Mercury distributions in the North Pacific Ocean20 years of observations

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  • sona, G.A. Gillb, L. Whalina

    remineralization of settling particles, the influence of diagenetic processes from continental margin sediments and seasonal

    0 (20concentrations 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.

    D 2004 Elsevier B.V. All rights reserved.

    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

    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&M University, Galveston, TX 77551, USA

    Received 30 August 2003; received in revised form 5 January 2004; accepted 16 February 2004

    Available online 15 June 2004


    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 (198687) 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 1.15F 0.86 pM with the highestconcentrations found within the Japanese coastal waters. The overall upper-water Hg concentration average, calculated for the

    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,

    ne associated with remobilization of mercury as a result ofF.J.G. Lauriera,*, R.P. MaMercury distributions in the North Pacific

    Ocean20 years of observationsMarine Chemistry 91. Introduction

    Mercury (Hg) concentrations in the low picomolar

    range have been reported for open-ocean environ-

    0304-4203/$ - see front matter D 2004 Elsevier B.V. All rights reserved.


    * Corresponding author.

    E-mail address: (F.J.G. Laurier)

    04) 319ments including the North and South Pacific Oceans

    (Fitzgerald et al., 1984; Gill and Fitzgerald, 1985,

    1987, 1988; Kim and Fitzgerald, 1986, 1988; Gill and

    Bruland, 1987; Mason and Fitzgerald, 1990, 1991,

    1993) and the North and South Atlantic Oceans

    (Olafsson, 1983; Dalziel and Yeats, 1985; Gill and

    Fitzgerald, 1988; Cossa and Martin, 1991; Cossa et

  • F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3194al., 1992; Mason et al., 1995a,b,1998, 2001; Cossa et

    al., 1996, 1997; Lamborg et al., 1999; Mason and

    Sullivan, 1999) for more than 20 years. Many current

    studies have focused on the speciation and the distri-

    bution of the different Hg species present in the water

    column, such as dissolved gaseous Hg, reactive Hg,

    dimethyl and methyl Hg. The specific interest in

    methylated Hg compounds is driven by its toxicity

    and ability to bioaccumulate through the food chain

    (Wiener et al., 2003). Consumption of marine fish and

    shellfish is a primary exposure pathway of Hg to

    humans (NRC, 2000). However, methylated Hg spe-

    cies typically represent only a few percent of the total

    Hg concentration present in the water column and

    hence will not significantly affect its overall distribu-

    tional features in the ocean.

    The oceans play a crucial role in the global Hg

    cycle; serving both as a source, as well as a sink, for

    atmospherically derived Hg (Mason et al., 1994a;

    Mason and Sheu, 2002; Laurier et al., 2003a,b).

    Mercury already present and/or deposited to the

    ocean, via wet and dry deposition, can be converted

    to dissolved gaseous Hg (elemental Hg) within the

    upper ocean. The elemental Hg evades to the atmo-

    sphere and this airsea exchange process constitutes a

    major flux of Hg to and from the ocean and is a major

    mechanism controlling the residence time of Hg in the

    surface ocean ( < 5 years, Mason et al., 1994a).

    Moreover, the coupling between atmospherically

    borne Hg contamination and high methylmercury

    concentration in fish has been recognized (Rolfhus

    and Fitzgerald, 1990).While the surface concentra-

    tions and distributions of Hg appear to be controlled

    mainly by airsea exchange processes, alterations can

    also occur due to upwelling and particle scavenging

    processes (Gill and Fitzgerald, 1987). Surface water

    depletion of Hg has been reported due to biological

    activities by scavenging and/or photoreduction (Kim

    and Fitzgerald, 1986; Mason et al., 1995a,b). Gill and

    Fitzgerald (1987) presented evidence for particle

    scavenging removal in productive surface waters of

    the equatorial Pacific, but very little direct information

    on particle scavenging of mercury in open ocean

    surface waters exists, making it difficult to assess

    the relative importance of this process on an ocean-

    wide basis. Nevertheless, Mason and Fitzgerald

    (1996) suggest that particle scavenging and sinkingof Hg from surface water is a relatively small flux.Indeed, the model proposed by Mason and Sheu

    (2002) shows that deep water mixing is a more

    important source of Hg to the deep ocean than particle


    High Hg concentrations have been observed within

    the mixed layer/thermocline region and could reflect a

    Hg enrichment due to the presence of high particle

    densities (Gill and Fitzgerald, 1988; Cossa et al., 1992;

    Mason and Fitzgerald, 1993; Mason et al., 1995a,b;

    Cossa et al., 1996). In coastal areas, this feature

    appears to be produced by margin sediment sources,

    and in the open ocean, it can arise from lateral

    advection when density surfaces outcrop at different

    latitudes where Hg deposition to the surface is elevated

    (Gill and Fitzgerald, 1988; Cossa et al., 2004).

    Below the thermocline, Hg concentrations typically

    decrease with depth, suggesting that scavenging re-

    moval processes occur and are important in maintain-

    ing low Hg concentrations at depth. Rapid scavenging

    removal along deep-water circulation pathways may

    also be responsible for depleting Hg in deep water of

    the North Pacific relative to the North Atlantic Ocean,

    similar to other particle reactive elements such as Al or

    Pb, whose inter-ocean fractionation patterns are simi-

    lar to Hg (Orians and Bruland, 1986; Schaule and

    Patterson, 1981). In addition, anthropogenic inputs

    have a relatively larger impact on the North Atlantic

    compared to the North Pacific.

    The first investigations of the Hg distribution in the

    open ocean (Olafsson, 1983; Dalziel and Yeats, 1985;

    Gill and Fitzgerald, 1985, 1987, 1988; Gill and Bru-

    land, 1987) and coastal waters (Dalziel, 1992; Cossa

    and Martin, 1991) reported mainly reactive and total

    Hg distributions. For the Pacific Ocean, the first

    research of Hg speciation was carried out in the

    equatorial Pacific (Kim and Fitzgerald, 1988; Mason,

    1991; Mason and Fitzgerald, 1991, 1993) and showed

    that in the Pacific Ocean environment strongly con-

    trasts, from a productivity perspective, with the more

    oligotrophic Atlantic regions covered by the more

    recent studies (Mason et al., 1995a,b, 1998; Cossa et

    al., 1997; Mason and Sullivan, 1999). The previously

    cited Pacific studies also demonstrated the importance

    of surface water productivity and deep-water reminer-

    alization, and associated microbial activity, in control-

    ling Hg concentration and speciation. Mason et al.

    (1994b) developed a budget for the Pacific Oceansurface waters, which indicated that Hg supply by

  • tion during transport due to particulate dissolution.

    In the past 20 years, there have been only a limited

    F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 319 5number of studies where the vertical distribution of

    total Hg concentrations (HgTOT) has been reported for

    the Pacific Ocean; three profiles in the South Pacific

    and two for the North Pacific (Gill and Bruland, 1987;

    Gill and Fitzgerald, 1988). According to Gill and

    Fitzgerald (1988), vertical Hg distributions in the

    North Pacific seem to be governed by an external

    cycling process, in which water column distributions

    reflect a rapid competition between the magnitude of

    the input source, primarily atmospheric and the inten-

    sity of the removal process occurring in the water

    column. This hypothesis is reinforced by the similarity

    between the vertical distribution features of Hg and

    other reactive elements, such as Al, Pb and Bi, which

    are introduced primarily by atmospheric input. These

    elements have been shown to have oceanic residence

    times shorter than the oceanic residence time.

    Here, we report new information on the vertical

    distribution of total Hg for the North Pacific Ocean

    obtained as a part of an IOC (Intergovernmental

    Oceanographic Commission) cruise in 2002 and com-

    pare these results with information obtained from the

    North Pacific Ocean for the past 20 years. Included in

    this comparison are the earlier studies noted above

    and previously unpublished work from the VERTEX

    program obtained in 19861987. Station locations

    include open ocean and coastal areas for both the

    west and the east North Pacific Ocean. The data are

    used to support the contentions regarding Hg cycling

    pointed out in previous studies and to report on other

    processes affecting the vertical distribution and cy-

    cling of mercury in the ocean from a temporal, spatial

    and seasonal perspective.

    2. Methods

    Samples were collected during the 2002 Interna-

    tional Oceanographic Commission (IOC) Baseline

    Trace Metal cruise on the R/V Melville on a transectupwelling of thermocline water in the equatorial

    region exceeded deposition but that deposition

    exceeded evasion at mid-latitudes. The thermohaline

    circulation thus transported this net mid-latitude Hg

    input to the equator with enhancement of concentra-across the central and western North Pacific. Thegeneral aim of the research proposed by the 2002

    IOC cruise was to examine the relationships between

    atmospheric dust deposition and reactive trace ele-

    ment additions to surface waters, and the impact of

    these inputs on the cycling and transport of a variety

    of biologically and geochemically significant trace

    elements (including Hg, Al, Fe, As, Sb and Se). The

    proposed cruise track provided the opportunity to

    collect seawater samples spanning a large range of

    dust deposition fluxes into a variety of hydrological-

    ly distinct biogeochemical zones. The vertical sam-

    pling at selected stations were performed in order to

    determine the penetration of these surface signals

    into the thermocline and intermediate waters of the

    North Pacific. Samples were collected using pre-

    cleaned Go-Flo bottles. Between 12 and 18 depths

    (for the deepest casts) were sampled per cast and

    water was decanted from the Go-Flo bottles into

    acid-cleaned Teflon bottles in a laminar flow hood as

    soon as possible after boarding the bottles. For

    surface water collections, samples were collected

    using a fish sampler (Cutter and Measures,

    1999). The fish sampler is a device that swims

    in the surface waters abreast of the ship while it is

    moving forward, thus removing the sampling inlet

    from the shipss contamination. Water was pumped

    into the laminar flow hood through cleaned tubing in

    a way to prevent any contamination of the collected


    Samples were analyzed in a cleanroom container

    for total mercury concentrations (HgTOT) on board, as

    soon as possible after collection (within 6 h). HgTOTmeasurements were performed using bromine mono-

    chloride (BrCl) pre-oxidation method (Mason and

    Fitzgerald, 1993) followed by the addition of acidic

    10% stannous chloride solution (SnCl2) for the reduc-

    tion. The samples were then bubbled using a 1-l glass

    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,

    by cold vapor atomic fluorescence (CVAFS), was

    achieved by heating the gold columns in a stream of

    argon. The released Hg vapor was flushed into a

    quartz cell of the atomic fluorescence detector

    (Bloom and Fitzgerald, 1988). The detection limit,

    corresponding to three times the standard deviation

    on the blank is 0.20 pM and the precision averages5% of the measurements.

  • 3. Result and discussion

    3.1. Hydrographic setting

    3.1.1. Mixed layer

    The circulation of the North Pacific Ocean upper

    water is characterized by two gyres, the Subtropical

    Gyre and the Subarctic Gyre. The Subartctic gyre is

    bordered by the North Equatorial Current (NEC) and

    the North Pacific Current (NPC) at its south end and

    north end, respectively (Fig. 1). The NEC flows west

    and on approaching the western boundary, it divides

    with some water going south to the NEC and some

    north. The latter continues northeast past Japan as the

    Kuroshio current. After its separation point, which is

    reached near 35jN, the Kuroshio leaves the Japanesecoast to flow east and becomes the Kuroshio exten-

    sion to about 170jE. From there, the Kuroshioextension is referred to as the North Pacific Current

    (Fig. 1). Similarly to all western boundary currents,

    the Kuroshio extends to great depth, well below the

    current characterized by warm and saline water con-

    trasting with the Oyashio water, which is typically

    cold (between 2.3 and 3.8 jC) and less saline (33x).As the NPC approaches the North American conti-

    nent, it divides into the California current, flowing

    southward while the remnant water swings north to

    form the Alaskan Gyre in the Gulf of Alaska. The

    Northern Gyre of the North Pacific Ocean is the

    Subpolar Gyre that forms the Polar Front by meeting

    the NPC. The Subpolar Gyre is delimited by the

    Alaskan Current and the NPC at its north end and

    south end, respectively.

    The different stations of the 2002 IOC cruise can

    be sorted in three main geographic groups (Fig. 1),

    including high (stations 2, 3 and 4), middle (stations

    1, 5 and 6) and low latitudes (stations 7, 8 and 9). The

    different IOC station locations are reported in Table 1.

    Fig. 2a,b,c corresponds to the hydrographic charac-

    teristic of the water masses along the cruise track, for

    each station. Referring to Figs. 1, 2a,b,c includes the

    two longitudinal transects: from station 1 to station 3

    ook E

    ). End

    F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3196thermocline which causes instabilities to develop

    along its path. Also contributing to NPC is the

    Oyashio, coming from the north from the Bering

    Sea and with some contribution of the sea of

    Okhotsk waters. The Kuroshio is a western boundary

    Fig. 1. IOC 2002 and VERTEX cruises track. IOC Cruise Track C

    Begin Date/Port: 01May02 Osaka, Japan (34j65VN, 135j42VW

    numbers from 1 to 9 and the relative dates correspond to the depth-profiland from station 3 to station 7, and one latitudinal

    transect: from station 7 to station 9. The temperature

    and salinity vertical profiles along the cruise crack are

    displayed in Fig. 2a and b. The VERTEX stations are

    located in the northwestern part of the North Pacific

    xpedition, Leg 23, [COOK23MV]. Chief Scientist: Chris Measure.

    Date/Port: 04Jun02 Honolulu, HA (24j15VN, 153j84VE). The

    e sampling stations with the test as a test station.

  • F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 319 7mainly under the influence of the Alaskan Gyres and

    the western Subtropical Gyre for the northern (V7

    T7, V7T8) and southern (V7T5, V7T6) stations,

    respectively (Fig. 1).

    3.1.2. Intermediate water masses

    Between a depth of about approximately 200 and

    1000 m, three main water masses can be distin-

    guished: the North Pacific Central Water (NPCW)

    and the North Pacific Intermediate Water (NPIW)

    masses at both middle and low latitudes, and the

    Pacific Subarctic Water (PSW) at high latitudes.

    The NPCW extends from the Equatorial Water to

    about 40jN and includes both Western and EasternNorth Pacific Central Water, representing the main

    upper water mass of the North Pacific Ocean. The

    VERTEX stations include the western NPCW for the

    southern stations and the NPIW. Apart from station

    9, the middle and low latitudes IOC stations are

    located within the eastern NPCW. The NPIW, evident

    as low salinities and a salinity minimum, is found

    below the NPCW. North of the NPCW is the Pacific

    Subarctic Water mass (Favorite et al., 1976), which

    Table 1

    Position of the 2002 IOC cruise over the North Pacific Ocean

    Stations Latitudes (j) Longitudes (j)

    Station 1 34.28 N 146.59 E

    Station 2 44.00 N 154.59 E

    Station 3 50.00 N 167.00 E

    Station 4 39.21 N 170.34 E

    Station 5 33.45 N 170.35 E

    Station 6 30.30 N 170.34 E

    Station 7 24.15 N 170.19 E

    Station 8 26.54 N 176.21 W

    Station 9 22.45 N 158.00 Wextends across the greater part of the North Pacific

    Ocean. Its characteristic properties are low salinity

    (33.534.5x) and relatively low temperature (24jC), which is typically found for IOC stations 2 and3 and the northern VERTEX stations. Beside IOC

    stations 2 and 3, there was a well defined thermo-

    clines between 250 and 1000 m at the other IOC

    stations (Fig. 2a).

    3.1.3. Deep and bottom water masses

    Below the thermocline, the water mass in the

    approximate depth range of 1000/1500 to 3000 m, is

    usually called the Pacific Deep Water (PDW) with theAntarctic Bottom Water (AABW) entering the north-

    ern hemisphere below 3000 m. The PDW does not

    participate to a large extend in the thermohaline

    circulation and its properties are determined nearly

    entirely through slow mixing processes (Tomczak and

    Godfrey, 1994). Between 2000 m and the bottom, the

    water is characterized by uniform properties. The

    circulation of the AABW is very sluggish with a slow

    advection from the south, mixing with the water

    above. As seen in Fig. 2a and b, the limit between

    upper and deep water masses appears to be located at

    higher depth for the high-latitude stations than for the

    rest of the cruise track. This is most likely due to the

    intrusion of PDW in the Northern part of the North

    Pacific Ocean.

    3.2. Total mercury in the North Pacific Ocean during

    the 2002 IOC cruise

    3.2.1. Underway samples

    Surface water total mercury (HgTOT) concentra-

    tions were sampled by the underway surface sampler

    (Fish sampling device described by Cutter and

    Measures, 1999) and analyzed on board along the

    cruise track (Fig. 3). The HgTOT concentrations varied

    from 0.34 to 4.43 pM and averaged 1.15F 0.86 pM.The highest HgTOT concentrations were found within

    the Japanese coastal waters, most likely due to local

    sources, and decreased as the cruise reached the more

    remote waters of the North Pacific Ocean. No signif-

    icant pattern in HgTOT concentrations was observed

    along the cruise track, but a slight local increase in

    HgTOT concentrations was found around station 7

    (Figs. 1 and 3). These higher HgTOT concentrations,

    compared to the rest of the values obtained in the

    remote parts of the North Pacific Ocean, could partly

    be explained by an higher reactive gaseous mercury

    deposition over this area, as shown by Laurier et al.

    (2003a). The distribution of surface water HgTOTconcentrations should relate in some degree to the

    atmospheric deposition input patterns as this is the

    main source of Hg to the oceans (Gill and Fitzgerald,

    1987, 1988; Mason et al., 1992; Mason and Fitzger-

    ald, 1991, 1993, 1996; Mason et al., 1994a, 1998,

    2001; Mason and Sheu, 2002). For the North Pacific

    Ocean, Laurier et al. (2003a) showed a low produc-

    tion and low deposition of reactive gaseous mercury athigh latitudes in contrast to lower latitudes where both

  • F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3198production and deposition were enhanced. Conversely,

    dissolved gaseous Hg showed a higher evasional flux at

    low latitude compared to higher latitudes. These high

    surface Hg concentrations could also be the result of a

    past dust deposition event originating from Southeast

    Fig. 2. Hydrographic properties dAsia, as seen by the concomitant increase in surface-

    water silver, aluminum and iron concentrations in the

    same area (Brown et al., 2002; Buck et al., 2002; Sato et

    al., 2002). Aeolian dust deposition together with sea-

    sonal stratification of the euphotic zone is known to

    uring the 2002 IOC cruise.

  • F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 319 9influence the surface water concentrations of these

    dissolved reactive trace metals (Bruland et al., 1994;

    Measures and Vink, 2000). However, it is known that

    Hg is not elevated in dust relative to other atmospheric

    particles. For example, Lamborg et al. (1999) found

    that Hg concentrations on atmospheric particles did not

    increase with the increase in dust concentration and

    indeed enrichment factors decreased during dust events

    over the Atlantic Ocean.

    Direct aeolian dust input is likely to influence Hg

    concentration in the surface waters but to a smaller

    extent compared to local reactive gaseous mercury

    wet and dry deposition which is determined by MBL

    photochemistry. The surface water Hg distribution

    appears to reflect various local and external input

    mechanisms and its reactivity in the upper water

    column, which includes photoreduction/evasion and

    particulate scavenging processes, all superimposed

    upon the general pattern of wind-driven ocean circu-

    lation and mixing.

    Fig. 3. Surface water total mercury concentrations (pM) along th3.2.2. Mixed layer and upper water masses

    The HgTOT concentrations depth profiles are

    grouped in three figures corresponding to middle

    (Fig. 4), high (Fig. 5) and low (Fig. 6) latitudes. Within

    the mixed layer (upper 150 m), HgTOT concentration

    averaged 0.46F 0.21 pM with station 7 exhibiting thehighest values. Besides station 7, mixed-layer HgTOTconcentrations showed little differences along the

    cruise track and overall displayed a similar trend with

    a rapid decrease within the first 100 m (Figs. 46). The

    relative decrease corresponded on average to

    65F 17% of the HgTOT concentrations present in thesurface water.

    As mentioned previously, enhanced Hg at the

    surface is due to atmospheric Hg deposition, and

    localized stratification, which can inhibit mixing near

    the seaair interface (Gill and Bruland, 1987). As

    discussed in Section 3.3, the effect of seasonal strat-

    ification in the euphotic zone could result in the

    subsurface Hg enrichment.

    e 2002 IOC cruise track, 1st May 2002 to 4th June 2002.

  • throughout the depth range of 2001000 m, which

    includes the mixed layerupper water interface and

    the upper water masses (Fig. 2a,b).

    Conversely, an increase in concentrations was ob-

    served for stations 1, 2, 3 and 4, with averages signif-

    icantly higher within in the upper water mass

    (0.76F 0.28 pM) in contrast to the mixed layer(0.45F 0.16 pM). This increase in concentration couldbe partly the result of mixing between the depleted

    mixed layer and the upper water masses (NPCW, PSW

    and PSW). Moreover, a rapid transfer of Hg from

    surface to deeper water that occurs during the genera-

    tion of settling particles could also result in an enrich-

    ment in the subsurface and the upper water mass. Thus,

    the combination of accumulation of organic matter

    within the pycnocline and particulate scavenging of

    mercury throughout the water column, and especially

    in the deep water, could result in the sub-surface

    F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 31910The decrease in the concentrations within the

    previously defined mixed layer (from 0 to f 150 mdeep), which is subject to both photochemistry and

    turbulence, can partly be explained by photoreduction

    of Hg in the surface water and subsequent loss by

    evasion. In addition, particulate scavenging processes

    will also remove Hg from the upper water. Below 150

    m, this tendency was interrupted in some cases by a

    re-increase, rapid for stations 5 and 6, or by fairly

    constant HgTOT concentrations, for most of the other

    stations (Figs. 46). Stations 5 and 6 revealed a

    similar increase in HgTOT concentrations (Fig. 4)

    corresponding to the mixed layer-NPCW interface

    (depth range 100200 m), which was characterized

    by a specific salinity of 34.6 (Fig. 2b). Similarly, one

    high value in HgTOT concentration is observed for

    station 7 at depth 300 m, which also corresponded to

    the same specific water layer (Figs. 2b and 4). For the

    rest of the stations, no significant trend was reported

    maximum. For station 1, the variation in the concen-

    Fig. 4. Vertical distribution of total mercury concentrations during

    the 2002 IOC cruise track, 1st May 2002 to 4th June 2002.Fig. 5. Vertical distribution of total mercury concentrations duringthe 2002 IOC cruise track, 1st May 2002 to 4th June 2002.

  • F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 319 11tration corresponded to the thermocline, which can, as

    discussed below, partly account to the sub-surface Hg

    maximum. However, for stations 2 and 3, no thermo-

    cline was present. For these two high-latitude stations,

    the Hg concentration enhancement in the below surface

    region corresponded to the O2 minimum zone. Partic-

    ulate sinking and dissolution is known to be an impor-

    tant source of Hg to the low-oxygen region.

    Calculations suggest that the Hg particulate Hg flux

    could account for about 60% of the Hg supplied to the

    low-oxygen region (Mason and Fitzgerald, 1991). An

    increased particulate flux could support higher micro-

    bial activity in the low-oxygen water and deliver more

    particulate associated Hg to this region (Mason and

    Fitzgerald, 1993). Consequently, changes in the supply

    of particulate matter to the low-oxygen zone associated

    with changes in productivity and latitudinal changes

    associated with horizontal advection most likely ac-

    count for the observed trend. For station 1, horizontal

    Fig. 6. Vertical distribution of total mercury concentrations during

    the 2002 IOC cruise track, 1st May 2002 to 4th June 2002.advection is likely playing an important role. The

    highest concentrations for station 1 corresponded ex-

    actly to the intrusion of low salinity, low temperature

    water observed between 400 and 800 m deep (Fig. 2a

    and b). The proximity to station 1 of both the Oyashio

    and the PSW could influence the hydrographic char-

    acteristic and, consequently, the HgTOT concentrations,

    as seen by the comparable values between stations 1

    and 2 for the considered depth range. However, the

    exact origin of this intrusive water mass is unknown.

    The Hg maximum, located within or near waters

    comprising the main thermocline region, was a char-

    acteristic feature of six of the nine stations. Vertical

    distributions with a subsurface maximum are well

    known and occur for several elements (Bruland,

    1980; Landing and Bruland, 1987; Bruland et al.,

    1994). Beside direct atmospheric deposition and dia-

    genetic remobilization of mercury (vertical transfer),

    horizontal advection along isopycnals of Hg enriched

    water can be an important factor on the distribution of

    mercury and other trace metals (Flegal et al., 1986;

    Mason and Fitzgerald, 1991; Mason et al., 1994b,

    1998). According to Gill and Fitzgerald (1988), dia-

    genetic reactions in margin sediments combined with

    such advective transport is likely to be responsible for

    Hg maximum for waters close to continental margin,

    which could also be an explanation for the feature

    observed for station 1.

    For the open ocean area, the accumulation of par-

    ticulate matter at the pycnocline can partly account for

    the subsurface Hg maximum. On the other hand, if this

    feature was ubiquitous in the open North Pacific

    Ocean, there should be a similar feature for all the

    stations, which was not the case. Lateral advection

    along the isopycnals is also likely to contribute to

    elevated Hg concentration when isopycnal surfaces

    from high Hg deposition area, outcrop at higher lat-

    itudes. Consequently, both lateral and vertical mixing

    can potentially control the distribution and the latitu-

    dinal variation of Hg concentration in the upper water.

    However, the signal from both processes is likely to be

    non-continuous as, for example, high particulate fluxes

    are not continuous but are related to seasonal surface

    water productivity. The transient nature of these ther-

    mocline maxima is evident in the data collected on the

    North Atlantic near Bermuda (Mason et al., 2001).

    Beside the discussed increase in the values withinthe upper water masses, HgTOT concentrations were

  • where the lower values were found (Figs. 46). For

    the rest of the stations, no significant trend either was

    observed in HgTOT concentrations between different

    or within each upper water mass. Other parameters,

    such as productivity combined with biological up-

    take, influence the distribution of Hg in the water

    column. Since productivity is season dependent, it

    could lead to seasonal differences in Hg, whether by

    increasing concentrations if the particulate matter

    accumulates within the thermocline; or by removing

    the Hg from the water column when the particles

    undergo sedimentation.

    3.2.3. Deep and bottom water masses

    Deep and bottom water Hg profiles were obtained

    for stations 2, 7 and 9 exclusively (Fig. 7). For the deep

    water mass, in the 15003000 depth range, HgTOTconcentrations averaged 1.19F 0.27 pM and weresignificantly higher ( p < 0.05) compared to those mea-

    F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 31912overly characterized by a low average of 0.65 pM and

    a low standard deviation of 0.26 pM. These values

    point out the much lower and less variable HgTOTcompared to other oceans, such as the North Atlantic

    Ocean (Mason et al., 1998) or other open ocean

    regions (Cossa et al., 1992). This feature also illus-

    trates the relative homogeneity of upper-water Hg

    concentrations throughout the remote parts of the

    North Pacific Ocean. In contrast to other oceans, (Gill

    and Fitzgerald, 1988; Mason et al., 1998; Mason and

    Fitzgerald, 1991), the presence or the absence of a

    thermocline did not seem to play a critical role on the

    distribution of HgTOT concentrations in the North

    Pacific Ocean. In previous studies, maximum Hg

    concentrations were observed in the thermocline zone

    suggesting a potential Hg enrichment in the particu-

    late matter that accumulates and decomposes within

    this zone (Mason et al., 1998).

    In the upper water, it appears that the vertical

    distribution of the hydrographic characteristics only

    influenced the HgTOT concentrations to a limited

    extent, since no general trend in HgTOT concentrations

    in relation to the hydrographic characteristics was

    observed within the upper water masses. The only

    significant tendencies in the concentrations, mainly

    observed within the mixed layer, were most likely due

    to the chemistry of Hg such as photo-reduction and

    evasion, particulate scavenging and dilution processes

    for the mixed layerupper water interface. Except

    for stations 1 and 2 that showed higher concentrations

    in the upper water and the mixed layer, respectively,

    no strong difference in HgTOT concentrations was

    observed between the geographic locations (high,

    middle and low latitudes).

    Between 800 and 1500 m, corresponding to the

    deeper parts of the upper water including the upper

    waterdeep water interface, HgTOT concentrations

    averaged 0.73F 0.23 pM and remained relativelyconstant except for slight increases observed for

    stations 2 and 3. As seen in Fig. 2a,b,c, these two

    stations were characterized by the proximity of the

    PSW in comparison to other location stations and the

    potential influence of the general overall upwelling of

    the PDW. This hypothesis is supported by the higher

    HgTOT concentrations found for stations 2 and 3, at a

    depth of 1500 m. Additionally, stations 1 and 4 are

    also likely to be subject to a stronger influence of thedeep water in contrast to the lower latitudes stations

    Fig. 7. Vertical distribution of total mercury concentrations duringthe 2002 IOC cruise track, 1st May 2002 to 4th June 2002.

  • were obtained. In 1980, the analytical reproducibility

    was only 2030% (Gill and Fitzgerald, 1988).

    In the mixed layer, VERTEX stations (V7T5,

    V7T6 and V7T7) exhibited the same trend as for

    the IOC data, with high Hg concentrations in the

    mixed layer followed by a decrease within the first

    100 m. For the upper water, the most striking feature

    for these stations is the constancy and the small

    variability in the concentrations, which is comparable

    to the IOC cruise data (Fig. 8). Conversely, the V7T8

    station displayed quite a different profile in Hg con-

    centrations with two important increases, first between

    the near surface and 250 m followed by constant

    values and a second increase between approximately

    1000 and 1500 m. Even though both increases result in

    higher Hg concentrations than those for the other

    VERTEX and IOC profiles within the same depth

    ranges, the upper Hg maximum resembles those

    reported for stations 5 and 6, while at the remaining

    F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 319 13sured for the mixed layer plus the upper water mass,

    which averaged 0.64F 0.26 pM. Furthermore, a gen-eral increase in the concentration was observed for all

    the three stations with a relative increase that averaged

    94F 21% within the considered depth range. Thehighest concentrations were at station 2, whereas

    stations 7 and 9 were characterized by a comparable

    distribution in the concentrations. Similar to the upper

    water masses, the concentrations in total Hg were low

    and showed a small variability within the North Pacific

    Ocean contrary to other ocean basin. Below 3000 m,

    where the water mass theoretically corresponded to the

    AABW, HgTOT concentrations averaged 1.06F 0.27and remained constant without exhibiting a significant

    tendency and difference in the concentrations com-

    pared to the deep water. They were, however, still

    higher than those measured in the upper water masses

    (Fig. 7). Higher particulate load at the interfaces

    between water masses and in the deep water could

    result in an increase in Hg concentrations by the same

    mechanism discussed above for the thermocline. More

    likely, the limited effect of mixing combined with

    particulate scavenging constantly removes Hg, there-

    fore maintaining a relative low and constant level for

    the bottom water. Given that the HgTOT concentrations

    were higher in the NPIW, upwelling of these deep

    waters is likely to supply additional Hg to the upper-

    water zone and enhance the transfer of Hg between the

    deeper water and the mixed layer.

    3.3. Spatial, temporal and seasonal variations in Hg

    distribution in the North Pacific Ocean

    This comparison is based on the Hg profiles

    obtained during the following campaigns: N. Pac

    1980 (Gill and Fitzgerald, 1988), VERTEX (1986

    87) and IOC (2002, this study). The measurements

    from the N. Pac 1980 cruise took place in the north

    central Pacific Ocean. For the methods used by Gill

    and Fitzgerald (1988), Hg levels were defined as

    the fraction of Hg that was SnCl2 reducible after

    24 h digestion with acid. These measurements are

    comparable to our HgTOT determinations since the

    waters were mostly oligotrophic, with low particulate

    matter and low dissolved organic matter concentra-

    tions. However, some caution should be exercised in

    the interpretation of the profile features, particularlygiven the time period in which these measurements

    Fig. 8. Vertical distribution of total mercury concentrations duringthe VERTEX cruise (VII) track, JulyAugust 1987.

  • concentration was less pronounced in the thermocline

    in December 1999 and in March 2000.

    Seasonal changes in mercury in the upper water

    column result mainly from the annual development of

    a near surface seasonal thermocline, which inhibits

    vertical mixing during summer months. Thus, as Hg

    input is mainly atmospheric, buildup of Hg could

    seasonally occur in this surface layer. The euphotic

    zone forms a shallow, oligothrophic, region overlying

    the subsurface waters and, according to Bruland et al.

    (1994), the physical, biological and chemical structure

    within a stratified euphotic zone enhanced the effect

    of atmospheric inputs of other reactive trace metals

    (Al, Fe and Mn) on mixed layer concentrations. In

    addition, the low summertime scavenging rate within

    the surface mixed layer allows the atmospheric input

    Fig. 9. Temporal and spatial comparison of vertical distribution

    of total mercury concentrations during the 2002 IOC cruise

    (MayJune 2002), VERTEX cruise (JulyAugust 1987) and the

    N. Pac station (14j40VN; 160j00VW) aboard the R/V T.G.Thompson (October 1980).

    F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 31914depths levels, Hg was not significantly different from

    those described previously for the IOC cruise.

    The same explanations, as discussed previously,

    could account for the presence of the Hg maxima in

    V7T8 station subsurface waters, particularly the

    influence of margin coastal sediment Hg sources

    combined with lateral advection. Martin et al. (1989)

    measured Fe during the same VERTEX (V7) cam-

    paign and reported that an increase in dissolved Fe

    concentrations around the 1000 m depth was depen-

    dent upon distance from the continental margin, where

    supplemental Fe was originally introduced, via Fe-

    rich particulates, into the water column.

    Contrary to the IOC results, there was no differ-

    ence in the concentration between the mixed layer and

    the upper water mass. This could be due to a stronger

    mixing within the subsurface waters in the higher

    latitudes of the northeast Pacific Ocean. Nevertheless,

    the overall upper-water Hg concentration average,

    calculated for VERTEX (0.58F 0.37 pM), remainsstatistically equivalent to the average for the IOC

    study (0.64F 0.26 pM) and the deep water profile(V7T7) also exhibits similar trend with an increase

    in the deep water (10003000 m deep) followed by

    the constancy in the concentration for the bottom

    water (Fig. 9). The relative increase in Hg concen-

    trations between 1000 and 3000 m represented up to

    213% of the concentration measured at 1000 m.

    Furthermore, for the N. Pac 1980 campaign, the Hg

    depth profile within the deep and bottom waters was

    characterized by comparable concentrations (Fig. 9),

    which reinforced the homogeneous characteristics of

    this particular water mass. However, higher Hg con-

    centrations were reported for the mixed layer and the

    upper water mass compared to both VERTEX and IOC.

    The Hg distribution of this remote location is likely to

    be influenced by lateral advection and ventilation, but

    could also be partly due to the influence of seasonal

    variation in the hydrographic properties of the subsur-

    face water since the measurement occurred at the end of

    the summer. Similarly, Gill and Fitzgerald (1988) and

    Mason et al. (2001) both reported seasonal transient

    high concentrations in the main thermocline and at

    comparable depth in the northwest Atlantic Ocean near

    Bermuda, in July 1979 and September 1983 for the first

    study and September 1999 for the second. These

    measurements took place during a period of highproductivity, and, in contrast, the maximum in Hg of dissolved metal to greatly accumulate.

  • Given the mercury input to surface waters by

    atmospheric wet and dry deposition, the presence of

    a near surface seasonal thermocline is likely to induce

    a temporary Hg enrichment in the mixed layer. The

    effect of such stratification on Hg distribution is

    clearly illustrated in Fig. 10, where the measurements

    were performed at one particular location, south of the

    VERTEX stations (33jN, 139jW). The mercurymaximum that developed within the near surface

    exhibited concentrations two to three times higher in

    late summer than spring. During the winter, the lack

    of stratification combined with a faster removal by

    mixing and the oligotrophic conditions lead to lower

    and less variable concentrations. On the other hand,

    higher atmospheric Hg deposition during the summer

    and the enhanced recycling of atmospheric mercury

    species in the lower latitudes of the North Pacific

    Ocean could additionally account for the surface

    enrichment (Laurier et al., 2003a).

    F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 319 15Fig. 10. Seasonal variation in vertical Hg distribution during theVERTEX campaign at station: 33jN; 139jW.According to their oceanic profiles, reactive trace

    metals can be classified as nutrient-type and as

    scavenged-type (Martin et al., 1989; Bruland et al.,

    1994). Nutrient-type metals are removed from surface

    water by biogenic particles and remineralized in inter-

    mediate and deep waters with dissolution of biogenic

    material. Internal biogeochemical cycles with physical

    mixing and circulation patterns control the distribution

    of the nutrient-type metals. Their concentrations in the

    relatively young deep waters of the North Atlantic are

    substantially less compared to the older, nutrient-rich,

    North Pacific deep water. Scavenged-type metals un-

    dergo net particulate removal throughout the water

    column, even from deep waters. They exhibit concen-

    tration maxima corresponding to their external sources

    and have deep-water concentrations higher in the North

    Atlantic compared to the North Pacific. External

    inputs, such as aeolian dust, wet and dry deposition,

    control the concentrations and the distribution of the

    scavenged-type metals. It must be noted that for trace

    metals with a large anthropogenic signal, differences

    between the deep waters of the Atlantic and the Pacific

    may also be due to the enhanced inputs in the last

    century. Thus, because of the ocean circulation pat-

    terns, the anthropogenic inputs would be seen as a

    signal in the deep North Atlantic waters but not to the

    same extent in the deep North Pacific.

    During both the Vertex and IOC cruises, Hg vertical

    profiles in the North Pacific showed interesting simi-

    larities with dissolved Fe and Mn characteristics as

    reported by Bruland et al. (1994). Both Fe and Mn

    exhibited a maximum concentration in the surface

    mixed layer, with a sharp gradient through the ther-

    mocline. Contrary to Fe, Hg does not display a

    nutrient-type distribution at greater depth, below the

    surface mixed layer (Martin et al., 1989; Bruland et al.,

    1994). However, Hg is known to exhibit comparable

    features to the Mn distribution (Cossa et al., 1988;

    Gobeil and Cossa, 1993; Laurier et al., 2003a,b). For

    example, in Figs. 4 and 5, the transient increase in Hg

    concentrations observed for the 2002 IOC cruise

    stations 1, 2 and 3 also correspond to low dissolved

    oxygen value (Fig. 2) and could be due partly to the

    release of Hg from Mn oxidesorganic complexes. In

    addition, Martin et al. (1985) showed that the distri-

    bution of Mn is also influenced by lateral transport of

    dissolved Mn-enriched water from suboxic boundaryregions. It appears that Hg has distributional features

  • and many other trace metals but longer than that of

    Pb (Broeker and Peng, 1982).

    The deep-water Hg concentration differences be-

    tween the North Atlantic and the North Pacific Ocean

    are the result of this fractionation process, which

    introduces or removes elements from the deep ocean

    as water moves from the Atlantic to the Pacific Ocean

    (Broeker and Peng, 1982). Based on the deepest depth

    sampled in the northwest Atlantic, Gill and Fitzgerald

    (1988) showed comparable results with Hg concen-

    trations two to three times higher than in the central

    North Pacific Ocean. The comparison between data

    collected in 1998 from the north, mid and south

    Atlantic (Mason et al., 1998, 2001) and our data from

    the North Pacific Ocean (this study) show a three- to

    sixfold higher HgTOT concentrations in the Atlantic. In

    Fig. 11, the data clearly illustrate the inter-ocean

    fractionation with Hg with higher average concentra-

    F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 31916that are partly characteristic of short time, scavenged-

    type, trace metal elements with an atmospheric input.

    In addition, the influence of the anthropogenic signal

    also affects its overall distribution.

    3.4. Inter-ocean comparison

    The differences in Hg concentrations within the

    upper water of the North Pacific Ocean can be consid-

    ered small when compared to differences reported for

    the Atlantic Ocean (Gill and Fitzgerald, 1988;Mason et

    al., 1998, 2001). The Hg concentrations are quite

    similar all throughout the North Pacific Ocean and

    appear to have remained fairly constant during the past

    20 years. Although the global mercury budget has been

    strongly impacted by anthropogenic activities within

    the past 100 years (Mason and Sheu, 2002), the time

    scale for the thermohaline circulation of 5001000

    years (Broeker and Peng, 1982) might not enable a

    mercury signal to be detected in the old deep waters of

    the North Pacific Ocean. These observations are in

    accordance with the model for Hg global cycle pro-

    posed by Mason and Sheu (2002), which suggests that

    average ocean Hg concentration has increased only

    10% between pre-industrial and industrial eras. Given

    the circulation patterns, this average increase in Hg

    concentration would be mostly perceptible within the

    North Atlantic water column because of water sinking

    and deep water formation. For the AABW, the increase

    is less likely to be noticeable since the South Hemi-

    sphere is less impacted by anthropogenic inputs. Fur-

    thermore, the role of fractionation processes occurring

    at depth (scavenging of dissolved and particulate Hg

    species from the water column, mainly via adsorption/

    sedimentation) that leads to differences in Hg con-

    centrations might also account for the lower Hg

    concentrations measured for the Pacific compared

    to the Atlantic. Gill and Fitzgerald (1988) pointed

    out the differences in Hg concentrations between the

    Atlantic and the Pacific Ocean using data from the

    early Eighties. With a steady-state oceanic box mod-

    el, they estimated that Hg has a mean residence time

    in seawater of approximately 350 years. This value is

    lower than the oceanic mixing time scales, 500-1000

    years (Broeker and Peng, 1982), suggesting that Hg

    will be removed from the water column, especially in

    deep waters, mainly by particulate scavenging. Theresidence time of mercury is comparable to that of FeFig. 11. Inter-ocean fractionation of deep water mercury concen-

    trations between the South Atlantic Ocean (1996 IOC cruise) andthe North Pacific Ocean (2002 IOC cruise).

  • neous and do not show a strong geographical pattern.

    The concentrations for the deep and the bottom

    Brown, M.T., Measure, C.I., Vink, S., 2002. Dissolved Fe in the

    Central and Western North Pacific: results from the 2002 IOC

    F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 319 17waters were very comparable. Nonetheless, variance

    in Hg concentration in the upper water, generally

    within or close to the main thermoclines, was ob-

    served among several stations, and for both VER-

    TEX and IOC campaigns, which covered the

    northwest, northeast and central Pacific Ocean. Hor-

    izontal advection of water along isopycnals, vertical

    isopycnal mixing, ventilation, the presence of a

    thermocline associated with remobilization of mer-

    cury as a result of remineralization of settling par-

    ticles, the influence of diagenetic processes from

    continental margin sediments and seasonal stratifica-

    tion all appeared to be factors that can account for

    the distribution of Hg in the upper water. The

    hydrographic characteristics could not solely explain

    the contrasts reported for the different Hg profiles

    displayed in this study.

    The results discussed here confirm the notion oftions in the deep water of the South Atlantic (Knorr

    cruise, 1996) compared to the North Pacific (IOC

    cruise, 2002).

    The inter-ocean Hg fractionation and distribution

    agrees quite well with its estimated residence time

    (f 350 years) and reinforces the fact that regenera-tion processes such as diagenetic processes at the

    sedimentwater interface that lead to a release of

    dissolved Hg into the water column, do not recycle

    large amounts of Hg back in the water column, as it is

    the case for biogeochemically cycled elements. Nev-

    ertheless, the constant flux of particles within the

    bottom water will result in an immediate removal of

    this newly introduced Hg via adsorption, keeping the

    Hg concentrations low in the deepest water. This

    feature could explain the relative constancy observed

    in HgTOT concentrations at the deepest depths for our

    profiles (Fig. 6).

    4. Summary and conclusions

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    Mercury distributions in the North Pacific Ocean-20 years of observationsIntroductionMethodsResult and discussionHydrographic settingMixed layerIntermediate water massesDeep and bottom water masses

    Total mercury in the North Pacific Ocean during the 2002 IOC cruiseUnderway samplesMixed layer and upper water massesDeep and bottom water masses

    Spatial, temporal and seasonal variations in Hg distribution in the North Pacific OceanInter-ocean comparison

    Summary and conclusionsReferences


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