UC Berkeley

Shales contain high levels of organic carbon (OC) and represent a large fraction of the Earth's reduced carbon stocks. While recent evidence suggests that shale-derived OC may be actively cycled in riverine systems, this process is poorly understood and not currently considered in global C models. Through the use of sediment density fractionations, extractions, radiocarbon measurements, and chemical characterization, we provide information on the abundance, chemistry, and mobility of shale-derived OC in floodplain sediments of a shale-rich mountainous watershed. The heavy fraction of the sediment, representing mineral-associated OC, is the largest (84 ± 6% of TOC) and oldest (Δ14C values −224 to −853‰) OC pool. Evidence of shale-derived OC is observed in all sediment C pools (i.e., occluded light fraction, water-soluble, and pyrophosphate-extractable) except the free light fraction, which is entirely modern. Relatively consistent chemistry was observed across samples for extracted and density-separated OC, despite wide ranges of Δ14C values. Carbon spectroscopy revealed that floodplain sediments had a higher degree of functionalized aromatic groups and lower carbonate content compared to shale collected nearby, consistent with chemical alteration and mixing with other C sources in the floodplain. We estimate that approximately 23–34% of sediment OC is derived from shale, with implications for other shale-derived elements (e.g., N). This study demonstrates the important contribution of shale-OC, particularly in environments with low litter inputs. The large impact of radiocarbon-dead shale-OC, which has a thermally altered chemical structure distinct from plant litter, on Δ14C values and reactivity of sediment-OC must be considered.