The underground weathering of Toarcian black shales from SE France and its paleoenvironmental, taphonomical and biogeochemical consequences

Abstract

Organic-rich strata deposited under poorly oxygenated waters known as ‘black shales’ constitute key targets for paleobiological and paleoenvironmental studies. Little is known, however, about how these oxidation-sensitive lithologies and their key paleoenvironmental information can be affected by recent chemical weathering. In this study, we present new geochemical (organic and inorganic stable isotopes, TOC, CaCO₃) and mineralogical (concentration and size distribution of pyrite framboids) data from weathered and unweathered black shales and limestone beds recording the Toarcian Oceanic Anoxic Event (T-OAE) from Beaujolais (SE France). These analyses show that the organic carbon of the weathered samples have decreased by 97 % and is generally ¹³C-enriched relative to coeval pristine samples. The resulting offset in organic carbon isotope (δ¹³C_org) varies from 0.1 to as much as 4 ‰ along the studied succession, likely reflecting temporal changes in the carbon isotope composition of the labile and refractory end-members across the T-OAE. Mildly weathered samples contain >50 % less pyrite framboids than pristine samples, but preserve their size distribution, making the latter a reliable proxy of original water column oxygenation. Pyrite dissolution was total in the highly weathered samples, which produced sulfuric acid and a moderate loss of the CaCO₃ fraction. The substantial ¹³C- and ¹⁸O-depletion recorded in the most weathered samples indicate that a part of the dissolved carbonate reprecipitated after exchanging with CO₂-rich meteoric waters. These results imply that underground continental weathering can dramatically alter the paleoenvironmental and taphonomical signals in organic rich-strata located at >40 m below the surface and should hence be more systematically considered when analyzing outcrop and subsurface data. In addition, our tentative estimates of petrogenic carbon oxidation rates at the study site are 2 to 10 times higher than that of other sedimentary rocks, suggesting that the fault-assisted oxidation of black shales in regions located away from active orogenic areas may contribute disproportionately to the global carbon cycle budget.