Searching for the 4.2 ka climate event at Lake Spore, Poland

Many regions around the world experienced abrupt climate change with far-reaching societal and environmental effects at ca. 4.2 cal. ka BP. As one of the most distinct climatic events of the last 11.7 ka, it has been used to establish the formal boundary between the mid- and late-Holocene. Although the 4.2 ka event was not pronounced in East-Central Europe, its exact impact on the area has yet to be evaluated. The aim of this paper is twofold: (1) to reconstruct the functioning of the selected lake and terrestrial ecosystems from northern Poland around the time of the 4.2 ka event, and (2) to disentangle the drivers of the recorded environmental changes (local factors vs. climate impact). We studied the sedimentary record of Lake Spore, focusing on the period between 3.8 and 4.5 cal. ka BP using numerous biotic and abiotic proxies, such as chironomids, cladocerans, pollen and spores, δ13Corg, δ15Norg, δ18Ocarb, δ13Ccarb, TOC/N, Fe, S, LOI550, and LOI950. The chironomid-derived reconstruction of mean July air temperature did not change during the 4.2 ka event. Pollen accumulation rate (PAR) data showed lower pollen production of some tree taxa during the period 4.0–4.25 cal. ka BP that is consistent with low PARs recorded for the corresponding period in the sediments of nearby (~75 km) Lake Suminko. This change might have occurred due to either regional-scale forest disturbance or cooler winters resulting from an exceptionally strong Siberian High during the 4.2 ka event. Distinct peaks of Fe and δ13Ccarb at 4.2 cal. ka BP likely record a pulse of podzolization. Although soil transformation processes were common in East-Central Europe after ca. 5 cal. ka BP due to general climate cooling, they could have been enhanced at Lake Spore during the 4.2 ka event, if winters were more severe. The δ13Corg, δ15Norg, TOC/N and δ18Ocarb records point to a water level rise after 4.2 cal. ka BP that most likely resulted from soil transformation in the catchment, not a change in effective precipitation. The composition of the terrestrial vegetation and the lake fauna and flora remained largely unaffected by the 4.2 ka event.