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Marine Chemistry 90 (2004) 3–19

Mercury distributions in the North Pacific

Ocean—20 years of observations

F.J.G. Lauriera,*, R.P. Masona, G.A. Gillb, L. Whalina

aChesapeake 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

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 1.15F 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 (0.64F 0.26 pM), was similar to the earlier VERTEX cruise (0.58F 0.37 pM) but was lower compared to the N. Pac

cruise (1.40F 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 1.10F 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.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Mercury; North Pacific Ocean; HgTOT

1. 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.

doi:10.1016/j.marchem.2004.02.025

* Corresponding author.

E-mail address: [email protected] (F.J.G. Laurier).

ments 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) 3–194

al., 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 air–sea 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 air–sea 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 sinking

of 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

sinking.

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 Ocean

surface waters, which indicated that Hg supply by

F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–19 5

upwelling 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-

tion during transport due to particulate dissolution.

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

number 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 1986–1987. 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 transect

across the central and western North Pacific. The

general 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 ships’s contamination. Water was pumped

into the laminar flow hood through cleaned tubing in

a way to prevent any contamination of the collected

water.

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-free

argon 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 averages

5% 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 Japanese

coast to flow east and becomes the Kuroshio exten-

sion to about 170jE. From there, the Kuroshio

extension 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

thermocline 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 Cook E

Begin Date/Port: 01–May–02 Osaka, Japan (34j65VN, 135j42VW). End

numbers from 1 to 9 and the relative dates correspond to the depth-profil

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

and 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: 04–Jun–02 Honolulu, HA (24j15VN, 153j84VE). Thee sampling stations with the ‘‘test’’ as a test station.

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 W


mainly under the influence of the Alaskan Gyres and

the western Subtropical Gyre for the northern (V7–

T7, V7–T8) and southern (V7–T5, V7–T6) 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 Eastern

North 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

extends across the greater part of the North Pacific

Ocean. Its characteristic properties are low salinity

(33.5–34.5x) and relatively low temperature (2–4

jC), which is typically found for IOC stations 2 and

3 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 the

Antarctic 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 at

high latitudes in contrast to lower latitudes where both

Fig. 2. Hydrographic properties during the 2002 IOC cruise.


production 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

Asia, 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

Fig. 3. Surface water total mercury concentrations (pM) along the 2002 IOC cruise track, 1st May 2002 to 4th June 2002.


influence 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.

3.2.2. 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 the

highest 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. 4–6). The

relative decrease corresponded on average to

65F 17% of the HgTOT concentrations present in the

surface water.

As mentioned previously, enhanced Hg at the

surface is due to atmospheric Hg deposition, and

localized stratification, which can inhibit mixing near

the sea–air 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.

Fig. 4. Vertical distribution of total mercury concentrations during

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




The decrease in the concentrations within the

previously defined mixed layer (from 0 to f 150 m

deep), 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. 4–6). Stations 5 and 6 revealed a

similar increase in HgTOT concentrations (Fig. 4)

corresponding to the ‘‘mixed layer-NPCW’’ interface

(depth range 100–200 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

throughout the depth range of 200–1000 m, which

includes the ‘‘mixed layer–upper 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 could

be 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

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




tration 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

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 within

the upper water masses, HgTOT concentrations were




overly 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 layer–upper 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

water–deep water’’ interface, HgTOT concentrations

averaged 0.73F 0.23 pM and remained relatively

constant 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 the

deep water in contrast to the lower latitudes stations

where the lower values were found (Figs. 4–6). 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 1500–3000 depth range, HgTOTconcentrations averaged 1.19F 0.27 pM and were

significantly higher ( p < 0.05) compared to those mea-


the VERTEX cruise (VII) track, July–August 1987.


sured 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. The

highest 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 0.27

and 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, particularly

given the time period in which these measurements

were obtained. In 1980, the analytical reproducibility

was only 20–30% (Gill and Fitzgerald, 1988).

In the mixed layer, VERTEX stations (V7–T5,

V7–T6 and V7–T7) 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 V7–T8

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

Fig. 9. Temporal and spatial comparison of vertical distribution

of total mercury concentrations during the 2002 IOC cruise

(May–June 2002), VERTEX cruise (July–August 1987) and the

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

Thompson (October 1980).


depths 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

V7–T8 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), remains

statistically equivalent to the average for the IOC

study (0.64F 0.26 pM) and the deep water profile

(V7–T7) also exhibits similar trend with an increase

in the deep water (1000–3000 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 high

productivity, and, in contrast, the maximum in Hg

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

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 mercury

maximum 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).

Fig. 10. Seasonal variation in vertical Hg distribution during the

VERTEX 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 oxides–organic 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 boundary

regions. It appears that Hg has distributional features

Fig. 11. Inter-ocean fractionation of deep water mercury concen-

trations between the South Atlantic Ocean (1996 IOC cruise) and

the North Pacific Ocean (2002 IOC cruise).


that 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 500–1000

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. The

residence time of mercury is comparable to that of Fe

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-


tions 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

sediment–water 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

For the past 20 years, Hg concentrations in the

North Pacific Ocean have remained stable, whether

in the mixed layer and upper waters or in the deep

and bottom waters. The levels of mercury found over

most of the North Pacific are also quite homoge-

neous and do not show a strong geographical pattern.

The concentrations for the deep and the bottom

waters 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 of

Gill and Fitzgerald (1988) that the vertical distribu-

tion of Hg is not controlled solely by internal bio-

geochemical ‘‘nutrient type’’ recycling and mixing

process. Rather, Hg is regulated by the local magni-

tude of external sources and the intensity of water

column processes. The distribution of Hg will be

modified by advective transport, upwelling and dif-

fusion. In addition, the horizontal advection in the

sub-thermocline region could be a significant process.

Thus, according to Fitzgerald and Mason (1997), the

atmospheric derived Hg entering and regenerating at

depths within the main thermocline will be carried to

lower latitudes by the advective waters associated

with the generalized meridional density flow in the

Pacific Ocean.

Furthermore, Hg fractionation between the Atlantic

and the Pacific deep water is evident and remained

comparable between the Atlantic and the North Pa-

cific Oceans for the last 20 years.

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