Circulation changes in the Mediterranean Sea over the last 30,000 years – Constraints from deep-water Nd isotopes

Abstract

The Mediterranean circulation is sensitive to ongoing climate change, and played a key role in the rhythmic deposition of organic-rich sedimentary layers, known as sapropels. These deposits represent periods of anoxic deep-water phases in the geologic past. However, many aspects of their interactions with climatic and oceanographic processes are unresolved. Here, we integrate a novel offshore record of Nd isotope composition (εNd) with authigenic εNd data from multiple Mediterranean sites and use a refined box model, to resolve mixing and advection of Mediterranean deep water over the last ∼30,000 years. During the Last Glacial Maximum, more unradiogenic εNd (−7.4 to −6.5) in the deep EMS corresponds to a 56 % ± 14 % decline in water exchange between the eastern and western Mediterranean Sea (EMS vs. WMS). This decline was caused by lower sea levels, which also led to distinct deep circulation modes in the two basins. Meanwhile, deep convection occurred in the Ionian Sea, implying a shift of EMS deep-water formation zone. The Heinrich Stadial 1 is characterized by increased εNd values, showing a strong intermediate-water outflow caused by deglacial sea-level rise, but the deep EMS circulation weakened. The more radiogenic and homogeneous εNd (−5.3 to −4.7) during sapropel S1 deposition suggest deep-water stagnation in the EMS. This was accompanied by enhanced Nile runoff and 44 % reduced EMS–WMS exchange that was limited to shallower depths, with weak but persistent outflow from the Adriatic Sea. Such basin-wide stagnation initiated ∼1000 years before the onset of S1 and terminated with full deep-water renewal during S1 ending.