A Saltier Glacial Mediterranean Outflow

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 degrees 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 O-18 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. Plain Language Summary The Gulf Stream is slowing down because of the meltdown of the Greenland ice sheet. In the past, such a slowdown often resulted in a brief but quite extreme climate cooling in the Northern Hemisphere. Fortunately, the Gulf Stream would eventually speed up again for reasons that remain poorly understood. It is thought that the exchange of water between the Atlantic Ocean and the Mediterranean Sea through the Strait of Gibraltar plays an important role in bringing the Gulf Stream back to speed. In order to test this idea, we need to know the strength of the Atlantic-Mediterranean exchange during times at which the Gulf Stream slowed down. Little shell-like organisms called benthic foraminifera, which live at the bottom of the ocean, record information about the properties of the water in which they grow within their shells. By analyzing a set of foraminifera living at a location close to the Strait of Gibraltar, we infer that it is indeed likely that the Atlantic-Mediterranean exchange changed significantly during a slowdown of the Gulf Stream. It is questionable whether or not the Gibraltar exchange will also intensify due to the current melting of ice.