HR: 16:45h
AN: H54A-04    [Abstracts]
TI: ENSO-Orchestrated Particle Supply, Deposition, and Carbon Sequestration in Amazonian River Basins by Erosion-Sedimentation Processes
AU: * Aalto, R
EM: aalto@u.washington.edu
AF: Dept. Earth and Space Sciences, University of Washington, Seattle, WA 98195-1310 United States
AU: Aufdenkampe, A K
EM: aufdenkampe@stroudcenter.org
AF: Stroud Water Research Center, 970 Spencer Rd., Avondale, PA 19311 United States
AU: Bourgoin, L M
EM: maurice@lmtg.obs-mip.fr
AF: Institut de Recherche pour le Developpement, Observatoire MIP LMTG UMR 5563, Toulouse, 31400 France
AB: Application of a new geochronological method quantifies century-scale floodplain sedimentation rates across a pristine 720,000 km$^{2}$ basin in northern Bolivia, covering the principal sediment and water sources for the Madeira River, the largest sediment source for the Amazon. Large, rapid-rise, cold-phase ENSO floods account for the preponderance of sediment accumulation and dominate sediment discharge from Andean tributaries into the large rivers of the Amazonian lowland. Discharge data indicate considerable inter-annual variation of sediment supply from the Andes, resulting from the interaction of Andean erosion and the dynamics of extreme climate. Transient, ENSO-driven processes therefore control both the formation of floodplains and sedimentary strata within the Bolivian foreland, and also modulate the efflux, transport, floodplain storage, exchange due to channel migration, and downstream delivery of sediment and associated carbon (entrained coarse material and sorbed molecules), nutrients, and pollutants to the Amazon main stem. Such infrequent, extreme mechanisms of sediment and carbon transport suggest that a three-step process could represent a major carbon sink in Amazonian foreland basins: 1) extensive Andean hillslope failure and channel migration during large La Ni¤a associated storms mobilizes vast quantities of fresh organic matter and sediment with low organic carbon (OC) content; 2) within the river, mineral surfaces acquire normal OC loadings via sorption as they are rapidly evacuated from the mountainous source basins to adjacent foreland depocenters; and 3) deposited sediments preserve "fresh" carbon within organo-mineral complexes and by deep burial in point bars and "crevasse-splay" deposits that have little potential for exchange with the biosphere and atmosphere. Calculations and new measurements suggest that this process could sequester 250-500 Mtonnes of carbon per event in the Amazonian foreland and regulate the supply of particulate OC to the lower Amazon. Extrapolated globally, this hypothesis could account for a significant fraction of the "missing carbon sink" and the atmospheric carbon dioxide anomalies typically observed during cold-phase ENSO.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1869 Stochastic processes
DE: 1030 Geochemical cycles (0330)
DE: 1615 Biogeochemical processes (4805)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting