Controls on barite precipitation and preservation in Mediterranean sediments: from sapropel deposition to modern sedimentation

Mediterranean sediments have registered some of the most exceptional Ba records in marine basins. Although Organic Rich Layers (ORLs) are less well studied, both sapropels and ORLs are characterized by marked increases in Ba content in response to productivity oscillations, as demonstrated by numerous data sets and sediment records. During sapropel deposition, barite was abundantly produced in the water column due to increased productivity and associated microbial processes involved in organic matter degradation. In this scenario, bacterial activity and extracellular polymeric substances (EPS) have been shown to be the main control of barite precipitation in the water column, which is further supported by experimental work and observations from microenvironments of intense organic matter mineralization in the ocean water column. Once accumulated in marine sediments, barite tends to be well preserved in both eastern and western Mediterranean basins, where the availability of sulphate in pore waters prevented dissolution. Thus, differences in barite abundance in sapropels compared to ORLs support differences in productivity rates, and also differences in primary producers and microbial processes. Indeed, differences in productivity types between modern eastern and western basins also support that such differences over time may have led to spatial differences in barite formation. The general decline in productivity, and hence microbial activity, across the Mediterranean basins during the Holocene is indicated by the remarkably low Ba content in recent sediments. Dissolution of barite through the water column is also important in modern environments and is still poorly understood in the past. Overall, a better understanding of the microbial processes involved in barite production and the factors controlling its preservation is required to further constrain the information captured by Ba proxies.