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

Abstract

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 127 kyr ago whereas several records of past climate suggest that change may have occurred up to 15 kyr earlier. 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 alpha-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.

Hemispheric roles of climate forcings during glacial‐interglacial transitions as deduced from the Vostok record and LLN‐2D model experiments

“…This is mainly due to the absence of a common chronology between the Vostok record and the orbital control on insolation, and also to the difficulty to choose the most relevant target insolation curves. Although the earlier temperature signal in the South, in phase with CO2, may suggest a dominant role of the southern insolation, which would be in agreement with the conclusion reached by Henderson and Slowey [2000] for the penultimate deglaciation (T2), the more traditional view of the northern insolation forcing the deglaciations cannot be ruled out. We can advance the hypothesis that low latitudes could drive a quasi-simultaneous change in both hemispheres, through for instance the orbital control on the tropical climate [Clement et al, 1999], and that the warming in the North was counterbalanced during several thousand years by some oceanic process in relation with a progressive melting of the northern ice.…”

Diagenetic rejuvenation of raised coral reefs and precision of dating. The contribution of the Red Sea reefs to the question of reliability of the Uranium-series datings of middle to late Pleistocene key reef-terraces of the world

“…A brief fluctuation in δ 18 O in the 5e record is also worthy of note. Another contribution (HENDERSON & SLOWEY, 2000) based on the same core from the southern slope of the Little Bahama Bank gives direct datings for termination II (Fig. 15).…”

“…Because modeling the South Polar ice cap and southern hemisphere insolation is beyond our competence we do not discuss the attempts at an explanation for this brevity, the timing of variations in insolation between the hemispheres (HENDERSON & SLOWEY, 2000), or the difference between earlier global atmospheric warming and the increase in the insolation of the high latitudes of the Northern hemisphere We insist only on their possible contribution to the discussion of the reliability of the dates derived from the SPECMAP time scale, in turn clearly relevant to the chronology of raised reefs in itself closely coordinated with the causes of changes in sea-level. (1) The last interglacial, most recently dated marine series (Australian reefs and JPC 152, Bahamas coring) (2) the calculated insolation curve of the northern-hemisphere (3) orbitally tunned δ 18 O curves, and (4) continental climate-linked curves.…”

Glacial-to-Holocene sedimentation on the western slope of Great Bahama Bank

“…Sediments deposited on the slopes of the Bahama Banks have also proven to be valuable archives of late Quaternary oceanographic conditions (Droxler et al, 1983;Slowey and Curry, 1995;Marchitto et al, 1998;Lynch-Stieglitz et al, 1999) and sea level (Slowey et al, 1996;Henderson and Slowey, 2000). Given their potentially high temporal resolution, it is important to investigate the paleoceanographic records preserved by the rapidly deposited Great Bahama Bank slope sediments.…”