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)  18 O 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  18 O (max.  18 O ~ 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.

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 modern calibration Stute et al., 1995 Brazil

 18 O 3 isotopes of oxygen – 16 O(99.759%) – 17 O (0.037%) – 18 O (0.204%) Mass (kinetic) differences between O isotopes result in fractionation during their incorporation in calcite (CaCO 3 )  18 O = ( 18 O / 16 O) sample ( 18 O / 16 O) standard * 1000

 18 O standards PDB (PeeDee Belemnite) SMOW (Standard Mean Ocean Water) Difference from standard is expressed as per mil (‰ )

Empirical relationships suggest a temperature control on  18 O calcite  temperature    18 O calcite Erez et al., 1983

But temperature is not the only control on  18 O Salinity has a major impact on  18 O seawater So to use  18 O as a temperature proxy you somehow have to separate the temperature and salinity effects continent ocean  18 O = 0‰  18 O = +1.1‰ ice  18 O = -30‰

How to get temperature from  18 O Early attempt: Broecker, 1986 –Assume that benthic  18 O only affected by ice volume (salinity) while planktonic  18 O 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  18 O ice-- estimate  18 O seawater Porewater  18 O in sediments should record  18 O seawater once corrected for advection/diffusion GENERAL CONCENSUS: ICE VOLUME CHANGES AT THE LGM ACCOUNT TO ~1.1 ‰ OF THE  18 O CHANGE

High(er) sedimentation rate site: Ceara Rise Curry and Oppo, 1997

 18 O planktonic  =2.1‰ if ice volume ~1.2‰  >4°C  T  18 O benthic warm cold Curry and Oppo, 1997

OTHER COMPLICATIONS WITH  18 O Vital effects Changing depth habitats Changing of water masses surface 100 m depth

Mg/Ca Assume: the relationship between Mg and Ca in seawater is ~constant in space and time Mg 2+ (or Cd 2+ Ba 2+ Sr 2+) can substitute for Ca 2+ in CaCO 3 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  18 O change smaller in W Eq. Pacific than E Eq. Pacific Lea et al., °E91°W  18 O ~1.3‰~2‰  SST ~3°C~3°C

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)

DISSOLUTION 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 P Decrease [CO 3 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  18 O and Sr/Ca LGM SST estimates  T ~ 5 to 6°C Guilderson et al., 1994 LGM

Deglacial/Early Holocene W Pacific SSTs: Coral Sr/Ca and Foram Mg/Ca bulk coral Sr/Ca probed coral Sr/Ca Mg/Ca coral Sr/Ca foram Mg/Ca YD

Sr/Ca ratios of pristine coral skeleton compared with bored skeleton and secondary infilling 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: U K 37 = [C37:2]-[C37:4]/[C37:2+C37:3+C37:4] (Brassell et al., 1986) Modified to: U K 37’ =[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: –U K 37’ = 0.033T (Prahl and Wakeham, 1987) –U K 37’ = 0.033T (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 CaCO 3 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°W19°N, 20°W

Comparison of estimates for Holocene- LGM 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  18 O Alkenone undersaturation ratios Planktonic foram Mg/Ca <-2 >4 <-3 -3 to to -4 CLIMAP Prell, 1985 Mix et al., 1999 Webster & Stretten, 1978 Rind & Peteet, 1985 Guilderson Stute et al Broecker, 1986 Stott & Tang, 1996 Curry & Oppo Bard et al., 1997 Lea et al., 2000 Hastings (rev. Lohmann) Atlantic Pacific Indian