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.

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 worlds oceans. Several geochemical proxy tools include long-chain organic molecules (e.g. alkenones), stable and radioactive isotopes, and trace metals [Henderson, 2002]. Additionally, sediment cores 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 [Cronin].

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 [Urey, 1947]. When these shells precipitate, they sink and form sediments on the ocean floor whose δ¹⁸O can be used to infer past SSTs [Emiliani, 1955]. 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 [Olausson, 1965; Shackleton, 1967].

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 [Cronin]. Other influences aside, Mg/Ca ratios have successfully quantified the tropical cooling that occurred during the last glacial period [Lea et al., 2003].

Alkenone is a long-chain, complex organic molecule produced by photosynthetic algae which is temperature sensitive and can be extracted from marine sediments. Use of alkenone 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 [Herbert, 2003]. Another advantage of using alkenone is that it is a product of photosynthesis and necessitates formation in the sunlight of the upper surface layers. As such, it is a better record of near-surface SST [Cornin].

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 [Cronin].

From Lear et al. [2002] Figure 8-B: Cibicidoides spp. Mg/Ca-temperature calibration. Compilation of corrected FAAS, ICP-AES, and ICP-MS data for C. wuellerstorfi, C. pachyderma, C. compressus, and a wuellerstorfi-like Cibicidoides. Triangles represent rejected data lying in the upper 5 percentile of the standard error.

One notable study is that by Lear et al. [2002] who worked to calibrate bottom water temperature to Mg/Ca ratios in 9 locations covering a variety of depths from up to six different benthic foraminifera (depending on location). The authors found an equation calibrating bottom water temperature ot Mg/Ca ratios that takes on an exponential form:

Mg/Ca = 0.867 +/- 0.049*exp{0.109 +/- 0.007*BWT}

where Mg/Ca is the Mg/Ca ratio found in the benthic foraminifera and BWT is the bottom water temperature. A plot of their calibration can be seen at left.

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 [Bauch and Polyakova, 2003].

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) [Cronin]. Carbon isotope and cadmium/calcium ratio proxies are used because variability in their ratios is due partly to changes in bottom-water chemistry, which is in turn related the source of deep-water formation [e.g. Oppo and Lehman 1993; Lehman and Keigwin 1992]. 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 [Cronin]. For example, McManus et al. [2004] used protactinium/thorium isotopes (²³¹Pa and ²³⁰Th) to show that the Atlantic Meridional Overturning Circulation had been nearly (or completely) shut off during the last glacial period. 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 [McManus et al., 2004]. 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 [Pelejero et al., 2005].

Screen Shot of Figure 1 from Pearson and Palmer [2000] showing surface ocean pH as reconstructed from Boron isotope analysis for the past 60 million years.

Another application of δ¹¹B in plankton shells can be used as an indirect proxy for atmospheric CO₂ concentrations over the past several million years (see figure on right) [Pearson and Palmer, 2000].

See also

• Oceanography
• Paleoclimatology

References

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• Cronin, Thomas M. Paleoclimates - Understanding Climate Change Past and Present. New York: Columbia University Press. 2010.
• Emiliani, C. 1955. Pleistocene temperatures. Journal of Geology. Vol. 63. pp. 538–578.
• Henderson, G.M. 2002. New oceanic proxies for paleoclimate. Earth and Planetary Science Letters. Vol. 203. pp. 1–13.
• Herbert, T.D. 2003. Alkenone Paleotemperature Determinations. In H. Elderfield and K.JK. Turekian, eds., Treatise in Marine Geochemistry. pp. 391–432. Amsterdam:Elsevier.
• Lea, D.W. et al. 2003. Synchroneity of Tropical and High-Latitude Atlantic Temperaatures over the Last Glacial Termination. Science. Vol. 301. pp. 1361–1364.
• Lear, C.H., Y. Rosenthal, N. Slowey. 2002. Benthic foraminiferal Mg/Ca-paleothermometry: A revised core-top calibration. Geochimica et Cosmochimica Acta. Vol. 66. pp. 3375–3387.
• Lehman, S.J. and L.D. Keigwin. 1992. Sudden changes in North Atlantic circulation during the last deglaciation. Nature. Vol. 356. pp. 757–762.
• McManus, J.F. et al. 2004. Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes. Letters to Nature. Vol. 428. pp. 834–837.
• Olausson, E. 1965. Evidence of climatic changes in North Atlantic deep-sea cores with remarks on isotopic paleotemperature analysis. Progress in Oceanography. Vol. 3. pp. 221–252.
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• Pearson, P.N. and M.R. Palmer. 2000. Atmospheric carbon dioxide concentrations over the past 60 million years. Nature. Vol. 406. pp. 695–699.
• Pelejero, C. et al. 2005. Preindustrial to Modern Interdecadal Variability in Coral Reef pH. Science. Vol. 309. pp. 2204–2207.
• Shackleton, N.J. 1967. Oxygen isotope analyses and Pleistocene temperature re-assessed. Nature. Vol. 215. pp. 15–17.
• Urey, H.C. 1947. The thermodynamic properties of isotopic substances. Journal of the Chemical Society (London). (April):562-581.