Assessing proxy signatures of temperature, salinity, and hypoxia in the Baltic Sea through foraminifera-based geochemistry and faunal assemblages

Abstract

Current climate and environmental changes strongly affect shallow marine and coastal areas like the Baltic Sea. This has created a need for a context to understand the severity and potential outcomes of such changes. The context can be derived from paleoenvironmental records during periods when comparable events happened in the past. In this study, we explore how varying bottom water conditions across a large hydrographic gradient in the Baltic Sea affect benthic foraminiferal faunal assemblages and the geochemical composition of their calcite tests. We have conducted both morphological and molecular analyses of the faunas and we evaluate how the chemical signatures of the bottom waters are recorded in the tests of several species of benthic foraminifera. We focus on two locations, one in the Kattegat (western Baltic Sea) and one in Hanö Bay (southern Baltic Sea). We show that Mn/Ca sw , Mg/Ca sw , and Ba/Ca sw variations are mainly controlled by dissolved oxygen concentration and salinity. Their respective imprints on the foraminiferal calcite demonstrate the potential of Mn/Ca as a proxy for hypoxic conditions, and Ba/Ca as a proxy for salinity in enclosed basins such as the Baltic Sea. The traditional use of Mg/Ca as a proxy to reconstruct past sea water temperatures is not recommended in the region, as it may be overprinted by the large variations in salinity, alkalinity, and Mg/Ca sw. Salinity is the main factor controlling the faunal assemblages; a much more diverse fauna occurs in the higher salinity Kattegat (~32) than in the low‐salinity Hanö Bay 2 (~15). Molecular identification shows that only Elphidium clavatum occurs at both locations, but other genetic types of both genera Elphidium and Ammonia are restricted to either low or high salinity. The combination between foraminiferal geochemistry and environmental parameters demonstrates that in a highly variable setting like the Baltic Sea, it is possible to separate the different environmental impacts and therefore use these proxies to reconstruct how specific conditions may have varied in the past.