River Floodplains, Organic Carbon, and Paleoclimate: How River Dynamics Impart Biases in the Biogeochemical Climate Record of Terrestrial Landscapes

Abstract

River floodplains can be both sources and sinks for organic carbon. In the sedimentary record, geochemical signatures of terrestrial water availability record climate and ecology along river corridors in deep time. However, paleoenvironmental reconstructions are often confounded by the complex dynamics of organic carbon residence, entrainment, and deposition in a fluvial system. As alluvial rivers migrate across their sedimentary basins, they erode older deposits, integrating their environmental record with that of other sediment in transport. Transported organic carbon deposited on floodplains integrates environmental information over spatiotemporal scales associated with sediment mobilized from the entire catchment. Carbon production from plant communities on stable floodplains contribute information about the local climate and ecology. If catchment-averaged, environmental signals may be integrated over more than ~106 years; if local, they may be integrated over just ~104 years. River dynamics therefore fundamentally impact the spatiotemporal scales of integration of environmental information encoded within the organic carbon stored on their floodplains. We used physical experiments to explore how river kinematics impact the time over which organic carbon can accumulate on floodplain surfaces. We found that doubling the ratio of water to sediment flux caused the most stable parts of the floodplain to remain immobile for twice as long. In situ carbon accumulated on stable floodplains can therefore integrate significant amounts of local environmental information along with catchment-averaged information. Conversely, in situ organic carbon that accumulates on dynamic floodplains in systems with relatively high sediment fluxes predominantly records basin-averaged information but relatively small amounts of local information. Changes in river dynamics during past thermal events on Earth, linked to changes in the hydrological cycle and alterations in water and sediment fluxes, can be expected to alter the duration over which organic carbon can accumulate on Earth’s river floodplains and therefore the spatial and temporal scales to which climate reconstructions apply.