CARBON ISOTOPE INDICATORS OF CATCHMENT VEGETATION IN THE BRAZILIAN AMAZON

Particulate organic carbon (POC) samples from rivers draining wholly forested catchments in the Amazon Basin exhibit carbon isotope values (delta C-13) of generally between -27 and -30%o. These values are distinct from those of POC from rivers draining nonforested (grassland/woodland) catchments, which are generally higher than -26%o. The difference is due to the presence in the nonforested regions, of grasses which assimilate carbon via the C-4 photosynthetic pathway (average delta C-13 = similar to-13(0)/(0o)), rather than the C3 pathway (average delta C-13 = similar to-28%o) utilized by forest vegetation, and to negligible utilization of low -delta C-13 respired carbon dioxide in regions of open vegetation. A change in the area of the Basin covered by closed forest will lead to a modification in the carbon isotope composition of exported POC as a result of changing (1) the proportion of C4 carbon in the total biomass (2) the relative proportion of the total POC derived from upstream, high-altitude (Andean) sources, (3) erosion rates, and (4) productivity. All of these factors will tend to reinforce each other in their effect on the carbon isotope composition of POC exported from the Basin, with a decrease in forest area leading to an increase in the delta C-13 value of POC (and vice versa). There is a sufficient difference between the delta C-13 values of POC exported from forested and non forested catchments to elucidate past vegetation and therefore climatic changes in the Amazon Basin (and other tropical river basins), using the delta C-13 record in POC from either terrestrial or offshore sedimentary sequences. The carbon isotope composition of POC in rivers draining recently deforested regions indicates that the isotopic composition of coarse (>63 mu m) riverine POC will respond to vegetation changes in a catchment within years to decades, whereas the delta C-13 value of fine-grained (<63 mu m) riverine POC may take longer to respond.