Interactions of thorium isotopes with colloidal organic matter in oceanic environments

Guo, Laodong; Santschi, Peter H.; Baskaran, M.

doi:10.1016/s0927-7757(96)03723-5

Cited by 66 publications

(36 citation statements)

References 36 publications

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“…Total recovery rate was 98% for the carbon and 90% for 234 Th, which lie within the range found in other studies (Dai and Benitez-Nelson, 2001;Guo et al, 1997). The directly measured colloidal fraction was quite small for both OC (6% of the b 0.2 Am fraction) and 234 Th (5% of the b 0.2 Am fraction), which lies within the lower end of the range found in other studies (Dai and BenitezNelson, 2001;Guo et al, 1997). The ultrafiltered fraction contributed 88% and 42% OC and 234 Th, respectively to the dissolved (b 0.2 Am) fraction.…”

Section: Methodological Concerns: Mass Balance For Ultrafiltrationsupporting

confidence: 89%

“…Colloidal matter using both cut-offs are included in the average values as there was no significant difference in the fractionation whether a 10 or 3 kDa filter membrane was used. Total recovery rate was 98% for the carbon and 90% for 234 Th, which lie within the range found in other studies (Dai and Benitez-Nelson, 2001;Guo et al, 1997). The directly measured colloidal fraction was quite small for both OC (6% of the b 0.2 Am fraction) and 234 Th (5% of the b 0.2 Am fraction), which lies within the lower end of the range found in other studies (Dai and BenitezNelson, 2001;Guo et al, 1997).…”

Section: Methodological Concerns: Mass Balance For Ultrafiltrationsupporting

confidence: 88%

“…A small fraction of the OC (4% of total) and a large fraction of 234 Th (42% of total) were subsequently recovered in the acid rinses of the membranes and containers. Total losses to the filter membranes and containers between 20% and 35% are common (Dai and Benitez-Nelson, 2001;Guo et al, 1997). The source of this fraction can't be determined unequivocally.…”

Section: Methodological Concerns: Mass Balance For Ultrafiltrationmentioning

confidence: 99%

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Organic carbon to 234Th ratios of marine organic matter

et al. 2006

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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.

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Section: Methodological Concerns: Mass Balance For Ultrafiltrationsupporting

confidence: 89%

Section: Methodological Concerns: Mass Balance For Ultrafiltrationsupporting

confidence: 88%

Section: Methodological Concerns: Mass Balance For Ultrafiltrationmentioning

confidence: 99%

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Organic carbon to 234Th ratios of marine organic matter

et al. 2006

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“…The ratios in this very small sub-micron size range seemed to fit a pattern of decreasing C / 234 Th with particle size (Burd et al, 2000;Guo et al, 1997Guo et al, , 2002a. A model that included uptake of Th by small and abundant colloids and production of larger aggregates using a particle aggregation model, predicted a decrease in C / 234 Th with size for the smallest size classes and relatively invariant or slowly decreasing C / 234 Th in the 1-100 Am range (Burd et al, 2000).…”

Section: Overview Of Processes Controlling the C / 234 Th Ratio Of Mamentioning

confidence: 86%

“…Other data sets followed that measured C / 234 Th in colloidal size classes using cross flow filtration (e.g., Moran and Buesseler, 1992;Guo et al, 1997;replotted in Santschi et al, 2006-this volume) or small volume filtration for the smaller particle size classes. The ratios in this very small sub-micron size range seemed to fit a pattern of decreasing C / 234 Th with particle size (Burd et al, 2000;Guo et al, 1997Guo et al, , 2002a.…”

Section: Overview Of Processes Controlling the C / 234 Th Ratio Of Mamentioning

confidence: 99%

An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of 234Th as a POC flux proxy

et al. 2006

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Thorium-234 is increasingly used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean. This requires determination of both the 234 Th activity distribution (in order to calculate 234 Th fluxes) and an estimate of the C / 234 Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C / 234 Th variability, results obtained using a single sampling method show the most predictable behavior. For example, in most studies that employ in situ pumps to collect size fractionated particles, C / 234 Th either increases or is relatively invariant with increasing particle size (size classes N 1 to 100s Am). Observations also suggest that C / 234 Th decreases with depth and can vary significantly between regions (highest in blooms of large diatoms and highly productive coastal settings). Comparisons of C fluxes derived from 234 Th show good agreement with independent estimates of C flux, including mass balances of C and nutrients over appropriate space and time scales (within factors of 2-3). We recommend sampling for C / 234 Th from a standard depth of 100 m, or at least one depth below www.elsevier.com/locate/marchem the mixed layer using either large volume size fractionated filtration to capture the rarer large particles, or a sediment trap or other device to collect sinking particles. We also recommend collection of multiple 234 Th profiles and C / 234 Th samples during the course of longer observation periods to better sample temporal variations in both 234 Th flux and the characteristic of sinking particles. We are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and expect the utility of this tracer to increase, not just for upper ocean C fluxes but for other elements and processes deeper in the water column. D

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Marine Colloids

Santschi

2004

Water Encyclopedia

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Aquatic colloids, nanoparticles and macromolecules in the 1 nm to 1 µm size range, are characterized by large interfacial energies and scatter light. Colloids are most often separated from seawater through ultrafiltration. They are characterized by physical techniques, such as diffusion, permeation, or light scattering, and can be imaged by atomic force and electron microscopy. Chemically, they are composed mostly of organic macromolecules, which make up 20–30% of marine dissolved organic carbon (DOC). Marine colloidal organic matter (COM) contains biologically resistant heteropolycondensations, such as humic‐type molecules (geopolymers), and products of carbon fixation and consumption such as exopolymeric substances (EPS) excreted by microorganisms. Both geopolymers and biopolymers are amphiphilic with charged and uncharged groups, and contain strong metal‐binding ligands, rendering them metal‐ and radionuclide sequestering agents. Therefore, colloids in the ocean can act as vectors of metal and radionuclide removal and of solubilization, depending on the biomolecule, as well as modifiers of metal bioavailability in delicately balanced marine ecosystems. In addition, marine colloids are also important in their response to global change, as they enhance the sinking flux of carbon, and thus, play a role in global carbon cycling.

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Interactions of thorium isotopes with colloidal organic matter in oceanic environments

Cited by 66 publications

References 36 publications

Organic carbon to 234Th ratios of marine organic matter

Organic carbon to 234Th ratios of marine organic matter

An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of 234Th as a POC flux proxy

Marine Colloids

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