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Environmental changes during isolation processes from the Litorina Sea as reflected by diatoms and geochemical parameters -a case study

Stockholm University, Department of Physical Geography and Quaternary Geology, SE 106 91 Stockholm, Sweden; per.westman{at}geo.su.se

Anna Hedenström

Stockholm University, Department of Physical Geography and Quaternary Geology, SE 106 91 Stockholm, Sweden

The aim with the present case study was to detect environmental changes during the isolation of a lake from the Baltic Sea. Elemental and stable-isotope ratios of carbon and nitrogen are presented as well as results of a diatom analysis of a sedimentary sequence from Lake Skären in southeastern Sweden. The dated part of the analysed sequence covers the period c. 8200 to 7000 cal. years BP, i.e., the late Ancylus Lake to early Litorina Sea stages. It is concluded that subtle environmental changes due to ‘ uctuations in input of organic material from different sources cannot be detected by diatom analysis alone, but that chemostratigraphy is an important complement to diatom stratigraphy when interpreting the isolation process. We also conclude that, even though ₁₃C is highly variable in Baltic Sea sediment, it is most useful in isolation studies for recording the contribution of organic carbon derived from benthic macro-algae and/or terrestrial sources, as a proxy for shore displacement. It is especially valuable in combination with the C/N ratio, which seems to be the most unambiguous parameter. The ₁₅N data showed that cyanobacterial blooms in the coastal zone of the early Litorina Sea were probably less pronounced than in the offshore areas of the Baltic Sea. The abrupt increase of Chaetoceros spp. resting spores slightly prior to isolation is interpreted as caused by a eutrophication process during the isolation: it is probably also due to nitrogen deficiency due to denitrification in the coastal areas.

Key Words: Diatoms, 13C • 15N • C/N ratio • lake isolation • Chaetoceros • early Holocene • Litorina Sea • Sweden

The Holocene, Vol. 12, No. 5, 531-540 (2002)
DOI: 10.1191/0959683602hl560rp


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