25th International Meeting on Organic Geochemistry IMOG 2011

P-505
Organic carbon composition and routing in alpine headwaters:
the importance of storms
Jo Smith 1 , Niels Hovius 1 , Albert Galy 1 , Jens Turowski 2
1 Department of Earth Sciences, University of Cambridge, Cambridge, United Kingdom, 2 Eidg.
Forschungsanstalt für Wald, Schnee und Landschaft, Birmensdorf, Switzerland (corresponding
author:jcs74@cam.ac.uk)
The erosion of continental biomass in the form of
particulate organic carbon (POC) is a poorly
constrained flux in the global carbon cycle. It has
received little attention in the past because its
potential for carbon dioxide drawdown was thought to
be insignificant in comparison to silicate weathering
and burial of marine organic matter. However, recent
studies suggest that storm-driven POC erosion is an
effective way of sequestering carbon in tectonically
and climatically extreme regimes.
In order to assess whether biomass erosion and
burial is a globally significant carbon sink, we need to
know how much POC is harvested from less intense
geomorphic settings. More insight is also needed into
the processes involved in mobilizing organic carbon in
catchment headwaters, including its sources and
pathways and how these change under different
hydrologic conditions.
To these ends, we have collected both weekly flowproportional
samples (over a period of two years) and
higher-frequency grab samples (during storms in July
2010) of riverine and runoff suspended sediment from
the Erlenbach, a small catchment in the Swiss
Prealps. We have obtained their C and N
concentrations and isotopic ratios using stable isotope
mass spectrometry.
Riverine suspended sediment contains a mixture of
POC derived from contemporary sources (soils and
vegetation) and fossil POC found in bedrock, which
are characterised by their N/C and δ 13 Corg. Only the
modern component will have a direct effect on
atmospheric CO2.
By determining the same chemical parameters on
soil, bedrock, channel material and vegetation
samples collected throughout the catchment, we
define a mixing line between endmember sources
dominated by modern and fossil carbon. We find that,
while POC as a percentage of suspended load
remains constant regardless of the suspended
sediment concentration, it contains a significantly
higher fraction of modern POC during storms than
under normal flow conditions. The effect is most
pronounced at higher suspended sediment
concentrations and during the rising limb of the
hydrograph.
The fact that rising limb samples are closer to the
modern endmember than storm samples overall
provides clues to the mechanisms by which POC is
harvested. We propose that, during the initial storm
phase, litter and topsoil are delivered to the channel
by runoff, but that as the water level and erosive
power of the stream increase, more of the fossil
carbon that forms the channel sides and bottom is
incorporated into the suspended load.
The fact that samples collected at high suspended
sediment concentrations are closer to the modern
endmember than storm samples overall is also
significant. Long-term records of discharge and
suspended sediment concentration from the
catchment show that they are related by a power law;
that is, the vast majority of sediment is transported by
just a few very large floods. Our results suggest that
these are optimal conditions for exporting modern
POC. If these relationships and processes are typical
of temparate montane forests globally, then the
erosion of continental biomass is a potentially
significant carbon sink
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