PaleoSST in Forams: 18O and MgCa

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Paleo-SST in Forams ?18O and Mg/Ca

• Jenny Arbuszewski and
• Louisa Bradtmiller

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Outline

• Principles
• Calibration
• Global vs. Regional?
• Agreement
• Other effects?

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The History of ?18O

• Harold Urey (1893-1981)
• Nobel prize (deuterium)
• ?18O in carbonate rocks to determine formation
  temperature
• large reservoir of oxygen in the oceans which
  cannot have changed.

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The History of ?18O

• Cesare Emiliani (1922-1995)
• Regular, cyclic downcore variations in ?18O of
  foram. shells from the Caribbean
• Attributed mostly to temperature
• Glacial tropical SSTs 6-8 cooler

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The History of ?18O

• Nicholas Shackleton (1937-2006)
• Benthic forams
• Variability of ?18Osw
• Tool for stratigraphy, combination with
  independent SST proxy

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Principles

• Thermodynamic fractionation between 16O, 18O
  during precipitation of calcite
• ?18O(18/16Osample/18/16Ostandard)-1x1000
• Calcite is offset by 30 relative to seawater,
  then 0.20-0.27/C
• 0.27 at 0C
• 0.20 at 25C

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Calibration
O. universa equations (from Lea 2003, after Bemis
et al. 1998)
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Calibration
(Ravelo and Fairbanks 1992)
(Bemis et al.1998)
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Calibration

• T a b (?18Ocalcite-?18Owater) c
  (?18Ocalcite-?18Owater)
• T 16.5 - 4.80 (?18Ocalcite-?18Owater)
• O. Universa (Bemis et al. 1998)
• T 16.9 - 4.38 (?18Ocalcite-?18Owater) 0.1
  (?18Ocalcite-?18Owater)2
• Uvigerina (Shackleton 1974)

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Global vs. Regional
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Salinity
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Ice Volume
(Shackleton 2000)
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Summary
dissolution, species, etc.
(Ravelo and Hillaire-Marcel 2007)
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Theory behind Mg/Ca

• Substitution of Mg2 into calcite is endothermic
  ? favored at high T
• Inorganic precipitation experiments show this, as
  do culturing experiments
• T presumed to be main control on Mg incorporation
  into foram shells (utility in combination with
  d18O)

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Comparison to Theory
Lea, 2003
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Calibrations
Dekens et al., 2002 Core top Calibration Pre-expon
ential constant 0.31-0.38 Exponential
constant0.084-0.09
Anand et al., 2003 Sediment trap
Calibration Mg/Ca0.38e0.09T
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Calibration Complications

• Basinal and species offsets? can develop
  calibrations for each
• Dissolution imprint on Mg signal? attempt to
  correct (i.e. Dekens et al., 2002)
• Which Calibration method is best?
• Seasonality? mean annual vs seasonal T
• Depth habitat? what T to apply?
• SST vs. d18O calcification T (for planktonics)

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Comparison to Uk37
Nurnberg et al., 2000
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Comparison to SST
SST (C)
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Other influences and issues

• Dissolution and CO32-
• pH
• Salinity
• Addition of gametogenic calcite/vertical
  migration
• Calibrations for low end of T spectrum (benthic)
• Different cleaning methods in different labs

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Salinity Influence on Mg/Ca
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Salinity influence on d18Osw
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A few last words

• Benthic studies also important
• Can use species with different depth habitats to
  reconstruct water column structure
• New calibrations accounting for Salinity effect

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Interannual variability
Figure 2 Anand et al., 2003
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Average interannual variability
Figure 3 Anand et al., 2003
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Size effects
Figure 7a Anand et al., 2003
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Influence of Symbionts
Figure 9 Anand et al., 2003
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Comparison of Mg/Ca T and d18O calcification T
Figure 11 Anand et al., 2003
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Comparison of d18O and Mg/Ca derived T
Figure 11 Dekens et al., 2002
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Basinal offsets
Figure 2 Dekens et al., 2002
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Influence of CO32- on shell weight and Mg/Ca
Rosenthal and Lohmann, 2002
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Dissolution influence on Mg/Ca derived SST
Rosenthal and Lohmann, 2002
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Evidence of Salinity Effect in Published
Calibrations