The fate of organic carbon during lowland river transport and transient floodplain storage

SCHEINGROSS, J., ; REPASCH, M.; HOVIUS, N.; SACHSE, D.; DELLINGER, M.; LUPKER, M.; HILTON, R.; EGLINTON, T.; GRÖCKE, D.; GOLOMBEK, N.; FUCHS, M.; ORFEO, O.; HAGHIPOUR, N.

Washington DC

Congreso; American Geophysical Union (AGU) Fall Conference; 2018

American Geophysical Union (AGU)

Lowland rivers transport particulate organic carbon (POC) from terrestrial sources to oceanic sinks. Potential transformation of this carbon during transit modulates atmospheric CO2 levels over geologic timescales via the oxidation of rock-derived POC (POCpetro), which provides a long-term CO2 atmospheric source, and the export of POC derived from the terrestrial biosphere (POCbio), which provides a long-term CO2 sink upon burial and incorporation into new rock. Despite the importance of POC oxidation to setting global climate, identifying the controls on the fate of POC during fluvial transit is difficult because the dendritic structure of river networks causes spatial and temporal mixing of POC from tributaries, obscuring clear signals. Here, we exploit a unique field location where the Rio Bermejo, Argentina traverses 700 km across the Chaco Plain with no tributary inputs. High lateral migration rates (>10 m/yr) allow exchange of sediment between the river and floodplain, allowing examination of the influence of sediment transport and storage on the composition and flux of POC to downstream basins. Low POCpetro concentrations (~0 ? 0.04%) further simplify the system, such that bulk POC changes primarily reflect changes in POCbio. Sampling suspended and bedload sediment at five locations along the Bermejo shows a systematic increase of bulk δ13C and POCbio radiocarbon age from -26.33 ± 0.61? and 730 ± 140 yr at the upstream extent of our study area to -25.44 ± 0.25? and 1150 ± 220 yr at the downstream extent. Increased δ13C values are consistent with a chronosequence of Bermejo floodplain cores which show up to ~10? increase in δ13C and up to 70% loss of POC over ~20 ky of floodplain storage, suggesting ongoing selective oxidation of POC during floodplain storage. Despite floodplain oxidation of POC, Rio Bermejo sediments maintain relatively constant bulk POC values (~0.3 ± 0.1%) moving downstream, suggesting POC oxidized in floodplain storage is replaced via erosion of biomass produced in floodplains. This balance allows total Bermejo POCbio flux to be independent of floodplain storage time, and we suggest more generally that POC fluxes in lowland river systems should be set by competition between floodplain oxidation rates, net primary productivity, and river sediment supply and lateral migration rate.