THE GEOCHEMICAL CYCLE OF RHENIUM - A RECONNAISSANCE

Rhenium (Re) is one of a suite of elements (including uranium and moly bdenum) that display conservative behavior in seawater and are enriche d in anoxic sediments. The decay of Re-187 to Os-187 provides a geochr onometer in ancient sedimentary rocks and gives rise to Os-isotopic va riations in nature. In order to better characterize its sources to sea water, Re was measured in three major rivers (Amazon, Orinoco, Ganges- Brahmaputra) and some of their tributaries. Re concentrations span fou r orders of magnitude (from < 0.02 to 400 pmol/kg), with the highest c oncentrations found in rivers draining black shales in the Venezuelan Andes. Mainstream Re levels in the three rivers are between 1 and 10 p mol/kg, with a flux weighted average of 2.3 pmol/kg. The residence tim e for Re in the oceans is estimated to be 750,000 yr with respect to r iver inputs. Re profiles from the Atlantic and Pacific Oceans confirm that Re behaves conservatively in seawater, with no significant uptake onto particles and/or recycling within the water column. This is also true in the anoxic water column of the Black Sea. Re removal into ano xic sediments occurs at or below the sediment water interface, as demo nstrated in sediment pore waters from Chesapeake Bay. In oxic sediment s, Re is not cycled with manganese oxides, and it is not enriched in v ery slowly accumulating pelagic sediments with a large hydrogenous iro n and manganese component, or in manganese nodules. Burial of Re in an oxic sediments, which accumulate on 0.3% of the ocean floor, removes a pproximately 50% of the riverine Re flux to the oceans. Hence, oceanic Re concentrations may be very sensitive to changes in the area of ano xic sedimentation.