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[1] The loss of Arctic sea ice has accelerated in recent years. With the decline in sea ice cover, the Arctic Ocean biogeochemistry is undergoing unprecedented change. A key question about the changing Arctic Ocean biogeochemistry is concerning the impact of the shrinking sea ice cover on the particulate organic carbon (POC) export from the upper Arctic Ocean. Thus far, there are still very few direct measurements of POC export in the permanently ice-covered central Arctic Ocean. A further issue is that the magnitude of the POC export so far documented in this region remains controversial. During the ARK-XXII/2 expedition to the Arctic Ocean from 28 July to 7 October in 2007, we conducted a high-resolution study of POC export using 234 Th/ 238 U disequilibrium. Depth profiles of total 234 Th in the upper 200 m were collected at 36 stations in the central Arctic Ocean and its adjacent seas, i.e., the Barents Sea, the Kara Sea and the Laptev Sea. Samples were processed using a small-volume MnO 2 coprecipitation method with addition of a yield tracer, which resulted in one of the most precise 234 Th data sets ever collected. Thorium-234 deficit with respect to 238 U was found to be evident throughout the upper 100 m over the Arctic shelves. In comparison, 234 Th deficit was confined to the upper 25 m in the central Arctic Ocean. Below 25 m, secular equilibrium was approached between 234 Th and 238 U. The observed 234 Th deficit was generally associated with enhanced total chlorophyll concentrations, indicating that in situ production and export of biogenic particles are the main mechanism for 234 Th removal in the Arctic Ocean. Thorium-234-derived POC fluxes were determined with a steady state model and pump-normalized POC/ 234 Th ratios on total suspended particles collected at 100 m. Results showed enhanced POC export over the Arctic shelves. On average, POC export fluxes over the various Arctic shelves were 2.7 ± 1.7 mmol m −2 d −1 (the Barents Sea), 0.5 ± 0.8 mmol m −2 d −1 (the Kara Sea), and 2.9 ± 1.8 mmol m −2 d −1 (the Laptev Sea) respectively. In comparison, the central Arctic Ocean was characterized by the lowest POC export flux ever reported, 0.2 ± 1.0 mmol m −2 d −1 (1 standard deviation, n = 26). This value is very low compared to prior estimates and is also much lower than the POC export fluxes reported in other oligotrophic oceans. A ThE ratio ( 234 Th-derived POC export/primary production) of <6% in the central Arctic Ocean was estimated using the historical measurements of primary production. The low ThE ratio indicates that like other oligotrophic regimes, the central Arctic Ocean is characterized by low POC export relative to primary production, i.e., a tightly coupled food web. Our study strongly suggests that the current role of the central Arctic Ocean in C sequestration is still very limited. Meanwhile, this role might be altered because of global warming and future decline in sea ice cover.

We examined the impact of a cyclonic eddy and mode-water eddy on particle flux in the Sargasso Sea. The primary method used to quantify flux was based upon measurements of the natural radionuclide, 234 Th, and these flux estimates were compared to results from sediment traps in both eddies, and a 210 Po/ 210 Pb flux method in the mode-water eddy. Particulate organic carbon (POC) fluxes at 150m ranged from 1 to 4 mmol C m -2 d -1 and were comparable between methods, especially considering differences in integration times scales of each approach. Our main conclusion is that relative to summer mean conditions at the Bermuda Atlantic Time-series Study (BATS) site, eddy-driven changes in biogeochemistry did not enhance local POC fluxes during this later, more mature stage of the eddy life cycle (>6 months old). The absence of an enhancement in POC flux puts a constraint on the timing of higher POC flux events, which are thought to have caused the local O 2 minima below each eddy, and must have taken place >2 months prior to our arrival. The mode-water eddy did enhance preferentially diatom biomass in its center where we estimated a factor of 3 times higher biogenic Si flux than the BATS summer 2 average. An unexpected finding in the highly depth resolved 234 Th data sets are narrow layers of particle export and remineralization within the eddy. In particular, a strong excess 234 Th signal is seen below the deep chlorophyll maxima which we attribute to remineralization of 234 Th bearing particles. At this depth below the euphotic zone, de novo particle production in the euphotic zone has stopped, yet particle remineralization continues via consumption of labile sinking material by bacteria and/or zooplankton. These data suggest that further study of processes in ocean layers is warranted not only within, but below the euphotic zone.

In this study, we utilize 234 Th/ 238 U disequilibrium to determine particulate organic carbon (POC) export from the euphotic zone in the South China Sea. Depth profiles of 234 Th, total chlorophyll, pigments, and POC were collected during four cruises from August 2009 to May 2011, covering an entire seasonal cycle of spring, summer, autumn, and winter. The extensive data set that was acquired allows for an evaluation of the seasonal variability of upper ocean POC export and its controls in a large marginal sea. The results show that 234 Th fluxes from the euphotic zone fall in the range of 52821550, 34022694, and 302-2647 dpm m 22 d 21 for the coastal, shelf, and basin regimes, respectively. In these regimes, POC/ 234 Th ratios at the base of the euphotic zone fall in the range of 5.7-58.2, 4.6-44.0, and 2.5-15.5 lmol dpm 21 , respectively. Accordingly, for the coastal, shelf, and basin regimes, the mean POC export fluxes from the euphotic zone are 24.3, 18.3, and 6.3 mmolC m 22 d 21 , respectively. Seasonal variations in POC export flux are remarkable in the study area, and POC export peaks were generally observed in autumn. We use a simple linear regression (LLS) method to examine the correlation of POC export versus POC stock and versus plankton community structure. We found a strong correlation (R 2 5 0.73, p < 0.005) between POC export flux and the fraction of diatom in the coastal area, indicating that POC export flux in this province is driven by large phytoplankton, in particular, diatoms. In the shelf area, a relatively strong correlation (R 2 5 0.54, p < 0.0001) was noted for POC export flux and POC stock in the euphotic zone. This indicates that POC export flux in the South China Sea shelf is primarily controlled by POC stock. In contrast, in the South China Sea basin, we identified a weak but intriguing correlation (R 2 5 0.26, p < 0.0001) between POC export flux and the fraction of haptophytes and prasinophytes that are typically < 5 lm in size. This suggests that mechanisms controlling POC export flux in the South China Sea basin are complicated. However, small phytoplankton may play a significant role in controlling POC export flux since they dominate the phytoplankton community structure in this region. (2015), Role of particle stock and phytoplankton community structure in regulating particulate organic carbon export in a large marginal sea, Journal of Geophysical Research: Oceans PUBLICATIONS 1. Detailed information about the sampling stations is provided in Table A1. A total of 123 stations were occupied during the four survey cruises. Depth profiles of 234 Th, total chlorophyll, pigments, and POC were collected at 33 stations during the spring survey, 41 stations during the summer survey, 19 stations during the autumn survey, and 30 stations during the winter survey. For most of the stations in the basin (water depth > 200 m), water samples were collected throughout the upper 150 m at 5, 25, 50, 75, 100, 125, and

[1] During a spring intermonsoon cruise in 2004, depth profiles of total and particulate 234 Th in the upper 100 m were collected at 36 stations in the southern South China Sea (SCS), covering a surface area of $1.0 Â 10 6 km 2 . Thorium-234 was sampled by using a modified small-volume MnO 2 co-precipitation technique, which allows mapping the 234 Th distribution with a high spatial resolution. A stratified structure of 234 Th/ 238 U disequilibria was generally observed in the upper 100 m water column, suggesting that the euphotic zone of the southern SCS in this season can be separated into two layers: an upper layer with low export production rates and a lower layer with high export production rates. At the same time, we observed extensive zones of 234 Th excess within the euphotic layer, which is possibly due to intense remineralization of particulate matter. Particulate organic carbon (POC) export was estimated from a three-dimensional steady state model of Th ratio on suspended particles. The POC export for this region varied from a low of À10.7 ± 1.5 mmolC m À2 d À1 to a high of 12.6 ± 1.1 mmolC m À2 d À1 , with an average of 3.8 ± 4.0 mmolC m À2 d À1 . A negative flux of POC export is interpreted as the result of lateral input of particulate matter from nearby waters. Regional patterns in POC export show enhanced fluxes along the western and southern boundaries of the study region, and a ''tongue'' of low export extending northwestward from $7°N 116°E to $10°N 111°E. This geographic distribution is consistent with the overall surface circulation pattern of the southern SCS in this season.

[1] In the past decade, a MnO 2 -impregnated cartridge technique has been widely used to extract 234 Th from seawater. One of the inherent assumptions associated with this technique is that all Th species in the dissolved phase are subject to extraction by the MnO 2 cartridges. In this study, an intercalibration between the cartridge technique and a small-volume MnO 2 coprecipitation technique was carried out to test this assumption. We demonstrated that the collection efficiency for 234 Th could be substantially overestimated by the MnO 2 cartridge technique. This may be the result of organic complexation of a significant portion of 234 Th in seawater, causing this fraction of Th to pass through the MnO 2 cartridges. The overestimate in collection efficiency may explain the deep-water 234 Th deficit observed in some oceanographic settings (e.g., the Gulf of Mexico, the Middle Atlantic Bight, and the Gulf of Maine). Sensitivity tests show that using the cartridge technique can yield 234 Th-based particulate organic carbon export rates that are overestimated by factors of up to 10. Furthermore, the frequent observed disagreements between Th fluxes recorded by shallow sediment traps and estimated using the cartridge method may be ascribed, at least partially, to this methodological issue.

The recently developed 2-5 L small-volume MnO2 coprecipitation method for determining 234 Th in seawater has provided a new way to substantially increase, both temporally and spatially, the sampling resolution of 234 Th and 234 Th-based particulate organic carbon export estimates in the upper ocean. In this study, we further optimize the technique by reducing reagent quantities and the use of an additional water bath heating step. This optimization allows the filtration of the MnO2 precipitate onto a 25-mm diameter, 1.0 μm pore size QMA filter to be completed within < 30 minutes for unfiltered waters from the South China Sea. In addition, we have modified the MnO2 purification procedure to allow for alpha spetrometric measurements of 234 Th recoveries.Results from recovery experiments suggest that reagent amounts can be reduced to 0.0375 mg KMnO4 and 0.1 mg MnCl2•4H2O per liter of sample, while still maintaining high 234 Th recovery. This study further confirms that the addition of a yield monitor is necessary for the application of this small-volume method.

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