Prolonged interglacial warmth during the Last Glacial in northern Europe

Abstract

Few fossil‐based environmental and climate records in northern Europe are dated to Marine Isotope Stage (MIS) 5a around 80 ka BP. We here present multiple environmental and climate proxies obtained from a lake sequence of MIS 5a age in the Sokli basin (northern Finland). Pollen/spores, plant macrofossils, NPPs (e.g. green algae), bryozoa, diatoms and chironomids allowed an exceptionally detailed reconstruction of aquatic and telmatic ecosystem successions related to the development of the Sokli Ice Lake and subsequent infilling of a relatively small and shallow lake confined to the Sokli basin. A regional vegetation development typical for the early half of an interglacial is recorded by the pollen, stomata and plant macrofossil data. Reconstructions of July temperatures based on pollen assemblages suffer from a large contribution of local pollen from the lake's littoral zone. Summer temperatures reaching present‐day values, inferred for the upper part of the lake sequence, however, agree with the establishment of pine‐dominated boreal forest indicated by the plant fossil data. Habitat preferences also influence the climate record based on chironomids. Nevertheless, the climate optima of the predominant intermediate‐ to warm‐water chironomid taxa suggest July temperatures exceeding present‐day values by up to several degrees, in line with climate inferences from a variety of aquatic and wetland plant indicator species. The disequilibrium between regional vegetation development and warm, insolation‐forced summers is also reported for Early Holocene records from northern Fennoscandia. The MIS 5a sequence is the last remaining fossil‐bearing deposit in the late Quaternary basin infill at Sokli to be studied using multi‐proxy evidence. A unique detailed climate record for MIS 5 is now available for formerly glaciated northern Europe. Our studies indicate that interglacial conditions persisted into MIS 5a, in agreement with data for large parts of the European mainland, shortening the Last Glacial by some 50 ka to MIS 4‐2.