讲座题目： CO2 threshold for precessional variability of subarctic sea-ice
讲座嘉宾：中国科学院广州地球化学研究所 同位素地球化学国家重点实验室 罗立 副研究员
讲座内容：Sea ice is a critical and sensitive component of the Earth’s climate system, and has undergone dramatic reductions in extent and thickness for much of the Arctic in the last few decades. However, studies covering orbital timescales are still lacking, largely due to the lack of a reliable sea ice proxy in the subarctic regions, or marine sedimentary archives with sufficient age control and temporal resolution. Here we reconstruct high-resolution subarctic Pacific sea ice and summer sea surface temperature records for the past 180,000 years using novel organic geochemical proxies in the central Okhotsk Sea. Our sea ice reconstruction shows significant precession (23-kyr) cycles, which are coupled to local autumn insolation during intervals of low-mid atmospheric CO2 concentrations (<~260 ppm). We also find that the Okhotsk Sea was ice-free during the mid-late Holocene and throughout the penultimate interglacial (Marine Isotope Stage 5e) when CO2 concentration exceeded this threshold, suggesting that both insolation and atmospheric CO2 levels are responsible for controlling sea ice variation in the Okhotsk Sea on orbital timescales. Our proxy reconstructions are reproduced well by a suite of transient Earth system model simulations of the last glacial cycle and we explore how changes in CO2 and insolation act in concert to drive sea ice variations on orbital timescales.