Mobility of rhenium, platinum group elements and organic carbon during black shale weathering

摘要

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 (～365 Myr) Ohio Shale show a pronounced decrease (～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 (～67% loss). Conversely, organic phosphorus (P_(org)) concentrations increase by ～59% near the soil surface. The decrease in C_(org) is accompanied by a pronounced decrease in Re (～99%) and, to a lesser extent, Os (～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 Myr with an initial ~(187)Os/~(188)Os of ～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 (~(187)Os/~(188)Os of ～7.8) than the average of the unweathered samples (~(187)Os/~(188)Os of ～6.4).Based on data from this study, the molar ratio of labile Re to C_(org) in Ohio Shale is estimated at 7*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 ～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 ～12% of the estimated annual CO_2 flux from oxidative weathering of sedimentary rocks. The labile molar Re/Os of ～270 indicates that black shale weathering releases ～130 mol Os per year, which accounts for ～7% of the riverine Os input to seawater. The data from this study support the notion that 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.