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Dimethyl sulfide (DMS) production in the northern Arctic Ocean has been considered to be minimal because of high sea ice concentration and extremely low productivity. However, we found DMS concentration (1-33 nM) in melt ponds on sea ice at a very high latitude (78°N) in the central Arctic Ocean to be up to ten times that in the adjacent open ocean (<3 nM). We divided melt ponds into three categories: freshwater melt ponds, brackish melt ponds, and open saline melt ponds. Melt ponds from each category had different formation mechanisms and associated DMS contents. Closed brackish ponds (salinity of >20) had particularly high DMS concentration. Water in brackish ponds was mixed with open ocean water in the past via a hole at the bottom of the floe that kept the pond open to the ocean; therefore, unlike freshwater melt ponds, brackish ponds became sites of DMS accumulation. Our results suggest that continuous increase in melt pond coverage on Arctic sea ice could considerably impact future Arctic climate as well as enhancing DMS concentration in the Arctic atmosphere.

To estimate the atmospheric deposition flux of 210Pb in the equatorial western Indian Ocean, we determined the dissolved (<0.45 μm) and particulate 210Pb (>0.45 μm) in the water column. In addition, we calculated the atmosphere-derived dissolvable Pb in seawater using the budget of 210Pb. The dissolved 210Pb and total 210Pb were higher in the surface layer and, overall, showed a decreasing distribution with depth. In particular, radioactive 210Pb activities in the surface-to-upper layer (<1000 m depth) were 1.5 to 2 times higher than those reported in the 1970s (in nearby regions), suggesting that there has been additional 210Pb input in recent years. Based on the mass balance of the total 210Pb budget in the water column, we estimated the atmospheric deposition flux of 210Pb and the residence time of Pb for the first time in this region. The atmospheric deposition flux of 210Pb was estimated to be 0.1–0.5 dpm cm−2 yr−1, and these values agreed with the general global estimations for the major oceans (0.1–0.7 dpm cm−2 yr−1). Considering the residence time of 210Pb (29–41 years) in the water column (estimated from the 210Pb inventory and 234Th-based Pb scavenging rate), the atmospheric input of seawater-dissolvable Pb was quantified to be 0.08–0.1 nmol cm−2 yr−1, which is about eight times higher than the estimated input in the early 1990s in the region. Therefore, these results imply that radioactive 210Pb could be a useful tracer for quantifying Pb flux in seawater. [ABSTRACT FROM AUTHOR]

Highlights: What are the main findings? The first radiocesium and Pu isotopes data in Korean seas were present in 2011–2012; Higher 134Cs were observed in Korean seas immediately after the Fukushima accident; Fukushima-derived nuclides were introduced to Korean seas by the atmosphere for up to 2 months. What is the implication of the main finding? The residence times of 137Cs in Korean seas water is estimated to be 8–25 years; The dataset could be used to predict the dispersion of anthropogenic pollutant; The dataset could also be used to validate the model for future NPP accident around the Korean seas. The distributions of artificial radionuclides, radiocesium (134Cs and 137Cs) and plutonium isotopes (238Pu and 239+240Pu), in the surface water around the Korean seas (East/Japan Sea and Yellow Sea) in 2011–2012 and in three sections in the North Pacific between 2011 and 2014 were examined. The 137Cs activities in the surface water in the Korean seas in 2011 (immediately after the Fukushima nuclear power plant (NPP) accident on 17 March 2011) were comparable or not significantly different relative to those in 2010 and 2012. However, 134Cs, which had been not detected in the study area before the Fukushima accident (under the detection limit of 0.1 mBq kg−1 level), were detected rapidly in 2011 after the accident (in about 60% of the 72 samples) and gradually disappeared due to their short half-life (t1/2 = 2.06 years) in 2012 (detected in about 16% of the 24 samples). In addition, the highest activities of radiocesium and Pu isotopes appeared locally in some stations of the Korean Strait region (located between Korea and Japan) within 1–2 months immediately after the accident. This suggests that the radioactive nuclides released immediately after the Fukushima accident were significantly introduced through the atmosphere, based on recent studies conducted in neighboring areas. We also showed that the spatial distribution of radiocesium in the North Pacific moved eastward from 2012 to 2014, and we attempted to quantify the residence time of radiocesium (137Cs) in the Korean seas based on the long-term (tens of years scale) temporal trends of 137Cs activity data, which have been collected since the 1960s and 1970s. The estimated retention time of 137Cs in the East/Japan Sea and Yellow Sea were 25 ± 0.6 and 8.0 ± 0.1 years, respectively. These results are expected to be used as a preliminary study for a potential future event of a marine radioactive accident (which, of course, cannot be predicted) and as basic data for predicting the influences of radionuclide releases in the ocean. [ABSTRACT FROM AUTHOR]

Oceanic dimethyl sulfide (DMS) is of interest due to its critical influence on atmospheric sulfur compounds in the marine atmosphere and its hypothesized significant role in global climate. High-resolution shipboard underway measurements of surface seawater DMS and the partial pressure of carbon dioxide ( pCO2) were conducted in the Atlantic Ocean and Indian Ocean sectors of the Southern Ocean (SO), the southeast Indian Ocean, and the northwest Pacific Ocean from February to April 2014 during the 30th Chinese Antarctic Research Expedition. The SO, particularly in the region south of 58°S, had the highest mean surface seawater DMS concentration of 4.1 ± 8.3 nM (ranged from 0.1 to 73.2 nM) and lowest mean seawater pCO2 level of 337 ± 50 μatm (ranged from 221 to 411 μatm) over the entire cruise. Significant variations of surface seawater DMS and pCO2 in the seasonal ice zone (SIZ) of SO were observed, which are mainly controlled by biological process and sea ice activity. We found a significant negative relationship between DMS and pCO2 in the SO SIZ using 0.1° resolution, [DMS] seawater = −0.160 [ pCO2] seawater + 61.3 ( r2 = 0.594, n = 924, p < 0.001). We anticipate that the relationship may possibly be utilized to reconstruct the surface seawater DMS climatology in the SO SIZ. Further studies are necessary to improve the universality of this approach. [ABSTRACT FROM AUTHOR]