Was there a 4.2 ka BP event in Sweden? Evidence from peat, tree-rings and lake sediments

Abstract

The onset of the Late Holocene subepoch, marked by the 4.2 ka BP event, is characterized by drought along the equatorial regions and cooling trends near the poles, as evidenced by lower sea surface temperatures in the northern North Atlantic. However, the occurrence and significance of this event remain controversial, with inconsistent verification across different regions. A comprehensive review of over 30 studies examining climate proxies such as peat deposits, lake sediments, and tree rings from Sweden, spanning 5000 to 3500 cal a BP, reveals some recurring patterns. These studies extend from northern to southern Sweden, and many of these suggest a shift towards wetter, colder, or both wetter and colder climate around 4500–4000 cal a BP. Despite these indications, variations in the reliability and replication of climate proxies introduce substantial uncertainty. Additionally, several parts of Sweden lack sufficient proxy data. As a result, there is no clear evidence of a widespread, uniform climate change across Sweden during 4500–4000 cal a BP. The temporal variability of these changes, ranging from decadal to millennial scales, further complicates the identification of a single climatic event. The climate patterns in Sweden during this period align with the broader northern North Atlantic region, where both supportive and contradictory evidence exists for climate changes around 4200 cal a BP. The absence of consistent findings across multiple proxies and regions suggests that the 4.2 ka BP event was not a globally synchronous event but may represent a complex interplay of regional climatic shifts.