The oxygen isotopic composition and temperature of Southern Ocean bottom waters during the last glacial maximum

Abstract

We provide two new determinations of the oxygen isotopic composition of seawater during the last glacial maximum (LGM). High-resolution oxygen isotopic measurements were made on interstitial waters from Ocean Drilling Program (ODP) Sites 1168 and 1170 in the southeast Indian Ocean sector of the Southern Ocean. We use a diffusion–advection numerical model to calculate the glacial–interglacial change in bottom-water δ 18O sw from the pore water δ 18O profiles; the first such determinations from this part of the oceans. Statistical analyses of the model runs indicate that Circumpolar Deep Water (CDW) δ 18O sw changed by 1.0–1.1±0.15‰ since the last glacial maximum (LGM). Our results are consistent with a previous calculation from a South Atlantic Southern Ocean location (ODP Site 1093) also situated within CDW. The new values determined in this study, together with previous estimates, are converging on a global average Δ δ 18O sw of 1.0–1.1‰. Using the calculated bottom-water δ 18O sw, we have extracted the temperature component from the benthic foraminiferal δ 18O record at Sites 1168 and 1170. Since the LGM, bottom waters at these two sites warmed by 2.6 and 1.9°C, respectively. The absolute temperature estimates for the LGM (−0.5°C [ Θ=−0.6°C] at Site 1168 and −0.2°C [ Θ=−0.4°C] at Site 1170) are slightly warmer than those reported from previous studies using the same technique, but are consistent with more homogenous deep-ocean temperatures during the LGM relative to the modern.