Thorium-234 as a tracer of spatial, temporal and vertical variability in particle flux in the North Pacific

Abstract

An extensive 234 Th data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical Transport in the Global Ocean) study. Total 234 Th:238 U activity ratios near 1.0 indicated low particle fluxes at ALOHA, while 234 Th: 238 U ~0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial variability was large at both sites-even greater than seasonal variability as reported in prior studies. This variability in space and time confounds the use of single profiles of 234 Th for sediment trap calibration purposes. At K2, there was a decrease in export flux and increase in 234 Th activities over time associated with the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux predictions. This variability in space and time confounds the use of single profiles of 234 Th for sediment trap calibration purposes. High vertical resolution profiles show narrow layers (20-30 m) of excess 234 Th below the deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at depth that may be missed with standard sampling for 234 Th and/or with sediment traps. Also, the application of 234 Th as POC flux tracer relies on accurate sampling of particulate POC/ 234 Th ratios and here the ratio is similar on sinking particles and mid-sized particles collected by in-situ filtration (>10-50 µm at ALOHA and >5-350 µm at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of 234 Th and nutrients is used to demonstrate that while coupled during export, their ratios in the water column will vary with time and depth after export. Overall these 234 Th data provide a detailed view into particle flux and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and 10's to 100's km at a resolution that is difficult to obtain with other methods.

Seasonal and interannual variability of the marine bacterioplankton community throughout the water column over ten years

“…Currents in the region vary with both depth and season, (Hickey, 1991(Hickey, , 1992 and likely cause spatial separation between the locations where particles are generated and where they settle. Particle flux variability across space (Buesseler et al, 2009) may interact with these seasonal currents, resulting in seasonally variable particle delivery to the sea floor at SPOT. Water below the sill of the San Pedro basin at 750 m is primarily trapped (Berelson, 1991), suggesting that currents alone seem like an unlikely explanation for the greater seasonality at 890 m than at intermediate depths.…”

Advances in measurements of particle cycling and fluxes in the ocean

“…In recent years, smaller volume methods that require as little as 1 L, but typically 4 L , were developed and have since been applied widely (Buesseler et al, 2008a;Buesseler et al, 2008b;Cai et al, 2008;Martin et al, 2011). This method requires knowledge of the 238 U activity in seawater.…”

“…As export flux is a product of complex interactions between mixing, phytoplankton, and zooplankton, it is likely that the scales of variability of export are of a 19 similar scale to phytoplankton or smaller. Results from studies using 234 Th and underwater cameras have hinted at this smaller scale variability of export (Guidi et al, 2007;Buesseler et al, 2008b; Guidi et al, 2008). A recent study modeling export flux and measurements of 234 Th has demonstrated the heterogeneity of export on scales of 100 km and less .…”

“…The mismatch of the temporal and spatial scales over which the two methods integrate has made this approach difficult, however. Lagrangian and multi-week sampling efforts are required to properly calibrate sediment traps Buesseler et al, 2008b;Lampitt et al, 2008; Cochran et al, 2009). In addition a calibration factor determined for 234 Th may not be the same as the calibration factor for other particulate species.…”

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

“…Numerous processes can impact POC/ 234 Th ratios in the ocean including particle surface-area-to-volume ratios , solution chemistry issues (Guo et al, 2002;Hung et al, 2004), the chemical composition of particles and their affinity for 234 Th (Szlosek et al, 2009), POC assimilation by food webs (Buesseler and Boyd, 2009), particle aggregation (Burd et al, 2000) and fragmentation (Maiti et al, 2010) and Th decay (Cai et al, 2006). …”