A dinoflagellate cyst record of Holocene climate and hydrological changes along the southeastern Swedish Baltic coast Shi-Yong Yu a,b, ⁎ , Björn E. Berglund b a Large Lakes Observatory, University of Minnesota Duluth, 2205 East 5th Street, Duluth, MN 55812, USA b GeoBiosphere Science Center, Department of Geology/Quaternary Sciences, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden Received 15 May 2006 Available online 7 February 2007 Abstract A high-resolution, well-dated dinoflagellate cyst record from a lagoon of the southeastern Swedish Baltic Sea reveals climate and hydrological changes during the Holocene. Marine dinoflagellate cysts occurred initially at about 8600 cal yr BP, indicating the onset of the Littorina transgression in the southeastern Swedish lowland associated with global sea level rise, and thus the opening of the Danish straits. Both the species diversity and the total accumulation rates of dinoflagellate cysts continued to increase by 7000 cal yr BP and then decreased progressively. This pattern reveals the first-order change in local sea level as a function of ice-volume-equivalent sea level rise versus isostatic land uplift. Superimposed upon this local sea level trend, well-defined fluctuations of the total accumulation rates of dinoflagellate cysts occurred on quasi- 1000- and 500-yr frequency bands particularly between 7500 and 4000 cal yr BP, when the connection between the Baltic basin and the North Atlantic was broader. A close correlation of the total accumulation rates of dinoflagellate cysts with GISP2 ice core sea-salt ions suggests that fluctuations of Baltic surface conditions during the middle Holocene might have been regulated by quasi-periodic variations of the prevailing southwesterly winds, most likely through a system similar to the dipole oscillation of the modern North Atlantic atmosphere. © 2007 University of Washington. All rights reserved. Keywords: Holocene; Baltic Sea; Dinoflagellate cysts; Littorina transgression; Climate changes; North Atlantic storminess Introduction Glacio-chemical measurements on Greenland ice cores reveal that the North Atlantic area has experienced millennial- scale changes in storminess (O'Brien et al., 1995), accompanied by periods of repeated cooling during the late glacial and Holocene as marked by episodic southward advection of polar drift ice (Bond et al., 1997). Persistent storm activities not only shaped the northwestern European coasts in the form of widespread beach-ridge progradations and dune buildings (Clemmensen et al., 2001; Wilson, 2002; Orford et al., 2003), but also exerted a great impact on coastal hydrology and ecology (Nordberg et al., 2000; Stenseth et al., 2002). Both observations and modeling (Orvik et al., 2001; Schrum, 2001; Lehmann et al., 2002) indicate that climate and hydrological conditions in the Baltic Sea are closely related to variations of the prevailing southwesterly winds (Fig. 1A), which are known to be a manifestation of the North Atlantic Oscillation (NAO) (Jacobi and Beckmann, 1999). Therefore, reconstructions of past hydrological conditions in the Baltic Sea are particularly important for understanding paleo-atmospheric dynamics in the North Atlantic realm. The brackish-water conditions in the landlocked Baltic basin are primarily maintained by the wind-driven water exchange with the North Sea as well as river discharge (Hanninen et al., 2000; Winsor et al., 2001; Gustafsson and Westman, 2002). Spatial variations of these two processes lead to a distinct salinity gradient between the Kattegat Sea and the Gulf of Bothnia (Fonselius and Valderrama, 2003). In the deep basins, a permanent halocline is present, separating the surface water mass from the dense, oxygen-depleted, and stagnant deep-water column. The deep water can be occasionally renewed by inflows of saline water in late autumn or winter, when a prolonged period of strong easterly winds is followed by stormy southwesterly winds as a low pressure develops over the North Sea (Lass and Quaternary Research 67 (2007) 215 – 224 www.elsevier.com/locate/yqres ⁎ Corresponding author. Large Lakes Observatory, University of Minnesota Duluth, 2205 East 5th Street, Duluth, MN 55812, USA. Fax: +1 218 726 6979. E-mail address: syu@d.umn.edu (S.-Y. Yu). 0033-5894/$ - see front matter © 2007 University of Washington. All rights reserved. doi:10.1016/j.yqres.2006.12.004