A multiphasic Younger Dryas cold period recorded in sediments of Lake Steisslingen, SW-Germany: A biomarker perspective

Abstract

The Younger Dryas (12,680–11,590 yr BP) was the last millennial scale cold event of the Quaternary, when a mean annual temperature decline of 3–5 °C impacted severely on the environment. Whereas pollen distributions document changes in land vegetation, details of limnic ecosystem responses to the cooling event and to its recovery are still not well known. Here, we studied the laminated sediments of Lake Steisslingen (SW-Germany) deposited between 13,500 and 11,100 yr BP in order to investigate the evolution of an aquatic ecosystem. Bulk sediment and organic matter composition mark onset and termination of the Younger Dryas, but remain monotonous through this interval, requiring application of organic and isotope geochemical techniques to follow past ecosystem dynamics. The Younger Dryas is characterized by both an abrupt appearance and demise of long-chain alkenones indicative for haptophyte algae. Based on algal and bacterial biomarker distribution, the Younger Dryas can be divided into three stages, each characterized by the dominance of a microorganism group. The first phase was dominated by nC17 alkanes and hopanoids indicative of cyanophytes and heterotrophic bacteria. The second phase shows a proliferation of cold-adapted haptophyte algae, while in the latest phase highly-branched isoprenoids (HBI) reveal an aquatic community shift towards diatoms. Heterocyte glycolipids document a moderate change in N2-fixing cyanobacterial community structure over the Allerød/Younger Dryas transition but show a marked collapse of the cyanobacteria association at the Younger Dryas/Preboreal climate recovery. Terrigenic wax lipids indicate significant changes in vegetation between the Allerød, Younger Dryas and Preboreal chronozones but remain homogeneous within the Younger Dryas.