE Atlantic deep-water
Sites 366 (2853 m), 397 (2900 m), and 544B (3607 m).
Additionally, these events coincided with an increased
advection of well-oxygenated deep water in the Northeast
Atlantic as indicated by a distinct increase of
o13 c
values, and cooling of NADW, as revealed by the drastic
increase in
o18 o
values.
Their conclusion.from these
simultaneous events was that marked intensification of NADW
production and advection occurred near 2.75 Ma, coincident
with an expansion of Northern Hemispheric glaciation and/or
a major cooling of NADW.
46
2.6 to 2.5 Ma
Another change in the character of deep-water
circulation occurred from 2.6 to 2.5 Ma.
Prior to this
time period, inferred pulses of increased paleospeeds
affected all depths.
During this time interval, however,
decreased paleospeeds are inferred at Site 517 and slightly
increased paleospeeds are inferred at Site 514.
A
significant increase in inferred paleospeeds occurs at Site
518, based upon coarsest one-percentile values that reach
far higher than those found in older sediment at this site.
Synchronous with this increase in coarsest
one-percentile values is a decrease in percent carbonate to
lowest values (62%) recorded at Site 518, and a depletion
in 13 c from 2.62 to 2.55 Ma. The grain-size statistics
and geochemical indicators recorded in sediment from Site
518 suggest that a fast-moving, nutrient-rich,
carbonate-depleted, deep-water mass was present at this
site.
As mentioned above, inferred paleospeeds decreased at
Site 517; unlike Site 518, however, mean and coarsest
one-percentile values of the silt fraction decreased
simultaneouly at this time. In addition, benthic
values were enriched 2.6 to 2.42 Ma as were
o13 c
o18 o values.
47
These combined factors indicate that a more slowly moving
water mass, derived from the North Atlantic, was present at
this site.
As illustrated in figure 9, a slight increase in
inferred paleospeeds at Site 514 was concurrent with a
minimum sample diversity of 50 radiolarian taxa (2.57 Ma)
and highest Antarctic factor loading (2.58, 2.56, 2.52,
2.47 Ma) at the same site (Ciesielski and Grinstead,
1986).
These statistics indicate that coolest Antarctic
surface waters for the Late Pliocene were present and
corresponded with movement of the Polar Front Zone north of
Site 514.
A northward shift of the polar front (Ciesielski and
Grinstead, 1986), enhanced AABW production (Bark, 1986;
Ledbetter and Ciesielski, 1986), and enriched o18 o values
at Site 518 marked the reintroduction of significant
volumes of bottom waters with an Antarctic source into the
Rio Grande Rise region.
The shallowing of AABW to depths
near Site 518'allowed for an injection of a coarse silt
sub-population into the sediment size distribution.
The
presence of a fine-silt tail, however, indicates that Site
518 was situated within the transitional water mass between
NADW and AABW.
Site 517 was most likely influenced by the
NADW as evidenced by the enriched o1 3 c values.
The
relatively depleted o18 o values at Site 517 would
48
indicate a slight Northern Hemispheric warming; this agrees
with the fine grain size and slow inferred paleospeeds as a
Northern Hemispheric warming is inferred to result in a
decrease in the production of North Atlantic Bottom Waters.
2.5 to 2.3 Ma
A trend from relatively slow to increasing inferred
paleospeeds occurs throughout the latest time interval.
Grain size is finer, and therefore inferred paleospeeds are
lower, for the 2.5 to 2.4 Ma interval than seen previously
for Sites 517 and 518.
The first significant and
continuous increase in grain size occurs at Site 514,
coarsening to values similar to those at Sites 517 and 518.
For the first time at Site 518, high carbonate values
coincide with depleted
o13 c values; this occurs from 2.5
to 2.4 Ma when coarsest one-percentile values at this site
are above their mean value but are low compared to the
previous two time intervals.
The o18 o values are
relatively enriched but do not reach a peak value until
approximately 2.42 Ma.
From 2.42 to 2.3 Ma, the opposite
is true for the above paleoceanographic indicators at Site
518: low carbonate values coincide with enriched o1 3 c and
o18 o values and relatively higher coarsest one-percentile
values.
From 2.5 to 2.4 Ma, relatively low coarsest
one-percentile values correspond with enriched
o13 c
49
values and depleted
however,
o18 o
o13 c
o18 o·values
at Site 517.
By 2.4 Ma,
values are significantly depleted while
values are enriched and coarsest one-percentile
values begin to approach their peak values.
By this time interval, coarsest one-percentile values
for Site 514 remain consistently high except for a brief
decrease from 2.4 to 2.45 Ma.
This brief lapse corresponds
approximately with a deviation from predominantly
"Antarctic" radiolarian assemblages to "upwelling" and
"mixed-zone" radiolarian assemblages at Site 514
(Ciesielski and Grinstead, 1986).
This deviation indicates
a temporary southward shift of the polar front to a
position near Site 514.
At 2.3 Ma, however, the Antarctic
radiolarian assemblage returned to Site 514 as the Polar
Front Zone regained its northerly position.
The anomalous relationship between
o13 c
and percent
carbonate values at Site 518 for this time interval
suggests that an appreciable change in the Calcite
Compensation Depth (CCD) was independent of a change in the
13 c.
The
o13 c
values of benthic foraminifera
apparently were affected by factors other than those
directly associated with the water mass.
Therefore,
percent carbonate is probably a better water-mass indicator
because it measures the carbonate undersaturation
associated with the influencing water mass.
From 2.5 to
50
2.42 Ma, the high percent-carbonate values and lack of a
coarse or fine tail in the silt distribution (at Site 518)
indicate the influence of a water mass similar to modern
NADW.
At 2.42 Ma, however, the percent-carbonate values
declined to a low (67%) and did not return to their high
values until approximately 2.2 Ma.
At the same time,
coarsest one-percentile and mean values of the silt
fraction decreased (slightly) until they reach a minimum
value at 2.3 Ma.
This would suggest the shallowing of NADW
and the return of a transitional water mass to depths of
Site 518.
Interestingly,
o13 c
values at Site 517 became
significantly depleted at the time of inferred shallowing
of NADW from Site 518.
Because it is documented that
glaciation had commenced in the Northern Hemisphere and
corresponded with significantly enriched
o18 o
values,
NADW must still have been a major component of the South
Atlantic deep waters.
Therefore, NADW is inferred to have
shallowed to depths near 517.
Again, a depleted
value associated with NADW seems anomalous.
a curious lowering of
a13 c
o13 c
Concurrently,
values and a dramatic increase
in the size and abundance of Globocassidulina subglobosa
was noted for the same time period at Site 548 (Loubere and
Jakiel, 1985) and Site 606 (Keigwin, 1986).
Keigwin (1986)
suggested that the microhabitat (Corliss, 1985) of
~
subglobosa was affected by increased flux of organic matter
51
to the deep sea approximately 2.4 Ma.
If true, this
increased flux of organic matter could have significantly
altered the 13 c fraction within the water masses causing
the observed anomalous values.
Unfortunately, the lack of a record for Site 516A
restricts the interpretation at this depth for this time
interval.
However, it is evident that accompanying this
time period of glacial advance was an increase in the
inferred paleospeeds of the deep waters and a fundamental
change in the relationship of
values.
o13 c
and percent carbonate
The northward advance of the Southern Hemispheric
polar front zone past its present position near Site 514
(Ciesielski and Grinstead, 1986) , coincident with the
extensive Northern Hemispheric glaciation (Shackleton et
al., 1984), demonstrates the degree of global cooling at
this time.
Increased inferred paleospeeds at Site 514 (2. 4
Ma) may indicate that AABW velocities had significantly
increased to affect grain size at this site.
Interpretation of Site 548
The stratigraphy for Site 548, unfortunately, is rather
ambiguous due to weak paleomagnetic intensity and lack of
warmer water, stratigraphically useful taxa (deGracianski
et al., 1985).
Ages are approximate, therefore, and were
not assigned to down-core depth values.
Ages are indicated
52
on the grain-size and isotopic plots (fig. 5) according to
information from Loubere (19S6), Loubere and Moss (19S6),
and Loubere and Jakiel (19S5).
Due to this ambiguity, the
direct correlation of ages between those of Site 54S and
the South Atlantic sites should be made with reservation.
A significant increase in the grain size of the silt
distribution occurs at approximately 2.S5 Ma, approximately
coincident to the date that Loubere (19S6) assigns for the
first appearance of MOW at Site 54S (2.9 Ma).
A comparison
of the oxygen isotope plot to the grain-size plot shows
a1 So values decrease with an increase in grain
that
size.
An influence of MOW would have such an effect on the
a1 So values because the higher-temperature of the water
mass would cause the benthic foraminiferal
ratio to be anomalously light.
lSo; 16o
The coarser grain-size
distribution would be the result of the replacement of a
more sluggish eastern basin North Atlantic intermediate
water mass, with a faster moving eastern boundary current
of Mediterranean origin (Loubere, 19S6).
Interestingly, this time of first significant influx of
MOW corresponds with the first Late Pliocene pulse of NADW
to depths of Site 517 (2963 m) and Site 518 (3944 m) as
shown in figure 10.
This seems reasonable if the
contribution of the salinity component of MOW is important
in the reinforcement and formation of a more dense NADW
53
{Reid, 1977; Johnson, 1982), allowing NADW to approach
deeper depths in the South Atlantic than previously.·
Accordingly, a peak in the grain size of the silt fraction
occurs approximately 2.7 Ma, when it appears from
grain-size and isotopic data at Site 518 that the core of
NADW deepened temporarily to this site.
Mean and coarsest one-percentile values remain
consistently high at Site 548 until 2.6 to 2.5 Ma, when
grain sizes were appreciably smaller and coincided with an
enrichment in alSo values.
This may correspond to a
shallowing of MOW from depths of Site 548 (Loubere, 1986).
This shallowing from 2.6 to 2.5 Ma corresponds with an
inferred shallowing of NADW at Site 517 and 518.
Thus,
there appears to be a direct link between increasing
glaciation, increased flow of MOW, and a deepening of NADW
in the South Atlantic.
CONCLUSIONS
The integration of geochemical, faunal, and grain-size
information for Site 516A, 517, 51S, 514 and 54S is useful
for the interpretaion of the changes in the deep-water mass
properties during the Late Pliocene climatic deterioration
leading to the initiation of Northern Hemispheric
glaciation.
Furthermore, the grain-size distribution of
the silt fraction provides an important tool in
understanding these water-mass interactions.
A combination of an increasingly cooled and glaciated
Northern Hemisphere and the introduction of a highly saline
MOW at approximately 2.9 Ma allowed the formation of a more
dense NADW with properties similar to those of today.
The
formation of this denser NADW allowed its subsequent
expansion to depths of 3900 m in the South Atlantic.
Consequently, with this addition of modified NADW at
approximately 2.9 Ma, water-mass stratification in the
South Atlantic began to approach that of today.
The
climatic forcing of the deep-water masses is best
illustrated by the coincidence of inferred Late Pliocene
pulses of deep-water paleospeeds with enrichment patterns
of benthic a1 So values (fig. 10).
This study also addresses the event related to the
excursion in alSo values at 3.2 Ma.
If bottom-water
55
production had increased, inferred paleospeeds for site 518
would have been higher than for any other site in this
study due to the encroachment of AABW to this depth
(3900 m).
Although a pulse of increased paleospeeds is
inferred for the 3.1 Ma event, inferred paleospeeds for
Site 518 are lower than for the shallower sites 517 and
516A.
This study supports the idea that the
o18 o
excursion, associated with a cooling of surface and bottom
waters, did not coincide with an increase in AABW
production.
The inferred pulse of increased paleospeeds at
3.1 Ma is interpreted here as an increase in the ACC, a
result of increased pole-to-equator temperature gradients
associated with a climatic cooling event.
Relatively slow
inferred paleospeeds returned immediately after the 3.1 Ma
event and continued to approximately 2.9 Ma, when increased
paleospeeds evidently occurred at all deep-water sites in
this study.
For this reason I conclude that the event at
3.1 Ma was a prominent, though not permanent, change.
The results of this study show that a step-wise change
of climate that ?egan approximately 2.9 Ma resulted in the
expansion of NADW into the South Atlantic.
establi~hment,
This led to the
by 2.4 Ma, of a deep-water stratification
similar to that of today.
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. APPENDIX A
SITE 516A
30°16'S 35°17'W
1313 METERS
AAIW-UCPW
Age
(my)
Silt-mean
Coarsest 1%
silt-fraction
(phi)
(phi)
2.77
2.80
2.91
2.95
2.99
:3.02
:3.06
:3.10
:3.14
3.17
3.23
3.25
3.30
3.34
3.36
3.41
6.315
6.595
6.690
6.650
6.690
6.670
6.650
6.740
6.620
6.645
6.670
6.635
6.610
6.585
6.610
6.660
4.12
4.30
4.96
4.9:3
4.80
5.00
4.41
5.16
5.05
4.78
5.09
5.05
4.89
4.71
4.80
5.05
62
SITE 517
30° 56.8 1 S 35° 02.5 'W
2963 METERS
NADW
~-----------------------------------
Age
(my)
Silt-mean
Coarsest 1%
silt-fraction
(phi)
(phi)
2.13
2.17
2.22
2.26
2.30
2.35
2.40
2.44
2.49
2.53
2.58
2.62
2.67
2.72
2.81
2.86
2.91
2.96
2.99
3.01
3.07
3.09
3.10
3.11
3.13
3.18
3.19
3.20
3.21
3.23
3.25
6.670
6.690
6.705
6.770
6.650
6.780
6.765
6.745
6.865
6.755
6.665
6.725
6.685
6.760
6.690
6.560
6.720
6.725
6.820
6.815
6.805
6.875
6.735
6.765
6.910
6.830
6.880
6.825
6.555
6.675
6.830
4.91
4.89
4.83
4.91
4.96
5.00
5.04
5.18
5.15
5.22
5.11
5.00
5.11
5.07
5.11
5.04
5.20
5.11
5.26
5.16
5.24
5.15
5.22
5.05
4.98
5.18
5.11
5.09
5.18
4.78
4.71
63
SITE 518
3 8° o8 . 1 'w
3944 METERS
2 9° 58 . 4 's
AABW-NADW
-----------------------------------Coarsest 1%
Age
(my)
Silt-mean
silt-fraction
(phi)
(phi)
2.24
2.28
6.825
6.845
t5.820
t5.745
6.815
t5.710
6.735
6.825
6.795
6.725
6.845
6.830
6.830
6.760
6.800
6.770
6.750
6.830
6.750
t5.795
6.775
6.870
6.835
6.850
6.840
6.880
6.850
6.780
6.840
6.880
6.790
6.720
6.860
6.750
2.::30
2.35
2.40
2.44
2.49
2.54
2.58
2.Ei2
2.66
2.71
2.75
2.80
2.84
2.86
2.90
2.95
2.97
2.99
3.03
3.06
3.11
3.1o
3.21
3.23
3.27
3.36
3.45
3.50
::3.55
3.59
3.64
3.72
5.13
5.20
5 .. 18
5.11
5.02
5.13
5.07
4.63
4.82
5.16
5.05
4.96
4.94
5.29
5.18
5.04
5.22
5.20
5.11
5.16
5.16
5.27
5.09
5.27
5.24
5.07
5.07
5.31
5.11
5.18
5.27
5.11
5.16
5.07
64
SITE 514
46°02.8'S 26°51.3'W
4318 METERS
AABW
---------~--------------------------
Age
(my)
silt-mean
Coarsest 1%
silt-fraction
(phi)
(phi)
--------~---------------------------
2.20
2.24
2.27
2.32
2.37
2.40
2.42
2.47
2.48
2.49
. 2.50
2.52
2.53
2.55
2.56
2.57
2.58
2.59
2.61
2.63
2.64
2.67
2.68
2.71
2.73
2.74
2.76
2.77
2.78
2.79
2.80
2.82
2.83
2.85
2.86
~ 2. 88
2.89
2.92
2.94
2.96
2.97
2.98
3.00
6.640
6.790
6.825
6.730
6.835
6.775
6.675
6.895
6.895
7.000
6.820
6.920
6.960
6.810
6.810
6.875
6.860
6.915
6.810
6.850
6.950
6.920
6.850
6.760
6.915
6.945
6.904
7.045
6.715
6.935
6.925
6.945
6.885
7.010
6.930
6.91.5
6.910
6.890
6.905
6.910
6.905
6.385
6.650
4.93
5.26
5.00
5.22
5.33
5.35
5.16
5. 31
5.20
5.46
5.20
5.44
5.44
5.31
5.26
5.38
5.27
5.40
5.27
5 •. 35
5.40
5.13
5.44
5.27
5.49
5.29
5.46
5.66
4.96
5.48
5.44
5.57
5.37
5.40
5.11
5.38
5.49
5.37
5.53
5.48
5.24
4.72
5.29
65
SITE 514 (CON'T)
------------------~-----------------
Age
(my)
Silt-mean
Coarsest 1%
silt-fraction
(phi)
(phi)
-----------------~--~-------------~-
3.00
3.04
3.06
3.07
3 .. 09
3.11
3.13
3.15
3.16
3.18
6.910
6.805
6.825
6.740
6.895
6.690
6.935
6.905
6 .. 930
6.890
5.44
5.20
5.37
5.42
5.31
5.40
5.48
5.53
5.42
5.49
66
SITE 548
48°54.9'N 12°09.8'W
1251 METERS
Mow·
-------~------------------------------
Depth
(em)
Silt-mean
Coarsest l%
silt-fraction
(phi)
(phi)
----~---------------------------------
142.67
142.87
14:JoJ:J
143.73
144.20
144.38
144.78
152.56
16:3.25
16:3.75
156.74
167.28
167.75
167.95
168.49
169.25
170.50
171.60
171.50
172.48
172.70
173.50
173.98
174.50
174.96
175.17
5.790
5.725
5.605
5.560
5.465
5.345
5.575
5.645
5.545
5.380
5.595
5.670
5.735
5.880
5.760
5.755
5.975
5.880
6.000
6.015
5.985
5.645
5.875
6.000
5.895
5.805
4.25
4.23
4.18
4.18
4.14
4.10
4.16
4.18
4.21
4.16
4.25
4.23
4.21
4.25
4.23
4.18
4.25
4.25
4.21
4.30
4.32
4.23
4.30
4.23
4.30
4.23