A Saltier Glacial Mediterranean Outflow

Abstract

The state of Atlantic Meridional Overturning Circulation (AMOC) is influenced by both the strength and the location of the Mediterranean Outflow Water (MOW) plume in the Gulf of Cadiz. To evaluate the influence of MOW on AMOC over deglaciations, precise and accurate salinity and temperature reconstructions are needed. For this purpose, we measured Mg/Ca and clumped isotopes of several benthic foraminiferal species at Integrated Ocean Drilling Program Site U1390 in the Gulf of Cadiz. The clumped isotope results of Cibicidoides pachyderma, Uvigerina mediterranea, and Pyrgo spp. are consistent between species and record no significant difference in Last Glacial Maximum to Holocene deep water temperature. Over the deglaciation, the Mg/Ca-based temperatures derived from U. mediterranea indicate three periods of MOW absence at Site U1390. Mg/Ca-based temperatures of Hoeglundina elegans and C. pachyderma are on average 6°C too cold when compared to the present core-top temperature, which we explain by a carbonate ion effect on these epibenthic species related to the high alkalinity of the MOW. Combining deep water temperature estimates with the benthic oxygen isotope data and considering different relationships between seawater oxygen isotopes and salinity, we infer a salinity decrease of MOW by three to eight units over the deglaciation and four units during Sapropel 1, accounting for the global δ18O depletion due to the decrease in ice volume. Our findings confirm that the Mediterranean Sea accumulates excess salt during a glacial low stand and suggest that this salt surged into the Atlantic over the deglaciation, presumably during Heinrich Stadial 1.