Were Pleistocene proglacial lakes biological deserts? Insights from varved clays in Lithuania

Varved clays from a Late Pleistocene proglacial lake in the Pašaminė sections (Lithuania) contain diverse organic matter of different origin, i.e. aquatic and allogenous macerals of the groups of liptinite, vitrinite and inertinite (their average contributions are 55%, 13.5% and 31.5%, respectively). The contents of TOC and TN vary from 0.51% to 1.01% and from 0.06% to 0.13%, respectively. Particulate organic matter could constitute the beginning of food chain for invertebrates represented by the trace fossils Cochlichnus anguineus, Glaciichnium liebegastensis, Gordia carickensis, Gordia isp., Helminthoidichnites isp., and Warvichnium ulbrichi. The accumulation of organic matter was higher during summers, as suggested by co-occurrence of TOC and quartz supplied to the lake mostly during ice melting. River inflow during humid seasons fed the lake and facilitated proliferation of algae, whereas, aeolian transportation of mineral (e.g., fine-grained quartz) and organic detritus (semifusinite) can be related to dry periods and the wildfires in vegetated areas. In spite of common thesis that Quaternary proglacial lakes were biological deserts, the Late Pleistocene proglacial lake is characterized by a food chain which starts at least from algae living in the lake and particulate organic matter transported from glaciated and non-glaciated, vegetated areas.