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

Millankovitch proposed that summer insolation at mid-latitudes in the:North ern Hemisphere directly causes the ice-age climate cycles'. This would impl y that times of ice-sheet collapse should correspond to peaks in Northern H emisphere June insolation. But the penultimate deglaciation has proved cont roversial because June insolation peaks 127 kyr ago whereas several records of past climate suggest that change may have occurred up to 15 kyr earlier (2-8). There is a dear signature of the penultimate deglaciation in marine oxygen-isotope records. But dating this event, which is significantly befor e the C-14 age range, has not been possible. Here we date the penultimate d eglaciation in a record from the Bahamas using a new U-Th isochron techniqu e. After the necessary corrections for or-recoil mobility of U-234 and Th-2 30 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 difficu lt to reconcile with Tune Northern Hemisphere insolation as the trigger for the ice-age cycles. Potential alternative driving mechanisms for the ice-a ge cycles that are consistent with such an early date for the penultimate d eglaciation are either the variability of the tropical ocean-atmosphere sys tem or changes in atmospheric CO2 concentration controlled by a process in the Southern Hemisphere.