BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-15-2309-2018Mercury distribution and transport in the North Atlantic Ocean along the
Geotraces-GA01 transectMercury distribution and transport in the North Atlantic OceanCossaDanieldcossa@ifremer.frHeimbürgerLars-EricPérezFiz F.https://orcid.org/0000-0003-4836-8974García-IbáñezMaribel I.https://orcid.org/0000-0001-5218-0064SonkeJeroen E.PlanquetteHélèneLherminierPascalehttps://orcid.org/0000-0001-9007-2160BoutorhJuliaCheizeMarieMenzel BarraquetaJan Lukashttps://orcid.org/0000-0002-9735-1231ShelleyRachelSarthouGéraldineISTerre, Université Grenoble Alpes, CS 40700, 38058 Grenoble Cedex 9, FranceAix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, Marseille, FranceInstituto de Investigaciones Marinas, CSIC, Eduardo Cabello 6, 36208 Vigo, SpainUni Research Climate, Bjerknes Centre for Climate Research, Bergen 5008, NorwayCNRS, GET-OMP, 14 Ave. E. Belin, 31240 Toulouse, FranceLEMAR, Université de Bretagne Occidentale, 29280 Plouzané, FranceIFREMER, Brittany Center, LPO, BP 70, 29280 Plouzané, FranceGEOMAR, Helmholtz Centre for Ocean Research, 24148 Kiel, GermanyDaniel Cossa (dcossa@ifremer.fr)19April20181582309232331October201713November201723February20183March2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://bg.copernicus.org/articles/15/2309/2018/bg-15-2309-2018.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/15/2309/2018/bg-15-2309-2018.pdf
We report here the results of total mercury (HgT) determinations
along the 2014 Geotraces Geovide cruise (GA01 transect) in the North
Atlantic Ocean (NA) from Lisbon (Portugal) to the coast of Labrador (Canada).
HgT concentrations in unfiltered samples (HgTUNF) were
log-normally distributed and ranged between 0.16 and 1.54 pmol L-1,
with a geometric mean of 0.51 pmol L-1 for the 535 samples analysed.
The dissolved fraction (< 0.45 µm) of HgT
(HgTF), determined on 141 samples, averaged 78 % of the
HgTUNF for the entire data set, 84 % for open seawaters
(below 100 m) and 91 % if the Labrador Sea data are excluded, where the primary
production was high (with a winter convection down to 1400 m).
HgTUNF concentrations increased eastwards and with depth from
Greenland to Europe and from subsurface to bottom waters. The
HgTUNF concentrations were similarly low in the subpolar gyre
waters (∼ 0.45 pmol L-1), whereas they exceeded
0.60 pmol L-1 in the subtropical gyre waters. The HgTUNF
distribution mirrored that of dissolved oxygen concentration, with highest
concentration levels associated with oxygen-depleted zones. The relationship
between HgTF and the apparent oxygen utilization confirms the
nutrient-like behaviour of Hg in the NA. An extended optimum multiparameter
analysis allowed us to characterize HgTUNF concentrations in the
different source water types (SWTs) present along the transect. The
distribution pattern of HgTUNF, modelled by the mixing of SWTs, show
Hg enrichment in Mediterranean waters and North East Atlantic Deep Water and low concentrations in young waters formed in the subpolar gyre and Nordic
seas. The change in anthropogenic Hg concentrations in the Labrador Sea
Water during its eastward journey suggests a continuous decrease in Hg
content in this water mass over the last decades. Calculation of the water
transport driven by the Atlantic Meridional Overturning Circulation across
the Portugal–Greenland transect indicates northward Hg transport within the
upper limb and southward Hg transport within the lower limb, with
resulting net northward transport of about 97.2 kmol yr-1.
Introduction
The ocean plays a central role in the global mercury (Hg) cycle. It receives
Hg mainly from atmospheric deposition, whereas it disposes of it in deep
marine sediments (e.g. Mason et al., 1994). In the meantime, the largest
part of Hg is recycled in the atmosphere, while a smaller fraction penetrates
the ocean interior via thermohaline circulation or the biological pump (see
reviews by Mason and Sheu, 2002; Fitzgerald et al., 2007; Mason et al.,
2012). Firstly, Hg re-injection to the atmosphere results from the formation
of volatile elemental Hg via photoreduction and microbiological reduction of
divalent Hg (e.g. Mason et al., 1995; Amyot et al., 1997). Secondly, Hg
integration into the thermohaline circulation involves its solubilization in
surface waters followed by the subduction of these water masses along
isopycnals (e.g. Gill and Fitzgerald, 1988). Thirdly, the biological pump
consists of Hg sorption onto biogenic particles produced in the euphotic
zone. Then it conveys sinking materials at depth with possible Hg
remobilization due to the particulate remineralization–dissolution process
(e.g. Mason and Fitzgerald, 1993). The shape of observed vertical oceanic Hg
profiles, characterized by increasing concentrations with depth, includes the
marks of these different routes and is akin to nutrient-type profiles
(Gill and Fitzgerald, 1988; Cossa et al., 2004; Lamborg et al., 2014; Bowman
et al., 2015, 2016). The Hg cycle is also known for being highly perturbed by
human activities (e.g. Mason et al., 2012; Lamborg et al., 2014; Zhang et
al., 2014; Amos et al., 2015). Modern Hg concentrations in the global
atmosphere are more than 3 times the pre-industrial Hg concentrations,
leading to increasing Hg concentrations in surface and intermediate
oceanic layers, which remain to be precisely estimated. Despite these
advances in knowledge of the Hg biogeochemical cycle, the key features of
the Hg distribution among the principal oceanic water masses are still poorly
documented. Recent enhancements in the precision of Hg analyses
allow more reliable vertical Hg profiles in the water columns (e.g. Cossa et
al., 2011; Lamborg et al., 2014; Heimbürger et al., 2015; Bowman et al.,
2015, 2016; Munson et al., 2015; Cossa et al., 2017a, b). In addition, an
original approach for the estimation of the anthropogenic fractions of Hg
concentrations in oceanic waters has been proposed (Lamborg et al., 2014).
Owing to these last methodological breakthroughs, significant advances in
detailed Hg oceanic distributions are possible.
Schematic view of the water circulation in the North Atlantic Ocean
adapted from García-Ibáñez et al. (2015) and Daniault et
al. (2016). Red lines indicate the circulation in surface, while blue lines
indicated circulation at depth. Black lines represent the Geovide
cruise that transects (Geotraces-GA01). The main geographical features, water
masses and currents are indicated: Newfoundland (NFL), United Kingdom (UK),
United States of America (USA), Denmark Straight Overflow Water (DSOW),
Iceland–Scotland Overflow water (ISOW), Labrador Sea Water (LSW), Lower North
East Atlantic Deep Water (NEADWL), Mediterranean waters (MW) and
North Atlantic Deep Water (NADW), Deep Western Boundary Current (DWBC),
East Greenland Current (EGC), Labrador Current (LC), North Atlantic
Current (NAC) and West Greenland Current (WGC).
The North Atlantic Ocean (NA) plays an active role in the cycling of chemical
species in the ocean because it is a region where deep water formation drives
the Atlantic Meridional Overturning Circulation (AMOC) (Kuhlbrodt et al.,
2007). Particularly in the subpolar NA, chemical properties, including Hg,
are transported to the ocean interior; thus, NA offers a unique opportunity
for studying the oceanic response to changes in atmospheric Hg deposition.
The Geotraces-GA03 zonal and meridional transects, sampled in 2010
and 2011, covered the NA from east to west between 18 and 40∘ N,
from the coast of Africa to the coast of the USA. Here, we report the results of the
Geovide cruise along the Geotraces-GA01 transect, which
targeted the NA from 40 to 60∘ N, from Portugal to Newfoundland via
the southern tip of Greenland (Fig. 1). This article provides (i) a
high-resolution description of the HgT distribution in the waters of the subpolar
and subtropical gyres of the NA, (ii) characterization the HgT
concentrations of the main water masses of the NA, (iii) an estimate of the
temporal change of anthropogenic Hg in LSW and (iv) quantification of the HgT
transport associated with the upper and lower limbs of the AMOC. These new
data contribute to a refinement of the depiction of the Hg distribution in the
NA waters and should allow further improvements in the oceanic Hg modelling.
Oceanographic context
A full description of the water masses along the Geotraces-GA01
transect can be found in García-Ibáñez et al. (2018). Briefly,
the North Atlantic Current (NAC) conveys the warm salty surface waters from
subtropical regions northwards to the subpolar regions, where they are cooled
down by heat exchange with the atmosphere (Fig. 1). The intermediate and deep
waters formed this way fill up the Global Ocean, initiating the
southward-flowing limb of the AMOC (e.g. McCartney and Talley, 1984;
Lherminier et al., 2010). In addition, the general circulation pattern is
characterized by the subtropical and the subpolar gyres (Fig. 1).
In the subtropical gyre (Fig. 1), several water masses can be identified.
They are listed from top to bottom: (i) the mixed layer, (ii) the eastern North
Atlantic Central Water (ENACW), (iii) the Mediterranean waters (MW), (iv) the
Labrador Sea Water (LSW) and (v) the Lower North East Atlantic Deep Water
(NEADWL), which contains about 30 % Antarctic Bottom Water
(AABW) (García-Ibáñez et al., 2015). The transformation of ENACW
leads to the formation of different mode waters including the Subpolar Mode
Waters (SPMWs) (McCartney and Talley, 1982; Tsuchiya et al., 1992; van Aken
and Becker, 1996; Brambilla and Talley, 2008; Cianca et al., 2009). SPMWs are
the near-surface water masses of the subpolar gyre of the NA characterized by
thick layers of nearly uniform temperature, often denoted with temperature
in subscript (SPMW8, for example). SPMWs are formed during winter
convection at high latitudes due to atmospheric freshening of surface waters
originating from the subtropical gyre (McCartney, 1992). SPMWs participate in
the upper limb of the AMOC and provide much of the water that is eventually
transformed into the several components of North Atlantic Deep Water (NADW;
Brambilla and Talley, 2008).
In the subpolar gyre, ocean–atmosphere interaction is particularly intense.
The cooling down of subtropical waters produces dense waters, triggering the
deepening of the mixed layer and further leading to deep convection. The main
NA convection zones are located in the Labrador (LS), Irminger (IrS) and
Nordic seas (NS) (Fig. 1). Convection in those zones leads to the formation
of intermediate and deep waters such as LSW, Denmark Strait Overflow Water
(DSOW) and Iceland–Scotland Overflow Water (ISOW). LSW and ISOW are the main
components of NEADW, and the all three are components of the NADW, which
constitutes the cold deep limb of the AMOC, flowing southward towards
the Southern Ocean in the western Atlantic basin. LSW has been variably
produced in the past 50 years, depending on the intensity of winter
convection, linked to the intensity of the North Atlantic Oscillation (e.g.
Rhein et al., 2002; Cianca et al., 2009; Yashayaev and Loder, 2016). Depths
of winter convection in the LS vary from a few hundred metres (the early
2000s) to over 2000 m (early 1990s). The LSW is a thick layer in the LS but
thins out as it travels south-westwardly. It spreads out into the entire NA,
filling the subpolar gyre and entering the subtropical gyre. Within the
subpolar gyre, LSW is marked by a salinity minimum above the ISOW. In both
gyres, the well-ventilated LSW has a marked oxygen maximum.
In order to access all Hg species, the release of Hg from its ligands was
achieved by a BrCl solution (50 µL of a 0.2 N solution is added to
a 40 mL sample), and then the Hg was reduced with an acidic SnCl2
solution (100 µL of a 1 M solution is added to a 40 mL sample).
Potassium bromide (Sigma Aldrich, USA) and potassium bromate (Sigma Aldrich,
USA) were heated for 4 h at 250 ∘C to remove Hg traces before
making up BrCl solution with freshly double-distilled HCl (Heimbürger et
al., 2015). The generated Hg vapour was amalgamated into a gold trap and then
released by heating into an atomic fluorescence spectrometer (AFS). We used
two AFS systems in parallel (Tekran® Model
2500, Brooks® Model 3), both calibrated
against the NIST 3133 (National Institute of Standards and Technology) certified reference material. This technique, initially
described by Bloom and Crecelius (1983) and subsequently improved by Gill and
Fitzgerald (1985), is now an authoritative procedure officialised by the
US EPA as method 1631 (Environmental Protection Agency, 2002). The definitions of detection limit (DL),
reproducibility and accuracy given here are adopted from Taylor (1987) and
Hewitt (1989). Using a mirrored quartz cuvette
(Hellma®) allowed for an “absolute DL”,
defined as 2 times the electronic noise magnitude, as low as
1.7 femtomoles. However, in practice for trace measurements, the DL is
governed by the reproducibility of the blank values and calculated as
3.3 times the standard deviation of blank values. The blank values were determined on
a purged Hg-free seawater sample spiked with reagents (i.e. BrCl and
SnCl2). The mean (± standard deviation) of blank values measured during
the Geovide cruise was 3.2 ± 1.0 femtomoles. Thus, for a
40 mL seawater aliquot, the DL expressed in HgT concentration was
0.07 pmol L-1. The reproducibility (coefficient of variation of six
replicate measures) varied according to the concentration level between 5 and
15 %. The accuracy of HgT measurements was tested using ORMS-5 certified
reference material (CRM) from the National Research Council of Canada
(http://www.nrc-cnrc.gc.ca/, last access: April 2018) and spiked to a purged Hg-free seawater sample. Measurements
were always within the given confidence interval. To ensure good data
quality and to continue previous efforts (Cossa and Courau, 1990;
Lamborg et al., 2012), we organized the 2014 Geotraces
intercalibration exercise for total HgT and methyl Hg as a part of the
Geovide cruise. The intercalibration sample was taken on
22 June 2014 in the LS at 49.093∘ W, 55.842∘ N, and
2365 m depth. The sample was sent out to 10 participating laboratories. This
station was also planned as crossover station within the 2015 Arctic
Geotraces effort (Canadian cruise) but has subsequently been changed
to another location. Our results compare well with the consensus values,
HgT = 0.63 ± 0.12 pmol L-1, n=8. We measured the 2014
Geotraces intercalibration sample twice for HgT and obtained 0.51
(22 June 2014, on board) and 0.58 pmol L-1 (30 October 2014, home
lab).
Extended optimum multiparameter analysis
We used an extended optimum multiparameter (eOMP) analysis to characterize
the water mass HgTUNF concentrations along the
Geotraces-GA01 transect (García-Ibáñez et al., 2015,
2018). The eOMP analysis quantifies the proportions of the different source water types (SWTs) that contribute to a given water sample. The
HgTUNF concentration of each SWT, [HgTUNF]i, was
estimated through an inversion of the SWT fractions given by the eOMP
analysis. This approach was successfully applied to
dissolved organic carbon water mass definitions in the NA (Fontela et al.,
2016) and to evaluate the impact of water mass mixing and remineralization
on the N2O distribution in the NA (de la Paz et al., 2017). Here, we
performed an inversion of a system of 430 equations (HgTUNF
samples) and 11 unknowns ([HgTUNF]i). Samples for which the
difference between the observed HgTUNF and the predicted
HgTUNF values by the multiple linear regression (Eq. 1 below) was
3 times greater than the standard deviation were removed from the
analysis. Nine samples were used: Station 2 (125 m), Station 11 (793 m),
Station 11 (5242 m), Station 13 (1186 m), Station 15 (170 m), Station 19 (99 m),
Station 26 (97 m), Station 32 (596 m) and Station 38 (297 m). The SWTs were
characterized by potential temperature, salinity, and macronutrients. The
eOMP was restricted to depths below 75 m in order to avoid air–sea
interaction effects. The eOMP gave us the fractions of the 11 SWTs, and we
resolved the following expression to estimate the [HgTUNF]i:
[HgTUNF]j=∑i=111SWTij×[HgTUNF]i+εj(j=1…430),
where [HgTUNF]j represents the measured HgTUNF
concentration for each sample j, SWTij the proportion of SWT
i to sample j (obtained through the eOMP),
[HgTUNF]i the HgTUNF concentration for each SWT
i (unknown), and εj the residual. The 430
εjs of the inversion presented a null mean and a standard
deviation of 0.085 pmol L-1 (R=0.84).
Distribution of unfiltered total mercury (HgTUNF)
concentrations along the Geotraces-GA01 transect. LS: Labrador Sea;
IrS: Irminger Sea; IcB: Iceland basin; ENABw: western part of the eastern North
Atlantic basin; ENABe: eastern part of the eastern North Atlantic basin; IAP:
Iberian Abyssal Plain.
Mercury transport calculation
Velocity fields across the Geotraces-GA01 transect were calculated
using an inverse model constrained by Doppler current profiler velocity
measurements (Zunino et al., 2017) an overall mass balance of 1 ± 3 Sv
to the north (Lherminier et al., 2007, 2010). The volume transport per SWT
was computed by combining these velocity fields with the results of the eOMP
(García-Ibáñez et al., 2018). Finally, the HgTUNF
transport per water mass was calculated through Eq. (2):
THgTUNF=∑i=111TSWTi×HgTUNFi×ρi,
where TSWTi is the volume transport of SWT i,
[HgTUNF]i is the HgTUNF concentration for each SWT
i (from Eq. 1), and ρi is the density of the SWT i.
The inverse model configuration for the Geovide cruise data is
described in Zunino et al. (2017). The inverse model is based on the
least squares formalism, which provides errors on the velocities and
associated quantities such as the magnitude of the AMOC (estimated in density
coordinate) and the heat flux (Lherminier et al., 2010). The inverse model
computes the absolute geostrophic transport orthogonal to the section. The
Ekman transport is deduced from the wind fields averaged over the cruise
period and added homogeneously to the upper 40 m (Mercier et al., 2015). The
transport estimates of the inverse model across the section have been
validated by favourable comparisons with independent measurements (Gourcuff et
al., 2011; Daniault et al., 2011; Mercier et al., 2015).
Results
Distributions of potential temperature, salinity, dissolved oxygen and
silicic acid are given in García-Ibáñez et al. (2018),
Mercury concentrations in filtered (HgTF) vs.
unfiltered (HgTUNF) samples (n=141) collected along the
Geotraces-GA01 transect.
HgTUNF concentrations along the Geotraces-GA01 transect
ranged from 0.16 to 1.54 pmol L-1 (n=535), these data being
log-normally distributed, positively skewed (Skewness = 1.1;
Kurtosis = 2.1; Fig. S3) and having 97 % of the values lower than
1.00 pmol L-1. The geometric mean and the median were
0.51 pmol L-1, whereas the arithmetic mean and standard deviation were
0.54 and 0.19 pmol L-1, respectively. These concentrations are within
the range found along the Geotraces-GA03 transect
(0.09–1.89 pmol L-1, n=605) that crossed the NA within the
subtropical gyre from 18 to 40∘ N (Bowman et al., 2015), but are lower
than the range and the unusually high arithmetic mean determined in the South
Atlantic along the Geotraces-GA10 transect
(0.39–3.39 pmol L-1, n=375; Arne Bratkič, personal communication, 2017 and
1.45 ± 0.60 pmol L-1; Bratkič et al., 2016).
The overall distribution of the HgTUNF concentrations along the
Geotraces-GA01 transect is represented in Fig. 2. The main features
of HgTUNF concentrations are an eastward increase from Greenland
to Europe and a downward increase from subsurface to bottom waters. In
addition, the lowest and highest (most variable) HgTUNF values were
encountered in surface and subsurface waters, where Hg evasion to the atmosphere
and high particulate matter concentrations may generate low and high
HgTUNF concentrations, respectively. Out of the 141 filtered
samples that were analysed, altogether, the filtered fraction of Hg
(HgTF) represents, on average, 78 % (range: 36–98 %) of
the HgTUNF (Fig. 3). Excluding the upper 100 m, where the
biogenic suspended particles are usually abundant, and the stations located
on the shelf and slope, where particulate matter from continental sources are
usually present, the HgTF fraction represents, on average,
84 % (range: 72–98 %) of the HgTUNF. In addition, in the
LS, HgTF/ HgTUNF mean ratios were rather low
ranging 62–92 %, with a mean of 76 %. In fact, the primary
production was high in spring 2014 in LS, and the winter convection, which
reached 1400 m, conveyed surface particles at depth (Yashayaev et al., 2015;
Lemaitre et al., 2017). If we exclude the LS from the HgTF mean
computation, we obtain a mean percentage
HgTF/ HgTUNF ratio of 91 %, which is similar
to values (∼ 90 %) obtained along the Geotraces-GA03 zonal
and meridional transects (Bowman et al., 2015). In the following
subsections, detailed descriptions of the HgTUNF profiles for
the five following oceanographic environments are given: LS, IrS, Iceland
basin (IcB), the eastern North Atlantic basin (ENAB) and Iberian Abyssal Plain
(IAP).
Total Hg in filtered samples (HgTF) vs. apparent
oxygen utilization (AOU) along the Geotraces-GA01
transect.
Labrador Sea (stations 61 to 78)
In the LS, the HgTUNF concentrations ranged from 0.25 to
0.67 pmol L-1, with a mean (± standard deviation) of
0.44 ± 0.10 pmol L-1 (n=113). Distribution, source and
cycling of Hg in the LS have been described and discussed in detail in a
companion paper (Cossa et al., 2017b). In summary, the highest HgTUNF
concentrations were found in the waters of the Labrador Current (LC)
receiving fresh water from the Canadian Arctic Archipelago and in the waters
over the Labrador shelf and continental rise. In the LSW that formed during the
2014 winter convection, HgTUNF concentrations were low
(0.38 ± 0.05 pmol L-1, n=23) and increased gradually with
depth (up to > 0.5 pmol L-1) in the North East Atlantic
Deep Water.
Irminger Sea (stations 40–60)
HgTUNF concentrations in the IrS waters varied from 0.22 to
0.76 pmol L-1, with a mean of 0.45 ± 0.10 pmol L-1 (n=103). In the IrSPMW, which was encountered in the upper 1000 m near
eastern Greenland and the upper 500 m in the rest of the IrS (Fig. 4a in
García-Ibáñez et al., 2018), HgTUNF values span
between 0.29 and 0.42 pmol L-1 (Fig. 2). Deeper HgTUNF
increased up to 0.50 and 0.63 pmol L-1 in LSW (∼ 1000 m) and
ISOW (∼ 2500 m). Lower HgTUNF concentrations
(0.40–0.50 pmol L-1) were associated with DSOW in the bottom
waters (stations 42–44, Fig. 2).
Iceland basin (stations 34–38)
HgTUNF concentrations in the IcB ranged from 0.18 to
0.65 pmol L-1, with a mean of 0.46 ± 0.10 pmol L-1 (n=51). In the top 100 m of the water column, HgTUNF
concentrations were quite variable (0.25–0.62 pmol L-1), probably as a
result of the counteracting importance of Hg evasion to the atmosphere,
high particulate matter and/or complexing substance concentrations. West of
the IcB (Station 38), contrasting HgTUNF levels were found on both
sides at 500 m, characterized by a thermohaline gradient (Fig. 2a and b in
García-Ibáñez et al., 2018). In the top waters,
HgTUNF levels were depleted to 0.18 pmol L-1, whereas
below 500 m, they were much higher and converge to values close to what we
found, at the same depths in the adjacent IrS (∼ 0.60 pmol L-1,
Station 40). In the bottom waters, constituted by more than 50 % of ISOW
(García-Ibáñez et al., 2018), HgTUNF concentrations
reached values > 0.50 pmol L-1.
Eastern North Atlantic basin (stations 17–32)
The HgTUNF concentrations in the ENAB varied from 0.18 to
1.14 pmol L-1, with a mean of 0.61 ± 0.18 pmol L-1 (n=174). The ENAB, also named western European basin, is characterized by
complex vertical stratification of the water column. The HgTUNF
vertical profiles at all the stations of the ENAB were characterized by a
complex but reproducible pattern depicting (i) two maxima peaks (the upper one at
the subsurface, the lower within the intermediate waters), and below, (ii) a
HgTUNF enhancement from 2500 m to the bottom
(Fig. 2). The position and intensity of the peaks vary with longitude. The
upper peak, which occurs within the top 200 m, is only 0.48 pmol L-1
at Station 29, but reaches 1.14 pmol L-1 at Station 19 (Fig. 2). The
vertical position of maxima of the lower peak deepens eastwards, from
200 m down to 800 m, concurrently with an increase in its magnitude
(Fig. 2). The position of the upper peaks suggests a relation with the position
of the fluorescence maximum (data not shown), whereas the position of the
lower peaks, which is close to the maximum of apparent oxygen utilization
(AOU) that rose above 70 µmol L-1 (Fig. 2), suggests a
dependence on the organic matter remineralization (see Discussion below).
Between 1400 and 2500 m, in the layer corresponding to LSW,
HgTUNF concentrations were quite uniform, with a mean
concentration of 0.54 ± 0.04 pmol L-1 (n=18).
The HgTUNF concentration increased from 3000 m downwards to the sea
bottom, consisting of NEADWL, where it reaches 0.95, 0.97, 1.03
and 1.13 pmol L-1 at stations 21, 19, 25 and 23, respectively.
Iberian Abyssal Plain (stations 1–15)
In the IAP, HgTUNF concentrations ranged from 0.19 to
1.54 pmol L-1, with a mean of 0.69 ± 0.23 pmol L-1 (n=94). The highest HgTUNF concentrations were measured in the
upper 100 m near the shelf slope. At Station 2, the only station on the
European shelf (bottom at 152 m), the HgTUNF concentrations
increased from 10 m to the bottom, from 0.38 to 0.86 pmol L-1, but
did not differ from the open NA ocean levels. Offshore, at stations 1, 11, 13
and 15 (Fig. 2), the vertical distributions of HgTUNF presented a
certain similarity with those of the eastern ENAB, but with an additional
third deep peak. As in the eastern ENAB, the upper peak is associated with
subsurface waters, and the second, centred around 800 m, is associated with
the oxygen minimum of SPMW8. The third peak, centred around
1100–1200 m, is associated with the salinity maximum of the core of MW
(Fig. S2). The presence of a HgTUNF peak in the MW was still
visible westwards, at stations 17, 19 and 23, near 1100 m, as a shoulder of the
main peak at 800 m (Fig. 2). Deeper within the water column, HgTUNF
increased gradually from 2000 m (LSW) to 3000 m (ISOW), 3500 m and below
(NEADWL), where HgTUNF concentrations reached 0.87 to
1.04 pmol L-1depending on the station.
In summary, the HgTUNF mean concentrations were low and similar
in the basins of the subpolar gyre (0,44, 0.45 and 0.46 pml L-1 for LS,
IrS and IcB respectively), whereas they exceeded 0.60 pml L-1 in the
subtropical gyre (0.61 and 0.69 pml L-1 for ENAB and IAP,
respectively). On the other hand, the profiles were rather homogenous in the
subpolar gyre compared to the multipeak vertical distribution observed in the
subpolar gyre (Fig. 2). A multipeak pattern was also observed in 1994 in the
eastern Atlantic slope water column in the Celtic Sea (Cossa et al., 2004).
The shape of the Hg profiles exhibited the same peaks in the same water
masses as the ones observed in this study (i.e. SPMWs and MW). However,
HgTUNF concentration levels measured 20 years ago were much
higher, varying most often from 0.3 pmol L-1 in subsurface waters to
more than 2.0 pmol L-1 at depth.
Total Hg in unfiltered samples (HgTUNF) vs. apparent
oxygen utilization (AOU) within the various source water types.
Anthropogenic HgT (HgTAnth) concentration distribution
in the core of the Labrador Sea Water (LSW) (S=34.9, σO= 27.74–27.82, 1200–2000 m) between the Labrador Sea and the eastern
North Atlantic basin. HgTAnth values were obtained according to
the model by Lamborg et al. (2014). Young LSW corresponds to the
LSW2014) formed during winter 2013–2014. The insert
shows the Hg concentration decrease in the troposphere over the North
Atlantic during the last 20 years according to Soerensen et al. (2012).
Total Hg in unfiltered samples (HgTUNF) vs. apparent
oxygen utilization (AOU) concentrations of each source water type (SWT),
calculated according to eOMP (Eq. 1) (see also García-Ibáñez et
al., 2018). Corrected HgTUNF is a theoretical HgT
concentration for a AOU concentration equal to zero, using the equation from
Fig. 5. ENACW12: eastern North Atlantic Central Water of 12 ∘C;
SPMW8 and SPMW7: Subpolar Mode Water of the Iceland basin of 7 and
8 ∘C; IrSPMW: Subpolar Mode Water of the Irminger basin; LSW:
Labrador Sea Water; MW: Mediterranean waters; ISOW: Iceland–Scotland Overflow
Water; NEADWL: Lower North East Atlantic Deep Water; DSOW:
Denmark Strait Overflow Water; PIW: Polar Intermediate Water; and SAIW6:
Subarctic Intermediate Water of 6 ∘C.
Water and total Hg in unfiltered samples (HgTUNF)
transported by the upper and lower limbs of the Atlantic Meridional Overturning
Circulation. Positive (negative) transport corresponds to northward
(southward) flow.
SWTEntire water column Upper limb Lower limb WaterHgTUNFWaterHgTUNFWaterHgTUNFtransporttransporttransporttransporttransporttransport(Sv)(mmol s-1)(Sv)(mmol s-1)(Sv)(mmol s-1)ENACW129.54.529.54.520.00.00SPMW84.13.023.72.700.40.31SPMW73.31.861.81.011.50.85IrSPMW-10.2-3.23-0.6-0.19-9.6-3.04SAIW61.00.412.41.04-1.5-0.62MW0.70.600.60.530.10.06LSW1.90.871.40.860.50.21NEADWL0.30.340.00.000.330.34PIW-2.2-1.18-0.1-0.08-2.1-1.10ISOW-4.9-3.040.00.00-4.9-3.04DSOW-2.5-1.090.00.00-2.5-1.09Total1.13.0818.710.2-17.7-7.12DiscussionBiogeochemical and hydrographical controls on HgT distribution
The main paths of the Hg cycle in the open ocean can be briefly summarized as
follows. Direct atmospheric deposition is the dominant source of Hg for the
oceans, most of the deposited Hg is re-emitted in the atmosphere, and a minor
Hg fraction is drawn down to the ocean interior with downwards convecting
waters or associated with sinking particles. At depth, the dissolution of
particulate matter, produced as a result of organic matter microbiological
remineralization, remobilizes Hg from particles produced in the euphotic
zone. The biological pumping/regeneration process results in a relationship
between Hg concentrations and nutrient or dissolved oxygen concentration (or
AOU), which are proxies of the organic matter remineralization (mainly the
microbial respiration) that the sample had experienced since it was last in
contact with the atmosphere. This biogeochemical behaviour, which is
qualified of nutrient-like behaviour, is observed in the present study
(Fig. 4). The correlation coefficient (R) between HgTF and the
AOU, obtained from in situ measurements of dissolved oxygen and
temperature, reached the highly statistically significant value of 0.87 (n=141, p<0.01). Similar behaviour was already observed in the
water column near the shelf edge of the western European margin (Cossa et
al., 2004) and elsewhere in the NA (Lamborg et al., 2014; Bowman et al.,
2015). Thus, the present results confirm that biological uptake and regenerative
processes appear to control a large part of the oceanic Hg distribution in
the subpolar and subtropical gyres of the NA.
Hydrological circulation may also impact the Hg distribution in the NA. We
estimated the HgTUNF (and AOU) values of each SWT using eOMP
(Table 1). The correlation coefficient between observed and predicted
(eOMP-based) values calculated with Eq. (1) (Materials and Methods section) for
HgTUNF is 0.71. The estimated HgTUNF concentrations
vary significantly between SWTs from 0.32 ± 0.03 to
1.04 ± 0.02 pmol L-1 for the IrSPM to the NEADWL.
However, a large part of the HgTUNF between SWTs is
due to the regeneration process as suggested by the correlation coefficient
(R=0.82) of the linear relationship between HgTUNF and AOU
(Fig. 5). Based on this model (HgTUNF=0.0043× AOU + 0.3547), we calculated mean corrected
HgTUNF concentrations for each identified SWT for a zero AOU
concentration. Corrected mean values range from 0.22 to
0.61 pmol L-1 in IrSPMW and NEADW, respectively (Table 1). This
variation should result from the origin, the route and the age of each SWT.
The corrected HgTUNF values of IrSPMW, PIM, SPMW, DSOW and LSW,
which formed in the subpolar gyre and in the NS last winter,
present very low and similar values, 0.22–0.31 (Table 1). The IrSPMW is
the youngest SPMW that has formed in the IrS as a result of interaction between the air and the
waters transported northwards by the NAC (e.g. McCartney and Talley, 1984);
the low HgTUNF value found in the IrSPMW may result from a net Hg
evasion in this region, consistently with the conclusion that western and
central NA are a net source of Hg in the atmosphere (Mason et al., 2017). On
the contrary, on the eastern NA side, where Hg deposition and evasion are
rather similar (Mason et al., 2017), the ENACW shows a higher corrected HgT
concentration (0.41 pmol L-1, Table 1). The highest corrected
HgTUNF mean concentration is calculated for NEADWL
(0.61 pmol L-1, Table 1), which is the dominant water mass in the
bottom IAP. Its main core is below ∼ 3500 m depth and spreads down
to the bottom (see Fig. 4 in García-Ibáñez et al., 2018). This
water mass contains a significant component from the Southern Ocean (AABW), which
is known to be Hg-rich (HgTAABW= 1.35 ± 0.39 pmol L-1, Cossa et al., 2011). The same
rationale can be drawn for the corrected HgTUNF concentration
in MW (0.41 pmol L-1, Table 1). Indeed, recent measurements in the
waters of the western Mediterranean give HgTUNF values
between 0.53 and 1.25 pmol L-1 within the layer that flows out of the
Mediterranean Sea at the Strait of Gibraltar (Cossa and Coquery, 2005; Cossa
et al., 2017a).
In summary, the distribution pattern of HgTUNF along the
Geotraces-GA01 transect, modelled by the mixing of SWTs (Fig. S4),
stresses the importance of Hg scavenging by plankton and organic matter
regeneration, but also shows that part of the Hg enrichment in certain SWTs,
including MW and NEADW, is due to preformed Hg outside the NA. This type of
result, characterizing the Hg concentrations in principal oceanic water
masses, should contribute to refinements in model formulation and
predictability.
Change in anthropogenic Hg in LSW
Evidence for a decrease in the Hg anthropization in the NA waters can be
obtained from the comparison of the present results with those obtained
20 years ago with similar clean sampling and analytical techniques. In a
companion paper (Cossa et al., 2017b), we have already compared the present
findings for the convection layer in the LS with the results of the 1993
International Oceanographic Commission cruise (Mason et al., 1998). Between
1993 and 2014 the decrease in HgTUNF concentrations would have
been more than a factor of 2 (1.14 ± 0.36 pmol L-1 vs.
0.40 ± 0.07 pmol L-1). However, bearing in mind the uncertainty
of the accuracy of early numbers, this magnitude of decrease cannot be
taken for granted. To circumvent this difficulty, the approach proposed by Lamborg et al. (2014) can be used to estimate the
anthropogenic Hg (HgAnth) concentrations in subsurface waters. HgAnth is
inferred from the difference between measured HgTUNF
concentrations and the concentrations predicted based on a worldwide
relationship between deep-ocean Hg concentrations and remineralised phosphate
(Lamborg et al., 2014). There is a Redfield ratio of 141 between AOU and
remineralized phosphate (Minster and Boulahdid, 1987), which is more representative
for the NA than the global value of 170 proposed by Anderson and
Sarmiento (1994). The LSW takes less than 20 years (Doney et al., 1997) to
flow more than 3000 km eastward from the LS to the subtropical gyre
of the NA. Along its path, LSW bears the record of Hg incorporation at the
time of its formation; thus sampling along its flow path allows the
observation of decadal variations in anthropogenic Hg inputs to the NA. In
the NA, estimation of HgAnth concentrations in the core of LSW,
defined within potential densities of 27.74 and 27.82, account for
36 ± 0.07 % of the HgTUNF, and are one-third lower for
younger waters (LS and IrS: 0.16 ± 0.11 pM) than for older waters (IcB
and ENAB: 0.24 ± 0.06 pM) (Fig. 6). This 30 % decrease in Hg
concentrations are consistent (i) with the observations of a temporal
decrease of Hg in the marine boundary layer of the NA (Sprovieri et al.,
2010; Weigelt et al., 2014) over the last two decades and with (ii) the
estimated decline in Hg concentrations in subsurface waters of the NA
estimated by models over the last few decades (e.g. Soerensen et al., 2012).
This means that LSW that formed in the 1990s in the LS and is currently present in
the ENAB received more HgAnth from the atmosphere than the
LSW2014-2015 that formed during the 2014 winter. These results contrast with what can be
deduced from the vertical profile of HgTUNF in the LS, where the
Hg regeneration in the water column is sufficient to account for the Hg
increase between the shallow LSW layer (LSW2014-2015) and the deep LSW
layer (LSW1987-1994) (Cossa et al., 2017b). This discrepancy between
these two deductions suggests that LSW, which is present in the eastern NA,
is likely older (and more imprinted by legacy HgAnth) than the
LSW currently present in the LS.
Latitudinal transport of Hg
The transport of HgTUNF per unit of water mass, calculated with
Eq. (2) (Materials and Methods section), are given in Table 2. We also applied
Eq. (2) separately to the upper and lower limbs of the AMOC and computed the
transport of HgTUNF per water mass for the two limbs. The
velocity fields across the Portugal–Greenland transect were calculated using
an inverse model constrained by Doppler current profiler velocity measurements
(Zunino et al., 2017). The volume transport per SWT was computed by combining
these velocity fields with the results of the eOMP (García-Ibáñez
et al., 2018).
The mean (velocity-weighted) HgTUNF concentration of the water
advected northwards within the upper limb of the AMOC is
0.55 pmol L-1, whereas the one advected southwards within the lower
limb of the AMOC is 0.40 pmol L-1. Across the Portugal–Greenland
transect, there is northward HgTUNF transport within the upper
limb of the AMOC (10.20 mmol s-1), and southward HgTUNF
transport within the lower limb (7.12 mmol s-1), resulting in a net
northward transport of 97.2 kmol yr-1. Most of the HgTUNF
southward transport is due to IrSPMW and ISOW displacements, whereas the
HgTUNF northward transport is associated with ENACW and SPMW
displacements (Table 2). The Hg exchange across the LS section can be roughly
estimated at 133 kmol yr-1, using the mean southward water transport
of the shelf edge LC as the Seal Island transect (Hamilton Bank near stations
77 and 78; 7.5 Sv, according to Han et al., 2008) and a mean HgT
concentration of 0.56 pmol L-1 (Cossa et al., 2017b). Thus, from our
snapshot study, the net Hg exchange across the Geovide
transect, which crosses the LS and the NA from Portugal and Greenland, would
mean a loss in the Arctic of 36 kmol yr-1. In comparison, Soerensen et
al. (2016), based on a mass balance budget, estimated that “Arctic seawater
is enriched in total Hg relative to inflowing waters from the North Atlantic
and North Pacific Oceans at all depths, resulting in a 26 Mg a-1 (i.e.
130 kmol a-1) net loss from the Arctic via circulation”.
Summary and conclusions
HgTUNF concentrations in the waters along the
Geotraces-GA01 transect, which crossed the NA from 40 to
60∘ N (Portugal to Canada), ranged from 0.16 to
1.54 pmol L-1, but with 97 % of the values lower than
1.00 pmol L-1 and a geometric mean of 0.51 pmol L-1 (n=535). The dissolved fraction (< 0.45 µm) of HgT,
determined on 141 samples, averaged 78 % of the HgTUNF for
the entire data set, 84 % for deep open seawaters and 91 % if the
Labrador Sea data, where the primary production was high, are excluded.
HgTUNF concentrations increased eastwards and downwards. The
HgTUNF concentrations were similarly low in the subpolar gyre
waters (∼ 0.45 pmol L-1), whereas they exceeded
0.60 pmol L-1 in the subtropical gyre waters, especially within
NEADWL. The relationship between HgTF and AOU, which
indicates a nutrient-like behaviour for Hg in the NA, attests to the influence
of organic matter regeneration on HgT mobilization. The distribution pattern
of HgTUNF along the transect, modelled by the mixing of SWTs, show
Hg enrichment in MW and NEADW, and low Hg concentrations in younger water
masses that formed last winter at high latitudes. Using the
HgTUNF fraction unexplained by regeneration processes as a proxy
for HgAnth, we observed geographical trend in the
HgAnth in the LSW along its eastward journey in the NA.
It was characterized by an eastward increase, which suggests that Hg incorporation
in the downwelling waters of the LS has decreased over the last 20 years,
parallel with the decrease in Hg concentrations in the NA troposphere. By
combining the velocity fields with the results of the eOMP, a net northward
Hg transfer of 97.2 kmol yr-1 across the Portugal–Greenland transect
can be calculated as a result of the AMOC. Taking into account the southern
Hg export with the LC, the net Hg exchange along the entire Geovide
transect would result in a loss in the Arctic of 36 kmol yr-1.
The data are available at the website:
www.eGEOTRACES.org.
AbbreviationsAABWAntarctic Bottom WaterAFSAtomic fluorescence spectrometerAMOCAtlantic Meridional Overturning CirculationAOUApparent oxygen utilizationCFCsChlorofluorocarbonsCRMCertified reference materialDLDetection limitDSOWDenmark Strait Overflow WaterDWBCDeep Western Boundary CurrentEGCEast Greenland CurrentENABEastern North Atlantic basinENACWEastern North Atlantic Central WatereOMPExtended optimum multiparameter analysisHgMercuryHgTAnthanthropogenic HgTHgTTotal mercuryHgTUNFunfiltered HgTHgTFfiltered HgTIAPIberian Abyssal PlainIcBIceland basinIOCInternational Oceanographic CommissionIrSIrminger SeaISOWIceland–Scotland Overflow WaterLCLabrador CurrentLSLabrador SeaLSWLabrador Sea WaterMWMediterranean watersNANorth Atlantic OceanNACNorth Atlantic CurrentNADWNorth Atlantic Deep WaterNEADWLLower North East Atlantic Deep WaterPIWPolar Intermediate WaterSPMWSubpolar Mode WaterSWTSource water typeWGCWest Greenland Current
The Supplement related to this article is available online at https://doi.org/10.5194/bg-15-2309-2018-supplement.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “GEOVIDE, an
international GEOTRACES study along the OVIDE section in the North Atlantic
and in the Labrador Sea (GA01)”. It is not associated with a
conference.
Acknowledgements
Thanks are due to members of the Geovide team for participating in
data acquisition: Fernando Alonso Pérez, Ryan Barkhouse, Vincent Bouvier,
Pierre Branellec, Lidia Carracedo Segade, Maxi Castrillejo, Leonardo Contreira, Nathalie Deniault,
Floriane Desprez de Gesincourt, Lorna Foliot, Debany Fonseca Pereira, Emilie Grossteffan,
Pierre Hamon, Catherine Jeandel, Catherine Kermabon, François Lacan, Philippe Le Bot, Manon Le Goff, Alison Lefebvre,
Stéphane Leizour, Nolwen Lemaitre, Olivier Menage, Fredéric Planchon, Arnout Roukaerts, Virginie Sanial,
Raphaelle
Sauzède, and Yi Tang. A special thank you is also due to the R/V
“Pourquoi Pas?” crew and Captain Gilles Ferrand, and the DT INSU
(Emmanuel de Saint Léger, Fabien Pérault), who organized the rosette
deployment/recovery processes. This research was funded by the French
National Research Agency (ANR-13-BS06-0014, ANR-12-PDOC-0025-01), the French
National Center for Scientific Research (Cnrs-Lefe-Cyber),
the LabexMER (ANR-10-LABX-19), the Global Mercury Observation System (GMOS,
no. 265113 European Union project), and the European Research
Council (ERC-2010-StG-20091028). For this work Maribel I. García-Ibáñez and Fiz F. Pérez were supported by the Spanish
Ministry of Economy and Competitiveness through the BOCATS (CTM2013-41048-P)
project, co-funded by the Fondo Europeo de Desarrollo Regional 2014–2020
(FEDER). Edited by: Catherine
Jeandel Reviewed by: two anonymous referees
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