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| Titel |
Reconstructing Mg/Ca ratios of seawater and implications for Mg/Ca based climate reconstructions |
| VerfasserIn |
Jos Wit, Jordahna Haig, Frans Jorissen, Ellen Thomas, Gert-Jan Reichart |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034112
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| Zusammenfassung |
| The temperature of the deep ocean plays a vital role in the Earth’s climate system.
Paleo-reconstructions of deep-sea temperatures are generally dependent on Mg/Ca
ratios measured from the carbonate tests of fossilized benthic foraminifera. Current
Mg/Ca-temperature calibrations are based on empirical relationships which have been
developed under present day environmental and chemical ocean conditions. However,
the incorporation of Mg (DMg) into foraminiferal calcite is not solely dependent
on temperature, but is also influenced by seawater chemistry. The Mg/Ca ratio of
seawater in particular, plays a crucial role in determining the degree of incorporation of
Mg in test carbonate. Due to its long oceanic residence time Mg concentrations
remain relatively constant over time scales of a few hundred thousand years, yet
can vary significantly over longer geological time scales. Therefore the accurate
reconstruction of past temperatures using foraminiferal Mg/Ca ratios hinges on our
understanding of Mg/Ca seawater changes over geological time. Here we present an
independent approach to reconstructing paleo-seawater Mg/Ca using the temperature
dependent offset in DMg incorporation between porcelaneous (high Mg) and hyaline
(low Mg) foraminifera. Porcelaneous foraminifera produce smooth opaque tests
made of needle shaped high Mg calcite, while hyaline foraminifera build perforate
tests made of radial low Mg calcite. As DMg values are species specific, using
foraminifera from the same time interval and environment (i.e. the same sample)
eliminates the effect of temperature and seawater chemistry on DMg. Subsequently,
combining a newly constructed Mg/Ca-temperature calibration for porcelaneous Pyrgo
spp.(one of the few porcelaneous taxe present in the deep sea) with an existing
calibration of hyaline Cibicides spp. allows us to mathematically solve for changes in
Mg/Ca seawater through time using the species specific offset in DMg. Our results
correspond well to changes in Mg/Ca of seawater as derived from geochemical models.
Applying the newly reconstructed Mg/Casw curve to the published fossil record
indicates that current Mg/Ca based temperature reconstructions prior to the mid
Pleistocene, are significantly underestimating absolute temperature and therefore
overestimating the growth of ice sheets via δ18Oc-Mg/Ca combined δ18Ow reconstructions. |
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