Warm summers and rich biotic communities during N-Hemisphere deglaciation

Detailed studies on fossil remains of plants or animals in glacial lake sediments are rare. As a result, environmental conditions right at the moment of deglaciation of the large N-Hemisphere ice-sheets remain largely unknown. Here we study three deglacial phases of the Fennoscandian Ice Sheet as a unique, repeated element in a long sediment record preserved at Sokli in northern Finland. We summarize extensive multi-proxy data (diatoms, phytoliths, chironomids, pollen, spores, non-pollen palynomorphs, macrofossils, lithology, loss-on-ignition, C/N) obtained on glacial lake sediments dated to the early Holocene (ca. 10 kyr BP), early MIS 3 (ca. 50 kyr BP) and early MIS 5a (ca. 80 kyr BP). In contrast to the common view of an unproductive ice-marginal environment, our study reconstructs rich ecosystems both in the glacial lake and along the shores with forest on recently deglaciated land. Higher than present-day summer temperatures are reconstructed based on a large variety of aquatic taxa. Rich biota developed due to the insolation-induced postglacial warming and high nutrient levels, the latter resulting from erosion of fresh bedrock and sediment, leaching of surface soils, decay of plant material under shallow water conditions, and sudden decreases in lake volume. Aquatic communities responded quickly to deglaciation and warm summers and reflect boreal conditions, in contrast to the terrestrial ecosystem which responded with some delay probably due to time required for slow soil formation processes. Birch forest is reconstructed upon deglaciation of the large LGM ice-sheet and shrub tundra following the probably faster melting smaller MIS 4 and MIS 5b ice-sheets. Our study shows that glacial lake sediments can provide valuable palaeo-environmental data, that aquatic biota and terrestrial vegetation rapidly accommodated to new environmental conditions during deglaciation, and that glacial lake ecosystems, and the carbon stored in their sediments, should be included in earth system modeling.