Understanding the formation of a major terrestrial carbon sink: A molecular isotopic study of an Andean watershed Angela F. Dickens, Department of Chemistry, Mount Holyoke College Lowland floodplain sediments adjacent to the Andes Mountains store vast amounts of organic carbon, but the origin of this carbon is poorly constrained. We are using isotopic analysis of plant-derived compounds to determine where in the watershed this carbon originated. Some preliminary results: Lignin phenols in large and small sediment particles derive from leaves and woods, not grasses, suggesting they originate in the mid- to lower-elevation forests, not the high elevation grasslands Carbon isotopes of lipids in the small fraction of one sediment suggest these lipids derive from mid- to upper-elevations in the watershed Map of the Bolivian Rio Beni watershed with sampling locations shown as yellow circles (from GoogleEarth and Guyot et al. (1993) Geodinamica Acta.)