The organic carbon cycle will be studied at two open ocean sites which reflect potential differences in carbon derived from marine and terrestrial sources. Studies of carbon isotopes and dissolved and particulate organic carbon will be conducted at Station "M" in the Northeast Pacific Ocean and in the Amazon River plume region of the Atlantic Ocean. The control of DOC concentrations and . 14C values by natural microbial assemblages will also be evaluated. Relationships between the different carbon pools, including sedimentary organic carbon (SOC), will be assessed at Station "M" during periods of high and low primary productivity and in a transect of Amazon River plume/Atlantic Ocean waters. The following questions will be addressed: (1) Are carbon species and . 14C gradients a function of seasonal carbon flux rates in the open ocean? (2) What is the contribution of terrestrially derived carbon species to the open ocean? (3) Are isotopes activities fractionated with respect to particle size and/or organic composition? (4) What fraction of the total DOC pool is potentially labile and truly refractory, and what are the . 14C signatures of each fraction?
