PREDICTIVE ISOTOPIC BIOGEOCHEMISTRY - HYDROCARBONS FROM ANOXIC MARINEBASINS

Carbon isotopic compositions were determined for individual hydrocarbo ns in water column and sediment samples from the Cariaco Trench and Bl ack Sea. In order to identify hydrocarbons derived from phytoplankton, the isotopic compositions expected for biomass of autotrophic organis ms living in surface waters of both localities were calculated based o n the concentrations of CO2(aq) and the isotopic compositions of disso lved inorganic carbon. These calculated values are compared to measure d delta values for particulate organic carbon and for individual hydro carbon compounds. Specifically, we find that lycopane is probably deri ved from phytoplankton and that diploptene is derived from the lipids of chemoautotrophs living above the oxic/anoxic boundary. Three acycli c isoprenoids that have been considered markers for methanogens, penta methyleicosane and two hydrogenated squalenes, have different delta va lues and apparently do not derive from a common source. Based on the c oncentration profiles and isotopic compositions, the C31 and C33 n-alk anes and n-alkenes have a similar source, and both may have a plankton ic origin. If so, previously assigned terrestrial origins of organic m atter in some Black Sea sediments may be erroneous.