1. Lamont-Doherty Earth Observatory of Columbia University, Route 9W, Palisades, New York 10964, USA
2. Department of Oceanography, Texas A&M University, College Station, Texas 77843-3146, USA
3. Present address: Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3PR, UK.

Correspondence to: Gideon M. Henderson^1,2 Correspondence and requests for materials should be addressed to G.M.H. (e-mail: Email: gideonh@earth.ox.ac.uk).

Abstract

Milankovitch proposed that summer insolation at mid-latitudes in the Northern Hemisphere directly causes the ice-age climate cycles¹. 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 127 kyr ago whereas several records of past climate suggest that change may have occurred up to 15 kyr earlier^{2, 3, 4, 5, 6, 7, 8}. There is a clear signature of the penultimate deglaciation in marine oxygen-isotope records. But dating this event, which is significantly before the ¹⁴C 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 -recoil mobility of ²³⁴U and ²³⁰Th 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 CO₂ concentration controlled by a process in the Southern Hemisphere.