Does ²³⁴Th/²³⁸U disequilibrium provide an accurate record of the export flux of particulate organic carbon from the upper ocean?

Abstract

The magnitude of the flux of biogenic particulate organic carbon (POC) exported from the surface waters of the world ocean and remineralized at depth is critical to constraining models of the global carbon cycle, yet remains controversial. The use of upper ocean sediment traps is still one of the primary tools for determining this export flux, although trap fluxes have been shown to vary significantly because of hydrodynamic and sample collection biases. Over the past decade, 234 Th increasingly has been used as a tracer to estimate POC export from the euphotic zone by multiplying the depth-integrated 234 Th flux by the POC/ 234 Th ratio of sinking particles. The accuracy of this technique is highly dependent on the natural variability in the POC/ 234 Th ratio and 234 Th flux, yet the significance of this variability to estimates of POC export remains uncertain. Based on an analysis of new 234 Th and POC data from the Labrador Sea and a review of 25 previous independent field studies, we report that POC export fluxes can vary 2-10 times or more solely because of variability in the POC/ 234 Th ratio and procedures used to estimate the 234 Th flux. Recommended improvements include studies of the biological, chemical, and physical mechanisms controlling 234 Th-organic matter interactions in seawater; detailed comparisons of POC/ 234 Th ratios in size-fractionated and sediment trap material; increased spatial and temporal sampling density of 234 Th; and more standardized procedures to calculate the 234 Th export flux.

Estimates of micro-, nano-, and picoplankton contributions to particle export in the northeast Pacific

“…The accuracy of 234 Th as a tracer of POC export depends on the assumption that 234 Th and POC are sinking on the same particles, and therefore sinking at the same rate (Moran et al, 2003;Smith et al, 2006;Speicher et al, 2006;Burd et al, 2007;Brew et al, 2009). A high degree of correlation between the size-fractionated distributions of 234 Th and POC ( Fig.…”

“…Recent studies have challenged this idea, suggesting that small phytoplankton contribute significantly to POC export, possibly through aggregation and incorporation into fecal pellets (Richardson and Jackson, 2007;Amacher et al, 2009;Stukel and Landry, 2010;Lomas and Moran, 2011;Stukel et al, 2013). A better understanding of the controls on the relative importance of small phytoplankton in POC export is needed to refine our understanding of the magnitude and mechanisms controlling the biological pump, particularly as recent climate models predict an expansion of the oligotrophic gyres where small cells dominate (Irwin et al, 2006;Polovina et al, 2008;Morán et al, 2010).…”

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

“…Various methods and studies have shown the C / 234 Th ratio to vary from 1 to N 100 Amol dpm À 1 (all references to C / 234 Th will be presented in units of lmol dpm À1 , typically determined for total organic C and 234 Th decay corrected to the time of sampling; one dpm = 1 disintegration per minute = 1 / 60 Bq = 1 / 60 disintegration per second). There is continued discussion regarding the significance of this variability for the accuracy of the 234 Th flux approach (e.g., Buesseler, 1998;Moran et al, 2003Moran et al, , 2004BenitezNelson and Charette, 2004). For the application of 234 Th as a POC flux proxy, we do not need to know the C / 234 Th ratio on all particles, just the ones sinking out of surface waters.…”

“…Examples can be found in the Arabian Sea (Moran et al, 2003), and elsewhere (Table 1). An exception to this pattern are recent studies of the Gulf of Mexico, where a single large volume in situ pumping system was used, but separate samples for POC and 234 Th were collected using separate filtration rigs (membrane filters for C and cartridges for 234 Th;Guo et al, 2002a;Hung et al, 2004).…”

Global database of surface ocean particulate organic carbon export fluxes diagnosed from the <sup>234</sup>Th technique

“…In 234 Th studies generally advection effects are neglected, as shown in Morris et al (2007), with the exception of upwelling regions or areas of strong advection (Murray et al, 1992;Buesseler et al, 1998). Using the SS model from a single profile of 234 Th activity needs to be justified as we assume that the initial activity does not change with time .…”