OCEAN PALAEOCIRCULATION

The ocean circulation in the geological record. plate tectonic movements in the past mean that the ocean circulation has not always had the same configuration as it has today. During warm greenhouse intervals of the cretaceous, for example, there were no subtropical or oceanic polar fronts (OPFs), the high- and low-latitude ocean gyres would have been weakened, or non-existent, and the thermal gradient from the tropics to the poles would have been reduced. With the tectonic opening of the Southern Ocean, ocean gateways between South America and Antarctica, and Australia and Antarctica, in the Early cenozoic (cainozoic), a more familiar pattern of [Page 760]surface ocean currents and deep water circulation (see thermohaline circulation) evolved. Cenozoic and quaternary reconstruction of ocean palaeocirculation is carried out using marine palaeoclimatic proxies from marine sediment cores. The protactinium/thorium ratio (231Pa/230Th) palaeocirculation proxy relies on the radioactive decay (see radioactivity, radionuclide) of uranium in the water column and can be used to reconstruct the rate of flow of deep water masses in the past, and the strength of the overturning circulation. Sortable silt mean size (mean grain size of the 10–63 μm sediment fraction) is also used as a proxy for reconstructing rates of deep water flow. nutrient proxies, such as carbon isotope ratios and cadmium:calcium ratios (Cd:Ca) measured in benthic foraminifera, have been used to monitor past changes in deep water and intermediate water formation. Stable carbon isotopes from marine sediments are also used to reconstruct ocean ventilation history. Water masses formed in different source regions are imprinted with distinct neodymium (Nd) isotope signatures, hence neodymium isotope ratios in foraminifera, the teeth of fossil fish, and ferromanganese crusts can all act as conservative tracers for ocean palaeocirculation in the open ocean. Neodymium isotopes have also been used to investigate the timing of opening of ocean gateways.

[See alsoforaminifera: analysis, meridional overturning circulation (moc), palaeoceanography, palaeoclimatology, proxy climatic indicator, stable isotope, tracers: in the oceans]