Evidence from U-Th dating against Northern Hemisphere forcing of the penultimate deglaciation

摘要

Milankovitch proposed that summer insolation at mid-latitudes in the Northern Hemisphere directly causes the ice-age climate cycles. This would imply that times of ice-sheet collapse should correspond to peaks in Northern Hemisphere June insolation. But the penultimate deglaciation has proved controversial because June insolation peaks 127kyr ago whereas several records of past climate suggest that change may have occurred up to 15kyr earlier. There is a dear signature of the penultimate deglaciation in marine oxygen-isotope records. But dating this event, which is significantly before the ~(14)C age range, has not been possible. Here we date the penultimate deglaciation in a record from the Bahamas using a new U-Th isochron technique. After the necessary corrections for α-recoil mobility of ~(234)U and ~(230)Th and a small age correction for sediment mixing, the midpoint age for the penultimate deglaciation is determined to be 135 ± 2.5 kyr ago. This age is consistent with some coral-based sea-level estimates, but it is difficult to reconcile with June Northern Hemisphere insolation as the trigger for the ice-age cycles. Potential alternative driving mechanisms for the ice-age cycles that are consistent with such an early date for the penultimate deglaciation are either the variability of the tropical ocean-atmosphere system or changes in atmospheric CO_2 concentration controlled by a process in the Southern Hemisphere.