The marine 234U/238U isotopic ratio in the past

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The seawater isotopic ratio of 234U/238U is often assumed to be constant and in stationary condition, since U behaves in seawater like a salt like element for which its sources and sinks are small compared to the dissolved amount of U. Dunk and co-workers (2000, Chemical Geology) have demonstrated that the marine U cycle bears numerous and partly very large uncertainties with the possibility of temporal changes of the seawater U isotopic composition. Climate driven changes of seawater 234U/238U might be driven by changes of freshwater fluxes and its respective U-isotopic composition, sea level variations due to the exposure of shelf areas in particular in mangrove forests, continental erosion due to change in the rate of physical and chemical weathering. Also sinks might have changes with respect to the redox sensitive nature of U. Previous work (and the here included data compilation) indicate that small changes of a few permil must have occurred at the end of the last glacial some 20000 years ago. However, due to the large uncertainties of the available data and the lack of a nearly continuous high resolution record it is most difficult to assess such changes in terms of climate driven variability of the above processes. Moreover, the 234U/238U ratio is used as quality control of U-series dating and thus to further improve the reconstruction of sea level or the marine radiocarbon calibration, a highly resolved high precision temporal record of sea water 234U/238U is urgently needed. Here we thus propose to use a set of unique sediment cores bearing fossil coral fragments that span in ages over 400000 years to produce such a unique record. High precision will be achieved from cutting edge Multi-collector ICPMAS technology. The documented isotopic variability will be explored with respect to the possible timing and amplitude of the above processes affecting the marine U budget.