Intellectual Merit: Data presented in this proposal challenge the conventional view of conservative behavior of rhenium in estuaries. Rhenium and molybdenum define the radiogenic Re-Os and the stable Mo isotope systems that have important basic and applied science applications. Both elements are thought to exhibit conservative behavior in oxic aquatic environments. They are characterized by long residence times in seawater that result from relatively high dissolved concentrations in seawater (Re: 40 pM; Mo: 105 nM) and low average riverine concentrations (Re: 2 pM; Mo: 5 nM). Both elements are redox-sensitive and accumulate in reducing sediments. They are easily mobilized from such sediments during oxic weathering and can be used as indicators of oxidation of old sedimentary organic matter.
The notion of conservative Re behavior is challenged by our finding of a pronounced nonconservative Re concentration profile in the Hudson River Estuary salinity profile. The projected freshwater concentration of ~60 pM exceeds the accepted world-average river concentration 30-fold. High Re concentrations of ~90 pM in fresh Mississippi River water indicate that the high Re concentrations in the Hudson River are no exception. Though similarly high Re concentrations were detected in the early 1990s in rivers draining into the Black Sea, these concentrations were interpreted as reflecting anthropogenic contamination (coal burning).
This proposal has three goals: 1) Surveys of Re (and Mo) concentrations will be conducted along the salinity gradient in the Hudson River estuary to determine whether the non-conservative Re profile is a steady-state feature. Two cruises will be conducted at very different flow stages to determine whether this feature is caused by a natural process or by anthropogenic contamination. 2) In collaboration with Prof. Franco Marcantonio (Tulane Univ.) the behavior of Re (and Mo) will be studied in the Mississippi River estuary. In addition, a year-long, bi-weekly time series of Re and Mo concentrations as well as a tri-monthly time series of Re and Mo concentrations in major Mississippi tributaries will be analyzed to identify source regions of high Re input. 3) Estimates of world-average riverine Re and Mo fluxes will be revised. This will be achieved through a collaborative research component that is based on the use of standardized, low-cost and well-tested sampling/filtration kits to be sent to collaborators world-wide, as well as the analyses of archived river water samples stored at WHOI and elsewhere (by collaborators).
Broader Implications: Constraining riverine fluxes of Re and Mo is critical for using the radiogenic Re-Os and stable Mo isotope systems as indicators of organic-matter cycling. The potentially wide-spread Re contamination of modern river systems may indicate a leaky industrial Re cycle or another anthropogenic source. The use of standardized sample kits aims at testing whether this method provides an economical way of improving global riverine fluxes for "hard to contaminate" elements, such as oxyanions Re, Mo, and U. If successful, this method will be used in the future as a combined educational and research tool for involving science teachers and their pupils in geochemical research. The project will support the scientific education of Research Assistant Tracy Abbruzzese and MIT/WHOI Joint Program student Christian Miller.