Paleoceanography

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Study of the oceans in the geologic past

title: "Paleoceanography" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["paleoceanography", "marine-geology", "paleogeography", "oceanographical-terminology"] description: "Study of the oceans in the geologic past" topic_path: "geography" source: "https://en.wikipedia.org/wiki/Paleoceanography" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Study of the oceans in the geologic past ::

Paleoceanography is the study of the history of the oceans in the geologic past with regard to biology, chemistry, circulation, geology, and patterns of biological productivity and sedimentation. 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.

Source and methods of information

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

Sea-surface temperature

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.

The substitution of magnesium in place of calcium in CaCO3 shells can be used as a proxy for the SST in which the shells formed. Mg/Ca ratios have several other influencing factors other than temperature, such as vital effects, shell-cleaning, and postmortem and post-depositional dissolution effects, to name a few.

Alkenones are long-chain, complex organic molecules produced by photosynthetic algae. They are temperature sensitive and can be extracted from marine sediments. Use of alkenones represents a more direct relationship between SST and algae and does not rely on knowing biotic and physical-chemical thermodynamic relationships needed in CaCO3 studies. Another advantage of using alkenones is that they are a product of photosynthesis, necessitating formation in the sunlight of the upper surface layers. As such, it better records near-surface SST.

Bottom-water temperature

The most commonly used proxy to infer deep-sea temperature history are the Mg/Ca ratios in benthic foraminifera and ostracodes. The temperatures inferred from the Mg/Ca ratios have confirmed an up to 3 °C cooling of the deep ocean during the late Pleistocene glacial periods. The authors found an equation calibrating bottom water temperature of Mg/Ca ratios that takes on an exponential form:

: \mathrm{Mg/Ca} = 0.867 \pm 0.049*\exp(0.109 \pm 0.007*\mathrm{BWT}) : where Mg/Ca is the Mg/Ca ratio found in the benthic foraminifera and BWT is the bottom water temperature.

Sediment Records

Sediment records have been used to make inferences about the past and predictions about the future, and has been used in Paleoceanography research since the 1930s. Modern time scale reconstructive research has advanced using sediment core-scanning methods. These methods have enabled research similar to that conducted with ice core records in Antarctica. These records can inform on the relative abundance of organisms present at a given time using paleoproductivity methods such as measuring the total diatom abundance. Records can also inform on historic weather patterns and ocean circulation such as Deschamps et al. described with their research into sediment records from the Chukchi-Alaskan and Canadian Beaufort Margins.Deschamps, Charles-Edouard & Montero-Serrano, Jean & St-Onge, Guillaume & Poirier, André. (2019). Holocene changes in deep-water circulation inferred from authigenic Nd and Hf isotopes in sediment records from the Chukchi-Alaskan and Canadian Beaufort margins Key points. Paleoceanography and Paleoclimatology. 34. 10.1029/2018PA003485.

Salinity

Salinity is a more challenging quantity to infer from paleorecords. Deuterium excess in core records can provide a better inference of sea-surface salinity than oxygen isotopes, and certain species, such as diatoms, can provide a semiquantitative salinity record due to the relative abundances of diatoms that are limited to certain salinity regimes. There have been changes to global water cycle and the salinity balance of the oceans with the North Atlantic and becoming more saline and the sub-tropical Indian and pacific oceans becoming less so.Vinogradova, N. & Ponte, R. In search of fingerprints of the recent intensification of the ocean water cycle. J. Clim. 30, 5513–5528 (2017).

With changes to the water cycle, there have also been variations with the vertical distribution of salt and haloclines.Liu, C., Liang, X., Ponte, R.M. et al. Vertical redistribution of salt and layered changes in global ocean salinity. Nat Commun 10, 3445 (2019).

Large incursions of freshwater and changing salinity can also contribute to a reduction in sea ice extent.

Ocean circulation

Several proxy methods have been used to infer past ocean circulation and changes to it. They include carbon isotope ratios, cadmium/calcium (Cd/Ca) ratios, protactinium/thorium isotopes (231Pa and 230Th), radiocarbon activity (δ14C), neodymium isotopes (143Nd and 144Nd), and sortable silt (fraction of deep-sea sediment between 10 and 63 μm). These ratios, however, are influenced by biological, ecological, and geochemical processes which complicate circulation inferences.

All proxies included are useful in inferring the behavior of the meridional overturning circulation. 231Pa and 230Th are both formed from the radioactive decay of dissolved uranium in seawater, with 231Pa able to remain supported in the water column longer than 230Th: 231Pa has a residence time ~100–200 years while 230Th has one ~20–40 years. In today's Atlantic Ocean and current overturning circulation, 230Th transport to the Southern Ocean is minimal due to its short residence time, and 231Pa transport is high. This results in relatively low 231Pa / 230Th ratios found by McManus et al. [2004] in a core at 33N 57W, and a depth of 4.5 km. When the overturning circulation shuts down (as hypothesized) during glacial periods, the 231Pa / 230Th ratio becomes elevated due to the lack of removal of 231Pa to the Southern Ocean. McManus et al. [2004] also note a small raise in the 231Pa / 230Th ratio during the Younger Dryas event, another period in climate history thought to have experienced a weakening overturning circulation.

Acidity, pH, and alkalinity

Boron isotope ratios (δ11B) can be used to infer both recent as well as millennial time scale changes in the acidity, pH, and alkalinity of the ocean, which is mainly forced by atmospheric CO2 concentrations and bicarbonate ion concentration in the ocean. δ11B has been identified in corals from the southwestern Pacific to vary with ocean pH, and shows that climate variabilities such as the Pacific decadal oscillation (PDO) can modulate the impact of ocean acidification due to rising atmospheric CO2 concentrations. Another application of δ11B in plankton shells can be used as an indirect proxy for atmospheric CO2 concentrations over the past several million years.

References

References

  1. (October 2002). "New oceanic proxies for paleoclimate". Earth and Planetary Science Letters.
  2. (2010). "Paleoclimates : understanding climate change past and present". Columbia University Press.
  3. (1947). "The thermodynamic properties of isotopic substances". Journal of the Chemical Society (Resumed).
  4. Emiliani, C.. (1955). "Pleistocene temperatures". Journal of Geology.
  5. (January 1963). "Evidence of climatic changes in North Atlantic deep-sea cores, with remarks on isotopic paleotemperature analysis". Progress in Oceanography.
  6. (1 July 1967). "Oxygen Isotope Analyses and Pleistocene Temperatures Re-assessed". Nature.
  7. (5 September 2003). "Synchroneity of Tropical and High-Latitude Atlantic Temperatures over the Last Glacial Termination". Science.
  8. Herbert, T. D.. (2003). "Treatise on geochemistry". Elsevier Science.
  9. (October 2002). "Benthic foraminiferal Mg/Ca-paleothermometry: a revised core-top calibration". Geochimica et Cosmochimica Acta.
  10. Piggot, Charles Snowden. “Core Samples of the Ocean Bottom and Their Significance.” ''The Scientific Monthly'', vol. 46, no. 3, 1938, pp. 201–217. ''JSTOR'', www.jstor.org/stable/16315. Accessed 24 Mar. 2021.
  11. S.L. Jaccard, E.D. Galbraith, D.M. Sigman, G.H. Haug, A pervasive link between Antarctic ice core and subarctic Pacific sediment records over the past 800kyrs, Quaternary Science Reviews, Volume 29, Issues 1–2, 2010, Pages 206-212, ISSN 0277-3791, {{doi. 10.1016/j.quascirev.2009.10.007.
  12. Sjunneskog, C., F. Taylor 2002.  Postglacial marine diatom record of the Palmer Deep, Antarctic Peninsula (ODP Leg 178, Site 1098) 1. Total diatom abundance.  VL  - 17.  DO  - 10.1029/2000PA000563.  Paleoceanography
  13. (June 2003). "Diatom-inferred salinity records from the Arctic Siberian Margin: Implications for fluvial runoff patterns during the Holocene". Paleoceanography.
  14. Yu, L. A global relationship between the ocean water cycle and near-surface salinity. ''J. Geophys. Res. -Oceans'' '''116''', C10025 (2011).
  15. Goosse, H. and Zunz, V.: Decadal trends in the Antarctic sea ice extent ultimately controlled by ice–ocean feedback, The Cryosphere, 8, 453–470, {{doi. 10.5194/tc-8-453-2014
  16. (19 February 1993). "Mid-Depth Circulation of the Subpolar North Atlantic During the Last Glacial Maximum". Science.
  17. (30 September 2005). "Preindustrial to Modern Interdecadal Variability in Coral Reef pH". Science.
  18. (17 August 2000). "Atmospheric carbon dioxide concentrations over the past 60 million years". Nature.

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