Quantitative salinity reconstructions of the Baltic Sea during the mid‐Holocene

Abstract

We reconstructed the past coastal environment of the Baltic Sea from c. 7300 to 3500 cal. a BP, with a focus on sea surface salinity (SSS). To quantitatively determine the SSS, two methods were employed: measurements of 87Sr/86Sr ratios in mollusc shells (SSSSr) and process length variations of dinoflagellate cysts Operculodinium centrocarpum (SSSpl). The SSSSr was ~6–7 between 6800 and 6400 cal. a BP, similar to modern conditions. Between 6000 and 3900 cal. a BP, SSSSr was consistently higher, ranging between ~9 and 13. Microfossils sensitive to salinity variations, such as Radiosperma corbiforum and Spiniferites spp., support the SSSSr estimate. In comparison with the SSSSr, the SSSpl values were consistently higher, with an average of ~14. We suggest SSSpl tend to overestimate salinity and are not as reliable as SSSSr. A multi‐proxy approach, including analysis of microfossil, organic carbon content, C/N ratios and grain‐size, allowed for a division of the study period into three zones (Zones I–III). Zone I (7300–6400 cal. a BP) is characterized by relatively O. centrocarpum and R. corbiforum concentrations, silt contents and C/N ratios, corresponding to the regional Littorina transgression maximum. Zone II (6400–3900 cal. a BP) is characterized by increased Spiniferites spp. and decreased R. corbiforum abundances, and by lowered C/N ratios and silt contents relative to Zone I. Zone III (3900–3500 cal. a BP) represents the start of the Post‐Littorina Sea phase, indicating a shift to the modern Baltic Sea phase with decreased salinity.

Reconstructing Holocene temperature and salinity variations in the western Baltic Sea region: a multi-proxy comparison from the Little Belt (IODP Expedition 347, Site M0059)

“…Thorsen et al, 1995;Thorsen and Dale, 1997;see below), evidenced here by high SSTs from TEX L 86 and the LDI in this interval. The Gymnodinium peak at ∼ 6800 cal yr BP has, to our knowledge, not yet been described from the Baltic Sea and does not seem to have occurred in the Baltic Proper (Brenner, 2005;Yu and Berglund, 2007;Ning et al, 2017). Thorsen et al (1995) and Har-land and Nordberg (2011) describe later Gymnodinium mass occurrences from the Skagerrak and the Kattegat, but sediments of equivalent age containing our Gymnodinium peak at ∼ 6800 cal yr BP were not recovered in the framework of those studies.…”

“…increasing CRS abundance, as most Chaetoceros require brackish to marine conditions (Snoeijs, 1993;Snoeijs and Vilbaste, 1994;Snoeijs and Potapova, 1995;Snoeijs and Kasperoviciene, 1996;Snoeijs and Balashova, 1998), and a significant rise in the DI values. Maximum abundances of dinocysts of phototrophic taxa occurred in EZ2 and are coeval with abundance maxima at the southern and southeastern coast of Sweden found by Ning et al (2017) or Yu and Berglund (2007) and in the Gotland and the Fårö basins (Brenner, 2005;Willumsen et al, 2013).…”

“…To improve our understanding of the impact and magnitude of future environmental changes in the Baltic Sea region, it is essential to generate high-resolution paleoreconstructions in order to investigate how salinity and temperature varied in the past (e.g. Zillén et al, 2008;Andrén et al, 2015a;Ning et al, 2017). A multi-proxy approach, comprising proxies representative for bottom water, surface water, and air (terrestrial) conditions, is used here to reconstruct a wide array of environmental change (in particular temperature and salinity) from the same or closely neighbouring samples and thus how conditions simultaneously changed within the same age constraints.…”

“…Diatoms and marine palynomorphs such as dinoflagellates, however, are common and often very good indicators of changes in salinity (Andrén et al, 2000;Snoeijs and Weckström, 2010;Ning et al, 2017).…”

The Baltic Sea: geology and palaeoenvironmental evolution – introduction

“…This region represents one important gateway between the Baltic Sea and the North Sea. A palaeoceanographical update on the entire Littorina Sea stage palaeoenvironmental Baltic Sea development is provided by Ning et al (2017) andH€ ausler et al (2017). Here, lagoonal marls were deposited along the developing Curonian Spit that were subsequently overridden by massive sand dunes and partly extruded by 'dune tectonic' processes.…”