Holocene environmental changes reflected by pollen, diatoms, and geochemistry of annually laminated sediments of Lake Suminko in the Kashubian Lake District (N Poland)

Abstract

Pollen, non-pollen palynomorphs (NPPs), diatoms, and geochemistry of lake sediments with partially preserved annual lamination were used to reconstruct post-glacial environmental changes of northern Poland in the Baltic region. The main stages in the lake's evolution indicate the following: (1) eutrophic conditions in the final stage of the Lateglacial, (2) very low trophy and strongly changeable water levels in the early Holocene, (3) slowly rising trophy through the mid- and late Holocene enabling persistence of the oligotrophic state of the lake up to c. AD 1450, (4) strong cultural eutrophication of the lake in the recent period (c. 150years), and (5) maximum Ca content in the early Holocene followed by a gradual decline to minimum values in recent time. Distinct, concurrent shifts in limnological proxies and tree pollen accumulation rates (PARs) enabled the identification of several potential Holocene cool climatic events of different magnitudes and durations. Strong reduction in Tetraëdron minimum and declines in tree PARs illustrate limitation of physiological processes in plants, which could result from shorter growing seasons and lower summer temperatures and insolation. Higher precipitation of Fe and Mn indicates more intensive weathering in the catchment, while peaks in Fe/Mn ratio and concurrent drops in Ca content suggest at least seasonal anoxia, which could result from a longer duration of ice cover and shorter periods of mixing. The data seem to display striking conformity of the main cooling events with the “Bond cycles” (Bond et al., 1997, 2001), but also indicate a more variable pattern that may reflect traces of a more complex cyclicity of climatic shifts.