Abstract
The cross-flow ultrafiltration technique and multiple isotopic tracers (13c, 14c, 23OTh, and 234 Th) were used to study the cycling of dissolved organic matter (DOM) in the Gulf of Mexico and the NEddle Atlantic Bight. A considerable fraction of the traditionally defined DOM was in a colloidal form. Colloidal organic carbon (COC,, I kDa-0.2 gm fraction) comprised -60% of the total DOC in estuarine waters, decreasing to -30-40% in oceanic waters. High molecular weight (HMW) COCIO (10 kDa-0.2 gm) made up 3-15% of the DOC pool, with highest abundances in estuarine waters and lowest ones in deep waters. A conservative mixing behavior of DOC was found in estuarine and oceanic waters. The fluxes of DOC by downward transport were comparable to those measured for POC by sediment traps. delta14C values of both COM, and COMIO in surface water were generally high, reflecting their contemporary inputs. Values of delta14C for bottom water COMIO, in contrast, could be much lower depending on its source functions. Thus, apparent 14 C ages of the bulk DOM are the weight-averaged ages of different DOM fractions with varying molecular weights and reactivities. Three colloidal end-members were identified: estuarine colloids with high delta14C values, high C/N ratios, and lower 813C values, offshore surface water (pelagic) colloids with intermediate, deltal4C values, lower C/N ratios, and higher 813C values, and offshore deep water colloids with low DELTA14C values, intermediate C/N ratios, and variable 513C values. Turnover times of COMIO and COM, derived from 23'Th were consistently short (1-65 days) regardless of apparent 14 C ages, indicating that HMW colloids are turning over more rapidly than the bulk DOM pool. The discrepancy between old 14 C ages and short turnover times suggests that 234 Th and 14C may trace different geochemical pathways of COM. In addition, it indicates some revisions in scavenging models currently used by oceanographers may be needed. The partitioning of 234Th was similar to that of organic carbon among dissolved, colloidal, and particulate phases. "Particle concentration effect", a negative correlation between the logs of distribution coefficients and the logs of particle or colloidal concentrations, was observed on both the long-lived 230Th and radiogenic 234 Th.
Guo, Laodong (1995). Cycling of dissolved and colloidal organic matter in oceanic environments as revealed by carbon and thorium isotopes. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1574734.