Abstract
Radiometric dating of glacial terminations over the past 640,000 years suggests pacing by Earth's climatic precession, with each glacial-interglacial period spanning four or five cycles of ~20,000 years. However, the lack of firm age estimates for older Pleistocene terminations confounds attempts to test the persistence of precession forcing. We combine an Italian speleothem record anchored by a uranium-lead chronology with North Atlantic ocean data to show that the first two deglaciations of the so-called 100,000-year world are separated by two obliquity cycles, with each termination starting at the same high phase of obliquity, but at opposing phases of precession. An assessment of 11 radiometrically dated terminations spanning the past million years suggests that obliquity exerted a persistent influence on not only their initiation but also their duration.
Highlights
A major challenge of testing the orbital (Milankovitch) theory of the ice ages is the uncertainty associated with the chronology of marine records
We find that duration is significantly correlated with caloric summer half-year energy (~equal contributions from obliquity and precession), integrated summer energy and Northern Hemisphere 150 (NH) summer insolation intensity at the commencement of a termination; the correlation with the precession index is much weaker but is remains significant for tilt
Our results show that the spacing of termination midpoints is consistent with obliquity forcing (Fig. 3b; Fig. 4a, c), and that the midpoints are most consistently aligned with peaks in an insolation forcing metric which integrates approximately equal amounts of obliquity and precession (Fig. 4a-c) (13, 18)
Summary
A major challenge of testing the orbital (Milankovitch) theory of the ice ages is the uncertainty associated with the chronology of marine records. We independently determined the age of terminations across the MPT by tying the radiometric chronology from a speleothem d18O time series to North Atlantic ocean-sediment records. The multi-proxy ocean data show that the commencement of large, near-monotonic benthic δ18O decreases for both terminations is approximately synchronous with rapid SST cooling and increased % C37:4 (Fig. 2) caused by meltwater from ice-sheet collapse reaching the Iberian margin. These terminal stadial events provide unequivocal evidence for the onset of the two terminations, as is the case with younger terminations recorded at the Iberian margin (23, 29).
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