Organic carbon to 234Th ratios of marine organic matter

Abstract

Uncertainties in the determinations of particulate organic carbon flux from measurements of the disequilibrium between 234 Th and its mother isotope uranium depend largely on the determination of the organic carbon to 234 thorium (OC : 234 Th) ratio. The variability of the OC : 234 Th ratio in different size fractions of suspended matter, ranging from the truly dissolved (b 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 234 Th was investigated through correlations. The observed decrease in the OC : 234 Th ratio with size, within the truly dissolved to small particle size range, is consistent with concepts of irreversible colloidal aggregation of non-porous nanoaggregates. No consistent trend in the OC :234 Th ratio was observed for particles between 1 or 10 to 6000 Am. Origin and fate of marine particles belonging to this size range are diverse and interactions with 234 Th too complex to expect a consistent relationship between OC :234 Th ratio and size, if all categories of particles are included. The relationship between OC and 234 Th 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 234 Th 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 234 Th to specific substance classes could explain the high variability in the relationship between OC and 234 Th. At 15 m, in the absence of lithogenic material, the OC : 234 Th ratio was a function of the fraction of TEP or TEP-precursors in OC, confirming that acidic polysaccharides have a high affinity for 234 Th and that TEP carry a ligand for 234 Th. Preferential binding to TEP might change distribution patterns of 234 Th 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 234 Th and OC was linearly related to the ratio between alumino silicates and C. The affinity of inorganic substances to 234 Th is known to be relatively low, suggesting that a coating of acidic polysaccharides was responsible for the apparently high affinity between 234 Th and lithogenic material. Overall, OC :234 Th ratios of all material collected during this investigation can best be explained by differential binding of 234 Th 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.

Direct comparison of 210Po, 234Th and POC particle-size distributions and export fluxes at the Bermuda Atlantic Time-series Study (BATS) site

“…The rationale is that particles larger than 53-mm are the dominant contributors to the downward flux of particles and therefore the POC/ 210 Po and POC/ 234 Th ratio in this size fraction should reflect that of sinking particles. Conversely, it is now recognized that aggregates and cells smaller than 53-mm may comprise a significant fraction of POC export (Pfannkuche and Lochte, 1993;Turley and Mackie, 1995;Waite et al, 2000;Brew et al, 2009;Lomas and Moran, in press). To obtain more accurate 210 Po-and 234 Th-derived estimates of POC export, it is necessary to know the size dependence of the POC/ 210 Po and POC/ 234 Th ratio and the size of the particles that are the dominant contributors to particle export.…”

Characterization of phytoplankton exudates and carbohydrates in relation to their complexation of copper, cadmium and iron

“…TEP and their dissolved precursors exhibit a high affinity to thorium and Fe (Honeyman & Santschi 1991, Quigley et al 2002, Passow et al 2006, Santschi et al 2006, Zhang et al 2008, and specifically alginic acid and carrageenans are known to bind metal ions (Gimenez et al 1995, Kim et al 1995. Concentrations of nickel (Ni), chromium (Cr) and Zn in the 0.2 to 0.8 µm size-range correlated well with TEP turnover rate in a lagoon off New Caledonia, suggesting that submicron TEP complex metals, but that hydrodynamically controlled reactivity affects their ability to adsorb metals (Mari et al 2009).…”

The role of polysaccharides and diatom exudates in the redox cycling of Fe and the photoproduction of hydrogen peroxide in coastal seawaters

“…(Tanaka et al, 1971). Transparent exopolymer particles (TEP), which are rich in acidic polysaccharides, are ubiquitous in the surface ocean (Passow, 2002), and have been shown to bind 234 Th (Passow et al, 2006) and are therefore a prime candidate to also bind iron.…”