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Zusammenfassung:
Different paleoclimate proxy records evidence repeated
abrupt climate transitions during previous glacial intervals.
These transitions are thought to comprise abrupt
warming and increase in local precipitation over Greenland,
sudden reorganization of the Northern Hemisphere atmospheric
circulation, and retreat of sea ice in the North Atlantic.
The physical mechanism underlying these so-called
Dansgaard–Oeschger (DO) events remains debated. A recent
analysis of Greenland ice core proxy records found that transitions
in NaC concentrations and 18O values are delayed
by about 1 decade with respect to corresponding transitions
in Ca2C concentrations and in the annual layer thickness during
DO events. These delays are interpreted as a temporal lag
of sea-ice retreat and Greenland warming with respect to a
synoptic- and hemispheric-scale atmospheric reorganization
at the onset of DO events and may thereby help constrain
possible triggering mechanisms for the DO events. However,
the explanatory power of these results is limited by the uncertainty
of the transition onset detection in noisy proxy records.
Here, we extend previous work by testing the significance
of the reported lags with respect to the null hypothesis that
the proposed transition order is in fact not systematically
favored. If the detection uncertainties are averaged out, the
temporal delays in the 18O and NaC transitions with respect
to their counterparts in Ca2C and the annual layer thickness
are indeed pairwise statistically significant. In contrast, under
rigorous propagation of uncertainty, three statistical tests
cannot provide evidence against the null hypothesis.We thus
confirm the previously reported tendency of delayed transitions
in the 18O and NaC concentration records. Yet, given
the uncertainties in the determination of the transition onsets,
it cannot be decided whether these tendencies are truly
the imprint of a prescribed transition order or whether they
are due to chance. The analyzed set of DO transitions can
therefore not serve as evidence for systematic lead–lag relationships
between the transitions in the different proxies,
which in turn limits the power of the observed tendencies to
constrain possible physical causes of the DO events.
