Significance of uncertain phasing between the onsets of stadial–interstadial transitions in different Greenland ice core proxies

Abstract

Abstract. Different paleoclimate proxy records evidence repeated abrupt climate\ntransitions during previous glacial intervals. These transitions are thought\nto comprise abrupt warming and increase in local precipitation over Greenland,\nsudden reorganization of the Northern Hemisphere atmospheric circulation, and\nretreat of sea ice in the North Atlantic. The physical mechanism underlying\nthese so-called Dansgaard–Oeschger (DO) events remains debated. A recent\nanalysis of Greenland ice core proxy records found that transitions in\nNa+ concentrations and δ18O values are delayed by\nabout 1 decade with respect to corresponding transitions in Ca2+\nconcentrations and in the annual layer thickness during DO events. These\ndelays are interpreted as a temporal lag of sea-ice retreat and Greenland\nwarming with respect to a synoptic- and hemispheric-scale atmospheric\nreorganization at the onset of DO events and may thereby help constrain possible triggering mechanisms for the DO events. However, the explanatory\npower of these results is limited by the uncertainty of the transition onset\ndetection in noisy proxy records. Here, we extend previous work by testing the\nsignificance of the reported lags with respect to the null hypothesis that the\nproposed transition order is in fact not systematically favored. If the\ndetection uncertainties are averaged out, the temporal delays in the\nδ18O and Na+ transitions with respect to their\ncounterparts in Ca2+ and the annual layer thickness are indeed\npairwise statistically significant. In contrast, under rigorous propagation of\nuncertainty, three statistical tests cannot provide evidence against the null\nhypothesis. We thus confirm the previously reported tendency of delayed\ntransitions in the δ18O and Na+ concentration\nrecords. Yet, given the uncertainties in the determination of the transition\nonsets, it cannot be decided whether these tendencies are truly the imprint of\na prescribed transition order or whether they are due to chance. The analyzed\nset of DO transitions can therefore not serve as evidence for systematic\nlead–lag relationships between the transitions in the different proxies, which\nin turn limits the power of the observed tendencies to constrain possible\nphysical causes of the DO events.\n