Calcium-rich rhodochrosite layers in sediments of the Gotland Deep, Baltic Sea, as indicators for seawater inflow

The formation of Ca-rich rhodochrosites in sediments of the predominantly anoxic Gotland Deep, Baltic Sea, has been assumed to reflect deepwater oxygenation events caused by inflows of large volumes of North Sea water into the Baltic Sea. Such episodically occurring seawater inflows are a product of specific North European/North Atlantic atmospheric conditions. In this work, the formation conditions of sub-recent Ca-rich rhodochrosites was analysed for the first time in surficial sediments of the Gotland Deep. The stratigraphy of Ca-rich rhodochrosite layers was directly compared to recent oceanographic records and the isotopic composition of the carbonates was analysed. The solid phase composition was compared to the porewater composition in order to analyse thermodynamic equilibria. Furthermore, the distribution of trace elements in these sediments was analysed.

The hypothesis of a synsedimentary formation of Ca-rich rhodochrosite as a product of North Sea water inflows was confirmed in this study. However, dependent on hydrographic conditions, surface sediments can be re-suspended or eroded and authigenic carbonates can be re-dissolved in undersaturated bottomwaters. ... mehrHence, the use of Ca-rich rhodochrosite layers in Holocene sediments of the central Baltic Deeps as a proxy for past seawater inflows is limited.

Considering a possible reconstruction of deepwater conditions during the formation of authigenic carbonates by studying their cationic composition, it has to be stated that cation ratios are influenced by early diagenetic changes in the porewaters due to continuous organic matter mineralisation leading to a disequilibrium of the porewaters and authigenic carbonates formed at the sediment/water interface.

The distribution of redox sensitive trace elements is affected by redox turnover events, which occur in the surficial sediment after seawater inflows, resulting in vertical shifts of possible palaeoredox proxies in the sediment column.