Important role of colloids in the cycling of 210Po and 210Pb in the ocean: Results from the East/Japan Sea
Introduction
Both 210Pb (half-life 22.3 years) and 210Po (half-life 138 days) belong to the 238U decay series. 210Pb is produced by 226Ra decay via short-lived daughters, including the chemically inert noble gas 222Rn (half-life 3.8 days). 210Po is produced by 210Pb decay via the short-lived intermediate 210Bi (half-life 5 days). Thus, in the upper ocean, 210Pb originates from both the atmosphere and in situ production, while most 210Po is produced by the in situ decay of 210Pb. Both 210Pb and 210Po are removed by adsorption onto particulate matter in seawater, but 210Po is more efficiently taken up and removed by biota (Fisher et al., 1983, Kadko, 1993, Wei and Murray, 1994, Kim, 2001, Stewart et al., 2005). This preferential uptake of 210Po by marine biota causes significant radioactive disequilibria between 210Pb and 210Po in the upper ocean (Bacon et al., 1988, Kadko, 1993, Kim, 2001).
Although 210Po removal is known to be associated with biological production in the water column, large deficiencies in 210Po have been observed in oligotrophic areas of the ocean, such as the East China Sea (Nozaki et al., 1990), the Sargasso Sea (Kim, 2001), and the northern South China Sea (Chung and Wu, 2005). Nozaki et al. (1990) have shown that such large deficiencies in the East China Sea are due to the focused input of atmospheric 210Pb rather than preferential scavenging of 210Po. However, Kim (2001) and Chung and Wu (2005) have suggested that such large deficiencies in 210Po in the oligotrophic ocean are likely to occur because of the effective uptake of 210Po by organisms such as cyanobacteria (i.e., Tricodesium), which are then transferred to the higher trophic levels through the food web. In the eutrophic ocean, however, 210Po may be released into the water through rapid decomposition or remineralization of organic matter, particularly below the mixed layer, resulting in a relatively small deficiency or even an excess (Kim, 2001).
Nevertheless, 210Pb and 210Po have been successfully used for studying particle scavenging and vertical fluxes (Cherry et al., 1975, Cochran et al., 1983, Fowler and Knauer, 1986, Nozaki et al., 1997, Masqué et al., 2002) and particulate organic carbon (POC) export (Shimmield et al., 1995, Friedrich and Rutgers van der Loeff, 2002, Murray et al., 2005, Stewart et al., 2010) in the upper ocean. However, there is no information on the role of colloids in the cycling of 210Pb and 210Po in the ocean although marine colloids are known to play an important role in the biogeochemical cycling of many particle-reactive nuclides (Guo and Santschi, 1997).
Thus, in this study, we have (1) determined colloidal 210Pb and 210Po activities in the open ocean, for the first time, (2) calculated the residence time of the truly dissolved, colloidal, and particulate phases of 210Po in the euphotic zone as possible proxies for other bio-reactive elements in the ocean, and (3) evaluated the biogeochemical factors controlling the partitioning and residence times of 210Po in the ocean. We used 10 kDa, instead of 1 kDa, cut-off size for colloidal separations in order to trace the turnover rate of 210Po in higher molecular weight colloidal fraction which turns over on timescale of <1 year, suitable for the half-life of 210Po. In general, lower molecular weight colloidal fraction (e.g., 1–10 kDa) turns over on timescales of >10 years (Guo and Santschi, 1997).
Section snippets
Study area
The East/Japan Sea (EJS) in the Northwest Pacific Ocean is a typical semi-closed marginal sea with a surface area of 1.01 × 106 km2 and a maximum depth of >3700 m. Because it has its own deep convection system (thermohaline conveyor belt), which is independent of the Pacific Ocean, the EJS has been regarded as a miniature of the global ocean in terms of deep water formation (Kim et al., 2001, Kim et al., 2002, Gamo et al., 2001). The EJS water is separated into colder water (North Korea Cold
Distributions of temperature, chlorophyll a, DOC, 210Pb, and 210Po
Temperatures decreased steeply from 26 to 1 °C in the surface layer (0–100 m) (Fig. 2). The mixed layer depth was within 10 m in the study region. The concentrations of chlorophyll a were in the range of 10–1130 ng/L in the surface layer. The highest concentrations of chlorophyll a were observed at depths between 25 and 50 m (Fig. 2). The concentrations of DOC in the upper 200-m layer were in the range of 64–86 μM (Fig. 2), which were similar to those in the world’s major oceans (Carlson and Ducklow,
Conclusions
The residence times of colloidal 210Po estimated using the net residence time model were in the range of 47–72 days, which were much longer than those of 234Th and HMW-DOM in the surface ocean. Such long colloidal residence times for 210Po, relative to 234Th indicate that Po, together with some proxy bio-reactive elements, turns over several times through HMW-DOM before they settle down from the euphotic zone. Thus, our results suggest that colloidal 210Po is potentially used as an excellent
Acknowledgements
We would like to thank I. T. Kim for helping with field sampling. This research was a part of the project titled “East Asian Seas Time series-I (EAST-I)” funded by The Ministry of Land, Transport and Maritime Affairs, Korea and The “National Research Foundation of Korea (NRF)” grant funded by The Korea government (MEST) (No. 2012-0006256). THK was partially supported by the BK21 scholarship from the School of Earth and Environmental Sciences, Seoul National University, Korea.
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