mercury distributions in the north pacific ocean—20 years of observations
TRANSCRIPT
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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.
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–196
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
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–19 7
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.
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–198
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.
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–19 9
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.
Fig. 5. Vertical distribution of total mercury concentrations during
the 2002 IOC cruise track, 1st May 2002 to 4th June 2002.
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–1910
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-
Fig. 6. Vertical distribution of total mercury concentrations during
the 2002 IOC cruise track, 1st May 2002 to 4th June 2002.
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–19 11
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
Fig. 7. Vertical distribution of total mercury concentrations during
the 2002 IOC cruise track, 1st May 2002 to 4th June 2002.
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–1912
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-
Fig. 8. Vertical distribution of total mercury concentrations during
the VERTEX cruise (VII) track, July–August 1987.
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–19 13
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).
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–1914
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.
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–19 15
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).
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–1916
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-
F.J.G. Laurier et al. / Marine Chemistry 90 (2004) 3–19 17
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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