paleothermometry. what is it? –determining past temperatures e.g. glacial-interglacial changes in...

PALEOTHERMOMETRY

PALEOTHERMOMETRY

• What is it?– Determining past temperatures

• e.g. Glacial-interglacial changes in sea surface temperature (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 (proxies)

• Sediment chemistry, physical properties• Fossil abundances• Shell chemistry

– Reconstruct past distributions of ocean temperature– Infer past processes from distributions

Modern SST from satellite data:AVHRR (Advanced very high resolution radiometer

Methods of paleothermometry

• Faunal assemblages – transfer functions (factor analysis)– Modern Analogue Technique (MAT)

18O

• Mg/Ca in foraminifera (Sr/Ca in corals)

• Alkenones

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

Abundance versus T for assemblages

Modern (core top) calibrations

Imbrie and Kipp, 1971

Polar assemblage is monospecific (100% N. pachyderma left-coiling)

“test” modern SST calculation

WINTER SUMMER

Identify 18 ka horizon using 18O (max. 18O ~ max. ice volume

Prell et al., 1980

McIntyre et al., 1976

Modern summer SST

LGM summer SST

Modern

CLIMAP SST>> Strong cooling at high latitudes and little change in the tropics.

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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

Groundwater noble gases

Controls: - excess air - fractionation - temperature

Global moderncalibration

Stute et al., 1995

Brazil

18O• 3 isotopes of oxygen

– 16O (99.759%)– 17O (0.037%)– 18O (0.204%)

• Mass (kinetic) differences between O isotopes result in fractionation during their incorporation in calcite (CaCO3)

18O =(18O / 16O)sample

(18O / 16O)standard

* 1000

18O standards

• PDB (PeeDee Belemnite)

• SMOW (Standard Mean Ocean Water)

• Difference from standard is expressed as per mil (‰)

Empirical relationships suggest a temperature

control on 18O calcite

temperature 18O calcite

Erez et al., 1983

But temperature is not the only control on 18O

• Salinity has a major impact on 18O seawater

• So to use 18O as a temperature proxy you somehow have to separate the temperature and salinity effects

continent

continentoceanocean

18O = 0‰ 18O = +1.1‰

ice

18O = -30‰

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

18Oplanktonic

=2.1‰ if ice volume ~1.2‰ >4°C T

18Obenthic

warm

cold

Curry and Oppo, 1997

OTHER COMPLICATIONS WITH 18O

• Vital effects

• Changing depth habitats

• Changing of water masses

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surface 100 m depth

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

Elderfield and Ganssen, 2000

Core top data

Hastings et al., 1998

• Zonal gradient increases during cold periods, decreases during warm periods

• Glacial-interglacial 18O change smaller in W Eq. Pacific than E Eq. Pacific

Lea et al., 2000

159°E 91°W18O ~1.3‰ ~2‰SST ~3°C ~3°C

Complications with Mg/Ca• Interspecies differences

• Dissolution affects Mg/Ca

Heterogeneity of Mg in Biogenic CalciteHeterogeneity of Mg in Biogenic CalciteWithin and Between Individual Calcite ChambersWithin and Between Individual Calcite Chambers

Eggins et al. (2003) - laser ablation Eggins et al. (2003) - laser ablation ICP-MS chamber wall profiles ICP-MS chamber wall profiles (planktonic foraminifera)(planktonic foraminifera)

W. Curry (WHOI) - Secondary Ionization W. Curry (WHOI) - Secondary Ionization Mass Spectrometry - point measurements Mass Spectrometry - point measurements (benthic foraminifera)(benthic foraminifera)

DISSOLUTIONDISSOLUTION

Davis et al. (2000)Russell et al. (1994)

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

Increase PDecrease [CO3

2-]

Decrease Mg/Ca

DISSOLUTION

Coral Sr/Ca

Linsley et al., 2004

Alibert and McCulloch, 1997

Regression choice matters for extrapolation to low T

Porites Sr/Ca SST on the Great Barrier Reef

Barbados coral 18O and Sr/Ca LGM SST estimates

T ~ 5 to 6°C

Guilderson et al., 1994

LGM

Deglacial/Early Holocene W Pacific SSTs:Deglacial/Early Holocene W Pacific SSTs:Coral Sr/Ca and Foram Mg/CaCoral Sr/Ca and Foram Mg/Ca

bulk coral Sr/Cabulk coral Sr/Ca

probed coral Sr/Caprobed coral Sr/Ca

Mg/CaMg/Ca

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4000 6000 8000 10000 12000 14000 16000 18000 20000

Years BP

paleoSST anomaly relative to present day

mean annual SST

Sulu Sea Mg/Ca (Rosenthal et a l. 2003) Makassar Strait Mg/Ca (Visser et a l. 2003)

Vanuatu coral Sr/Ca (Correge et a l. 2004) Vanuatu Coral Sr/Ca (Beck et a l. 1997)

Series6

coral Sr/Ca

foram Mg/Ca

YD

20

21

22

23

24

25

26

27

28

29

0 1000 2000 3000 4000 5000 6000 7000

sampling distance (microns)

Sr/Ca-derived SST

8.1

8.6

9.1

9.6

10.1

10.6

11.1

Sr/Ca (mmol/mol)

modern Bolling-Allerod (13.1 kyr BP)

19A10R_11.8kyr borings/infilling

Sr/Ca ratios of pristine coral skeleton Sr/Ca ratios of pristine coral skeleton compared with bored skeleton and secondary compared with bored skeleton and secondary

infillinginfilling

YD

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:– UK

37’= 0.033T+0.043 (Prahl and Wakeham, 1987)– UK

37’ = 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) non-upwelling 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

38°N, 10°W 19°N, 20°W

Comparison of estimates for Holocene-LGM SST difference

Faunal estimates CLIMAP Modern analogue Downcore assemblages

Terrestrial tree line and snow lineHawaiiPapua New GuineaEast AfricaColombia

Coral Sr/CaBarbados

Groundwater noble gas ratiosBrazil

18O

Alkenone undersaturation ratiosPlanktonic foram Mg/Ca

-2 -1 -1-2 -1 -1

-4.7-3.9-3.3-4.6

-5 -2

-5.4-3 1 -1<-2>4

<-3-3 to -3.5

-3 to -4

CLIMAPPrell, 1985Mix et al., 1999

Webster & Stretten, 1978Rind & Peteet, 1985

Guilderson

Stute et alBroecker, 1986Stott & Tang, 1996Curry & OppoBard et al., 1997Lea et al., 2000Hastings (rev. Lohmann)

Atlantic Pacific Indian