Constraints on the ocean oxygen isotopic enrichment between the Last Glacial Maximum and the Holocene: Paleoceanographic implications

Abstract

We review the various methods which have been applied to estimate the change of seawater δ 18O (δw) between the Last Glacial Maximum (LGM) and the Holocene. The most accurate constraints on these estimates are provided by the measurement of pore waters δ 18O and by high resolution records of benthic foraminifer δ 18O in the high latitude oceans of both hemispheres. They show that the δ 18O of seawater in the deep ocean during the LGM was 1.05±0.20‰ heavier than today, with significant regional variations. Constraints resulting from ice sheet models are less accurate, because both the volume and isotopic composition of each ice sheet are still poorly known. The amplitude of the benthic δ 18O change between the LGM and the Holocene, together with the δ 18O and δ 13C values of the benthic foraminifera genus Cibicides during the LGM, show that the Southern Ocean deep waters were extremely cold, close to the freezing point. During this time, deep waters of the South Atlantic and the Pacific oceans were at least 1.3°C warmer than those of the Southern Ocean. Overall, the glacial deep ocean, below 2500 m, was characterized by extremely cold temperatures, everywhere lower than 0°C. δ 18O values of benthic foraminifer from the North Atlantic are highly variable. This variability suggests that deep Atlantic waters were not homogeneous, probably because they resulted from the sinking of different surface water masses at various locations during winter. The deep waters in the North Atlantic were at most 2°C warmer than in Southern Ocean. Alternatively, they could have been nearer the freezing point with a δ 18O value lighter than the mean ocean water. Brine formation during winter would preserve such light δ 18O values of the northern North Atlantic surface water.