Shale as a Source of Organic Carbon in Floodplain Sediments of a Mountainous Watershed

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 (Delta C-14 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 Delta C-14 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 Delta C-14 values and reactivity of sediment-OC must be considered. Plain Language Summary Shales contain high levels of organic carbon (OC) and represent a large fraction of the Earth's total carbon stocks. While recent evidence suggests that shale-derived OC, which is millions of years old, may be actively cycled in riverine systems, this process is poorly understood and not currently considered in global C models. In this study, we analyze sediments collected from the floodplain of the East River, CO, located in a high-elevation mountainous watershed underlain by shale bedrock, to determine the importance and mobility of shale-derived OC in this environment. OC closely associated with sediment minerals is the largest (84 6%) and oldest OC pool, containing a large, but variable, amount of shale-derived OC. Evidence of shale-derived OC is also observed in other sediment OC pools which are considered to be more mobile and more easily degraded to carbon dioxide by bacteria (e.g., water-soluble). This study concludes that there are two primary OC sources in floodplain sediments, plant-litter and shale-derived OC, each with distinct chemical characteristics and reactivity. We estimate that 23-34% of the sediment OC is derived from shale, demonstrating the important contribution of shale-OC to the carbon cycle at this site, particularly in environments with low plant-litter inputs. Key Points Shale-derived organic carbon (OC) accounts for an estimated 23-34% of total OC in floodplain sediments of a shale-dominated landscape The mineral fraction contains the largest and oldest pool of OC, and shale-derived OC is also observed in pools considered to be actively cycled Implications for shale as an unrecognized source of global C and other rock-derived elements and for interpretation of bulk C-14 data