Black Shale: A Source of Dissolved Organic Matter in Natural Waters

Black shales are fine-grained laminated sedimentary rocks that are rich in organic matter (OM). Exposure of shales to earth surface environments results in oxidative weathering of the shale. While prior studies have demonstrated a loss of organic carbon during weathering of shales, other efforts have shown that rivers draining watersheds underlain with black shale transport significantly aged (¹⁴C-depleted) dissolved and particulate OM when compared to rivers draining other lithologies. Consequently, it is uncertain if complete oxidation of ancient sedimentary OM occurs within an outcrop during weathering, or whether black shales can be a source to rivers of OM that is distinct in composition and isotopic character from other pools such as soils, decaying vegetation, and authochthonous production. To address this, column experiments were initiated in which sterile, air-saturated water was passed through shale substrates packed in glass flow-through cells. A Late Devonian black shale from the Appalachian Basin, USA, (7.60% TOC) was used. The Type-II kerogen contained in this shale is representative of kerogen compositions found in most marine sedimentary rocks. Three parallel column experiments including crushed shale, solvent-extracted crushed shale, and a baked-sand blank have been continually monitored for several months. Effluent from the columns has been collected and analyzed daily for dissolved organic carbon concentrations (DOC). Solution pH, conductivity, major cations/anions, and ¹³C NMR spectra of effluent collected on solid-phase-extraction disks have also been monitored. Within several days after initiation of experiments, DOC concentrations exiting both columns stabilized at 0.5 mg/L, which equates to 3.2 x 10^-3 mg_carbon/g_toc/hr or 2.2 x 10^-4 mg_{carbon}/m^{2}/hr. At this rate, organic carbon in the 82g shale column will fully dissolve in 36 years. However, release of kerogen into solution is not likely to be simple dissolution, but instead dominated by absorption/desorption exchange with the kerogen and mineral matrix. Streams draining black shales exhibit DOC concentrations that are similar to what is observed in these column experiments, which suggests that during weathering, sedimentary rock rich in organic carbon may be a source of OM to river ecosystems. If this OM is not remineralized during transport, these waters may be one source of ^{14}$C-depleted organic matter to seawater that is distinct in composition from other terrestrial sources.