25th International Meeting on Organic Geochemistry IMOG 2011

P-500
Geochemical characterization of the impact of landuse change
on soil and riverine organic matter dynamics in a pristine
tropical rainforest, Guyana
Ryan Pereira 1 , Robert Spencer 3 , Peter Hernes 2 , Rachael Dyda 2 , Isabella Bovolo 4 , Geoff
Parkin 1 , Thomas Wagner 1 , Nikolai Pendentchouk 5
1 Newcastle University, Newcastle upon Tyne, United Kingdom, 2 University of California Davis, Davis, United
States of America, 3 Woods Hole Research Centre, Falmouth, United States of America, 4 Iwokrama
<strong>International</strong> Centre for Rain Forest Conservation and Development, Georgetown, Guyana, 5 University of
East Anglia, Norwich, United Kingdom (corresponding author:Ryan.Pereira@ncl.ac.uk)
The two greatest threats to terrestrial
ecosystems are land-use and climate change.
Tropical forest ecosystems store ~50% of the World‘s
living terrestrial carbon pool and �12% soil carbon
(SOM) pool. With respect to Amazonia, seasonal
variability can trigger an almost instantaneous and
large increase in the release of carbon from soils into
rivers when linked to transitional periods of extreme
precipitation. 1,2 Rivers reflect this intense cycling, with
the largest fluxes of water and carbon to the ocean
coming from tropical rivers such as the Amazon,
Congo, or Orinoco Rivers, exporting 26-28% of global
riverine dissolved organic carbon 3 (DOC).
Iwokrama <strong>International</strong> Centre for Rainforest
Conservation and Development (www.iwokrama.org)
is a 371,000 hectare reserve situated within the
largely pristine rainforest of the Guiana Shield.
Iwokrama is defined by the catchment of the Burro
Burro River, which flows into the Essequibo River,
(av.discharge ~5000m 3 /s; 4 ).However, no geochemical
information exists to gauge future changes.
A new geochemistry program established in
2010 examines the capacity of pristine tropical
rainforest and its soils to generate, store and recycle
carbon and water; and to address the issue of the
forests‘ response to natural climate variability and
land-use, thereby characterising the current state of
the forest and how it responds to external forcing.
Here we present geochemical data for soils
and river water collected from two small headwater
catchments during the peak dry and wet seasons of
2010. The data are used to provide a direct
comparison of two river catchments situated on
nutrient-poor bedrock with similar vegetation but with
pristine and perturbed land uses (sustainable timber
harvesting) to examine and quantify the differences in
terrestrial carbon cycling. Using lignin-phenol
biomarkers and stable isotope (δ 18 O, δD & δ 13 C)
analyses, we characterize the sources and
processing of dissolved organic matter (DOM).
Evaporation dynamics in rivers of both catchments
show seasonal transitions of δ 18 O isotopes (dry: -2.1
to -3.9‰, wet: -4.6 to -6.3‰) and increased humidity
in the wet season (δ 18 O vs δD slope