HR: 17:15h
AN: H24B-06    [Abstracts]
TI: Riverine Particulate Organic Carbon From a Pristine, Active Mountain Belt: The Importance of Vegetation and Landslides
AU: * Hilton, R G
EM: rgh31@esc.cam.ac.uk
AF: Department of Earth Sciences University of Cambridge, Downing Street, Cambridge, CB2 3EQ
AU: Hovius, N
EM: nhovius@esc.cam.ac.uk
AF: Department of Earth Sciences University of Cambridge, Downing Street, Cambridge, CB2 3EQ
AU: Galy, A
EM: albert00@esc.cam.ac.uk
AF: Department of Earth Sciences University of Cambridge, Downing Street, Cambridge, CB2 3EQ
AB: The erosion and transfer of terrestrial organic carbon is an important component in the carbon cycle. The sequestration of this material in sedimentary basins may influence global climate. Mountain rivers play a crucial role in the routing of Particulate Organic Carbon (POC) because they have high sediment production rates, small storage potential and tendency to discharge sediment to the oceans at very high concentrations. We present new constraints on the sourcing and transfer of POC in the tectonically active Southern Alps of New Zealand, where anthropogenic disturbance is minimal. Riverine POC can be derived from bedrock (in the form of sedimentary kerogen), standing biomass and soils. In active mountain belts, landsliding dominates sediment production and transfer of POC from hillslopes to river channels. Landslide debris fans, river suspended load and bedload, and bedrock were sampled in catchments draining the rapidly eroding western Southern Alps, in order to assess the sourcing and transfer of POC. Samples were analyzed for percent organic carbon (C_org), percent nitrogen (N_ {org) and δ¹³C. Using C/N and δ¹³C as source proxies, we have determined the main sources of the POC. Landslide debris has C_org ~ 0.15% to 2.77%. The sand, silt and clay fractions of this material all have C/N ~ 10 to 40 and δ¹³C ~ -20‰ to -28‰. This indicates that POC in landslide debris is dominated by soil carbon (C/N ~ 16 to 25) mixed with hillslope vegetation (C/N ~ 40+) and diluted by bedrock (C_org = 0.01%, C/N = 4.7). Variations in the vegetation type of the pre-slide hillslopes and the depth of failure (i.e. volume of bedrock) give rise to variability of carbon concentration and source signature between landslides. In river suspended load, C_org ~ 0.5%, C/N ~ 15.6 and δ¹³C ~ - 23.5‰ at mean flow. These values are relatively uniform along the mountain belt. C_ {org} decreases with increasing suspended sediment concentration. We conclude that the bulk of the suspended load POC is sourced from landslide debris material, and dominated by modern, soil-derived organic carbon. Based on our measurements, we estimate that catchments draining the western Southern Alps transfer between 283 and 3873 x 10⁶ moles yr^-1 of CO₂ from atmosphere to sediment via the erosion of terrestrial POC. Normalized to catchment area, this represents a carbon flux of 1.3 to 8.6 x 10⁶ moles km^-2 yr^-1. This is an order of magnitude greater than the published CO₂ flux due to silicate weathering for the same catchments (0.14 to 0.56 x 10⁶ moles km^{- 2} yr^-1). If more than ~ 10% of the POC transported to the ocean is preserved in marine sediments, then this process could be the most significant way in which the Southern Alps and other active mountain belts alter global climate.
UR: http://www.esc.cam.ac.uk/esp
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0428 Carbon cycling (4806)
DE: 1051 Sedimentary geochemistry
DE: 1862 Sediment transport (4558)
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: Fall Meeting 2005