Mercury sources, distribution, and bioavailability in the North Pacific Ocean: insights from data and models.

Fish harvested from the Pacific Ocean are a major contributor to human methylmercury (MeHg) exposure. Limited oceanic mercury (Hg) data, particularly MeHg, has confounded our understanding of linkages between sources, methylation sites, and concentrations in marine food webs. Here we present methylated (MeHg and dimethylmercury (Me2Hg)) and total Hg concentrations from 16 hydrographic stations in the eastern North Pacific Ocean. We use these data in combination with information from previous cruises and coupled atmospheric-oceanic modeling results to better understand controls on Hg concentrations, distribution, and bioavailability. Total Hg concentrations (average 1.14 ± 0.38 pM) are elevated relative to previous cruises. Modeling results agree with observed increases and suggest that at present atmospheric Hg deposition rates, basin-wide Hg concentrations will double relative to circa 1995 by 2050. Methylated Hg accounts for up to 29% of the total Hg in subsurface waters (average 260 ± 114 fM). We observed lower ambient methylated Hg concentrations in the euphotic zone and older, deeper water masses, which likely result from decay of MeHg and Me2Hg when net production is not occurring. We found a significant, positive linear relationship between methylated Hg concentrations and rates of organic carbon remineralization (r2 = 0.66, p < 0.001). These results provide evidence for the importance of particulate organic carbon (POC) transport and remineralization on the production and distribution of methylated Hg species in marine waters. Specifically, settling POC provides a source of inorganic Hg(II) to microbially active subsurface waters and can also provide a substrate for microbial activity facilitating water column methylation.

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title: Mercury Exposure from Domestic and Imported Estuarine and Marine Fish in the U.S. Seafood Market
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title: Picomolar mercury measurements in seawater and other materials using stannous chloride reduction and two-stage gold amalgamation with gas phase detection
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title: Factors influencing the oxidation, reduction, methylation and demethylation of mercury species in coastal waters
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title: Mercury speciation in open ocean waters
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title: Sources and variations of mercury in tuna.
abstract: While the bulk of human exposure to mercury is through the consumption of marine fish, most of what we know about mercury methylation and bioaccumulation is from studies of freshwaters. We know little of where and how mercury is methylated in the open oceans, and there is currently a debate whether methylmercury concentrations in marine fish have increased along with global anthropogenic mercury emissions. Measurements of mercury concentrations in Yellowfin tuna caught off Hawaii in 1998 show no increase compared to measurements of the same species caught in the same area in 1971. On the basis of the known increase in the global emissions of mercury over the past century and of a simple model of mercury biogeochemistry in the Equatorial and Subtropical Pacific ocean, we calculate that the methylmercury concentration in these surface waters should have increased between 9 and 26% over this 27 years span if methylation occurred in the mixed layer or in the thermocline. Such an increase is statistically inconsistent with the constant mercury concentrations measured in tuna. We conclude tentatively that mercury methylation in the oceans occurs in deep waters or in sediments.

title: Geochemical and biological controls over methylmercury production and degradation in aquatic ecosystems
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title: Determination of mercury methylation potentials in the water column of lakes across Canada.
abstract: A stable isotope technique was used to trace the formation of methylmercury in lake water incubation assays at in situ conditions in five lakes across Canada. Methylation activity was only detected in the anoxic hypolimnia of lakes. The stable isotope was methylated at varying rates between lakes and depths within lakes ranging from 0.56%/day to 14.8%/day. A peak in methylation potential was typically observed just below the oxycline, which decreased with increasing depth. The depth and rates of methylation potential changed seasonally with no methylation activity occurring after fall turnover. A decrease in the sulfate concentration was concomitant with the zone of mercury methylation potential indicating the likely involvement of sulfate reducing bacteria in the methylation process. A simple correlation test between DOC concentrations and methylation rates indicated a positive relationship (r2=0.62; p=0.006; n=27). The demethylation rate constant in the anoxic hypolimnia was less than 0.12 d(-1).

title: Geochemical controls on the production and distribution of methylmercury in near-shore marine sediments.
abstract: We examined temporal differences in sedimentary production of monomethylmercury (MMHg) at three sites in Long Island Sound (LIS). Sediment-phase concentrations of Hg species decreased from west to east in LIS surface sediments, following the trend of organic matter. However, Hg methylation potentials, measured by incubation with an isotopic tracer (200Hg), increased from west to east. 200Hg methylation potentials were enhanced in August relative to March and June, attributable to differences in activity of sulfate-reducing bacteria. Organic matter and acid-volatile sulfide influenced the distribution coefficient (KD) of inorganic Hg (Hg(II) = total Hg - MMHg) and inhibited 200Hg methylation in surface sediments. 200Hg methylation varied inversely with the KD of Hg(II) and positively with the concentration of Hg(II), mostly as HgS0, in LIS pore waters. Accordingly, we posit that a principal control on MMHg production in low-sulfide, coastal marine sediments is partitioning of Hg(II) between particle and dissolved phases, which regulates availability of Hg substrate to methylating bacteria. Most of the partitioning in LIS sediments is due to Hg-organic associations. This suggests that reductions in the organic content of coastal sediment, a potential result of nutrient abatement programs intended to inhibit eutrophication of near-shore waters, could enhance MMHg production by increasing the bioavailability of the large reservoir of "legacy Hg" buried within the sediment.

title: Mercury Methylation by Dissimilatory Iron-Reducing Bacteria
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title: Speciation and bioavailability of mercury in well-mixed estuarine sediments
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title: Bioaccumulation and Trophic Transfer of Methylmercury in Long Island Sound
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title: Methylmercury cycling in sediments on the continental shelf of southern New England
abstract: Exposure of humans to monomethylmercury (MMHg) occurs primarily through consumption of marine fish, yet there is limited understanding concerning the bioaccumulation and biogeochemistry of MMHg in the biologically productive coastal ocean. We examined the cycling of MMHg in sediments at three locations on the continental shelf of southern New England in September 2003. MMHg in surface sediments is related positively to inorganic Hg (Hg(II) = total Hg − MMHg), the geographical distribution of which is influenced by organic material. Organic matter also largely controls the sediment–water partitioning of Hg species and governs the availability of dissolved Hg(II) for methylation. Potential gross rates of MMHg production, assayed by experimental addition of 200 Hg to intact sediment cores, are correlated inversely with the distribution coefficient ( K D ) of Hg(II) and positively with the concentration of Hg(II), most probably as HgS 0 , in 0.2-μm filtered pore water of these low-sulfide deposits. Moreover, the efflux of dissolved MMHg to overlying water (i.e., net production at steady state) is correlated with the gross potential rate of MMHg production in surface sediments. These results suggest that the production and efflux of MMHg from coastal marine sediments is limited by Hg(II), loadings of which presumably are principally from atmospheric deposition to this region of the continental shelf. The estimated diffusive flux of MMHg from the shelf sediments averages 9 pmol m −2  d −1 . This flux is comparable to that required to sustain the current rate of MMHg accumulation by marine fish, and may be enhanced by the efflux of MMHg from near-shore deposits contaminated more substantially with anthropogenic Hg. Hence, production and subsequent mobilization of MMHg from sediments in the coastal zone may be a major source of MMHg to the ocean and marine biota, including fishes consumed by humans.

title: Methylated mercury species in marine waters of the Canadian high and sub Arctic.
abstract: Distribution of total mercury (THg), gaseous elemental Hg(0) (GEM), monomethyl Hg (MMHg), and dimethyl Hg (DMHg) was examined in marine waters of the Canadian Arctic Archipelago (CAA), Hudson Strait, and Hudson Bay. Concentrations of THg were low throughout the water column in all regions sampled (mean +/- standard deviation; 0.40 +/- 0.47 ng L(-1)). Concentrations of MMHg were also generally low atthe surface (23.8 +/- 9.9 pg L(-1)); however at mid- and bottom depths, MMHg was present at concentrations sufficient to initiate bioaccumulation of MMHg through Arctic marine foodwebs (maximum 178 pg L(-1); 70.3 +/- 37.3 pg L(-1)). In addition, at mid- and bottom depths, the % of THg that was MMHg was high (maximum 66%; 28 +/- 16%), suggesting that active methylation of inorganic Hg(II) occurs in deep Arctic marine waters. Interestingly, there was a constant, near 1:1, ratio between concentrations of MMHg and DMHg at all sites and depths, suggesting that methylated Hg species are in equilibrium with each other and/or are produced by similar processes throughout the water column. Our results also demonstrate that oceanographic processes, such as water regeneration and vertical mixing, affect Hg distribution in marine waters. Vertical mixing, for example, likely transported MMHg and DMHg upward from production zones at some sites, resulting in elevated concentrations of these species in surface waters (up to 68.0 pg L(-1)) where primary production and thus uptake of MMHg by biota is potentially highest. Finally, calculated instantaneous ocean-atmosphere fluxes of gaseous Hg species demonstrated that Arctic marine waters are a substantial source of DMHg and GEM to the atmosphere (27.3 +/- 47.8 and 130 +/- 138 ng m(-2) day(-1), respectively) during the ice-free season.

title: Mercury methylation, demethylation and reduction rates in coastal and marine surface waters of the Mediterranean Sea
abstract: In situ experiments using isotopically labeled mercury species ( 199 Hg(II) and Me 201 Hg) are used to investigate mercury transformation mechanisms, such as methylation, demethylation and reduction, in coastal and marine surface waters of the Mediterranean Sea. The aim of this work is to assess the relative contribution of photochemical versus biological processes to Hg transformation mechanisms. For this purpose, potential transformation rates measured under diurnal and dark incubation conditions are compared with major biogeochemical parameters (i.e. hydrological and biological data) in order to obtain the relative contribution of various biotic and abiotic mechanisms in both surface (high light) and bottom (low light) waters of the euphotic zone. The results demonstrate that coastal and marine euphotic zones are significant reactors for all Hg transformations investigated (i.e. methylation, demethylation, reduction). A major outcome demonstrates that Hg methylation is taking place in oxic surface seawater (0.3–6.3% day − 1 ) and is mainly influenced by pelagic microorganism abundance and activities (phyto- and bacterioplankton). This evidences a new potential MeHg source in the marine water column, especially in oligotrophic deep-sea basins in which biogeochemistry is mostly governed by heterotrophic activity. For coastal and marine surface waters, although MeHg is mainly photochemically degraded (6.4–24.5% day − 1 ), demethylation yields observed under dark condition may be attributed to microbial or chemical pathways (2.8–10.9% day − 1 ). Photoreduction and photochemical reactions are the major mechanisms involved in DGM production for surface waters (3.2–16.9% day − 1 ) but bacterial or phytoplanktonic reduction of Hg(II) cannot be excluded deeper in the euphotic zone (2.2–12.3% day − 1 ). At the bottom of the euphotic zone, photochemical processes are thus avoided due to the attenuation of UV-visible sunlight radiation allowing biotic processes to be the most significant. These results suggest a new potential route for Hg species cycling in surface seawater and especially at the maximum biomass depth located at the bottom of the euphotic zone (i.e. maximum chlorophyll fluorescence). In this environment, DGM production and demethylation mechanisms are thus probably reduced whereas Hg methylation is enhanced by autotrophic and heterotrophic processes. Experimental results on mercury species uptake during these investigations further evidenced the strong affinity of MeHg for biogenic particles (i.e. microorganisms) that correspond to the first trophic level of the pelagic food web.

title: Methylated mercury species in Canadian high Arctic marine surface waters and snowpacks.
abstract: We sampled seawater and snowpacks in the Canadian high Arctic for methylated species of mercury (Hg). We discovered that, although seawater sampled under the sea ice had very low concentrations of total Hg (THg, all forms of Hg in a sample; on average 0.14-0.24 ng L(-1)), 30-45% of the THg was in the monomethyl Hg (MMHg) form (on average 0.057-0.095 ng L(-1)), making seawater itself a direct source of MMHg for biomagnification through marine food webs. Seawater under the ice also contained high concentrations of gaseous elemental Hg (GEM; 129 +/- 36 pg L(-1)), suggesting that open water regions such as polynyas and ice leads were a net source of approximately 130 +/- 30 ng Hg m(-2) day(-1) to the atmosphere. We also found 11.1 +/- 4.1 pg L(-1) of dimethyl Hg (DMHg) in seawater and calculated that there could be a significant flux of DMHg to the atmosphere from open water regions. This flux could then resultin MMHg deposition into nearby snowpacks via oxidation of DMHg to MMHg in the atmosphere. In fact, we found high concentrations of MMHg in a few snowpacks near regions of open water. Interestingly, we discovered a significant log-log relationship between Cl- concentrations in snowpacks and concentrations of THg. We hypothesize that as Cl- concentrations in snowpacks increase, inorganic Hg(II) occurs principally as less reducible chloro complexes and, hence, remains in an oxidized state. As a result, snowpacks that receive both marine aerosol deposition of Cl- and deposition of Hg(II) via springtime atmospheric Hg depletion events, for example, may contain significant loads of Hg(II). Overall, though, the median wet/dry loads of Hg in the snowpacks we sampled in the high Arctic (5.2 mg THg ha(-1) and 0.03 mg MMHg ha(-1)) were far below wet-only annual THg loadings throughout southern Canada and most of the U.S. (22-200 mg ha(-1)). Therefore, most Arctic snowpacks contribute

title: Role of the ocean in the global mercury cycle
abstract: Air-sea exchange of mercury (Hg) is a critical part of the global Hg cycle as it determines, to a large degree, the response time of the biosphere to changes in mercury inputs. Recent measurements have demonstrated that the cycling of Hg between the ocean and atmosphere is complex, principally because of the enhanced oxidation of elemental Hg (Hgo), and the formation of reactive gaseous Hg (RGHg) in the marine boundary layer. We estimate that the dry deposition of RGHg to the ocean, which has not been previously considered in global budgets, is 35% of the total Hg input to the ocean. A further reevaluation of the global Hg cycle suggests that there is a net transfer of Hg from the terrestrial environment to the ocean and that the deep ocean Hg concentration is increasing by a few percent per year. Similarly, anthropogenic inputs on land have increased Hg on the Earth's surface layer with accumulation in the terrestrial environment accounting for nearly 80% of the net input from man's activities. Dry deposition of RGHg is important for the terrestrial realm but because of its relatively short residence time in the atmosphere, it is the oxidation of Hgo over the ocean, rather than RGHg transport offshore, which is primarily contributing to oceanic RGHg deposition.

title: Human impacts on open ocean mercury concentrations
abstract: We develop an empirically constrained multicompartment box model for mercury cycling in open ocean regions to investigate changes in concentrations resulting from anthropogenic perturbations of the global mercury cycle. Using Monte Carlo simulations, we explicitly consider the effects of variability in measured parameters on modeled seawater concentrations. Our simulations show that anthropogenic enrichment in all surface (25%) and deep ocean waters (11%) is lower than global atmospheric enrichment (300–500%) and varies considerably among geographic regions, ranging from >60% in parts of the Atlantic and Mediterranean to <1% in the deep Pacific. Model results indicate that open ocean mercury concentrations do not rapidly equilibrate with atmospheric deposition and on average will increase if anthropogenic emissions remain at their present level. We estimate the temporal lag between changes in atmospheric deposition and ocean mercury concentrations will vary from decades in most of the Atlantic up to centuries in parts of the Pacific.

title: Mercury distributions in the North Pacific Ocean—20 years of observations
abstract: Vertical mercury (Hg) distributions for the North and Central Pacific Ocean are reported here for three different cruises over a time period of 20 years: N. Pac (1980), VERTEX (1986–87) and IOC (2002). The vertical distribution was not controlled solely by the hydrographic characteristics or by internal biogeochemical “nutrient type” recycling and mixing processes. Rather, Hg distribution appeared to be regulated by the local magnitude of external sources and the intensity of water column processes. During the 2002 IOC cruise, the total mercury (HgTOT) concentrations averaged 1.15±0.86 pM with the highest concentrations found within the Japanese coastal waters. The overall upper-water Hg concentration average, calculated for the IOC study (0.64±0.26 pM), was similar to the earlier VERTEX cruise (0.58±0.37 pM) but was lower compared to the N. Pac cruise (1.40±0.34). Variance in Hg concentrations in the upper water, generally within or close to the main thermocline, was observed among several stations and for both VERTEX and IOC campaigns. Horizontal advection of water along isopycnals, vertical mixing, ventilation, the presence of a thermocline associated with remobilization of mercury as a result of remineralization of settling particles, the influence of diagenetic processes from continental margin sediments and seasonal stratification of the euphotic zone all appeared to be factors that can account for the distribution of Hg in the upper water. The deep and bottom waters of the North Pacific Ocean averaged 1.10±0.31 pM and were characterized by comparable Hg concentrations among the different cruises, which reinforced the homogeneous characteristics of this particular water mass. In addition, the comparison between the Atlantic and the Pacific deep water showed the presence of inter-ocean Hg fractionation resulting in Hg concentrations in the deep North Pacific Ocean being three- to sixfold lower than in the deep Atlantic.

title: A commercially available rosette system for trace metal—clean sampling
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title: Decadal changes in Pacific carbon
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title: The distribution and speciation of mercury in the South and equatorial Atlantic
abstract: Mercury concentration and speciation were measured in surface and deep ocean waters of the South and equatorial Atlantic. In the surface waters, total Hg was 2.9±1.7 pM on average, with a significant fraction present as reactive Hg (1.7±1.2 pM). The reactive Hg fraction consisted of elemental Hg (Hg°) as the dominant species (1.2±0.8 pM). Measurements in surface waters also showed that Hg partitioned to the “colloidal” phase (0.33±0.28 pM) and was associated with particulate matter (0.1±0.05 pM). No dimethylmercury (DMHg;

title: Comparison of distillation with other current isolation methods for the determination of methyl mercury compounds in low level environmental samples
abstract: In the present paper two isolation procedures for the separation of methyl mercury compounds (MeHg) from natural water samples, followed by aqueous phase ethylation, precollection on the Carbotrap, isothermal gas chromatography and cold vapour atomic fluorescence (CV-AFS) detection were compared and evaluated. The first isolation technique is based on extraction of MeHg into methylene chloride and back extraction into water by solvent evaporation. The second is based on the distillation of MeHg compounds. A comparison of these two isolation procedures was performed on 110 water samples of various origin (oxic, anoxic fresh water and sea water) and containing a wide MeHg concentration range (from 0.01 to 25 ng l−1. A relatively good agreement of the results was obtained in the concentration range below 1 ng l−1 as Hg (r2=0.919, n=61). In anoxic, sulphide rich and organic matter rich water samples that usually contain higher MeHg values, significantly higher values (approx. 30%) were obtained by distillation. This is partly due to sulphide interference during the ethylation step, as well as incomplete release of MeHg from bound sites. Distillation was found advantageous to solvent extraction. It gives consistent and high recoveries (80–95%) for various water samples, and achieves lower detection limits (0.006 ng l−1 for 50 ml water sample). It is also less laborious and without the use of organic solvent. Additionally, distillation provides the specific separation of MeHg and dimethylmercury, if present.

title: Application of stable isotopes in environmental tracer studies – Measurement of monomethylmercury (CH3Hg+) by isotope dilution ICP-MS and detection of species transformation
abstract: The monovalent cation monomethylmercury (CH3Hg+) was determined in certified reference materials by isotope dilution GC/ICP-MS and good agreement between measured and certified values has been found. The use of enriched stable isotopes with subsequent detection by ICP-MS is a powerful tracer technique to study dynamic environmental processes. For the first time, it was possible to monitor opposite processes like Hg2+ methylation and CH3Hg+ demethylation at ambient tracer levels simultaneously in the same sample. A scheme for calculating the formation of new species from stable tracers used in environmental studies is presented. The sensitivity of stable tracer methods is superior to traditional tracer or radiotracer techniques. In case of mercury methylation, where the generation of a new compound is monitored, the limit of detection depends only on the precision of the isotope ratio measurements and the concentration of the ambient CH3Hg+ already present in the sample, not on the absolute detection limit of the GC/ICP-MS technique used for analyzing CH3Hg+. A 0.25% change in concentration of CH3Hg+ is detectable. In the case of CH3Hg+ demethylation, where the decrease of the added tracer is monitored, the detection limit again depends on the precision of the isotope ratio measurement and the ambient CH3Hg+ concentration but additionally on the amount of the added tracer as well. A decrease in the CH3Hg+ level of 2% of the added tracer is detectable. The validity of the calculation scheme was tested and no difference was found between individually measured isotope concentrations and calculated concentrations from solutions with multiple stable tracers.

title: Mercury in the North Atlantic
abstract: Reactive mercury (Hg) concentrations (0.80±0.44 pM), measured during a cruise in the sub-polar North Atlantic (50–70°N) in August 1993 were lower those of more southerly regions of this ocean. A large fraction of the Hg in the surface waters was elemental Hg (Hg°; 88% of the reactive Hg, on average; 0.65±0.50 pM). Little dimethylmercury (DMHg) or monomethylmercury (MMHg) was found in surface waters but higher concentrations were found at depth. The high surface water Hg° concentrations likely reflect production of Hg° in concert with the short season of primary production in these waters. Overall, gas exchange plays a dominant role in the cycle of Hg in the upper waters of this region. For the sub-thermocline waters, a comparison of the Hg speciation in different water masses suggests that surface waters accumulate reactive Hg and methylated Hg—due to input of reactive Hg via remineralization and conversion of reactive Hg to methylated Hg—after sinking during winter to form North Atlantic deep water. Reactive Hg concentrations varied between water masses with the lowest concentrations being found in the waters north of Iceland (the source waters for deep water formation). DMHg was found throughout the sub-thermocline water column and the limited data set for MMHg suggests that this species was also prevalent. These observations suggest that methylated Hg production in the ocean is not confined to low oxygen or anoxic regions, as has been found in lakes. Concentrations of DMHg (0.08±0.07 pM, on average) were lower than those previously measured in the equatorial Pacific. Total Hg concentrations averaged 2.4±1.6 pM but little of this was particulate Hg (

title: Redfield ratios of remineralization determined by nutrient data analysis
abstract: A nonlinear inverse method is applied to nutrient data upon approximately 20 neutral surfaces in each of the South Atlantic, Indian, and Pacific basins, between 400 and 4000 m depth. By accounting for the gradients in nutrients due to the mixing of [open quotes]preformed[close quotes] concentrations of the major water masses, the nutrient changes due to biological activity are examined, and the time-mean, basin-wide Redfield ratios calculated. It is found that the P/N/C[sub org]/[sup O][sub 2] ratios of nutrient regeneration between 400 and 4000 m (corrected for the effect of denitrification) are approximately constant with depth and basin, at a value of 1/16[+-]1/117[+-]14/170[+-]10. These ratios agree with those of fresh organic matter, suggesting that the flux of organic material to the deep ocean may be dominated by fast-sinking matter produced by sporadic, high-productivity events. Sedimentary denitrification reduces the N/P utilization ratio to 12 [+-] 2 between 1000 and 3000 m. In the Indian and Pacific basins the C[sub org]/C[sub inorg] regeneration ratio decreases from approximately 7 [+-] 13 at 400 m to 3 [+-] 1 at 1000 m and to 1 [+-] 0.5 at 4000 m, suggesting a significant amount of calcium carbonate dissolution above the calcite lysoclines in themore » Indian and Pacific oceans. 74 refs., 8 figs., 3 tabs.« less

title: Redfield ratio based on chemical data from isopycnal surfaces
abstract: The composition of the organic matter oxidized within the thermocline of the Atlantic and Indian oceans has been estimated from the chemical data along the σθ 27.0 and 27.2 horizons. These estimates are based on the differences between the preformed and observed concentrations of PO4, NO3, total inorganic CO2, alkalinity, and O2. Since these isopycnal horizons are ventilated both from the north and the south, the preformed concentration estimates take into account the relative contributions of these two end-members. The proportions of these end-members are estimated on the basis of the potential temperature of the water, assuming that no cross-isopycnal mixing occurred. Except for the northern Indian Ocean, the composition of the waters entering the isopycnal horizons is established through linear extrapolation of the property-oxygen trends to oxygen saturation. The σθ 27.0 and 27.2 waters do not outcrop in the northern Indian Ocean, and these horizons receive waters spilling into the Indian Ocean from the Red Sea. Therefore the composition of the Red Sea overflow water with these densities is taken to be the northern Indian end-member. The results of this study indicate that the accepted Redfield P:N:C: −O2 ratio of 1:16:106:138 requires revision. Our analysis yields a ratio of 1:16:103:172 if the carbon value is represented by the observed increase in the total CO2 concentration. On the other hand, if the carbon value is assumed to be represented by the oxygen utilization minus the oxygen used for oxidation of NH3 with two moles of O2 per nitrogen atom, a ratio of 1:16:140:172 is obtained. Thus the P: −O2 ratio lies between 1:103 and 1:140. This discrepancy may be accounted for by the increased CO2 concentrations in the source waters as a result of the uptake of anthropogenic CO2 or by an excess demand of oxygen for oxidation of hydrogenated organic molecules. Therefore, without a firm knowledge of either the corrections for the anthropogenic CO2 effect or the hydrogen content of the biological residues, the true P:C ratio cannot be obtained. The ratio of P:CaCO3 dissolution has been estimated to be about 1:12. This indicates that the CO2 produced by the oxidation of organic carbon to that derived from the dissolution of CaCO3 is bout 10:1 at these density horizons.

title: Oxygen Utilization and Organic Carbon Remineralization in the Upper Water Column of the Pacific Ocean
abstract: As a part of the JGOFS synthesis and modeling project, researchers have been working to synthesize the WOCE/JGOFS/DOE/NOAA global CO2 survey data to better understand carbon cycling processes in the oceans. Working with international investigators we have compiled a Pacific Ocean data set with over 35,000 unique samples analyzed for at least two carbon species, oxygen, nutrients, chlorofluorocarbon (CFC) tracers, and hydrographic parameters. We use these data here to estimate in-situ oxygen utilization rates (OUR) and organic carbon remineralization rates within the upper water column of the Pacific Ocean. OURs are derived from the observed apparent oxygen utilization (AOU) and the water age estimates based on CFCs in the upper water and natural radiocarbon in deep waters. The rates are generally highest just below the euphotic zone and decrease with depth to values that are much lower and nearly constant in water deeper than 1200 m. OURs ranged from about 0.02–10 μmol kg−1yr−1 in the upper water masses from about 100–1000 m, and averaged = 0.10 μmol kg−1yr−1 in deep waters below 1200 m. The OUR data can be used to directly estimate organic carbon remineralization rates using the C:O Redfield ratio given in Anderson and Sarmiento (1994). When these rates are integrated we obtain an estimate of 5.3 ± 1 Pg C yr−1 for the remineralization of organic carbon in the upper water column of the Pacific Ocean.

title: Global net community production estimated from the annual cycle of surface water total dissolved inorganic carbon
abstract: Global net community production is determined, for the first time, from the decrease in salinity (S)-normalized total dissolved inorganic carbon (NCT 5 CT 3 35/S) inventory in the surface mixed layer corrected for changes due to net air-sea CO2 exchange and diffusive carbon flux from the upper thermocline. Changes in the mixed layer NCT inventory are estimated using a derived annual cycle of NCT and global records of the mixed layer depth. The annual NCT cycle is deduced from regional algorithms relating NCT to sea surface temperature (SST) and nitrate (NO ), along with global records of seasonal mean SST and NO , and from the monthly mean surface partial 2 2 3 3 pressure of CO2 and total alkalinity fields using thermodynamic models. The two methods show similar regional trends and yield global net community production estimates of 6.7 and 8.0 Gt C (1 Gt C 5 1 3 10 12 kg carbon), respectively. The two global estimates are not significantly different and represent an 8-month period of 1990 (warming period) during which the mixed layer NCT concentration decreases. However, the estimates do not account for net community production during a 4-month cooling period. Ratios of net community production during the warming and cooling periods are estimated from multiyear sediment trap data at the Hawaii Ocean Time-series (228N, 1588W) and Ocean Weather Station P (508N, 1458W) sites. Global extrapolation of these ratios yields annual rates of net community production of 9.1 6 2.7 and 10.8 6 2.7 Gt C yr 21

title: Oxygen solubility in seawater : better fitting equations
abstract: We examined uncertainties associated with the routine computation of O2 solubility (Co*) at 1 atm total pressure in pure water and seawater in equilibrium with air as a function of temperature and salinity. We propose formulae expressing C*(at STP, real gas) in cm3 dm−3 and µmol kg−1 in the range (tF ≥ t 40°C; 0 ≥ S 42‰) based on a fit to precise data selected from the literature.

title: A chlorofluorocarbon section in the eastern North Atlantic
abstract: We present the distributions of two chlorofluorocarbons (CFCs) CFC-11 and CFC-12, measured as part of a hydrographic section between Iceland and the equator during July and August 1988. CFC-tagged water has filled the entire subpolar water column and subtropical thermocline in the eastern North Atlantic. Measurable CFC concentrations are observed at the ocean bottom as far south as 35°N, and the CFC penetration depth shoals to ≈750 m in the tropics. Specific features in the CFC distributions include a clear signal of Labrador Sea mid-depth ventilation, a CFC-enriched overflow water boundary current along the Iceland slope, and a mid-depth, equatorial plume of upper North Atlantic Water. The CFC data are used, in conjuction with the hydrographic data from the cruise, to illustrate the ventilation time-scales and pathways for the water masses in the eastern basin. A subsurface CFC maximum at about 100–200 m depth in the subtropics is shown to be a by-product of the heating and degassing of the seasonal thermocline and of the temperature sensitivity of CFC solubility. The CFC concentrations in the subpolar mode water are undersaturated by 15–18% relative to the atmosphere, reflecting the age of the mode waters and the very deep winter mixed layers in the eastern subpolar gyre. The CFC concentrations in the oxygen minimum off tropical Africa are much lower than the concentrations in the subtropical gyre, supporting previous work that suggests that isolation and enhanced productivity both contribute to the formation of the tropical oxygen minimum. In addition, the CFC inventories at the tropical stations have increased between 1982 and 1983 (TTO/TAS) and the summer of 1988 at a slower rate relative to the subtropical inventories over the same period. Thermocline oxygen utilization rates calculated from the CFC concentration data range from 5 to 10 μmol kg −1 y −1 and are in line with previous estimates for the eastern subtropical thermocline. The low CFC concentrations in Mediterranean Water, about one-quarter those in the Labrador Sea Water, are shown to result from entrainment near the Straits of Gibraltar of a large component of low CFC, lower Atlantic thermocline water. Based on the CFC and other transient tracer distributions, the deep eastern basin can be divided into two regions: the Iceland Basin and surrounding area influenced by Iceland-Scotland Overflow Water that is ventilated on a decadal time-scale, and the area south of ≈50°N that has little or no CFC and is ventilated from a southern source on a much longer time-scale. A northward flowing boundary current of low CFC, modified Eastern Basin Bottom Water is also found along the Rockall Plateau and in the Iceland Basin.

title: A new transient tracer: Measured vertical distribution of CCl2FCClF2 (F‐113) in the North Pacific Subarctic Gyre
abstract: The first quantitative marine measurements of trichlorotrifluoroethane (F-113) were made in the North Pacific subarctic gyre in July of 1986, using a modification of the analytical procedure developed for the transient tracers F-11 and F-12 (Gammon et al., 1982; Wisegarver and Cline, 1985). The measured mixed layer concentration was 0.29±0.02 pM/L; the corresponding atmospheric mixing ratio was 35±1 pptv (Rasmussen scale). 
 
Below a subsurface maximum both F-11 and F-113 were found to fall off exponentially with increasing depth, F-113 reaching effective blank levels first. The significance of adding F-113 to the existing suite of measurable oceanic transient tracers rests in the possibility of age-dating water masses by their (F-113/F-11) ratio with near annual resolution for the period since 1977.

title: Comparison of Time-Dependent Tracer Ages in the Western North Pacific: Oceanic Background Levels of (SF6, CFC-11, CFC-12 and CFC-113
abstract: To verify the actual usefulness of time-dependent tracer dating techniques in the ocean, we simultaneously obtained two cross sections of sulfur hexafluoride (SF6) and chlrofluoromethanes (CFC-11, trichlorofluoromethane; CFC-12, dichloro-difluromethane; CFC-113, trichlorotrifluoroethane) in the western North Pacific in 1998. The vertical distribution patterns of SF6 and CFC-113 were similar in shape to those of CFC-11 and CFC-12. Maximum penetration depths of SF6 and CFC-113 remained around 800 m in the subpolar region and 400 m in the tropical region, while the maximum penetration depths of CFC-11 and CFC-12 were still found below 1000 m depth. We also found all maximum contents of these tracers around 26.6−26.8σθ with a gradual decrease southward. This suggested that a new subsurface water mass in the subpolar region spread out over the entire North Pacific, which agrees closely with previous studies based on the salinity minimum. Moreover, we compared the tracer ages (the elapsed period of a water mass from when the water mass left from the ocean surface) using ten time-dependent tracer dating techniques, CFC-11, CFC-12, CFC-113, SF6, CFC-11/CFC-12, CFC-113/CFC-11, CFC-113/CFC-12, SF6/CFC-11, SF6/CFC-12 and SF6/CFC-113. This quantitative evaluation of multiple tracer dating techniques in the ocean was the first confirmation of its usefulness based on the observational data on the ocean basin-wide scale. We conclude that SF6/CFC-11, SF6/CFC-12, SF6/CFC-113 and SF6 dating techniques would be the most promising tools for determining the age of water mass not only just for the past several decades but for the future, too.

title: Solubilities of chlorofluorocarbons 11 and 12 in water and seawater
abstract: The solubilities of dilute mixtures of the chlorofluorocarbons CCl3F (CFC-11) and CCl2F2 (CFC-12) in pure water and seawater over the temperature range 0 to 40°C have been measured by gas chromatography. The data have been fitted to equations in temperature and salinity of the form used previously to fit the solubilities of other gases. The fitted values have an estimated accuracy of ∼1.5% and a relative precision of ∼0.7% for both chlorofluorocarbons. The nonideal behavior of these compounds in dilute air mixtures is discussed, and their solubilities from a water-saturated atmosphere are presented in parametric form.

title: Mapping the spatial distribution of global anthropogenic mercury atmospheric emission inventories
abstract: This paper describes the procedures employed to spatially distribute global inventories of anthropogenic emissions of mercury to the atmosphere, prepared by Pacyna, E.G., Pacyna, J.M., Steenhuisen, F., Wilson, S. [2006. Global anthropogenic mercury emission inventory for 2000. Atmospheric Environment, this issue, doi:10.1016/j.atmosenv.2006.03.041 ], and briefly discusses the results of this work. A new spatially distributed global emission inventory for the (nominal) year 2000, and a revised version of the 1995 inventory are presented. Emissions estimates for total mercury and major species groups are distributed within latitude/longitude-based grids with a resolution of 1×1 and 0.5×0.5°. A key component in the spatial distribution procedure is the use of population distribution as a surrogate parameter to distribute emissions from sources that cannot be accurately geographically located. In this connection, new gridded population datasets were prepared, based on the CEISIN GPW3 datasets (CIESIN, 2004. Gridded Population of the World (GPW), Version 3. Center for International Earth Science Information Network (CIESIN), Columbia University and Centro Internacional de Agricultura Tropical (CIAT). GPW3 data are available at http://beta.sedac.ciesin.columbia.edu/gpw/index.jsp ). The spatially distributed emissions inventories and population datasets prepared in the course of this work are available on the Internet at www.amap.no/Resources/HgEmissions/

title: Global Ocean Circulation Modes Derived from a Multiple Box Model
abstract: A 16-box model of the global ocean circulation is constructed to investigate the sensitivity of the thermohaline circulation to upper-ocean density perturbations. The model attempts to represent the main geographical components of the global ocean and is tuned to give a present-day circulation broadly compatible with observed volume fluxes. Extensive tests are made of the model's sensitivity to upper-ocean density perturbations, equivalent to a range of ±3 psu in salinity about the current climate. It is found that there are seven separate modes of thermohaline circulation that can occur within the model's constraints, as well as the possibility of significant variations in the strength of the current circulation mode. These modes are related to possible changes in surface forcing, such as the abrupt increase in freshwater from ice-sheet melting, where such a possibility exists. A mode with North Atlantic upwelling rather than deep- or bottom-water formation was found to occur with large addition...

title: Vertical mercury distributions in the oceans
abstract: The vertical distribution of mercury (Hg) was determined at coastal and open ocean sites in the northwest Atlantic and Pacific Oceans. Reliable and diagnostic Hg distributions were obtained, permitting major processes governing the marine biogeochemistry of Hg to be identified. 
 
The northwest Atlantic near Bermuda showed surface water Hg concentrations near 4 pM, a maximum of 10 pM within the main thermocline, and concentrations less than or equal to surface water values below the depth of the maximum. The maximum appears to result from lateral transport of Hg enriched waters from higher latitudes. In the central North Pacific, surface waters (to 940 m) were slightly elevated (1.9 ± 0.7 pM) compared to deeper waters (1.4 ± 0.4 pM), but no thermocline Hg maximum was observed. At similar depths, Hg concentrations near Bermuda were elevated compared to the central North Pacific Ocean. This inter-ocean pattern results from a combination of enhanced supply of Hg to the northwest Atlantic by rainfall and scavenging removal along deep water circulation pathways. An upper water column Hg maximum was also observed at a Slope Water station in the northwest Atlantic and in the Tasman Sea. 
 
We hypothesize that the source of this Hg comes from diagenetic reactions in oxic margin sediments, releasing dissolved Hg to overlying water. In contrast, a broad minimum (1.5 ± 0.2 pM) in the vertical distribution was observed in the coastal waters off Peru which coincided with the intense oxygen minimum zone in the water column. Removal of dissolved Hg from solution during contact with highly anoxic shelf and slope sediments is proposed as the mechanism for producing this unique profile. 
 
Geochemical steady-state box modeling arguments predict a relatively short (~350 years) mean residence time for Hg in the oceans, demonstrating the reactive nature of Hg in seawater and precluding significant involvement in “nutrient-type” recycling. Mercury's distributional features and reactive nature suggest that interaction of Hg with settling particulate matter and margin sediments play important roles in regulating oceanic Hg concentrations. Oceanic Hg distributions are governed by an “external cycling process”, in which water column distributions reflect a rapid competition between the magnitude of the input source and the intensity of the (water column) removal process. In this regard, Hg shares many distributional features with other elements, such as Pb and Bi, which have very short oceanic residence times.

title: Reactive gaseous mercury formation in the North Pacific Ocean's marine boundary layer: A potential role of halogen chemistry
abstract: Reactive gaseous mercury (RGHg), atmospheric elemental mercury (Hg0), and ozone (O3) along with other ancillary parameters have been measured simultaneously in the marine boundary layer (MBL) over the North Pacific Ocean during the 2002-IOC cruise between Osaka, Japan, and Honolulu, Hawaii. Atmospheric Hg0 concentrations varied from 1.6 to 4.7 ng m−3 with an average of 2.5 ng m−3 and did show significant variations along the cruise track with a slight diel cycle in Hg0 concentrations. RGHg concentrations varied from 0.15 to 92.4 pg m−3 with an average of 9.5 pg m−3. The data strongly suggest photochemical in situ formation of RGHg with increasing concentrations in the tropical regions concomitant to a dramatic decrease in O3 (from over 50 to less than 5 ppbv). A distinct diurnal variation in RGHg concentrations with maxima at midday suggests a photochemically driven oxidation of marine boundary layer Hg0. In the tropical area, enhanced RGHg formation and daily variation in O3 could both be related to reactive halogen chemistry. Flux calculations also emphasize the role of the tropical marine boundary layer in mercury cycling over the North Pacific Ocean.

title: Mercury in the Atlantic Ocean: factors controlling air–sea exchange of mercury and its distribution in the upper waters
abstract: To understand further the cycling of mercury at the earth's surface we discuss the results of recent measurements of Hg concentration and speciation in the upper ocean and marine boundary layer of the Atlantic Ocean. In water, dissolved gaseous Hg (DGHg) and total Hg measurements are reported; for the atmosphere, total gaseous Hg, reactive gaseous Hg (RGHg) and particulate Hg measurements were made. These measurements allow estimation of gas evasion to the atmosphere and deposition to the ocean. In conjunction with the field collections, incubation experiments both on board ship and in the laboratory have examined further the processes controlling the oxidation and reduction of Hg species in water. Our results suggest that dry deposition of RGHg. could be significant.

title: Distribution and Formation of North Pacific Intermediate Water
abstract: The North Pacific Intermediate Water (NPIW), defined as the main salinity minimum in the subtropical North Pacific, is examined with respect to its overall property distributions. These suggest that NPIW is formed only in the northwestern subtropical gyre; that is, in the mixed water region between the Kuroshio Extension and Oyashio front. Subsequent modification along its advective path increases its salinity and reduces its oxygen. The mixed water region is studied using all bottle data available from the National Oceanographic Data Center, with particular emphasis on several winters. Waters from the Oyashio, Kuroshio, and the Tsugaru Warm Current influence the mixed water region, with a well-defined local surface water mass formed as a mixture of the surface waters from these three sources. Significant salinity minima in the mixed water region are grouped into those that are directly related to the winter surface density and are found at the base of the oxygen-saturated surface layer, and thos...

title: Intermediate water circulation in the North Pacific subarctic and northern subtropical regions
abstract: The intermediate water circulation in the North Pacific subarctic and northern subtropical regions is investigated through inverse analysis, focusing on the volume and heat transports from the subtropical to the subarctic regions. The inverse method we adopted is a hybrid method of β-spiral and box inverse methods which permits diapycnal flux. The isopycnal velocities estimated through the inverse analysis are mostly consistent with the oxygen distribution and support the hypothesis that warm and saline intermediate water is transported from the transition domain east of Japan to the northern Gulf of Alaska. The northward volume transport across 46°N between 158°E and 130°W is estimated to be −0.2 to 5.3 Sv in the density range of 26.7–27.2σθ. The upward diapycnal transports in the open subarctic North Pacific (region N) across 26.7 and 27.2σθ isopycnal surfaces are estimated to be 0.2 to 1.5 Sv and −0.2 to 0.9 Sv, respectively. Part of the water transported upward across 26.7σθ might outcrop and be carried to the subtropical region by the southward Ekman drift. Through the examination of heat balance of the intermediate layer in the subarctic region, it is suggested quantitatively that the intermediate heat transport from the south plays an essential role in maintaining the heat of the mesothermal waters in the subarctic region.

title: Formation and variation of temperature inversions in the eastern subarctic North Pacific
abstract: Hydrographic data from profiling float observations for 2001–2005 and from expendable bathythermograph observations for 1993–2005 were analyzed to study the formation and variation of temperature inversions (T-inversions) in the eastern subarctic North Pacific (SNP). The formation and variation of T-inversions differed significantly between the northern and southern regions of the eastern SNP. In the northern region, the temperature minimum (T-min) at the top of T-inversions outcropped to the sea surface and was cooled in the mixed layer nearly every winter. This process caused a seasonal cycle in the magnitude of T-inversions (ΔT), with a maximum in winter. In the southern region, the winter T-min outcropped relatively infrequently and the ΔT did not exhibit a significant maximum in winter during most years. The T-min in the southern region was likely to outcrop upstream near the date line roughly one year earlier and was then advected to the southern region.

title: The distribution and cycling of mercury species in the western Mediterranean
abstract: Total mercury concentrations were determined in water samples from ten vertical profiles in the western Mediterranean. Most Hg concentrations ranged from 0.5 to 4 pM, with a geometric mean of 2.26 pM. Such concentrations are in the same range as those measured in the North Atlantic and equatorial Pacific waters. There is no indication that the presence of geothermal activity or cinnabar deposits around the Mediterranean basin induces higher mercury concentrations in waters. Vertical concentration profiles were characterized by a maximum just below the thermocline, which is mainly developed in regions with relatively high primary production in the overlying waters. These observations support the remobilization model according to which, the mercury, associated with degradable organic matter, is solubilized from the particles accumulated in the thermocline layer. Additional measurements of certain mercury species, including reactive mercury (HgR) and gaseous species [elemental mercury (Hg°) and dimethylmercury (DMHg)], were performed on three profiles in the Alboran Sea and the Strait of Gibraltar. While 50% of the total mercury consisted of unidentified organic association, a maximum of the three determined species were observed below the thermocline: up to 0.71 pM, 0.43 pM and 0.30 pM for HgR, Hg° and DMHg, respectively. DMHg and Hg° appear to be formed in the low oxygen zone. A specific methylation rate of 3 × 10−9 s−1 can be estimated, which is six times higher than the values proposed for the North Atlantic waters (Mason et al., 1995a, Water, Air and Soil Pollution, 80, 665–677). 
 
The mass balance budget in the western Mediterranean shows that, while the total mercury exchanges at the straits are not unbalanced, mercury enters the Mediterranean as inorganic mercury and is exported to the Atlantic Ocean partially as methylated species. Riverine input is mainly composed by particulate mercury which is readily accumulated in coastal sediments. The dissolved mercury flux from non-marine sources is largely dominated by atmospheric deposition, which is one order of magnitude higher than riverine input. The mercury accumulation rate in the sediments is quantitatively equivalent to the atmospheric inputs, but mainly constituted by terrestrial material deposited nearshore. On the basis of ☐-core analyses and the global model of Mason et al. (1994a, Geochimica et Cosmochimica Acta, 58, 3191–3198) and present results the pre-industrial Hg riverine, atmospheric and sedimentation fluxes are estimated to have been about one-third the current one.

title: Speciation of mercury in surface and deep-sea waters in the Mediterranean Sea
abstract: A summary of data recently obtained for mercury analysis and speciation (reactive Hg, total Hg and monomethylmercury (MMHg)) in filtered and non-filtered seawater samples, dissolved gaseous mercury (DGM) and dimethylmercury (DMHg) in open and coastal waters of the Mediterranean Sea is presented. The majority of the results were obtained during an oceanographic cruise aboard the research vessel Urania from 14 July to 9 August, 2000, as part of the MED-OCEANOR Project funded by the National Research Council of Italy. The results are compared with those obtained in contaminated coastal environments of the Adriatic (The Gulf of Trieste and Kastela Bay) and non-contaminated coastal waters of the eastern Adriatic coast obtained in 1998. Total mercury concentrations in surface ocean waters are relatively low with an average of 0.81 pM (0.49–1.91 pM). Reactive Hg represents a substantial part with an average of 57% of total Hg (15–97%). Most mercury in open ocean waters was present in the dissolved form (32–95%, av. 70%), which is mainly due to the low abundance of particulate matter, a phenomenon well known for the Mediterranean open ocean waters. On average the percentage of Hg as MMHg was about 20%, of which about 66% was present in the dissolved form. The percentage of DGM in the surface ocean waters represents about 9% of total Hg (2.5–24.5%) and may originate from photochemical, biologically mediated mechanisms or diffusion from deeper layer either due to biological and/or to tectonic activity which is typical of the Mediterranean region. The presence of DMHg was confirmed only in waters below 20 m (up to 12 fM), while in surface waters DMHg was below the limit of detection (

title: Sea-Air Partitioning of Mercury in the Equatorial Pacific Ocean
abstract: The partitioning of gaseous mercury between the atmosphere and surface waters was determined in the equatorial Pacific Ocean. The highest concentrations of dissolved gaseous mercury occurred in cooler, nutrient-rich waters that characterize equatorial upwelling and increased biological productivity at the sea surface. The surface waters were supersaturated with respect to elemental mercury; a significant flux of elemental mercury to the atmosphere is predicted for the equatorial Pacific. When normalized to primary production on a global basis, the ocean effluxes of mercury may rival anthropogenic emissions of mercury to the atmosphere.

title: Mercury speciation in surface and deep waters of the Mediterranean Sea
abstract: Mercury speciation and its distribution in surface and deep waters of the Mediterranean Sea were studied during two oceanographic cruises on board the Italian research vessel URANIA in summer 2003 and spring 2004 as part of the Med Oceaneor and MERCYMS projects. The study included deep water profiles of dissolved gaseous Hg (DGM), reactive Hg (RHg), total Hg (THg), monomethyl Hg (MeHg) and dimethyl Hg (DMeHg) in open ocean waters. Average concentrations of measured Hg species were characterized by seasonal and spatial variations. Overall average THg concentrations ranged between 0.41 and 2.65 pM (1.32 ± 0.48 pM) and were comparable to those obtained in previous studies of the Mediterranean Sea. A significant fraction of Hg was present as “reactive” Hg (average 0.33 ± 0.32 pM). Dissolved gaseous Hg (DGM), which consists mainly of Hg0, represents a considerable proportion of THg (average 20%, 0.23 ± 0.11 pM). The portion of DGM typically increased towards the bottom, especially in areas with strong tectonic activity (Alboran Sea, Strait of Sicily, Tyrrhenian Sea), indicating its geotectonic origin. No dimethyl Hg was found in surface waters down to the depth of 40 m. Below this depth, its average concentration was 2.67 ± 2.9 fM. Dissolved fractions of total Hg and MeHg were measured in filtered water samples and were 0.68 ± 0.43 pM and 0.29 ± 0.17 pM for THg and MeHg respectively. The fraction of Hg as MeHg was in average 43%, which is relatively high compared to other ocean environments. The concentrations reported in this study are among the lowest found in marine environments and the quality of analytical methods are of key importance. Speciation of Hg in sea water is of crucial importance as THg concentrations alone do not give adequate data for understanding Hg sources and cycling in marine environments. For example, photoinduced transformations are important for the presence of reactive and elemental mercury in the surface layers, biologically mediated reactions are important for the production/degradation of MeHg and DGM in the photic zones of the water column, and the data for DGM in deep sea indicate the natural sources of Hg in geotectonicaly active areas of the Mediterranean Sea.

title: Export Production in the Equatorial and North Pacific Derived from Dissolved Oxygen, Nutrient and Carbon Data
abstract: A global ocean inverse model that includes the 3D ocean circulation as well as the production, sinking and remineralization of biogenic particulate matter is used to estimate the carbon export flux in the Pacific, north of 10°S. The model exploits the existing large datasets for hydrographic parameters, dissolved oxygen, nutrients and carbon, and determines optimal export production rates by fitting the model to the observed water column distributions by means of the “adjoint method”. In the model, the observations can be explained satisfactorily with an integrated carbon export production of about 3 Gt C yr−1 (equivalent to 3⋅1015 gC yr−1) for the considered zone of the Pacific Ocean. This amounts to about a third of the global ocean carbon export of 9.6 Gt C yr−1 in the model. The highest export fluxes occur in the coastal upwelling region off northwestern America and in the tropical eastern Pacific. Due to the large surface area, the open-ocean, oligotrophic region in the central North Pacific also contributes significantly to the total North Pacific export flux (0.45 Gt C yr−1), despite the rather small average flux densities in this region (13 gC m−2yr−1). Model e-ratios (calculated here as ratios of model export production to primary production, as inferred from satellite observations) range from as high a value as 0.4 in the tropical Pacific to 0.17 in the oligotrophic central north Pacific. Model e-ratios in the northeastern Pacific upwelling regions amount to about 0.3 and are lower than previous estimates.

title: Concentration, distribution, and bioavailability of mercury and methylmercury in sediments of baltimore harbor and Chesapeake Bay, Maryland, USA
abstract: For the Chesapeake Bay, sediments in regions such as Baltimore Harbor have total mercury (Hg) concentrations that exceed environmental effects guidelines. However, fish concentrations do not appear elevated. Indeed, the factors controlling the transfer of sedimentary Hg, especially as monomethylmercury (MMHg), the most bioaccumulative form of Hg, to these aquatic organisms are poorly understood. To examine this, we have investigated the distribution and bioavailability of Hg and MMHg to benthic organisms in Baltimore Harbor and the Chesapeake Bay, in Maryland, USA. The results discussed here show that sediment concentration for both total Hg and MMHg covaries with sediment organic content and that this parameter is a better predictor, for surface sediments, of concentration than iron content, acid volatile sulfide (AVS), or other factors. Furthermore, correlations between inorganic Hg and MMHg in benthic biota with sediment levels suggest that variation in the bioaccumulation factor (SBAF) for invertebrates is best explained in terms of sediment organic content. Thus, the results from this study emphasize the importance of organic matter in regions removed from point source input in controlling both the concentration and bioavailability of MMHg to organisms. Because of the exponential nature of the SBAF/organic content relationship, there is a nonlinear organism response to MMHg in sediments that must be considered in any estimation of the toxic effect of sediment MMHg. Also, as a result of the decoupling between total Hg and MMHg concentration and bioavailability in surface sediments, any remediation evaluation of bioavailability and/or toxicity that is based only on total Hg concentration is unlikely to provide a reliable prediction.

title: Mercury Methylation from Unexpected Sources: Molybdate-Inhibited Freshwater Sediments and an Iron-Reducing Bacterium
abstract: Methylmercury has been thought to be produced predominantly by sulfate-reducing bacteria in anoxic sediments. Here we show that in circumneutral pH sediments (Clear Lake, CA) application of a specific inhibitor of sulfate-reducing bacteria at appropriate concentrations typically inhibited less than one-half of all anaerobic methylation of added divalent mercury. This suggests that one or more additional groups of microbes are active methylators in these sediments impacted by a nearby abandoned mercury mine. From Clear Lake sediments, we isolated the iron-reducing bacterium Geobacter sp. strain CLFeRB, which can methylate mercury at a rate comparable to Desulfobulbus propionicus strain 1pr3, a sulfate-reducing bacterium known to be an active methylator. This is the first time that an iron-reducing bacterium has been shown to methylate mercury at environmentally significant rates. We suggest that mercury methylation by iron-reducing bacteria represents a previously unidentified and potentially significant source of this environmental toxin in iron-rich freshwater sediments.

title: Mercury methylation in estuaries: Insights from using measuring rates using stable mercury isotopes
abstract: Rates of mercury (Hg) methylation and methylmercury (MeHg) demethylation in sediment of the Hudson River, Chesapeake Bay and Bay of Fundy were measured using stable isotopes of mercury (Hg) and methylmercury (MeHg). Methylation of the isotope correlated well with in situ MeHg concentration, and MeHg turnover times were on the order of days. It was concluded that methylation was more important than demethylation in controlling the differences in concentrations of MeHg among ecosystems. Also in situ MeHg concentration appeared to be a good indicator of methylation activity in sediment across ecosystems. Examination of a temporal data set, collected from the vicinity of Hart Miller Island, Chesapeake Bay between 1998 and 2002, and a spatial data set of a longitudinal transect of the main stem of the Chesapeake Bay, is used to further examine the controls on Hg methylation and MeHg concentration. Microbial activity and the formation of sulfide appear to be at least as important as Hg concentration in controlling MeHg concentration in estuarine sediment.

title: A non-steady-state compartmental model of global-scale mercury biogeochemistry with interhemispheric atmospheric gradients
abstract: A box model of mercury (Hg) cycling between the atmosphere and ocean is described and used to estimate Hg fluxes on a global scale (The Global/Regional Interhemispheric Mercury Model, GRIMM). Unlike previous simulations of this system, few assumptions are made concerning the rate of prominent marine biogeochemical processes affecting Hg (e.g., evasion, particle scavenging, and deep ocean burial). Instead, consistency with two observed atmospheric distributions was required: the interhemispheric gradient in total atmospheric Hg and the value for changes in the deposition of Hg from the atmosphere since industrialization observed in both hemispheres. Sensitivity analyses underscore the importance to modeling of the atmospheric lifetime of Hg, the magnitude of the interhemispheric gradient, the historical changes in Hg concentrations of various reservoirs, and vertical exchange between the surface ocean and the permanent thermocline. Results of the model indicate: lower evasional fluxes of Hg from the global ocean than previous estimates; a prominent role for particle scavenging as a removal mechanism from the surface ocean; a modest influence of dry processes (dust and gas) on Hg removal from the atmosphere; and an estimate of natural land-based sources of Hg to the atmosphere that is no more than about half that of anthropogenic sources.

title: Air-Sea Exchange in the Global Mercury Cycle
abstract: We present results from a new global atmospheric mercury model coupled with a mixed layer slab ocean. The ocean model describes the interactions of the mixed layer with the atmosphere and deep ocean, as well as conversion between elemental, divalent, and nonreactive mercury species. Our global mean aqueous concentrations of 0.07 pM elemental, 0.80 pM reactive, and 1.51 pM total mercury agree with observations. The ocean provides a 14.1 Mmol yr−1 source of mercury to the atmosphere, at the upper end of previous estimates. Re-emission of previously deposited mercury constitutes 89% of this flux. Ocean emissions are largest in the tropics and downwind of industrial regions. Midlatitude ocean emissions display a large seasonal cycle induced by biological productivity. Oceans contribute 54% (36%) of surface atmospheric mercury in the Southern (Northern) Hemisphere. We find a large net loss of mercury to the deep ocean (8.7 Mmol yr−1), implying a ∼0.7%/year increase in deep ocean concentrations.

title: Global anthropogenic mercury emission inventory for 2000
abstract: The paper reviews the current state of knowledge regarding global emissions of mercury and presents a new inventory of global emissions of mercury to the atmosphere from anthropogenic sources for the year 2000. The largest emissions of Hg to the global atmosphere occur from combustion of fossil fuels, mainly coal in utility, industrial, and residential boilers. As much as two-thirds of the total emission of ca. 2190 ton of Hg emitted from all anthropogenic sources worldwide in 2000 came from combustion of fossil fuels. Emissions of Hg from coal combustion are between one and two orders of magnitude higher than emissions from oil combustion, depending on the country. Various industrial processes account for additional 30% of Hg emissions from anthropogenic sources worldwide in 2000. Major contribution to emissions from this source category comes from gold production using Hg technology. The Asian countries contributed about 54% to the global Hg emission from anthropogenic sources in 2000, followed by Africa (18%) and Europe, including the European part of Russia (11%). China heads the list of the 10 countries with highest Hg emissions from anthropogenic activities. With more than 600 ton of Hg, China contributes about 28% to the global emissions of mercury. It is expected that future changes of Hg emissions from anthropogenic sources worldwide until the year 2020 should be within ±20% of the current estimates, although this assessment should be treated with great caution. Emission estimates for various continents presented in this paper were used to prepare global emission maps. These maps are presented in a companion paper (Wilson et al., 2005. Spatial distribution of global anthropogenic mercury atmospheric emissions. Atmospheric Environment, in this issue).