A 5-million-year record of (de)oxygenation and associated changes in basin restriction in the Mediterranean Sea

Restricted basins are susceptible to develop anoxia because these land-locked basins can trap nutrients and the water column can easily become stratified. The Mediterranean basin has a limited connection to the open ocean and became increasingly restricted over time, making it suitable to study effect of basin configuration on anoxia development. Enhanced runoff, related to increased North-African monsoon intensity, regularly resulted in increased productivity and/or deep-water anoxia and deposition of organic-rich sediments, the so-called sapropels. Most depositional models for sapropel formation focus on the most recent sapropels (e.g. S1 and S5), outcrop samples or individual Pliocene sapropels from ODP leg 160. Until recently, continuous and high-resolution records were lacking, whereas such records can provide important constraints on the relationship between sapropel deposition and its environmental driving forces, such as climate and basin configuration. Here we present newly acquired XRF-scanning data of redox-sensitive trace elements and estimates for total organic carbon (TOC) from a 5 Myr record of the Eastern Mediterranean Sea (ODP Site 967), where reoccurring sapropels are recorded from 3.2 Ma onwards. Based on our geochemical proxies, we reconstruct (de)oxygenation and associated basin restriction over the last 5 Myr. This record allows us to elucidate which primary processes drove sapropel formation in this basin and whether these processes changed over time. We show that the first preserved Pliocene sapropels (~3.2-3.0 Ma) are highly enriched in redox-sensitive trace elements and have TOC values up to 25%, and likely formed in a much more open (i.e. with increased water renewal) depositional environment. Such a model of Mediterranean sapropel deposition differs significantly from that of the more recent sapropels, which are deposited in a much more restricted environment. Hence, the depositional model for recent sapropel formation cannot be directly extrapolated to these older sapropel deposits.