Learning from past changes in Mediterranean Thermohaline Circulation

Mediterranean thermohaline circulation is a very sensitive system to changes in regional climate conditions. The current warming conditions have already been transferred into the deep water properties, and future projections indicate an overall weakening of this circulation system. In this context, it becomes extremely relevant understanding the natural range of variability of this system and the interplay between the different basins and sub-basins controlling deep and intermediate convection in the Mediterranean Sea. Here we use the past record as a natural laboratory to learn on the impact that different climate forcings had in this circulation system, and also evaluating its consequences in the Mediterranean Outflow Water (MOW) and thus in the Atlantic oceanography. These are the main results of the ERC-Consolidator grant TIMED, devoted to understanding of past changes in Mediterranean thermohaline circulation. This new data set is based in a variety of proxies that are sensitive to ocean circulation changes such as U/Mn ratios measured in the foraminifera diagenetic coatings, Nd isotopes, absolute dating on deep sea corals, among other geochemical and sedimentological tools applied in sediment cores from both E- and W-Med. The obtained results indicate the deglacial development of an intense minimum oxygen zone in the W-Med associated to the LIW which extended down to at least 950m. These evidences support by the first time, that the formation of the deglacial Organic Rich Layer was also connected to a weakening of E-Med convection, this would indicate a long pre-conditioning prior to the last Sapropel (S1) that would start with last Heinrich event in the North Atlantic and fully develop with the onset of the African humid period. We identify that an intense aridification and cooling of the E-Med driven by the AMOC weakening of the Younger Dryas was responsible of a strong reactivation of E-Med convection that resulted in a stronger MOW, and also triggered enhanced deep intermediate convection in the W-Med. This circulation change pushed out an old water mass previously accumulated in the Med for several centuries with major consequences in deep ecosystems sustained by deep sea corals. The onset of the last S1 in the E-Med led to major changes in deep convection in both E and W-Med, but with opposite sign in their response. These results highlight the tide connection between the AMOC and E-Med convection and rise questions on the potential role that the associate changes in the MOW could have on the AMOC, particularly during weak stages.