The climatic and oceanographic setting during the Messinian in Northern Adriatic Sea: what can be learned for present and future deoxygenation dynamics?

During the Messinian, the Mediterranean Basin became highly sensitive to environmental changes due to the gradual restriction of water exchange with the Atlantic Ocean. This led to the widespread deposition of organic-rich layers known as sapropels, indicating significant disturbances in the carbon and oxygen cycles. These sediments formed under conditions of oxygen depletion, likely due to periodic weakening of the thermohaline circulation. Understanding the causes and extent of this circulation weakening in the past is crucial for predicting present and future deoxygenation trends in the Mediterranean under climate warming.

For this purpose, we investigate a Messinian sapropel-bearing succession cropping out at Monte dei Corvi (Ancona, central Italy) with mineralogical, petrographic, micropaleontological and stable carbon and oxygen isotopic analyses. Our findings reveal that sapropel deposition occurred due to increased sea surface buoyancy, which inhibited thermohaline circulation, consequently reducing bottom-water oxygen content and impacting bioturbating organisms. Within the lithological cycle, the recovery of an efficient thermohaline circulation is recorded by thin packstone layers underlying the marly limestone/marlstone, which record intense bottom currents activity. The accumulation of marly limestone/marlstone during periods of high primary productivity and organic carbon export to the seafloor led to bottom hypoxia but not organic matter preservation. Furthermore, the latter was deposited with surficial seawater density in the range of modern Adriatic, suggesting that primary productivity can promote bottom hypoxia even with similar modern deep oxygen renewal rates. These lithological changes were likely influenced by variations in the Adriatic Deep Water formation system paced by precession-driven climatic and oceanographic changes.

Integration of previously published Sea Surface Temperature (SST) data with our new isotopic data indicates that variations in Sea Surface Salinity (SSS) primarily controlled sapropel deposition, with the SSTs of sapropel deposits aligning closely with projected SST in the Eastern Mediterranean at the end of this century under climate warming. In this future scenario, warming will be coupled with an SSS increase, which likely counteract the density loss provided by temperature, making the bottom deoxygenation in the Eastern Mediterranean abysses unlikely. However, we caution that additional factors such as winter heat waves and eutrophication could exacerbate Mediterranean oxygen depletion and should be considered in model-based projections.