Variability in the uranium isotopic composition of the oceans over glacial-interglacial timescales

Uranium-series mass spectrometric analyses of corals from the uplifted last glacial terraces at Huon Peninsula, Papua New Guinea, that grew from 50,000 years ago to 30,000 years ago show systematically low values of U-234/U-238, at the time of coral growth, compared with modern corals. When combined with coral data from other studies a systematic trend emerges indicating shifts in the U-234/U-238 ratio at times of major glacial-interglacial transitions that involve large variations in sea-levels. From last glacial to Holocene, the rate of change in delta U-234 is approximately 1 parts per thousand per thousand years. The variations in the U budget of the oceans appear to be due to accumulation of excess U-234 in near shore areas in anoxic and suboxic sediments, in salt marshes and mangroves, in estuaries, and in continental margins during periods of warm climate and high sea-levels. These near-shore areas are exposed during periods of low sea level resulting in rapid oxidation of U into highly soluble phases. The subsequent release of U-234-enriched uranium into the oceans occurs over a sustained period, in step with rising sea-levels. (c) 2006 Elsevier Inc. All rights reserved.