Sedimentary trace elements as proxies to depositional changes induced by aHolocene fresh-brackish water transition

A halocline developed in the Gotland Deep, Baltic Sea, at c. 8.0 C-14 ky BP , as the result of a transition from fresh to brackish water. The sediment- water interface changed from oxic to predominantly anoxic, depositional con ditions were periodically euxinic and pyrite formation was extensive. This environmental change led to pyritization of the upper part of earlier depos ited sediments. This study discusses how the distribution of trace elements (As, Ba, Cd, Cu, Co, Mo, Mn, Ni, Pb, U, Zn and V) were affected by the cha nging redox conditions, productivity and salinity. The reducing conditions led to pyritization of Cu, Co, Ni, Cd, Mo, Mn and As. Lead and Zn concentra tions are very low in pyrite, in agreement with their coordination to sulfi de being tetrahedral. Certain elements are enriched in those sediments depo sited under euxinic conditions. This enrichment was caused by scavenging of elements dissolved in the water column and is restricted to elements that have a comparably long residence time in the Baltic Sea. Molybdenum appears to be the most unambigious proxy for euxinic conditions, whereas enrichmen t of U also requires brackish water in the productive zone. In the brackish environment, enrichment of Ba and V are linked to the cycling of organic c arbon. Manganese and As are the only elements that have been significantly remobilised due to the downward moving pyritization front.