Seasonality of Abrupt Climate Change over Greenland: Direct Tests for the Younger Dryas and 8.2 ka event using Paleolimnology

The Younger Dryas (YD) is the best documented example of abrupt climate change in Earth’s history, yet fundamental traits of the YD remain enigmatic. Greenland sat at the epicenter of YD climate change, with intense cooling registered in ice cores, yet mysteriously the Greenland Ice Sheet retreated during the YD. An influential hypothesis suggests that North Atlantic abrupt climate changes were characterized by extreme seasonality, with mild summers that explain counterintuitive YD glacier retreat. Ultimately, resolving seasonality is crucial to understanding the impacts of past and future climate change on all aspects of the Arctic system – including the ice sheet, because summer temperatures drive surface melt.The proposed study will directly assess seasonality of the YD on Greenland for the first time by developing quantitative season-specific temperature reconstructions. This study will also assess seasonality of the major cold snap over Greenland 8,200 years ago. Intriguingly, despite being shorter and smaller in annual magnitude than the YD, that event drove glacier advances around Greenland. This project is designed to be low-risk, utilizing classic sites that preserve rare, readily dateable YD lake sediment records. These lakes were not revisited for twenty years, until the PIs recovered YD sediments from one of them in summer 2019. Multiple well-calibrated, independent proxies – including insect and pollen assemblages, terrestrial leaf wax δ2H, and aquatic insect δ18O – will test and complement one another and provide new comparisons to annually integrated records from ice cores. Thiswork will build upon the research team’s experience calibrating and applying these proxies in Greenland, and a new calibration study will verify the seasonality of leaf wax δ2H in a range of Greenland climates.