Borehole Paleothermometry
Uses a calibrated thermistor suspended on a
cable.
Obtained from boreholes drilled in continental rock
and ice sheets
Terrestrial Boreholes:
The temperature profile through a terrestrial borehole is influenced by
ground surface temperature, subsurface fluid flow, vertical and lateral
inhomogeneities in bedrock properties, snow cover, and variable
topography
Ice Sheet Boreholes:
The temperature profile through an ice sheet provides a record of
past air temperature, modified by heat diffusion and ice flow, and by
the small and well characterized heat generation from ice
deformation
Borehole Paleothermometry
Because temperature
changes at the surface
affect the distribution
of temperature in the
subsurface, ground
temperatures comprise
an archive of signal of
past climate.
http://www.ncdc .noaa.gov/paleo/borehole/approach.html
1
Calibrating Greenland δ 18O using
borehole measurements
δ18 O = αTs + β
Optimal model:
δ18 O = 0.327Ts – 24.8
Smoothed curve: isotope history
is filtered to mimic the thermal
averaging in ice sheets
Cuffey et al., 1995
Calibrating
Greenland ∆T
using borehole
measurements
0.5‰/°C for Holocene, and
0.33‰/°C for glacial
∆T glacial-interglacial > 20°C
Uses a forward thermal model
incorporating conductive and
advective heat fluxes, forced by T =
aδ18O +b. Invert for optimal values
of a and b by comparing model
equation solutions with borehole T
data (estimated error is ±0.001‰/°C
for a, and ± 0.05‰ for b)
Cuffey et al., 1995; Johnsen et al ., 1995
2
Terrestrial Borehole
Paleothermometry
Huang et al., 2000
The numbers of boreholes on each continent
are respectively 245 (North America), 16
(South America), 146 (Europe), 92 (Africa),
60 (Asia), and 57 (Australia).
1.0 K warming over past 500 years
(1.2 K NH, 0.8 K SH), 0.5 K in 20th
century alone à generally higher
than other proxy based estimates
Shaded areas represent 1 standard error
about the mean. Superimposed are the
corresponding series of instrumental
surface air temperatures (SAT)
Comparison with other proxy indicators
Mann et al., 2003
Huang et al.
[2000] borehole
show NH T
increases of ~1 K
over the past 500
years
Mann et al.
[2003]
“optimized”
borehole show
NH T increases
of ~0.5 K over the
past 500 years
3
Advantages/Disadvantages of
Borehole Paleothermometry
Advantages:
“Direct” temperature study
Extensive geographic coverage (tens of thousands of boreholes
throughout the world)
Can be used to calibrate other paleoclimatic indicators
Disadvantages:
Low resolution and comparatively “short memory”
Signal damped by heat diffusion processes and do not contain high
frequency details
Relationship between surface and below-ground temperature is not
simple, and varies geographically
Ability to resolve details of the surface temperature history diminishes
with time
Noble Gas Paleothermometry in
Groundwaters
Atmospheric noble gas
abundances and
isotope ratios have
remained essentially
constant for at lest the
last 3 million years
[Phillips, 1981]
He is not used
because of sources in
the aquifer related to α
decay of U and Th.
4
Temperature dependence of solubility
High temperature sensitivity
The dissolved noble gas
concentration ratios of
water in solubility
equilibrium with the
atmosphere deviate
significantly from those in
the atmosphere as a result
of the different solubilities of
different gases.
Low temperature sensitivity
Stute et al., 1992
Ground water dynamics
Gases exchanged continuously between water
and ground air
Gas exchange is
limited to diffusion
across the water table
in the saturated zone
5
Noble gases record mean annual
temperature at the water table
Below 10 m, there is
almost no seasonal
variation (Ta < 0.2°C)
Stute and Schlosser, 1993
“Excess Air”
Noble gas concentrations are determined mainly by:
1) Solubility equilibrium (temperature)
2) Amount of “excess air”
“Excess air”: Dissolution of small air bubbles by very rapid
infiltration or fluctuation of the water table.
Assumed that gases of excess air are not fractionated relative to the
atmosphere (unlike due to solubility) à can correct for excess air
component using a set of equations with the ground T and amount of
excess air as unknowns
(1 − F ) Az
C (T , S, P, A, F ) = C (T , S , P ) +
FAz
1+
C
*
i
i
t
i
*
t
i = Ne, Ar, Kr, Xe; T = temperature, S = salinity, P = atmospheric pressure, A =
initial amount of excess air, F = fraction of excess air, zi = noble gas fractions
in dry air, C *t = solubility equilibrium concentrations as a function of T, S, and P
6
Dating Groundwater
14C
dating up to
~30,000 ya
•May be biased high (old)
due to secondary
exchange with CaCO3
minerals (can estimate
using measured variables
such as alkalinity, total
dissolved carbon, and
δ13 C)
•Error of ±2,000 years
14C:
•Sufficient for last glacial
– present interglacial
differences
τ1/2 = 5730 years
∆Tglacial-interglacial using noble gas
paleothermometry
9°C
5.4°C
5.5°C
7
Advantages/Disadvantages of Noble
Gas Paleothermometry
Advantages:
Based on a simple physical principle (temperature dependence of
the solubility of noble gases in water)
Reflects mean annual temperature
Accuracy of ~±0.5°C (1 σ error)
Disadvantages:
Uncertainty of
14C
dating in groundwater
Loss of high frequency climate signals due to dispersion during
transit (low temporal resolution) à used to determine reliable
temperature differences between the last glacial and present
interglacial which can be used to calibrate other
paleothermometers with higher time resolution.
Dendroclimatology
The use of tree rings as a proxy
indicator of climate
Earlywood
(light): large,
thin-walled cells
Latewood
(dark): densely
packed, thickwalled cells
1.
variation in total ring width:
a light and a dark band
2.
variation in latewood width:
just the dark bands
3.
variation in latewood density:
darkness of dark band
Variation in these rings is due to variation in environmental conditions
when they were formed. Thus, studying this variation leads to
improved understanding of past environmental conditions and is the
basis for many research applications of dendrochronology.
from the Univ. of Arizona’s Laboratory of Tree Ring Research
8
Principles of Dendroclimatology
The principle of limiting factors:
states that rates of plant processes are constrained by the
primary environmental variable that is most limiting
e.g., precipitation is often the most limiting factor to plant
growth in arid and semiarid areas.
e.g., in the higher latitudes, temperature is often the most
limiting factor that affects tree growth rates.
The principle of site selection:
This principle states that sites useful to dendrochronology can be
identified and selected based on criteria that will produce tree-ring series
sensitive to the environmental variable being examined.
http://web.utk.edu/~grissino/principles.htm
Calibration of Tree Ring Data
The principle of aggregate tree growth: Any individual
tree growth series can be decomposed into an
aggregate of environmental factors that affected the
patterns of tree growth over time.
Rt = At + Ct +δD1t + δD2t + Et
t = any one year; A = age related growth trend due to normal
physiological aging processes; C = climate; D1 = disturbance factors
within the forest stand; D2 = disturbance factors from outside the forest
stand (e.g. insects); δ = 0 for no disturbance, 1 for a disturbance; E =
random error processes not accounted for in the other processes
Climate (C) = f(sunshine, precipitation, temperature, wind speed,
humidity, and their distribution throughout the year)
Principle components analysis is used to select variables that account for
most of the variance in the measured data.
http://web.utk.edu/~grissino/principles.htm
9
Sample collection
Coring a live tree
Sampling a dead tree
Archeological samples
http://web.utk.edu/~grissino/gallery.htm
Dating Tree Rings
Ring counting is done by crossdating
•matching ring-growth characteristics across many samples from a
homogeneous area (area of similar environmental conditions)
•permits identification of exact year of formation for each ring
•Can eliminate errors from:
Locally absent rings
and false bands
http://www.ltrr.arizona.edu/skeletonplot/introcrossdate.htm
10
The Principle of Crossdating
The procedure of matching ring width variations among trees that have
grown in nearby areas, allowing the identification of the exact year in
which each ring formed.
Nash, 2002
Climatic Information
Climate information is obtained from measurements
of:
1) Ring width
2) Ring density (measured on x-ray negatives of prepared core
sections)
3) Isotopic variations
The 3 approaches are
complementary
from Bradley, 1999
11
Uniformitarian Principle
This principle states that physical and biological processes that
link current environmental processes with current patterns of tree
growth must have been in operation in the past. In other words,
"the present is the key to the past”.
Precipitation reconstruction for Northern New Mexico
Calibration period
Rigorous statistical methods are used to determine the relations hip
between climatic variables and the proxy.
Temperature Reconstructions
Land only extratropical
expression of NH temperature
variability
Anomalies from 1900-1999
instrumental mean
Recent annual
temperatures have
exceeded earlier
reconstructed warm
intervals by ~0.3ºC
Cook et al ., 2004
12
Temperature Reconstructions
Based on
tree ring
width and
density
data
“Late 20th century warmth is unprecedented at hemispheric, and
likely, global scales”
Jones and Mann, 2004
Isotope Dendroclimatology
Empirical studies have demonstrated that variations in the isotopic
content of tree rings (δ13 C, δ18O, δD) can be highly correlated to
climate.
For example, δD in modern plants is strongly correlated with δD in
associated environmental waters. δD in plants can provide a proxy
for spatial variations in δD of meteoric precipitation, assuming this
holds constant over time.
Problem: additional isotopic fractionation occurs within trees during
the synthesis of woods material and these biological fractionations
are themselves dependent on many factors, including temperature,
humidity, and wind speed.
13
Advantages/Disadvantages of
Dendroclimatology
Advantages:
Through cross-dating of multiple cores, the absolute age of
a sample can be established (unique in this way). Long tree
ring series are so accurate that they are used to calibrate
the radiocarbon scale.
Disadvantages:
Limited in time (up to several millenia)
Biological processes within the tree are extremely complex,
making interpretation difficult
CO2 fertilization effect?
Snow Lines (Glacial Moraines)
Moraines = bodies of glacially eroded and transported rock left
behind when a body of ice advances and then retreats
http://www.fettes .com/Cairngorms/moraine.htm
14
Glacier Fluctuations
Glacier fluctuations result from
changes in the mass balance of
glaciers
à Increases in net accumulation
leads to glacier thickening and
advance
àIncreases in net ablation leads to
glacier thinning and recession
(Ablation = reduction in volume of
glacial ice by the combined processes
of melting, evaporation and calving )
There are many climate
conditions which can lead to a
net change in glacier mass
balance
Glacier Response Time
Changes in mass balance are not immediately transformed into changes in
glacier front positions. Glacier front positions will lag behind climatic
fluctuations with varying response times (10s of years to millenia).
The response time depends on a number of
factors including the glacier length, basal
slope, ice thickness and temperature, and
overall geometry of the glacier itself. Glacier
front variations are thus a rather complex
integration of both short and long term climatic
fluctuations, so that one should not be
surprised to see some larger glaciers (longer
response times) advancing at the same time
that smaller glaciers (shorter response times)
are retreating.
The south cascade glacier (WA) has
a response time of 25-30 years
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Dating Glacial Moraines
Absolute dating: 14C dating of organic material in sediments (in some
cases not much organic material present)
Surface exposure dating: 10Be, 26Al, and
rock surfaces (102 to 107 years)
36Cl
surface exposure dating of
Terrestrial in situ cosmogenic nuclides (TCN)
(distinct from nuclides produced in the atmosphere)
Gosse and Phillips, 2001
Problems with surface exposure dating
Atmospheric/meteoric
contamination:
Erosion will make it
appear younger than it is
must take steps to ensure the
atmospheric component doesn’t
contaminate the terrestrial
component
e.g. 10Be is produced in the
atmosphere ~103 times greater
than the average rate of
production in rocks on Earth, and
is highly reactive with mineral
surfaces (in contrast to 36Cl)
Gosse & Phillips, 2001
16
Some considerations for surface
exposure dating
Estimating production rates via
geological calibration,
experimentally, and numerical
simulation can give very different
results
Snow coverage
Must determine altitude and latitude
dependence
Shielding of sloping surface
(topographic obstructions)
Surface coverage (snow, sand, soil,
peat)
Surface exposure dating with
26Al, and 36Cl
10Be,
Gosse & Phillips, 2001
17
Last Glacial Maximum Climate
The boundary between elevations where snow and ice persist for the entire
year and elevations experiencing only seasonal snow cover corresponds
roughly to the position of the mean annual 0ºC isotherm.
950 ± 70 m
Using today’s
lapse rate of 5°C
km-1, estimate
~5°C cooler in
LGM than today
45°N
45°S
Figure from “ The Glacial World According to Wally”
Problems Interpreting Paleotemperatures from Snowlines
Difficulty in identifying the response of snowlines to temperature
Snowline elevation also depends on:
- Variation of accumulation with elevation (accumulation gradient)
- Radiation balance
- Wind speed
- Humidity
- Variation of albedo with temperature
Lapse rates – may not be a reasonable assumption to use modern lapse rates, as
both temperature and moisture conditions in the past would have been different from
today.
Imprecise dating of glacial deposits
Snowlines in different regions are controlled by different climatic
parameters and that these must first be understood in order to use
paleosnowlines in paleoclimatic reconstructions.
18
Pollen Analysis (Palynology)
Where pollen has been preserved
over time, in lakes, bogs, estuaries,
etc., it provides a record of past
vegetation changes that may be due
to changes in climate.
Methods: requires rigorous
chemical treatment by hydrochloric,
sulfuric and hydrofluoric acid. Pollen
grains and spores are stained and
mounted on slides for microscopic
analysis.
Uniformitarian principal: By using
spatial relationships in modern pollen
distribution and their relationship to
modern climate as a guide to
interpreting pollen patterns recorded
in the past, paleoclimatic
reconstructions can be made.
Paleoclimatic Reconstruction using
Pollen Grains
4 basic attributes of pollen grains making paleoclimatic
reconstruction possible:
1) they possess morphological characteristics that are specific to a
particular genus or species of plant
2) they are produced in vast quantities by wind-pollinated plants,
and are distributed widely from their sources
3) they are extremely resistant to decay in certain sedimentary
environments
4) they reflect natural deposition at the time of pollen deposition
Pollen falling on sites where organic or inorganic sediments are
accumulating will become part of the stratigraphic record. The principal
sources of paleoclimatic information are peat from bogs and marshes.
19
Dating and Resolution
Dating:
• 14C analysis
Resolution:
•Time lag response of vegetation to climate change on the order
of 100-1500 years à represents a running mean of climatic
variation
•Complications due to burrowing worms and other mud dwellers,
or resuspension and redeposition during periods of turbulent
mixing
Pollen Diagram
A pollen spectrum consists of the number of different pollen grains at a particular level
expressed as a percentage of the total pollen count. In the pollen diagram, changes in the
percentage of one species are assumed to reflect similar changes in the vegetation
composition.
20
Quantitative Paleoclimatic
Reconstructions
More quantitative reconstructions of past climate can be obtained by
mathematically relating modern climatic conditions to modern pollen
rain, and using these relationships to convert the fossil pollen record
into specific paleoclimate estimates.
Multiple linear regression of modern conditions:
Cm = Tm *Pm (m = modern)
Cm = modern climate data; Tm = transfer function; Pm = modern pollen rain
Former climatic conditions use modern transfer function:
Cf = Tm *Pf (f = fossil)
Quantitative Paleoclimatic
Reconstructions
Relationships not always
linear
July Tmean (ºC) = 17.76 +
1.73(Quercus)0.25 +
0.09(Juniperus) +
0.51(Tsuga)0.25 –
0.41(Pinus)0.5 – 0.12(Acer) –
0.04(Fagus)
As more pollen types are
added, the number of
possible climate options
becomes more limited.
Bartlein et al . [1984]
July temperatures 6 ka BP over the north-central US and southern
Canada were 1-2ºC warmer than modern temperatures
21
Advantages/Disadvantages of using
Pollen for Paleoclimatic
Recontstruction
Advantages:
Large databases of surface pollen are available covering a large geographical
range
Pollen analysis can provide climate information and, in archeological sites, can
be used to understand human activities.
Disadvantages:
Must eliminate anthropogenic interferences such as disturbance and CO2
fertilization.
No-analogue problem (uniformitarian principle may not apply)
Vegetation responds to many climate (temperature, precipitation,soil
moisture) and non-climate variables (humans, insect infestations, fire, plant
successional change)
Pollen spectra may depend on preceding vegetation state, as well as climate
variables (autocorrelation in that is not accommodated by the transfer or
response function approaches)
22