Mercury distributions in the North Pacific Ocean—20 years of observations

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<ul><li><p>sona, G.A. Gillb, L. Whalina</p><p>remineralization of settling particles, the influence of diagenetic processes from continental margin sediments and seasonal</p><p>0 (20concentrations among the different cruises, which reinforced the homogeneous characteristics of this particular water mass. In</p><p>addition, the comparison between the Atlantic and the Pacific deep water showed the presence of inter-ocean Hg fractionation</p><p>resulting in Hg concentrations in the deep North Pacific Ocean being three- to sixfold lower than in the deep Atlantic.</p><p>D 2004 Elsevier B.V. All rights reserved.</p><p>Keywords: Mercury; North Pacific Ocean; HgTOTdeep and bottom waters of the North Pacific Ocean averagstratification of the euphotic zone all appeared to be factors that can account for the distribution of Hg in the upper water. The</p><p>ed 1.10F 0.31 pM and were characterized by comparable Hgvertical mixing, ventilation, the presence of a thermocliaChesapeake Biological Laboratory, University of Maryland, Solomons, MD 20688, USAbDepartment of Marine Sciences, Texas A&amp;M University, Galveston, TX 77551, USA</p><p>Received 30 August 2003; received in revised form 5 January 2004; accepted 16 February 2004</p><p>Available online 15 June 2004</p><p>Abstract</p><p>Vertical mercury (Hg) distributions for the North and Central Pacific Ocean are reported here for three different cruises over</p><p>a time period of 20 years: N. Pac (1980), VERTEX (198687) and IOC (2002). The vertical distribution was not controlled</p><p>solely by the hydrographic characteristics or by internal biogeochemical nutrient type recycling and mixing processes.</p><p>Rather, Hg distribution appeared to be regulated by the local magnitude of external sources and the intensity of water column</p><p>processes. During the 2002 IOC cruise, the total mercury (HgTOT) concentrations averaged 1.15F 0.86 pM with the highestconcentrations found within the Japanese coastal waters. The overall upper-water Hg concentration average, calculated for the</p><p>IOC study (0.64F 0.26 pM), was similar to the earlier VERTEX cruise (0.58F 0.37 pM) but was lower compared to the N. Paccruise (1.40F 0.34). Variance in Hg concentrations in the upper water, generally within or close to the main thermocline, wasobserved among several stations and for both VERTEX and IOC campaigns. Horizontal advection of water along isopycnals,</p><p>ne associated with remobilization of mercury as a result ofF.J.G. Lauriera,*, R.P. MaMercury distributions in the North Pacific</p><p>Ocean20 years of observationsMarine Chemistry 91. Introduction</p><p>Mercury (Hg) concentrations in the low picomolar</p><p>range have been reported for open-ocean environ-</p><p>0304-4203/$ - see front matter D 2004 Elsevier B.V. All rights reserved.</p><p>doi:10.1016/j.marchem.2004.02.025</p><p>* Corresponding author.</p><p>E-mail address: (F.J.G. Laurier)</p><p>04) 319ments including the North and South Pacific Oceans</p><p>(Fitzgerald et al., 1984; Gill and Fitzgerald, 1985,</p><p>1987, 1988; Kim and Fitzgerald, 1986, 1988; Gill and</p><p>Bruland, 1987; Mason and Fitzgerald, 1990, 1991,</p><p>1993) and the North and South Atlantic Oceans</p><p>(Olafsson, 1983; Dalziel and Yeats, 1985; Gill and</p><p>Fitzgerald, 1988; Cossa and Martin, 1991; Cossa et</p></li><li><p>F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3194al., 1992; Mason et al., 1995a,b,1998, 2001; Cossa et</p><p>al., 1996, 1997; Lamborg et al., 1999; Mason and</p><p>Sullivan, 1999) for more than 20 years. Many current</p><p>studies have focused on the speciation and the distri-</p><p>bution of the different Hg species present in the water</p><p>column, such as dissolved gaseous Hg, reactive Hg,</p><p>dimethyl and methyl Hg. The specific interest in</p><p>methylated Hg compounds is driven by its toxicity</p><p>and ability to bioaccumulate through the food chain</p><p>(Wiener et al., 2003). Consumption of marine fish and</p><p>shellfish is a primary exposure pathway of Hg to</p><p>humans (NRC, 2000). However, methylated Hg spe-</p><p>cies typically represent only a few percent of the total</p><p>Hg concentration present in the water column and</p><p>hence will not significantly affect its overall distribu-</p><p>tional features in the ocean.</p><p>The oceans play a crucial role in the global Hg</p><p>cycle; serving both as a source, as well as a sink, for</p><p>atmospherically derived Hg (Mason et al., 1994a;</p><p>Mason and Sheu, 2002; Laurier et al., 2003a,b).</p><p>Mercury already present and/or deposited to the</p><p>ocean, via wet and dry deposition, can be converted</p><p>to dissolved gaseous Hg (elemental Hg) within the</p><p>upper ocean. The elemental Hg evades to the atmo-</p><p>sphere and this airsea exchange process constitutes a</p><p>major flux of Hg to and from the ocean and is a major</p><p>mechanism controlling the residence time of Hg in the</p><p>surface ocean ( &lt; 5 years, Mason et al., 1994a).</p><p>Moreover, the coupling between atmospherically</p><p>borne Hg contamination and high methylmercury</p><p>concentration in fish has been recognized (Rolfhus</p><p>and Fitzgerald, 1990).While the surface concentra-</p><p>tions and distributions of Hg appear to be controlled</p><p>mainly by airsea exchange processes, alterations can</p><p>also occur due to upwelling and particle scavenging</p><p>processes (Gill and Fitzgerald, 1987). Surface water</p><p>depletion of Hg has been reported due to biological</p><p>activities by scavenging and/or photoreduction (Kim</p><p>and Fitzgerald, 1986; Mason et al., 1995a,b). Gill and</p><p>Fitzgerald (1987) presented evidence for particle</p><p>scavenging removal in productive surface waters of</p><p>the equatorial Pacific, but very little direct information</p><p>on particle scavenging of mercury in open ocean</p><p>surface waters exists, making it difficult to assess</p><p>the relative importance of this process on an ocean-</p><p>wide basis. Nevertheless, Mason and Fitzgerald</p><p>(1996) suggest that particle scavenging and sinkingof Hg from surface water is a relatively small flux.Indeed, the model proposed by Mason and Sheu</p><p>(2002) shows that deep water mixing is a more</p><p>important source of Hg to the deep ocean than particle</p><p>sinking.</p><p>High Hg concentrations have been observed within</p><p>the mixed layer/thermocline region and could reflect a</p><p>Hg enrichment due to the presence of high particle</p><p>densities (Gill and Fitzgerald, 1988; Cossa et al., 1992;</p><p>Mason and Fitzgerald, 1993; Mason et al., 1995a,b;</p><p>Cossa et al., 1996). In coastal areas, this feature</p><p>appears to be produced by margin sediment sources,</p><p>and in the open ocean, it can arise from lateral</p><p>advection when density surfaces outcrop at different</p><p>latitudes where Hg deposition to the surface is elevated</p><p>(Gill and Fitzgerald, 1988; Cossa et al., 2004).</p><p>Below the thermocline, Hg concentrations typically</p><p>decrease with depth, suggesting that scavenging re-</p><p>moval processes occur and are important in maintain-</p><p>ing low Hg concentrations at depth. Rapid scavenging</p><p>removal along deep-water circulation pathways may</p><p>also be responsible for depleting Hg in deep water of</p><p>the North Pacific relative to the North Atlantic Ocean,</p><p>similar to other particle reactive elements such as Al or</p><p>Pb, whose inter-ocean fractionation patterns are simi-</p><p>lar to Hg (Orians and Bruland, 1986; Schaule and</p><p>Patterson, 1981). In addition, anthropogenic inputs</p><p>have a relatively larger impact on the North Atlantic</p><p>compared to the North Pacific.</p><p>The first investigations of the Hg distribution in the</p><p>open ocean (Olafsson, 1983; Dalziel and Yeats, 1985;</p><p>Gill and Fitzgerald, 1985, 1987, 1988; Gill and Bru-</p><p>land, 1987) and coastal waters (Dalziel, 1992; Cossa</p><p>and Martin, 1991) reported mainly reactive and total</p><p>Hg distributions. For the Pacific Ocean, the first</p><p>research of Hg speciation was carried out in the</p><p>equatorial Pacific (Kim and Fitzgerald, 1988; Mason,</p><p>1991; Mason and Fitzgerald, 1991, 1993) and showed</p><p>that in the Pacific Ocean environment strongly con-</p><p>trasts, from a productivity perspective, with the more</p><p>oligotrophic Atlantic regions covered by the more</p><p>recent studies (Mason et al., 1995a,b, 1998; Cossa et</p><p>al., 1997; Mason and Sullivan, 1999). The previously</p><p>cited Pacific studies also demonstrated the importance</p><p>of surface water productivity and deep-water reminer-</p><p>alization, and associated microbial activity, in control-</p><p>ling Hg concentration and speciation. Mason et al.</p><p>(1994b) developed a budget for the Pacific Oceansurface waters, which indicated that Hg supply by</p></li><li><p>tion during transport due to particulate dissolution.</p><p>In the past 20 years, there have been only a limited</p><p>F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 319 5number of studies where the vertical distribution of</p><p>total Hg concentrations (HgTOT) has been reported for</p><p>the Pacific Ocean; three profiles in the South Pacific</p><p>and two for the North Pacific (Gill and Bruland, 1987;</p><p>Gill and Fitzgerald, 1988). According to Gill and</p><p>Fitzgerald (1988), vertical Hg distributions in the</p><p>North Pacific seem to be governed by an external</p><p>cycling process, in which water column distributions</p><p>reflect a rapid competition between the magnitude of</p><p>the input source, primarily atmospheric and the inten-</p><p>sity of the removal process occurring in the water</p><p>column. This hypothesis is reinforced by the similarity</p><p>between the vertical distribution features of Hg and</p><p>other reactive elements, such as Al, Pb and Bi, which</p><p>are introduced primarily by atmospheric input. These</p><p>elements have been shown to have oceanic residence</p><p>times shorter than the oceanic residence time.</p><p>Here, we report new information on the vertical</p><p>distribution of total Hg for the North Pacific Ocean</p><p>obtained as a part of an IOC (Intergovernmental</p><p>Oceanographic Commission) cruise in 2002 and com-</p><p>pare these results with information obtained from the</p><p>North Pacific Ocean for the past 20 years. Included in</p><p>this comparison are the earlier studies noted above</p><p>and previously unpublished work from the VERTEX</p><p>program obtained in 19861987. Station locations</p><p>include open ocean and coastal areas for both the</p><p>west and the east North Pacific Ocean. The data are</p><p>used to support the contentions regarding Hg cycling</p><p>pointed out in previous studies and to report on other</p><p>processes affecting the vertical distribution and cy-</p><p>cling of mercury in the ocean from a temporal, spatial</p><p>and seasonal perspective.</p><p>2. Methods</p><p>Samples were collected during the 2002 Interna-</p><p>tional Oceanographic Commission (IOC) Baseline</p><p>Trace Metal cruise on the R/V Melville on a transectupwelling of thermocline water in the equatorial</p><p>region exceeded deposition but that deposition</p><p>exceeded evasion at mid-latitudes. The thermohaline</p><p>circulation thus transported this net mid-latitude Hg</p><p>input to the equator with enhancement of concentra-across the central and western North Pacific. Thegeneral aim of the research proposed by the 2002</p><p>IOC cruise was to examine the relationships between</p><p>atmospheric dust deposition and reactive trace ele-</p><p>ment additions to surface waters, and the impact of</p><p>these inputs on the cycling and transport of a variety</p><p>of biologically and geochemically significant trace</p><p>elements (including Hg, Al, Fe, As, Sb and Se). The</p><p>proposed cruise track provided the opportunity to</p><p>collect seawater samples spanning a large range of</p><p>dust deposition fluxes into a variety of hydrological-</p><p>ly distinct biogeochemical zones. The vertical sam-</p><p>pling at selected stations were performed in order to</p><p>determine the penetration of these surface signals</p><p>into the thermocline and intermediate waters of the</p><p>North Pacific. Samples were collected using pre-</p><p>cleaned Go-Flo bottles. Between 12 and 18 depths</p><p>(for the deepest casts) were sampled per cast and</p><p>water was decanted from the Go-Flo bottles into</p><p>acid-cleaned Teflon bottles in a laminar flow hood as</p><p>soon as possible after boarding the bottles. For</p><p>surface water collections, samples were collected</p><p>using a fish sampler (Cutter and Measures,</p><p>1999). The fish sampler is a device that swims</p><p>in the surface waters abreast of the ship while it is</p><p>moving forward, thus removing the sampling inlet</p><p>from the shipss contamination. Water was pumped</p><p>into the laminar flow hood through cleaned tubing in</p><p>a way to prevent any contamination of the collected</p><p>water.</p><p>Samples were analyzed in a cleanroom container</p><p>for total mercury concentrations (HgTOT) on board, as</p><p>soon as possible after collection (within 6 h). HgTOTmeasurements were performed using bromine mono-</p><p>chloride (BrCl) pre-oxidation method (Mason and</p><p>Fitzgerald, 1993) followed by the addition of acidic</p><p>10% stannous chloride solution (SnCl2) for the reduc-</p><p>tion. The samples were then bubbled using a 1-l glass</p><p>bubbler for 15 min at 100 ml min 1 with Hg-freeargon in the laminar flow hood and the HgTOTreleased was trapped on gold columns. Quantification,</p><p>by cold vapor atomic fluorescence (CVAFS), was</p><p>achieved by heating the gold columns in a stream of</p><p>argon. The released Hg vapor was flushed into a</p><p>quartz cell of the atomic fluorescence detector</p><p>(Bloom and Fitzgerald, 1988). The detection limit,</p><p>corresponding to three times the standard deviation</p><p>on the blank is 0.20 pM and the precision averages5% of the measurements.</p></li><li><p>3. Result and discussion</p><p>3.1. Hydrographic setting</p><p>3.1.1. Mixed layer</p><p>The circulation of the North Pacific Ocean upper</p><p>water is characterized by two gyres, the Subtropical</p><p>Gyre and the Subarctic Gyre. The Subartctic gyre is</p><p>bordered by the North Equatorial Current (NEC) and</p><p>the North Pacific Current (NPC) at its south end and</p><p>north end, respectively (Fig. 1). The NEC flows west</p><p>and on approaching the western boundary, it divides</p><p>with some water going south to the NEC and some</p><p>north. The latter continues northeast past Japan as the</p><p>Kuroshio current. After its separation point, which is</p><p>reached near 35jN, the Kuroshio leaves the Japanesecoast to flow east and becomes the Kuroshio exten-</p><p>sion to about 170jE. From there, the Kuroshioextension is referred to as the North Pacific Current</p><p>(Fig. 1). Similarly to all western boundary currents,</p><p>the Kuroshio extends to great depth, well below the</p><p>current characterized by warm and saline water con-</p><p>trasting with the Oyashio water, which is typically</p><p>cold (between 2.3 and 3.8 jC) and less saline (33x).As the NPC approaches the North American conti-</p><p>nent, it divides into the California current, flowing</p><p>southward while the remnant water swings north to</p><p>form the Alaskan Gyre in the Gulf of Alaska. The</p><p>Northern Gyre of the North Pacific Ocean is the</p><p>Subpolar Gyre that forms the Polar Front by meeting</p><p>the NPC. The Subpolar Gyre is delimited by the</p><p>Alaskan Current and the NPC at its north end and</p><p>south end, respectively.</p><p>The different stations of the 2002 IOC cruise can</p><p>be sorted in three main geographic groups (Fig. 1),</p><p>including high (stations 2, 3 and 4), middle (stations</p><p>1, 5 and 6) and low latitudes (stations 7, 8 and 9). The</p><p>different IOC station locations are reported in Table 1.</p><p>Fig. 2a,b,c corresponds to the hydrographic charac-</p><p>teristic of the water masses along the cruise track, for</p><p>each station. Referring to Figs. 1, 2a,b,c includes the</p><p>two longitudinal transects: from station 1 to station 3</p><p>ook E</p><p>). End</p><p>F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3196thermocline which causes instabilities to develop</p><p>along its path. Also contributing to NPC is the</p><p>Oyashio, coming from the north from the Bering</p><p>Sea...</p></li></ul>