ISOTOPIC EVIDENCE FOR THE CONTEMPORARY ORIGIN OF HIGH-MOLECULAR-WEIGHT ORGANIC-MATTER IN OCEANIC ENVIRONMENTS

Previous work has suggested that apparent old C-14 ages for oceanic DOC are the result of mixing of different organic carbon fractions. This report provides direct evidence for a contemporary C-14 age of a high-molecular-weight (HMW) fraction of colloidal organic carbon (greater than or equal to 10 kD). Colloidal organic matter, COM(10) (from 10 kDaltons (kD) to 0.2 mu m), isolated from the upper water column of the Gulf of Mexico and the Middle Atlantic Bight (MAB) region, generally has a contemporary age (i.e., younger than a few decades), while COM(1) (from 1 kD to 0.2 mu m), is apparently old: 380-4500 y BP. Thus, HMW COM(10) (3-5% of DOC) from the upper water column is derived from living particulate organic matter (POM) and cycles rapidly, while a significant fraction of low-molecular-weight (less than or equal to 1 kD) DOM is likely more refractory, and cycles on much longer time scales. The presence of pigment biomarker compounds in COM(1) from the upper water column points to selected phytoplankton species as one of the sources of COM. Terrestrial carbon as another source of COM is suggested from the inverse correlation between Delta(14)C and delta(13)C values, as well as the increasing delta(13)C values with increasing salinity. Th-234-derived turnover times of COM(10) and COM(1) from both the Gulf of Mexico and MAB are consistently short, 1-20 and 3-30 days, respectively. These short residence times support the hypothesis that C-14 ages of colloidal fractions of DOC are the result of COM fractions being a mixture of several endmembers with fast and slow turnover rates.