Global database of ratios of particulate organic carbon to thorium-234 in the ocean: improving estimates of the biological carbon pump

Abstract

Abstract. The ocean's biological carbon pump (BCP) plays a major role in the global carbon cycle. A fraction of the photosynthetically fixed organic carbon produced in surface waters is exported below the sunlit layer as settling particles (e.g., marine snow). Since the seminal works on the BCP, global estimates of the global strength of the BCP have improved but large uncertainties remain (from 5 to 20 Gt C yr−1 exported below the euphotic zone or mixed-layer depth). The 234Th technique is widely used to measure the downward export of particulate organic carbon (POC). This technique has the advantage of allowing a downward flux to be determined by integrating the deficit of 234Th in the upper water column and coupling it to the POC∕234Th ratio in sinking particles. However, the factors controlling the regional, temporal, and depth variations of POC∕234Th ratios are poorly understood. We present a database of 9318 measurements of the POC∕234Th ratio in the ocean, from the surface down to >5500 m, sampled on three size fractions (∼>0.7 µm, ∼1–50 µm, ∼>50 µm), collected with in situ pumps and bottles, and also from bulk particles collected with sediment traps. The dataset is archived in the data repository PANGAEA® under https://doi.org/10.1594/PANGAEA.911424 (Puigcorbé, 2019). The samples presented in this dataset were collected between 1989 and 2018, and the data have been obtained from published papers and open datasets available online. Unpublished data have also been included. Multiple measurements can be found in most of the open ocean provinces. However, there is an uneven distribution of the data, with some areas highly sampled (e.g., China Sea, Bermuda Atlantic Time Series station) compared to some others that are not well represented, such as the southeastern Atlantic, the south Pacific, and the south Indian oceans. Some coastal areas, although in a much smaller number, are also included in this global compilation. Globally, based on different depth horizons and climate zones, the median POC∕234Th ratios have a wide range, from 0.6 to 18 µmol dpm−1.

Transient Enhancement and Decoupling of Carbon and Opal Export in Cyclonic Eddies

“…Calculated 234 Th fluxes are converted to other elemental fluxes (e.g., POC and opal) by multiplying the element to 234 Th ratio on sinking particles (Benitez‐Nelson et al, 2007; Maiti et al, 2008; Weinstein & Moran, 2005). Here bottle POC (and opal) / 234 Th ratios were measured in the upper 100 m of the water column (Figure S6) and showed similar patterns to previous observations (Black et al, 2018; Buesseler et al, 2006; Cai et al, 2006; Puigcorbé et al, 2020): high and variable at the surface and reaching more constant ratios (1.6–5.3 μmol C dpm −1 with a mean of 3.5 ± 0.97 μmol C dpm −1 ) by 100 m.…”

High-resolution spatial and temporal measurements of particulate organic carbon flux using thorium-234 in the northeast Pacific Ocean during the EXport Processes in the Ocean from RemoTe Sensing field campaign

“…In most prior studies, C/Th ratios decrease with depth due to the preferential heterotrophic utilization of C as a food source (e.g., see reviews by Buesseler et al, 2006;Puigcorbé et al, 2020). The high and invariant C/Th ratio with depth in the >51 mm pump samples suggests pervasive contamination by zooplankton swimmers.…”

“…Low particulate C/Th ratios are hypothesized to be due to some combination of efficient recycling of organic matter and/or smaller particle sizes (Buesseler et al, 2006). In contrast, the higher C/Th ratios (>10 mmol dpm À1 ) typical of the Southern Ocean and phytoplankton blooms are often associated with larger diatoms species and high POC export (Buesseler et al, 2001(Buesseler et al, , 2006Puigcorbé et al, 2020). These larger size classes of plankton were not dominant at Station P during EXPORTS, nor are they common at other sites with low BCP efficiencies (Puigcorbé et al, 2015).…”

Review of the analysis of 234Th in small volume (2–4 L) seawater samples: improvements and recommendations

“…The deficit of 234 Th ( 234 Th < 238 U) can therefore be used to measure the net flux of 234 Th on sinking particles out of a given depth interval. The 234 Th flux is subsequently used to derive a C flux by multiplying a known C: 234 Th ratio of sinking particles, often collected by sediment traps or in situ pumps [e.g., [5][6][7][8]. This approach has since been expanded to other elements and compounds, such as particle reactive organic contaminants and trace metals [e.g., [9][10][11][12][13].…”