Oxygen isotopic composition of the Mediterranean Sea since the Last Glacial Maximum: constraints from pore water analyses

Interstitial waters recovered from Ocean Drilling Program, Leg 161, site 976 in the western Mediterranean Sea are used in conjunction with a numerical model to constrain the δ18O of seawater in the basin since the Last Glacial Maximum, including Sapropel Event 1. To resolve the oxygen isotopic composition of the deep Mediterranean, we use a model that couples fluid diffusion with advective transport, thus producing a profile of seawater δ18O variability that is unaffected by glacial–interglacial variations in marine temperature. Comparing our reconstructed seawater δ18O to recent determinations of 1.0‰ for the mean ocean change in glacial–interglacial δ18O due to the expansion of global ice volume, we calculate an additional 0.2‰ increase in Mediterranean δ18O caused by local evaporative enrichment. This estimate of δ18O change, due to salinity variability, is smaller than previous studies have proposed and demonstrates that Mediterranean records of foraminiferal calcite δ18O from the last glacial period include a strong temperature component. Paleotemperatures determined in combination with a stacked record of foraminiferal calcite depict almost 9°C of regional cooling for the Last Glacial Maximum. Model results suggest a decrease of ∼1.1‰ in seawater δ18O relative to the modern value caused by increased freshwater input and reduced salinity accompanying the formation of the most recent sapropel. The results additionally indicate the existence of isotopically light water circulating down to bottom water depths, at least in the western Mediterranean, supporting the existence of an ‘anti-estuarine’ thermohaline circulation pattern during Sapropel Event 1.