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

Abstract

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

High particulate organic carbon export during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean

“…Multiple factors may have played a role in shaping the C/Th and N/Th ratios, such as particle source, sinking velocity, Th speciation and remineralisation of C, N and 234 Th associated with sinking particles . Buesseler (1998) and Buesseler et al (2006) hypothesised that high C/Th ratios encountered in productive regimes and high latitude areas may be related to the dominance of sinking of large cells, such as diatoms. This was explained by the fact that diatoms have a high volume relative to surface area (V:SA), and hence high C/Th ratios because C varies as a function of volume, whereas Th adsorption varies as a function of surface area.…”

“…This type of array was deployed a total of 11 times, including 7 deployments at the central station (C91, C98, C99, C114, C128, C136, C140) between 3 and 17 February. One of the cylinders for biogeochemical analyses was used after picking off swimmers under a binocular microscope: (i) one half was filtered through a pre-combusted QMA filter to analyse 234 Th, POC and PON on the same filter as recommended by Buesseler et al (2006); and (ii) the other half split was filtered using a pre-combusted GF/F filter to analyse POC and PON. The total POC and PON fluxes collected with the ST were determined as the average of the fluxes obtained from samples (i) and (ii).…”

“…Thus, we consider that the SS model gives the best estimate of 234 Th export in this study. Buesseler et al, 2006), are presented in Table 5.…”

Particle fluxes associated with mesoscale eddies in the Sargasso Sea

“…V = 0 in equation (Buesseler et al, 1992). This is an empirical approach that has inherent strengths and limitations that have been recently reviewed in some detail by Buesseler et al (2006). Briefly, variations in POC/ 234 Th can result from a variety of geochemical and biological processes, such as particle aggregation/disaggregation, changes to surface:volume ratios (i.e.…”

Latitudinal distributions of particulate carbon export across the North Western Atlantic Ocean

“…Basic questions such as how to accurately sample the particles that are responsible for that export remain enigmatic and controversial (i.e., Lampitt et al, 2008;Burd et al, 2010;Bishop et al, 2012). Besides the technical difficulties in particle collection, the questions of how and why C/ 234 Th ratios vary in marine systems have been raised by several studies for more than a decade (see review by Buesseler et al, 2006), with still no clear agreement. Nonetheless, one of the most common methods is to use large particles (> 50 μm) collected using in situ filtration pumps placed below the depth of the Z eu (usually standardized as 100-150 m), where C/ 234 Th variability is minimized and similar across techniques, e.g.…”