Geomorphic and geochemical controls on leaf wax biomarker transport and preservation in alluvial river systems: Rio Bermejo, Argentina

REPASCH, MARISA; SACHSE, D.; HOVIUS, N; SCHEINGROSS, J.S. ; SZUPIANY, RICARDO N.

Washington

Encuentro; AGU 2018 Fall Meeting; 2018

American Geophysical Union

Rivers are the primary conduits for organic carbon (OC) transfer from vegetation-rich uplands to long-term sinks, and thus are responsible for significant fluxes among different reservoirs of the carbon cycle. Fluxes of terrestrial OC out of river systems are generally less than fluxes into the systems, indicating loss of OC either during active fluvial transport, during residence in the active channel belt, or in older deposits outside of the active channel belt. Sedimentary biomarkers can be used to elucidate the mechanisms of transport, preservation, and/or transformation of OC during its passage from source to sink.In this study we evaluate the influence of fluvial sediment transport on terrestrial leaf wax n-alkanes in river suspended load. Our natural laboratory is the Rio Bermejo in northern Argentina, which transports sediment and organic matter from the central Andes in northern Argentina over 700 km across the foreland basin without receiving input of foreign material from tributaries. In this river system, high channel migration rates in a region of flexural foreland basin uplift (the forebulge) are responsible for remobilization of floodplain sediment and terrestrial organic matter.By sampling suspended sediment, river bank sediment, and soil from several locations along the length of the Rio Bermejo, and analyzing the dissolved chemistry, biomarker composition, and compound-specific stable isotopes (13C and H), we are able to evaluate the geomorphic and geochemical processes that act to influence the preservation of terrestrial biomarkers through the river system. Preliminary data suggest that concentrations of long-chain terrestrial (C25-C33) alkanes decrease downstream, while concentrations of short-chain (C15-C19) alkanes increase. This trend is corroborated by a downstream increase in bulk suspended sediment 13C values, suggesting a replacement of terrestrial OC by microbial OC. It is likely that microbial degradation is responsible for loss of terrestrial biomarkers as their residence time in the river system increases. Controlled laboratory experiments and analysis of recently deposited and aged river bank sediment samples test the hypothesis that oxidation of leaf wax alkanes by microorganisms occurs during floodplain storage, and will determine the timescales over which degradation occurs. With these data, we will be able to quantify the loss of OC during fluvial transit and determine the mechanisms responsible, enabling carbon cycle models to account for these losses.