Abrupt Mid-Holocene Cooling — Insights from Sub-Annually Resolved Reconstructions of Alkenone Lake Temperature

Summary Improved knowledge of past abrupt climate transitions on regional scales is essential for understanding current abrupt climate change and its repercussions. In the Northern Great Plains, we lack highly sensitive temperature proxies and validated records of past environmental change. In this study, we develop and apply a new, quantitative, high-resolution algal paleothermometry approach based on analysis of lacustrine alkenones preserved in sediments using Mass Spectrometry Imaging (MSI). We demonstrate that the alkenone summer temperature proxy and MSI technology a) offer unprecedented insights during an abrupt climate event (5500 BP) and b) perform well in modern validation exercises. Sub-annual resolution of the mid-Holocene Cooling (MHC, ∼5500 ka BP) with MSI alkenone paleothermometry offered unprecedented insights that would have remained hidden with conventional biomarker analysis. Application of additional high-resolution proxies including X-Ray Fluorescence and conventional biomarker analysis also captured the general pattern of environmental changes during the MHC phase but lacked evidence of a marked increase in variability characteristic of sudden state change. This study presents the first fully calibrated, high-resolution, lacustrine alkenone record of past lake water temperature. These analyses allow the development of better analogs for current climate change, especially in context of tipping points and climate system state change.