The foraminiferal record in the Holocene evolution of the Mecklenburg Bay (south-western Baltic Sea)

Foraminiferal assemblages were analyzed in a 620-cm long core retrieved from the central part of the Mecklenburg Bay (MB, south-western Baltic Sea) to aid in the reconstruction of environmental changes occurring in the area during the Holocene and to complement a set of previously investigated palaeoenvironmental proxies. A total of five foraminifera-based stratigraphic units were identified, including an initial 80-cm thick layer devoid of foraminifera. The next two units featured an increasing abundance of the foraminiferal assemblage dominated by the calcareous Ammonia group species. Nearly all the calcareous foraminifera found in the core were decalcified. Following the maximum abundance within the 470–410 cm layer, the foraminiferal abundance declined sharply and the assemblage’s dominance structure changed to domination of the agglutinated foraminiferal species, Eggerelloides scaber, which continued up to the top of the core and marked a pronounced shift in environmental conditions (shallower depth, lower salinity, more dynamic sedimentation conditions). The foraminifera-based stratigraphy of the core proved to be complementary to that emerging from previous analyses of diatoms and sediment geochemistry.

Conclusions

Analysis of Core 317980-3 retrieved from the Mecklenburg Bay (MB) in the south-western Baltic Sea revealed the presence of a total of 13 foraminiferal taxa (including the collective Ammonia group). Most of the calcareous forms represented the genus Ammonia (most likely A. beccarii and A. batava). The vast majority of specimens occurred as organic shell linings, and decalcification prevented species identification. Representatives of the Ammonia group occurred throughout most of the foraminifera-containing part of the core. Other species appeared either intermittently or were observed in some sediment layers only. The agglutinating species Eggerelloides scaber became a permanent member of the foraminiferal assemblages from the 200–195 cm layer (c. 3500 cal yrs BP) upward and was virtually the only taxon present in the uppermost 45 cm layer.
The taxonomic richness of foraminifera was low throughout most of the core length, with basically monogeneric or monospecific assemblages occurring at the opposite ends of the foraminifera-containing section.
A total of five foraminifera-based stratigraphic units were identified, including an initial 80-cm thick layer devoid of foraminifera, and containing abundant cladoceran remains, indicating freshwater conditions of the Ancylus Lake. The next two units featured an increasing abundance of the foraminiferal assemblage dominated by the calcareous Ammonia group species. After reaching the maximum value within the 470–410 cm layer (c. 6900–6300 cal yrs BP), the foraminiferal abundances decreased abruptly and the assemblage became dominated by the agglutinated species Eggerelloides scaber up to the top of the core. This was an indication of a pronounced shift in environmental conditions (shallower depth, lower salinity, more dynamic sedimentation conditions).
The proportion of calcareous to agglutinated foraminifera (C/A ratio) showed a drastic reversal, at about 3300 cal yrs BP, from the assemblage dominated by calcareous forms to that dominated by agglutinated ones, which may signify a transition to a less saline and more stressful (in terms of e.g. reduced oxygenation of the near-bottom water layer and pore water) environment in MB prevailing to this day.
The foraminifera-based stratigraphy of Core 317980-3 was broadly similar to that based on the diatomological analysis of the same core, but comprised more units, which refined the division resulting from the diatom analysis. On the other hand, the foraminifera-based stratigraphy coincided with that based on geochemical proxies.
The comparison of the foraminiferal record emerging from Core 317980-3 (MB, south-western Baltic Sea) with the results of similar studies in the Bornholm Basin (BB) showed differences and similarities in foraminiferal assemblages in the two areas, reflecting more general similarities and differences in the evolution of the Baltic Sea. The similarities involved a generally low taxonomic richness (although higher in MB than in BB), a sediment layer lacking foraminifera and rich in Cladocera dating to the Ancylus Lake stage, the domination of low-diversity assemblages of calcareous foraminifera (strongly decalcified in both cases) throughout the Littorina Sea stage, and the prevalence of a single, agglutinated species in the late phase of the Littorina Sea stage to date. The differences involved the dominant taxa in BB and MB, both calcareous (Elphidium spp. and Ammonia spp., respectively) and agglutinated (Reophax dentaliniformis and Eggerelloides scaber, respectively), but also reflected different timing of major palaeo-oceanographic events, with the onset of the Littorina Sea stage occurring earlier in MB than in BB. Our results therefore support the notion that the Littorina transgression occurred earlier in MB than in other parts of the Baltic Sea.

Acknowledgements

We thank Dr. Matthias Moros of the Leibniz Institute of the Baltic Sea Research (IOW) Warnemünde (Germany) for placing materials from Core 317980-3 at our disposal for the analysis of foraminifera. Our thanks are due to Ms. Anna Skrzypacz for supplying dry sediment weights, to Ms. Aleksandra Kaniak for identifying the cladocerans, and to Dr. Sławomir Dobosz for providing photographs of the foraminiferal test linings. We are grateful to Dr. Anna Binczewska for making the results of her research available to us prior to publication. We would also like to thank Professor Petra Heinz and Professor Peter Frenzel for critical reading of an earlier version of the manuscript and for helpful comments. Our thanks are due to an anonymous reviewer whose comments allowed us to improve the original version of the submitted paper. We are grateful also to the Oceanological and Hydrobiological Studies’ editorial team for their contribution to the improvement of the submission.
The research described in this study was supported by the Polish National Science Centre grant No. 2011/01/B/ST10/06497 awarded to RK. TR acknowledges the support from the research subsidy provided by the Institute of Marine and Environmental Sciences, University of Szczecin, Poland.

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