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

Tezer M. Esat 1 , and Yusuke Yokoyama 2

1 Research School of Earth Sciences and Department of Nuclear Physics, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia
2 Department of Earth and Planetary Sciences, Graduate School of Science , University of Tokyo , Tokyo 113-0033, Japan

 

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 234 U/ 238 U, 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 234 U/ 238 U 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 d 234 U is approximately 1‰ per thousand years. The variations in the U budget of the oceans appear to be due to accumulation of excess 234 U 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 234 U-enriched uranium into the oceans occurs over a sustained period, in step with rising sea-levels.

Fig. A compilation of d 234 U values in U-series dated corals from the Last Interglacial, the last glacial, and the Holocene. For each data set representative uncertainties are as shown. The sea-level curve is intended only as a guide for the eye in contrasting the trend in d 234 U with changes in sea-level. The shaded band represents the uncertainty in sea-level estimations. The Last Interglacial sea-level was 3–5 m above the current sea-level and we have placed it at d 234 U ˜ 153‰. The general resemblance of the d 234 U variations to the sea-level curve for this period is remarkable. In living coral d 234 U = 149 ± 2‰. There is a systematic rise in d 234 U from a low of 132‰, during the last glacial, up to about 153‰ at the start of the Holocene, corresponding to an extra ˜20 Mmol of 234 U. There is considerable variability during the 30–60 ka period over a range of 10‰ in d 234 U. The prominent sea-level high-stands around 80–110 ka display a large range of low to high values of d 234 U. The Last Interglacial is represented by a mass of points that appear randomly distributed, and may reflect increased variability in measurements due to older ages and digenesis in comparison to samples from the Holocene.