Marine Micropaleontology 37 (1999) 313–325 www.elsevier.com/locate/marmicro Ostracod assemblages in late Quaternary varved glaciomarine clay of the Baltic Sea Yoldia stage in eastern middle Sweden Kristian Schoning L , Stefan Wastega ˚rd Department of Quaternary Research, Stockholm University, S-106 91 Stockholm, Sweden Received 31 July 1997; accepted 9 November 1998 Abstract Four cored sites in eastern middle Sweden have been analysed for their content of calcareous fossils. The investigated sediments consist of late Quaternary varved clay deposited during the Baltic Sea Yoldia stage. Four regional palaeoecolog- ical units, A–D, have been established. Unit A reflects ice-proximal conditions, unit B optimal conditions for the benthic fauna and unit C the termination of a saline phase. In unit C oxygen deficiency might have occurred. In the fourth unit, D, no fossils were found. The fauna recorded is poor in taxa and indicates a weak marine influence. The salinity was higher in the western part of the investigated area due to the proximity to the inlet of marine water. The meltwater influx was high and the salinity probably fluctuated considerably. The ostracod Cytheromorpha macchesneyi (Brady and Crosskey) is reported for the first time in Quaternary deposits from northwestern Europe and indicates that a connection could have existed between the Baltic and the White Sea in Late Glacial times.  1999 Elsevier Science B.V. All rights reserved. Keywords: Baltic Sea; Yoldia Sea; ostracods; palaeosalinity; varved clay; Quaternary 1. Introduction Since Late Glacial times the Baltic Sea has under- gone several stages with alternating fresh- and brack- ish water conditions: the Baltic Ice Lake (fresh); Yoldia Sea (partly brackish); Ancylus Lake (fresh); and the Litorina Sea (brackish). In this investigation the marine calcareous fauna, i.e. foraminifera, mol- luscs and ostracods, has been examined in glacioma- rine varved clay deposited during the Yoldia Sea stage in central and eastern middle Sweden. The Yoldia Sea was formed when the Baltic Ice Lake was finally drained westwards and lowered ca. 26 m L Corresponding author. Fax: C46-8-6747895; E-mail: kristian.schoning@geo.su.se at 10,300 14 C yr B.P. (e.g. Stro ¨mberg, 1977; Svens- son, 1989; Wohlfarth et al., 1993). The stage ended with the transition to the freshwater Ancylus Lake stage at 9600 14 C yr B.P., due to the uplift of the strait relative to the sea (Bjo ¨rck, 1995). Although the Baltic Basin was in contact with the sea, through the Va ¨nern Basin, it took about 300 14 C yr from the drainage until marine water could enter, at ap- proximately 10,000 yr B.P. (Svensson, 1989). The cause of the saline ingression is not clear but it was probably an effect of the deglaciation of the Na ¨rke Strait (Fig. 1) in central Sweden (Wastega ˚rd et al., 1998). The strait was ca. 40 km wide and ca. 55 m deep in its deepest parts (Frede ´n, 1988). A tem- porary cooling has been recognised at the time of the saline ingression (e.g. Bjo ¨rck et al., 1996, 1997), 0377-8398/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII:S0377-8398(99)00027-4