No abstract

The years following the Fukushima Dai-ichi nuclear power plant (FDNPP) accident, the distribution of (90)Sr in seawater in the coast off Japan has received limited attention. However, (90)Sr is a major contaminant in waters accumulated within the nuclear facility and in the storage tanks. Seawater samples collected off the FDNPP in September 2013 showed radioactive levels significantly higher than pre-Fukushima levels within 6 km off the FDNPP. These samples, with up to 8.9 ± 0.4 Bq·m(-3) for (90)Sr, 124 ± 3 Bq·m(-3) for (137)Cs, and 54 ± 1 Bq·m(-3) for (134)Cs, appear to be influenced by ongoing releases from the FDNPP, with a characteristic (137)Cs/(90)Sr activity ratio of 3.5 ± 0.2. Beach surface water and groundwater collected in Sendai Bay had (137)Cs concentrations of up to 43 ± 1 Bq·m(-3), while (90)Sr was close to pre-Fukushima levels (1-2 Bq·m(-3)). These samples appear to be influenced by freshwater inputs carrying a (137)Cs/(90)Sr activity ratio closer to that of the FDNPP fallout deposited on land in the spring of 2011. Ongoing inputs of (90)Sr from FDNPP releases would be on the order of 2.3-8.5 GBq·d(-1) in September 2013, likely exceeding river inputs by 2-3 orders of magnitude. These results strongly suggest that a continuous surveillance of artificial radionuclides in the Pacific Ocean is still required.

Atmospheric deposition is an important input route of trace elements (TEs) to the global ocean. As atmospheric inputs impact phytoplankton community health and dynamics, atmospheric TE fluxes, and in particular atmospheric iron fluxes, are a key component of marine biogeochemical models. Trace element concentrations were determined in dry (aerosols) and wet (precipitation) deposition samples from the North Atlantic, north of 40°N, during the GEOVIDE cruise (GEOTRACES cruise GA01) in May/June 2014. Atmospheric aerosol loading in the study region was low (~2-500 ng m -3 ) throughout the cruise, as inferred from the very low aerosol Ti concentrations determined (0.0084-1.9 ng m -3 ). Wet deposition appeared to be of roughly equal or greater importance than dry deposition to the total depositional flux of TEs, which is consistent with other regions of the Atlantic Ocean outside of the influence of the Saharan plume.It can be challenging to convert aerosol chemical composition data into reliable flux estimates, due to the uncertainties associated with the parameterisation of dry deposition velocity, and precipitation rate.Therefore, the goal of this study was to compare TE flux estimates derived from two different techniques: (1) the traditional approach of summed wet and dry deposition TE fluxes, using concentration data, precipitation rates, and dry deposition velocities and, (2) using the inventory of the cosmogenic radioisotope beryllium-7 ( 7 Be) in the upper ocean as a proxy for atmospheric deposition. These two approaches yielded TE flux estimates that were in excellent agreement (within one standard deviation) for about half of the TEs under investigation. However, for the remaining TEs differences between the flux estimates ranged from two to forty times, with the traditional approach generally being the higher of the two estimates. Therefore, factors that may contribute to this variation, such as differences in the timescale of integration and selection of representative deposition velocities and precipitation rates, are discussed. Our results suggest that the 7 Be approach continues to show promise in this application, particularly in regions where precipitation samples cannot be routinely collected.

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Abstract. The island of South Georgia is situated in the iron (Fe)-depleted Antarctic Circumpolar Current of the Southern Ocean. Iron emanating from its shelf system fuels large phytoplankton blooms downstream of the island, but the actual supply mechanisms are unclear. To address this, we present an inventory of Fe, manganese (Mn), and aluminium (Al) in shelf sediments, pore waters, and the water column in the vicinity of South Georgia, alongside data on zooplankton-mediated Fe cycling processes, and provide estimates of the relative dissolved Fe (DFe) fluxes from these sources. Seafloor sediments, modified by authigenic Fe precipitation, were the main particulate Fe source to shelf bottom waters as indicated by the similar Fe / Mn and Fe / Al ratios for shelf sediments and suspended particles in the water column. Less than 1 % of the total particulate Fe pool was leachable surface-adsorbed (labile) Fe and therefore potentially available to organisms. Pore waters formed the primary DFe source to shelf bottom waters, supplying 0.1-44 µmol DFe m −2 d −1 . However, we estimate that only 0.41 ± 0.26 µmol DFe m −2 d −1 was transferred to the surface mixed layer by vertical diffusive and advective mixing. Other trace metal sources to surface waters included glacial flour released by melting glaciers and via zooplankton egestion and excretion processes. On average 6.5 ± 8.2 µmol m −2 d −1 of labile particulate Fe was supplied to the surface mixed layer via faecal pellets formed by Antarctic krill (Euphausia superba), with a further 1.1 ± 2.2 µmol DFe m −2 d −1 released directly by the krill. The faecal pellets released by krill included seafloor-derived lithogenic and authigenic material and settled algal debris, in addition to freshly ingested suspended phytoplankton cells.The Fe requirement of the phytoplankton blooms ∼ 1250 km downstream of South Georgia was estimated as 0.33 ± 0.11 µmol m −2 d −1 , with the DFe supply by horizontal/vertical mixing, deep winter mixing, and aeolian dust estimated as ∼ 0.12 µmol m −2 d −1 . We hypothesize that a sub-