A reevaluation of the oceanic uranium budget for the Holocene

We present a new assessment of the pre-anthropogenic U budget for the Holocene ocean. We find that the gross input of U to the ocean lies in the range 53 +/- 17 Mmol/year, where the dominant source is river runoff (42.0 +/- 14.5 Mmol/year) and the direct discharge of groundwater could represent a significant additional input (9.3 +/- 8.7 Mmol/year). The soluble U flux associated with the aeolian input of crustal dust is minor (1.8 +/- 1.1 Mmol/year), falling well within the errors associated with the riverine flux. Removal of U to the organic rich sediments of salt marshes and mangrove swamps during river-sea mixing may significantly modify the riverine flux, such that the net U input is reduced to 42 +/- 18 Mmol/year. Evaluation of the U isotope budget demonstrates that the limits we have established on the U input flux are reasonable and suggests that direct groundwater discharge may play a significant role in maintaining the oceanic excess of U-234. The total sink of U from the ocean lies in the range 48 +/- 14 Mmol/year. We find that three major processes control the magnitude of this flux: (1) removal to oxygen-depleted sediments (26.9 +/- 12.2 Mmol/year); (2) incorporation into biogenic carbonate (13.3 +/- 5.6 Mmol/year); and (3) crustal sequestration during hydrothermal alteration and seafloor weathering (5.7 +/- 3.3 Mmol/year). The removal of U to opaline silica (0.6 +/- 0.3 Mmol/year) and hydrogenous phases (1.4 +/- 0.8 Mmol/year) is minimal, falling well within the errors associated with the other sinks. That the input and output fluxes balance within the calculated errors implies that U may be in steady state in the Holocene ocean. In this case, the input and output fluxes lie in the range 34-60 Mmol/year, giving an oceanic U residence time of 3.2-5.6 x 10(5) years. However, given the large uncertainties, a significant imbalance between the Holocene input and output fluxes cannot be ruled out. The constancy of the ancient seawater U concentration implies that the U budget is in steady state over the time period of a glacial-interglacial climate cycle ( similar to 10(5) year). A Holocene flux imbalance must, therefore, be offset by an opposing flux imbalance during glacial periods or at the interglacial-glacial transition. We suggest that the storage of U in the coastal zone and shallow water carbonates during interglacial periods and the release of that U at or following the interglacial-glacial transition could be sufficient to affect the short-terin stability of the U budget. Providing tighter constraints on U fluxes in the Holocene ocean is a prerequisite to understanding the U budget on the time scale of a glacial-interglacial climate cycle and using this element as a valuable palaeoceanographic proxy. (C) 2002 Elsevier Science B.V. All rights reserved.