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.
[1] 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 (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.
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.
[3] When these shells precipitate, they sink and form sediments on the ocean floor whose δ18O can be used to infer past SSTs.
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.
[2] 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.
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.