Mobility of rhenium, platinum group elements and organic carbon during black shale weathering
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This study investigates the effects of black shale weathering on the Re-Os isotope system, platinum group element concentrations and the degradation of organic matter, Samples from a weathering profile in Late Devonian (similar to 365 Myr) Ohio Shale show a pronounced decrease (similar to 77%) in organic carbon (C-org) near the present soil surface, relative to the interior portion of the outcrop. A similar trend is observed for total N (similar to 67% loss). Conversely, organic phosphorus (P-org) concentrations increase by similar to 59% near the soil surface. The decrease in C-org is accompanied by a pronounced decrease in Re (similar to 99%) and, to a lesser extent, Os (similar to 39%). Palladium and Pt do not appear to be significantly mobile. The effects of Re and Os mobility on the Re-Os isotope system are significant: none of the samples plots on a 365 Myr isochron. Rather, the samples define a trend with a slope corresponding to an age of similar to 18 Myr with an initial Os-187/Os-188 of similar to 6.1. This indicates recent disturbance of the Re-Os system. Isotope mass balance calculations imply that the labile fraction of Os is significantly more radiogenic (Os-187/Os-188 of similar to 7.8) than the average of the unweathered samples (Os-187/Os-188 of similar to 6.4). Based on data from this study, the molar ratio of labile Re to C-org in Ohio Shale is estimated at 7 x 10(-8). We estimate the present-day riverine. black shale-derived Re flux to seawater using literature data on Re burial in anoxic marine sediments, and assuming steady-state between Re release during black shale weathering and Re burial in anoxic marine sediments, Then, the labile Re/C-org observed in this study implies that similar to 0.5 Tmol of C-org is released annually from weathering of black shales, a trace lithology in the continental crust. This flux corresponds to similar to 12% of the estimated annual CO2 flux from oxidative weathering of sedimentary rocks. The labile molar Re/Os of similar to 270 indicates that black shale weathering releases similar to 130 mol Os per year, which accounts for similar to 7% of the riverine Os input to seawater. The data from this study support the notion that the crustal cycles of labile C-org, Re and Os are tightly coupled. Gray shales, which are less C-org-, Re- and Os-rich, but much more abundant in the continental crust than black shales, are likely to be even more important continental sources of Re and Os to seawater. (C) 2002 Elsevier Science B.V. All rights reserved.