Paleoceanography

Paleoceanography is the study of the history of the oceans in the geologic past with regard to circulation, chemistry, biology, geology and patterns of sedimentation and biological productivity. Paleoceanographic studies using environment models and different proxies enable the scientific community to assess the role of the oceanic processes in the global climate by the re-construction of past climate at various intervals. Paleoceanographic research is also intimately tied to paleoclimatology.

Paleoceanography makes use of so-called proxy methods as a way to infer information about the past state and evolution of the worlds oceans. Several geochemical proxy tools include long-chain organic molecules (e.g. alkenones), stable and radioactive isotopes, and trace metals. Additionally, sediment cores can also be useful; the field of paleoceanography is closely related to sedimentology and paleontology.

Sea-surface temperature (SST) records can be extracted from deep-sea sediment cores using oxygen isotope ratios and the ratio of magnesium to calcium (Mg/Ca) in shell secretions from plankton, from long-chain organic molecules such as alkenone, from tropical corals near the sea surface, and from mollusk shells.

Oxygen isotope ratios (δ¹⁸O) are useful in reconstructing SST because of the influence temperature has on the isotope ratio. Plankton take up oxygen in building their shells and will be less enriched in their δ¹⁸O when formed in warmer waters, provided they are in thermodynamic equilibrium with the seawater. When these shells precipitate, they sink and form sediments on the ocean floor whose δ¹⁸O can be used to infer past SSTs. Oxygen isotope ratios are not perfect proxies, however. The volume of ice trapped in continental ice sheets can have an impact of the δ¹⁸O. Freshwater characterized by lower values of δ¹⁸O becomes trapped in the continental ice sheets, so that during glacial periods seawater δ¹⁸O is elevated and calcite shells formed during these times will have a larger δ¹⁸O value.

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