The Marine Geochemistry of Rhenium, Iridium and Platinum

The platinum group elements occur in low concentrations in most crustal materials. Sedimentary enrichments above these low background levels are thought to arise only from very specific processes, and thus are relatively easy to interpret. The thesis describes aspects of the marine geochemistry of two of these elements, iridium and platinum, as well as a neighbor in the periodic table, rhenium. Because Pt and Jr are highly enriched in meteorites compared to the Earth's crust, variations in their concentrations have been interpreted as reflecting changes in the amount of cosmic material in sediments. Re is also scarce (<O.l ppb) in most crustal materials, but is highly enriched in anoxic sediments, making it a very sensitive indicator of anoxic conditions. Our ability to interpret the sedimentary concentrations of these elements has been hampered by our lack of knowledge about their marine geochemistry and their behavior during sediment diagenesis. The thesis therefore attempts to answer two general questions: 1. Can Re, Ir and Pt be redistributed by the geochemical changes associated with early diagenesis of marine sediments? 2. What are the processes controlling Re enrichment in anoxic sediments? The main conclusions of the study are: 1. Changing redox conditions in sediments can mobilize Re, Ir and Pt, leading to variations in Pt and Ir concentrations that should not be confused with changes in cosmic flux. 2. The marine geochemistry of Re appears to be relatively straightforward. It behaves as a conservative element in oxic seawater and is removed only under anoxic conditions, within reducing sediments. Rhenium Iridium Platinum