Organic carbon to Th ratios of marine organic matter

Uncertainties in the determinations of particulate organic carbon flux from measurements of the disequilibrium between ²³⁴Th and its mother isotope uranium depend largely on the determination of the organic carbon to ²³⁴thorium (OC : ²³⁴Th) ratio. The variability of the OC : ²³⁴Th ratio in different size fractions of suspended matter, ranging from the truly dissolved (< 3 or 10 kDa) fraction to several millimeter sized marine snow, as well as from sediment trap material was assessed during an eight-day cruise off the coast of California in Spring 1997. The affinity of polysaccharide particles called TEP (transparent exopolymer particles) and inorganic clays to ²³⁴Th was investigated through correlations. The observed decrease in the OC : ²³⁴Th ratio with size, within the truly dissolved to small particle size range, is consistent with concepts of irreversible colloidal aggregation of non-porous nano-aggregates. No consistent trend in the OC : ²³⁴Th ratio was observed for particles between 1 or 10 to 6000 μm. Origin and fate of marine particles belonging to this size range are diverse and interactions with ²³⁴Th too complex to expect a consistent relationship between OC : ²³⁴Th ratio and size, if all categories of particles are included. The relationship between OC and ²³⁴Th was significant when data from the truly dissolved fraction were excluded. However, variability was very large, implying that OC flux calculations using different collection methods (e.g. sediment trap, Niskin bottles or pumps) would differ significantly. Therefore a large uncertainty in OC flux calculations based on the ²³⁴Th method exist due to individual decisions as to which types or size classes of particles best represent sinking material in a specific area. Preferential binding of ²³⁴Th to specific substance classes could explain the high variability in the relationship between OC and ²³⁴Th. At 15 m, in the absence of lithogenic material, the OC : ²³⁴Th ratio was a function of the fraction of TEP or TEP-precursors in OC, confirming that acidic polysaccharides have a high affinity for ²³⁴Th and that TEP carry a ligand for ²³⁴Th. Preferential binding to TEP might change distribution patterns of ²³⁴Th considerably, as TEP may sink when included in large aggregates, or remain suspended or even ascend when existing as individual particles or microaggregates. In the presence of lithogenic matter, at depths below 30 m, the ratio between ²³⁴Th and OC was linearly related to the ratio between alumino silicates and C. The affinity of inorganic substances to ²³⁴Th is known to be relatively low, suggesting that a coating of acidic polysaccharides was responsible for the apparently high affinity between ²³⁴Th and lithogenic material. Overall, OC : ²³⁴Th ratios of all material collected during this investigation can best be explained by differential binding of ²³⁴Th to both TEP and TEP-precursors, as well as to lithogenic minerals, which were very abundant in an intermediate nepheloid layer between 50 and 90 m.