Transcript Methods of paleotemperature reconstruction

PALEOTHERMOMETRY
PALEOTHERMOMETRY
• What is it?
– Determining past ocean temperature
• Glacial-interglacial changes in SST
• Why do it?
– Key climate parameter
• Controls heat and moisture fluxes (air-sea exchange)
– Key boundary condition for GCM’s
• Target for coupled ocean-atmosphere models
– Influence on deep circulation and chemistry
– Physical (T, S) and chemical (nutrient, metabolite)
distributions in the oceans reflect
physical/chemical/biological processes
PALEOTHERMOMETRY
• What’s the approach to determining past
temperature?
– Try to identify faithful geological recorders
• Sediment chemistry
• Fossil abundances
• Shell chemistry
– Reconstruct past distributions of ocean temperature
– Infer paleo processes
Modern SST
from shipboard
measurements
and AVHRR
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Reynolds and Smith, 1995
Methods of paleothermometry
• Faunal assemblages
– transfer functions (factor analysis)
– Modern Analogue Technique (MAT)
• 18O
• Mg/Ca in foraminifera (Sr/Ca in corals)
• Alkenones
Sea surface temperature
reconstructions
• CLIMAP show strong high-latitude cooling at
LGM and little low-latitude cooling at LGM
• BUT there is other evidence FOR low latitude
cooling
–
–
–
–
Snow line drop
Noble gases in groundwater
Vegetation changes (pollen)
Coral Sr/Ca
• RE-EVALUATE faunal estimates, and compare
with 18O, alkenone, and trace element records
CLIMAP
(Climate: Long-range Investigation,
Mapping And Prediction)
(CLIMAP, 1981)
• Modern (core-top) planktonic foraminiferal (and
other*) abundances
• Factor analysis to identify a few assemblages
which represents the faunal data
• Correlate assemblages to environmental
parameters
• Use fossil assemblages to infer
paleoenvironmental conditions
*also radiolaria, coccolithophorids, diatoms
Transfer functions
• Basic idea: there are assemblages of
planktonic foraminifera species that can be
identified by multivariate statistics
• Assume: the relationship between the
assemblage and a physical property (e.g.,
temperature) does not change through time
• Factor analysis assumptions:
– Core-top fauna related to surface water
properties
– SST is ecologically important
– Abundance variations can be represented by
linear mixing of a few assemblages
– Ecosystem remains ~constant through the time
studied
Imbrie and Kipp, 1971
Modern (core top)
calibrations
Polar assemblage is
monospecific (100% N.
pachyderma left-coiling)
Abundance versus
T for assemblages
Imbrie and Kipp, 1971
“test” modern SST calculation
WINTER
SUMMER
Identify 18 ka horizon using
18O (max. 18O ~ max. ice
volume
Prell et al., 1980
LGM summer SST
Modern summer SST
McIntyre et al., 1976
CLIMAP SST
>> Strong cooling at high latitudes and little change in the tropics.
Modern
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Advantages/Disadvantages of the
transfer function method
• It is reasonable to believe
that foram species
composition is related to T
(look at global maps)… so
it is reasonable to try to
get T from species data
• The method is objective
• By defining “factors” you
can throw away
information that is not
relevant
• Regression equations
are arbitrary-- no basis
in theory
• Species can migrate
due to changes in
environment
• Samples can fall out of
calibration range
Are CLIMAP tropical SSTs
too warm?
• Webster and Streten 1978 (QR)
• Rind and Peteet 1985 (QR)
• Tropical snow lines
– Snowline elevation drops
– Vegetation zones drop
• Glacial extent (date moraines)
• Lake and bog pollen records vegetation
zone depression
Comparison of estimates for HoloceneLGM SST difference
Faunal estimates
CLIMAP
Modern analogue
Downcore assemblages
Terrestrial tree line and snow line
Hawaii
Papua New Guinea
East Africa
Colombia
Coral Sr/Ca
Barbados
Groundwater noble gas ratios
Brazil
18O
Atlantic Pacific Indian
-2
-2
-1
-1
-1
-1
CLIMAP
Prell, 1985
Mix et al., 1999
-4.7
-3.9
-3.3
-4.6
Webster & Stretten, 1978
Rind & Peteet, 1985
-5
Guilderson
-5.4
-3
<-2
>4
Stute et al
Broecker, 1986
Stott & Tang, 1996
Curry & Oppo
Bard et al., 1997
Lea et al., 2000
Hastings (rev. Lohmann)
Alkenone undersaturation ratios
Planktonic foram Mg/Ca
1
-1
<-3
-3 to -3.5
-3 to -4
Groundwater noble gases
Global modern
calibration
Controls:
- excess air
- fractionation
- temperature
Brazil
Stute et al., 1995
Modern Analogue Technique
(MAT)
• Basic idea: Find modern samples that are
similar in species composition to ancient
samples --> temperature at modern site =
paleotemperature
• Disadvantage: Temperatures are limited to
range of modern variability
Primary issue with both MAT and transfer functions:
Do modern assemblages really represent the range of
past faunal variations in a given setting?
Modified transfer function
(Mix et al., 1999)
–Define faunal assemblage
factors using downcore
data
–Using these factors,
regress core-top
assemblages against SST
Faunal factors (e.g.)
-warm tropical
-upwelling
-E. boundary
Mix et al., 1999
CLIMAP
MAT
Mix et al., 1999
Modified transfer function technique suggests greater
equatorial cooling at LGM than CLIMAP (transfer
function method), though gyre centers are similarly stable
Modified approach
advantages/disadvantages
• Circumvents the “no
analog” problem
• Can exclude species
that are unusually
dissolution resistant
• Limited geographical
applicability (Mix et
al.’s study was only in
the tropics)
18O
• 3 isotopes of oxygen
– 16O
– 17O
– 18O
(99.759%)
(0.037%)
(0.204%)
• Kinetic differences between O isotopes
result in fractionation during their
incorporation in calcite (CaCO3)
18O / 16O)
(
sample
18O =
* 1000
18
16
( O / O)standard
18O standards
• PDB (PeeDee Belemnite)
• SMOW (Standard Mean Ocean Water)
• Difference from standard is expressed as per
mil ( ‰)
Empirical relationships suggest a temperature
18
control on  O calcite
temperature  
18O calcite
Erez et al., 1983
How to get temperature from 18O
• Early attempt: Broecker, 1986
– Assume that benthic 18O only affected by ice volume
(salinity) while planktonic 18O affected by both ice
volume and temperature
• Subtract the “ice volume” component to get T
• But the deep ocean does cool during glacial periods
• Estimate ice volume during glacials-- assume or
measure 18O ice-- estimate 18O seawater
• Porewater 18O in sediments should record 18O
seawater once corrected for advection/diffusion
• GENERAL CONCENSUS: ICE VOLUME
CHANGES AT THE LGM ACCOUNT TO ~1.1
‰ OF THE 18O CHANGE
High(er) sedimentation rate site: Ceara Rise
Curry and Oppo, 1997
Curry and Oppo, 1997
18O
planktonic
=2.1‰ if ice volume ~1.2‰  >4°C T
18O
warm
benthic
cold
But temperature is not the only
control on 18O
• Salinity has a major impact on 18O seawater
ice
continent
18O = -30‰
ocean
18O = 0‰
continent
ocean
18O = +1.1‰
• So to use 18O as a temperature proxy you
somehow have to separate the temperature and
salinity effects
OTHER COMPLICATIONS
WITH 18O
• Vital effects
• Changing depth habitats
surface
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
100 m depth
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Mg/Ca
• Assume: the relationship between Mg and Ca in
seawater is ~constant in space and time
• Mg2+ (or Cd2+ Ba2+ Sr2+) can substitute for Ca2+ in
CaCO3
• For inorganic calcite, the substitution of Mg is
governed by thermodynamics
– Amount of Mg in calcite increases exponentially
between 0° and 30° C
Core top data
Elderfield and Ganssen, 2000
Hastings et al., 1998
• Zonal gradient increases
during cold periods,
decreases during warm
periods
18O
SST
159°E
~1.3‰
~3°C
91°W
~2‰
~3°
• Glacial-interglacial 18O
change smaller in W Eq.
Pacific than E Eq. Pacific
Lea et al., 2000
Complications with Mg/Ca
• Interspecies differences
• Dissolution affects Mg/Ca
Heterogeneity of Mg in Biogenic Calcite
Within and Between Individual Calcite Chambers
Eggins et al. (2003) - laser ablation
ICP-MS chamber wall profiles
(planktonic foraminifera)
W. Curry (WHOI) - Secondary Ionization
Mass Spectrometry - point measurements
(benthic foraminifera)
Heterogeneity: A Close-Up of Different Calcite Phases
1°
2°
Primary Calcite - microgranular calcite
that is precipitated first and reflects
temperature (higher Mg/Ca)
Globigerinoides sacculifer
(planktonic foraminifer that lives in
the warm upper mixed layer)
Secondary Calcite - thick outer “crust”
that is precipitated after the primary calcite
(lower Mg/Ca)
Other calcite phases: gametogenic
(reproductive), post-depositional
(authigenic, diagenetic)
DISSOLUTION
Mg incorporation increases
dissolution susceptibility of
inorganic and biogenic calcite
This depth transect of core tops,
all from one area, should reflect
constant temperatures, but
Mg/Ca decreases with increasing
water depth of core
Decrease
Mg/Ca
Increase P
Decrease [CO32-]
DISSOLUTION
Davis et al. (2000)
Russell et al. (1994)
Coral Sr/Ca
Linsley et al., 2004
Porites Sr/Ca SST on the Great Barrier Reef
Regression choice matters
for extrapolation to low T
Alibert and McCulloch, 1997
Barbados coral 18O and Sr/Ca
LGM SST estimates
LGM
• T ~ 5 to 6°C
Guilderson et al., 1994
Alkenones
• What they are: long
chain organic
compounds produced
by coccolithophorids
(prymnesiophyte
algae)
• 10-20% of algal C
(membrane lipids)
Types of alkenones
Alkenone undersaturation as an
Indicator of SST
• FUNDAMENTAL RELATIONSHIP: a
DECREASE in temperature leads to an
INCREASE in the degree of undersaturation
• Initial ratio: UK37 =
[C37:2]-[C37:4]/[C37:2+C37:3+C37:4]
(Brassell et al., 1986)
• Modified to: UK37’ =[C37:2]/[C37:2+C37:3] (Prahl
and Wakeham, 1987)
• Ratio can be measured very precisely by GC-FID
(Gas Chromatography with Flame Ionization
Detector)
Alkenone calibration
• Most commonly used:
– UK37’= 0.033T+0.043 (Prahl and Wakeham, 1987)
– UK37’ = 0.033T+0.044 (core-top calibration of Muller et
al, 1998)
• Accuracy of SST estimation: ±1°C (in open
ocean, temperate and sub-polar waters)
• Assumptions:
– Production ratio is linearly correlated with growth
temperature
– There is no alteration in this ratio during sedimentation
Advantages of the alkenone
method
• Abundance, structural, and isotopic properties encode
multiple lines of information
• Can be widely applied (most oceanographic regions,
possibly in lakes)
• Can be measured in regions where conventional proxies
based on calcareous microfossils are limited (e.g., where
there is high CaCO3 dissolution, time periods with no
modern analogue)
• Potential to derive new information when used in concert
with other proxies (e.g., paleosalinity)
Disadvantages of the alkenone
method
•
•
•
•
Method of measurement
– Co-elution problems when
alkenones are embedded in
complex mixtures or when
concentrations are very low
Variations in calibrations between
species/strains
Location in the water column
– Variations in water depth for
alkenone production observed
Seasonality of alkenone UK37’
– Alkenone production under (cold)
upwelling vs (warm) nonupwelling conditions can lead to
bias in sedimentary record
•
•
•
•
•
•
Influence of other environmental
factors on UK37’
– Nutrient/light availability
Diagenetic alteration
– Some evidence for differential
preseveration of alkenones (Gong
and Hollander, 1999)
Sample preservation and storage
– Potential oxidation of double
bonds?
Cold water calibration
– Apparent non-linearity at high and
low extremes of calibration
SST calibration for sediments deposited
prior to known emergence of E.
huxleyii
Sediment redistribution
Carribean Sea
alkenone record
Rühlemann et
al., 1999
38°N, 10°W
19°N, 20°W
Comparison of estimates for HoloceneLGM SST difference
Faunal estimates
CLIMAP
Modern analogue
Downcore assemblages
Terrestrial tree line and snow line
Hawaii
Papua New Guinea
East Africa
Colombia
Coral Sr/Ca
Barbados
Groundwater noble gas ratios
Brazil
18O
Atlantic Pacific Indian
-2
-2
-1
-1
-1
-1
CLIMAP
Prell, 1985
Mix et al., 1999
-4.7
-3.9
-3.3
-4.6
Webster & Stretten, 1978
Rind & Peteet, 1985
-5
Guilderson
-5.4
-3
<-2
>4
Stute et al
Broecker, 1986
Stott & Tang, 1996
Curry & Oppo
Bard et al., 1997
Lea et al., 2000
Hastings (rev. Lohmann)
Alkenone undersaturation ratios
Planktonic foram Mg/Ca
1
-1
<-3
-3 to -3.5
-3 to -4