Pinghe Cai

Low export flux of particulate organic carbon in the central Arctic Ocean as revealed by 234Th:238U disequilibrium

Alexander von Humboldt Foundation; National Basic Research Program (973 Program) of China [2009CB421203]; International Bureau of the Federal Ministry of Education and Research of Germany [IB-CHN 06/018]

A high‐resolution study of particle export in the southern South China Sea based on 234Th:238U disequilibrium

During a spring intermonsoon cruise in 2004, depth profiles of total and particulate Th-234 in the upper 100 m were collected at 36 stations in the southern South China Sea (SCS), covering a surface area of similar to 1.0 x 10(6) km(2). Thorium-234 was sampled by using a modified small-volume MnO2 co-precipitation technique, which allows mapping the Th-234 distribution with a high spatial resolution. A stratified structure of Th-234/U-238 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 Th-234 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-234 fluxes combined with measurements of the POC/Th-234 ratio on suspended particles. The POC export for this region varied from a low of -10.7 +/- 1.5 mmolC m(-2) d(-1) to a high of 12.6 +/- 1.1 mmolC m(-2) d(-1), with an average of 3.8 +/- 4.0 mmolC m(-2) d(-1). 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 similar to 7 degrees N 116 degrees E to similar to 10 degrees N 111 degrees E. This geographic distribution is consistent with the overall surface circulation pattern of the southern SCS in this season.

Cyclone-driven deep sea injection of freshwater and heat by hyperpycnal flow in the subtropics

China (973 Program) [2009CB421200]; Program of Introducing Talents of Discipline to Universities [B07034]; Academia Sinica Thematic Program AFOBi, Taiwan [NSC 98-2116-M-001-005]

Role of particle stock and phytoplankton community structure in regulating particulate organic carbon export in a large marginal sea

In this study, we utilize 234Th/238U disequilibrium to determine particulate organic carbon (POC) export from the euphotic zone in the South China Sea. Depth profiles of 234Th, 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 234Th fluxes from the euphotic zone fall in the range of 528−1550, 340−2694, and 302–2647 dpm m−2 d−1 for the coastal, shelf, and basin regimes, respectively. In these regimes, POC/234Th 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 μmol dpm−1, 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−2 d−1, 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 (R2u2009=u20090.73, pu2009<u20090.005) 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 (R2u2009=u20090.54, pu2009<u20090.0001) 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 (R2u2009=u20090.26, pu2009<u20090.0001) between POC export flux and the fraction of haptophytes and prasinophytes that are typically <u20095 μm 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.

On the importance of the decay of 234Th in determining size-fractionated C/234Th ratio on marine particles

proxy for the export of particulate organic carbon (POC) from the upper ocean. Previous studies have shown that POC/ 234 Th ratios may vary by as much as two orders of magnitude, but the mechanism for this variability remains poorly understood. In this study, POC/ 234 Th and POC/ 228 Th ratios in three particle size classes were examined for samples collected from the upper 500 m at a station in the South China Sea. Our results indicate that changes in the POC/ 234 Th ratio with particle size at a given depth are mostly driven by simple decay of 234 Th. In fact, various trends in the POC/ 234 Th ratio versus particle size are explained by a combination of 234 Th decay and the preferential remineralization of POC relative to Th during particle aggregation. We also observed a trend of decreasing 234 Th/ 228 Th ratios with particle size. This trend is consistent with results expected from a Brownian-pumping model. Citation: Cai, P., M. Dai, W. Chen, T. Tang, and K. Zhou (2006), On the importance of the decay of 234 Th in determining size-fractionated C/ 234 Th ratio on marine particles, Geophys. Res.

Shelf‐basin exchange times of Arctic surface waters estimated from 228Th/228Ra disequilibrium

The transpolar drift is strongly enriched in 228Ra accumulated on the wide Arctic shelves with subsequent rapid offshore transport. We present new data of Polarstern expeditions to the central Arctic and to the Kara and Laptev seas. Because 226Ra activities in Pacific waters are 30% higher than in Atlantic waters, we correct 226Ra for the Pacific admixture when normalizing 228Ra with 226Ra. The use of 228Ra decay as age marker critically depends on the constancy in space and time of the source activity, a condition that has not yet adequately been tested. While 228Ra decays during transit over the central basin, ingrowth of 228Th could provide an alternative age marker. The high 228Th/228Ra activity ratio (AR = 0.8–1.0) in the central basins is incompatible with a mixing model based on horizontal eddy diffusion. An advective model predicts that 228Th grows to an equilibrium AR, the value of which depends on the scavenging regime. The low AR over the Lomonosov Ridge (AR = 0.5) can be due to either rapid transport (minimum age without scavenging 1.1 year) or enhanced scavenging. Suspended particulate matter load (derived from beam transmission and particulate 234Th) and total 234Th depletion data show that scavenging, although extremely low in the central Arctic, is enhanced over the Lomonosov Ridge, making an age of 3 years more likely. The combined data of 228Ra decay and 228Th ingrowth confirm the existence of a recirculating gyre in the surface water of the eastern Eurasian Basin with a river water residence time of at least 3 years.

How accurate are 234Th measurements in seawater based on the MnO2‐impregnated cartridge technique?

In the past decade, a MnO2-impregnated cartridge technique has been widely used to extract 234Th 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 MnO2 cartridges. In this study, an intercalibration between the cartridge technique and a small-volume MnO2 coprecipitation technique was carried out to test this assumption. We demonstrated that the collection efficiency for 234Th could be substantially overestimated by the MnO2 cartridge technique. This may be the result of organic complexation of a significant portion of 234Th in seawater, causing this fraction of Th to pass through the MnO2 cartridges. The overestimate in collection efficiency may explain the deep-water 234Th 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 234Th-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.

Reply to comment by Chin‐Chang Hung et al. on “How accurate are 234Th measurements in seawater based on the MnO2‐impregnated cartridge technique?”

[1] We disagree that the comparison of the smallvolume MnO2 coprecipitation method and the MnO2 cartridge method of Hung et al. [2008] is adequate. This is because at least some of the small-volume Th data used for their comparison relied on an early protocol of the smallvolume method that did not include a recovery correction. Thus results are potentially biased. New data from the South China Sea confirm our earlier conclusion that the extraction efficiency for Th is substantially overestimated by the cartridge method. We therefore restate that the small-volume Thmethod that includes a recovery correction is advantageous over the cartridge technique and is recommended to more accurately quantify Th activities used to constrain the export of particulate organic carbon in the upper ocean. [2] The comment by Hung et al. [2008] argued that the analytical procedures applied by Cai et al. [2006a] are not typical of other workers and that previous applications of the MnO2 cartridge method have found it to give good agreement with U in deep water and to agree with Th measured on small volume samples. They attempted to suggest that the two methods yield comparable results, within errors, when properly applied. We restate that the analytical procedures adopted by Cai et al. [2006a] is preferable by providing additional experimental data. Furthermore, we point out that the comparison of the small-volume MnO2 coprecipitation method with the MnO2 cartridge method of Hung et al. [2008] is inadequate. This is because at least some of the small-volume Th data cited by the authors were based on an early protocol of the method that did not include a recovery correction and thus could be biased (see below). Therefore the G Geochemistry Geophysics Geosystems

Mercury flux from salt marsh sediments: Insights from a comparison between 224Ra/228Th disequilibrium and core incubation methods

In aquatic environments, sediments are the main location of mercury methylation. Thus, accurate quantification of methylmercury (MeHg) fluxes at the sediment-water interface is vital to understanding the biogeochemical cycling of mercury, especially the toxic MeHg species, and their bioaccumulation. Traditional approaches, such as core incubations, are difficult to maintain at in-situ conditions during assays, leading to over/underestimation of benthic fluxes. Alternatively, the 224Ra/228Th disequilibrium method for tracing the transfer of dissolved substances across the sediment-water interface, has proven to be a reliable approach for quantifying benthic fluxes. In this study, the 224Ra/228Th disequilibrium and core incubation methods were compared to examine the benthic fluxes of both 224Ra and MeHg in salt marsh sediments of Barn Island, Connecticut, USA from May to August, 2016. The two methods were comparable for 224Ra but contradictory for MeHg. The radiotracer approach indicated that sediments were always the dominant source of both total mercury (THg) and MeHg. The core incubation method for MeHg produced similar results in May and August, but an opposite pattern in June and July, which suggested sediments were a sink of MeHg, contrary to the evidence of significant MeHg gradients between overlying water and porewater at the sediment-water interface. The potential reasons for such differences are discussed. Overall, we conclude that the 224Ra/228Th disequilibrium approach is preferred for estimating the benthic flux of MeHg and that sediment is indeed an important MeHg source in this marshland, and likely in other shallow coastal waters.