A statistical approach to the phasing of atmospheric reorganization and sea ice retreat at the onset of Dansgaard-Oeschger events under rigorous treatment of uncertainties

Abstract

Abstract. For previous glacial intervals, concomitant shifts in different proxy records from Greenland ice cores indicate repeated abrupt climate transitions comprising – among others – abrupt warming, a sudden reorganization of the atmospheric circulation, and a retreat of perannial sea ice. The physical mechanism underlying these so-called Dansgaard-Oeschger (DO) events remains debated. Former studies have made an effort to deduce the progression of temperature, circulation, and sea-ice changes at the onset of DO events from paleoclimate proxy records to constrain potential triggering mechanisms. In this context, recent research reports on systematically delayed transitions in Na+ concentrations and δ18O values compared to Ca2+ concentrations and the annual layer thickness by about one decade. This is interpreted as a temporal lag of sea ice retreat and Greenland warming with respect to atmospheric reorganization at the onset of DO-events. Here, we present a comprehensive statistical analysis of the relative phasing of DO transitions in Ca2+ and Na+ concentration records from the NGRIP ice core for the period 60–10 kyr BP. Regarding the time lags identified in this period as a sample generated from an unknown population, we derive probability density functions for the sample and population mean and test the null-hypothesis of a simultaneous transition. Special attention was paid to the uncertainties inherent to the transition onset detection in noisy data. Their rigorous propagation changes the test results from significant to non-significant and therefore a purely stochastic origin of the observed tendency for Ca2+ to lead the transition cannot be ruled out. In fact, we show that the data is very likely to comprise both: DO events that were led by a Ca2+ transition, as well as events led by a Na+ transition. Together, these findings clearly contradict a systematic lead or lag between the DO transitions in the two proxies, and the apparent Ca2+ lead should therefore not be interpreted as indication of a causal relationship. Under the assumption that all DO events followed the same physical mechanism and that the proxy interpretation holds true, the we conclude that at DO transition onsets, neither was the atmospheric reorganization caused by sea ice retreat, nor was the sea ice retreat triggered by atmospheric reorganization.