Abstract
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 seawater Mn∕Ca, Mg∕Ca, and Ba∕Ca (Mn∕Casw, Mg∕Casw, and Ba∕Casw) 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 seawater temperatures is not recommended in the region, as it may be overprinted by the large variations in salinity (specifically on Bulimina marginata), Mg∕Casw, and possibly also the carbonate system. Salinity is the main factor controlling the faunal assemblages: a much more diverse fauna occurs in the higher-salinity (∼32) Kattegat than in the low-salinity (∼15) Hanö Bay. 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 locations. The combination of foraminiferal geochemistry and environmental parameters demonstrates that in a highly variable setting like the Baltic Sea, it is possible to separate different environmental impacts on the foraminiferal assemblages and therefore use Mn∕Ca, Mg∕Ca, and Ba∕Ca to reconstruct how specific conditions may have varied in the past.
Highlights
Shelf and fjord environments are known to be sensitive to and directly affected by global climate change
Paleoenvironmental reconstructions have demonstrated that large environmental changes in the Baltic Sea have occurred, and how the system switched from fresh water lake stages (e.g., Ancylus Lake) at the end of the last deglaciation to more marine conditions during the Littorina Sea stage (∼ 8.5–3 ka; Björck, 1995, 2008; Jakobsson et al, 2007; Andrén et al, 2015; Kotthoff et al, 2017)
In this study we explore how the differing water column chemistry between two contrasting locations (the saline Kattegat and the brackish Hanö Bay (Fig. 1) affects the composition of the benthic foraminiferal assemblages using morphological and molecular identification techniques, and how each site’s different chemical signatures are incorporated into the foraminiferal tests of key species, including an exercise to determine the possible effects of seasonal variations
Summary
Shelf and fjord environments are known to be sensitive to and directly affected by global climate change. Paleoenvironmental reconstructions have demonstrated that large environmental changes in the Baltic Sea have occurred, and how the system switched from fresh water lake stages (e.g., Ancylus Lake) at the end of the last deglaciation to more marine conditions during the Littorina Sea stage (∼ 8.5–3 ka; Björck, 1995, 2008; Jakobsson et al, 2007; Andrén et al, 2015; Kotthoff et al, 2017). Even during the saline Littorina Sea stage, variability in salinity remained high and several periods occurred when large areas of bottom waters became fully anoxic and even euxinic (Björck, 1995; Gustafsson and Westman, 2002; Zillén et al, 2008; Widerlund and Andersson, 2011; Jilbert and Slomp, 2013; Kotilainen et al, 2014; Lenz et al, 2015; Ning et al, 2016; Van Helmond et al, 2017). The proxy variations are fixed into the foraminiferal tests during calcification (as opposed to a secondary diagenetic imprint which may occur after deposition) and representative for the bottom water conditions for that particular period, while proxies based on bulk sediment composition may be altered over time by continuing (bio)geochemical processes within the sediment
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