The distribution of dissolved aluminium (dAl) in the water
column of the North Atlantic and Labrador Sea was studied along GEOTRACES
section GA01 to unravel the sources and sinks of this element. Surface water
dAl concentrations were low (median of 2.5 nM) due to low aerosol deposition
and removal by biogenic particles (i.e. phytoplankton cells). However,
surface water dAl concentrations were enhanced on the Iberian and Greenland
shelves (up to 30.9 nM) due to continental inputs (rivers, glacial flour,
and ice melt). Dissolved Al in surface waters scaled negatively with
chlorophyll
Aluminium (Al) in the oceans has been used as a tracer for mineral dust
deposition (Han et al., 2008; Measures and Vink, 2000; Measures and Brown,
1996) and water masses (Measures and Edmond, 1990). Aluminium is the third
most abundant element in the Earth's crust (Rudnick and Gao, 2003), but
concentrations of dissolved Al (dAl; filtered through 0.4 or 0.2
A major source of Al to the surface ocean is dry atmospheric deposition of terrigenous material (Kramer et al., 2004; Measures et al., 2005; Orians and Bruland, 1986), which can be carried thousands of kilometres in the atmosphere before deposition into the ocean (Duce et al., 1991; Prospero and Carlson, 1972). Wet atmospheric deposition (rain, fog, and snow) also plays an important role in supplying Al to both the North Atlantic (Schlosser et al., 2014; Shelley et al., 2017) and the global ocean (Guerzoni et al., 1997; Vink and Measures, 2001). Glacial runoff has been reported as a pronounced source for Arctic and Antarctic surface waters (Brown et al., 2010; Statham et al., 2008), but its impact beyond the immediate source regions has not yet been established. Fluvial inputs were historically considered a dominant source of Al to the surface ocean (Stoffyn and Mackenzie, 1982), but Al removal through particle scavenging during estuarine mixing processes appears to strongly reduce the riverine Al outflows (Hydes, 1989). However, recent publications have indicated significant fluvial sources for Al (Brown and Bruland, 2009; Brown et al., 2010; Grand et al., 2015). Sediment resuspension represents an important source of Al to the deep ocean, especially along ocean margins with strong boundary currents (Jeandel et al., 2011) and in areas with benthic nepheloid layers (Middag et al., 2015b; Moran and Moore, 1991). Recently, hydrothermal vents (Measures et al., 2015; Resing et al., 2015) were noted as Al sources to the deep Atlantic and Pacific oceans, with plumes extending at depth over 3000 km in the Pacific Ocean.
Removal of Al in oceanic waters occurs through particle scavenging with
subsequent sinking of the particulate matter (Orians and Bruland,
1986). This removal occurs via both active and passive scavenging processes.
Active scavenging occurs when dAl is incorporated into the atomic structure
of opaline diatom frustules, a process which has been demonstrated in
laboratory experiments and is also supported by positive correlations
between silicic acid (
In the North Atlantic (specifically 40–65
This manuscript provides an overview of the surface and water column
distribution of dAl in the North Atlantic Ocean and Labrador Sea along
GEOTRACES section GA01. The sources and sinks of Al for the surface and deep
ocean are discussed, and the controls that regulate dAl are examined in
light of
The GEOVIDE cruise was conducted as part of the GEOTRACES programme (GA01
section), and sailed on 15 May (2014) from Lisbon (Portugal), passed by the
southern tip of Greenland (16, 17 June), and arrived in St. John's (Canada) on 30 June (Fig. 1a). A total of 32 stations
were sampled for dissolved and particulate trace metals. Seawater was
collected using a trace metal clean rosette (TMR, General Oceanics Inc.
model 1018 intelligent rosette) attached to a Kevlar line and fitted with
Dissolved Al samples were filtered using 0.2
Dissolved seawater samples were analysed using flow injection analysis (FIA)
with fluorescence detection as developed by Resing and Measures (1994), and
modified by Brown and Bruland (2008). A slight modification of the method
published by Brown and Bruland (2008) is the use of a 2 M ammonium acetate
buffer instead of a 4 M buffer in the reaction stream. In short, acidified
samples were buffered online to pH
Calibration was undertaken using standard additions prepared in low trace
metal seawater. The different standards were prepared from a stock standard
solution of 1
The accuracy and precision of the measurements were evaluated by analysis of
consensus seawater samples as well as internal reference seawater. GEOTRACES
deep (GD) and SaFe
Suspended particles were digested at LEMAR following the protocol of
Planquette and Sherrell (2012) and analysed for pAl
exactly as described in Gourain et al. (2018).
Silicic acid concentrations were analysed on board using a Bran
A SeaBird sensor package 911 mounted to the CTD frame recorded pressure, temperature, and salinity data, while a SeaBird 43 was used for dissolved oxygen. Salinity and oxygen data were calibrated using analysis of discrete samples with a salinometer (Guildline) and the Winkler method (Carpenter, 1965), respectively.
Along the GEOVIDE section, three biogeochemical provinces defined by
Longhurst (2010) were studied: (i) the North Atlantic Subtropical (NAST)
region, including the Iberian Basin (IB, stations 1 to 19); (ii) the North
Atlantic Drift (NADR) region, including the Eastern North Atlantic Basin
(ENAB, stations 21 to 26), and the Iceland Basin (IcB, stations 29 to 38);
(iii) the subarctic North Atlantic (SANA), including the Irminger Basin (IrB,
stations 40 to 60) and the Labrador Basin (LB, stations 61 to 71) (Fig. 1a).
The salinity distribution and the main water masses in the North Atlantic and
Labrador Sea are shown in Fig. 1b. The main water masses used in the
discussion of the dAl distribution are the (i) MOW which originates in the
Mediterranean Sea, is present at intermediate layers (
Figure 2 shows average dAl concentrations in surface waters (
Average surface distribution (
Surface concentration of dAl in the Iberian Basin during GEOVIDE. Label GA03 refers to GEOTRACES section GA03 (Measures et al., 2015); 64PE370 refers to GEOTRACES section GA04N (Rolison et al., 2015). Black arrows show the cruise tracks. Red arrows show the location of the Tagus estuary and the northward direction of the Tagus plume. Plot created in Ocean Data View (Schlitzer, 2018).
Surface concentration around the southern tip of Greenland of:
In the North Atlantic atmospheric aerosol loading declined, in a
northwestward direction, with increasing
distance from African dust source regions (e.g. Sahara and Sahel; Duce et
al., 1991; Jickells et al., 2005). The low dAl surface concentrations
observed (Fig. 2) in the different basins suggest a low aerosol deposition to
the study area, which is consistent with low aerosol deposition fluxes
reported for the GEOVIDE cruise by Shelley et al. (2017) and Menzel
Barraqueta et al. (2018). Aerosol deposition is considered as a major source
of Al to the surface ocean and modelling studies on global dust deposition
indicate a 10-fold decrease in atmospheric dust deposition fluxes between
Portugal and the Labrador Sea (
Biogenic opal production and biogenic particles play an important role in the
removal of dAl in the surface ocean as a result of the high particle surface
affinity of dAl (Moran and Moore, 1988b). Removal of dAl by particles
therefore represents a mechanism which reduces dAl and increases pAl
concentrations in surface waters (Moran and Moore, 1988a). During the GEOVIDE
cruise, Chl
Cross-section plot of the pAl to dAl ratio (mol : mol) over the full depth of the water column. Plots created in Ocean Data View (Schlitzer, 2018).
Correlations between salinity (
Enhanced surface water (ca. 15 m) dAl concentrations were observed on the
Iberian shelf (stations 1, 2, and 4), with average dAl concentrations of
Concentrations of dAl in surface water samples collected on the Greenland
shelf ranged between 2 and 7 nM, and coincided with reduced salinities (down
to 32.2) and enhanced pAl concentrations (up to 62 nM; Fig. 4 a, b, and c).
Linear regressions between dAl, pAl, and salinity for surface samples
collected SE and SW of Greenland had coefficients of
Freshwater endmembers (salinity 0) for Al were determined from linear
regressions between dAl, pAl, and salinity for the eastern stations (49, 53,
56, and 60; dAl
The section of dAl showed low concentrations in surface waters and an
increase with depth (Fig. 6a, b, c), therefore resembling a nutrient-type
distribution. The IB formed an exception with maximum dAl (up to 38 nM)
observed at intermediate depths associated with the MOW (Fig. 6a, b, c and
Fig. S4; see Sect. 3.4.1). Average dAl depth profiles as well as maximum,
minimum, median, and quartile (1st and 3rd) dAl values per basin are
presented in Fig. 6b. In subsurface waters between 50 and 500 m depth the
median dAl concentrations were lowest in the LB (4.3 nM) and highest in the
ENAB (7.6 nM) associated with North East Atlantic Central
Waters (Fig. 6c), with an overall median concentration along
the full transect of 5.9 nM (
Section plot of particulate Al along the GEOVIDE section. For station numbers, please refer to Fig. 6. Data are from Gourain et al. (2018).
In the remote oligotrophic regions of the North Atlantic Ocean with enhanced
Saharan dust inputs, dAl shows enhanced surface water concentrations with
depletion at depth (Measures et al., 2015), typical for a scavenged-type
element (Bruland et al., 2014). A scavenged-type distribution for dAl has
also been described for the Pacific Ocean (Orians and Bruland, 1985). In
contrast, a nutrient-type depth distribution of dAl has been reported for the
Arctic Ocean (Middag et al., 2009), Mediterranean Sea (Hydes et al., 1988;
Rolison et al., 2015), North Atlantic (40–50
The Mediterranean Sea receives large inputs of aerosols which result in
elevated surface dAl concentrations of up to 174 nM, as reported by Hydes et
al. (1988). The presence of the MOW was indicated by a mid-depth maximum in
salinity (
Dissolved Al and particulate Al (pAl; Gourain et al., 2018)
concentrations against salinity for the Mediterranean Outflow Water (MOW)
between the neutral-density layer 27.6 and 27.8 kg m
Sediment resuspension and transport due to physical forcings, such as internal waves, tides, and currents, as well as diffusion of Al-rich pore waters, are all deemed to increase Al levels in bottom waters (Stoffyn-Egli, 1982; Van Beueskom et al., 1997). Enhanced dAl levels have been observed as a result of continental margin inputs for the Drake Passage (Middag et al., 2012) and European shelf (de Jong et al., 2007; Moran and Moore, 1991). Moreover, in the North Atlantic, resuspension of sediments associated with benthic nepheloid layers has been shown to elevate dAl concentrations in comparison to overlying waters (Middag et al., 2015b; Moran and Moore, 1991; Sherrell and Boyle, 1992).
On the Iberian and Greenland shelves and margins we observed both enhanced
dAl concentrations and pAl to dAl ratios (Figs. 3, 4, and 5). In contrast, on
the Newfoundland shelf no enhanced dAl concentrations were observed. On the
Iberian shelf and margin, enhanced dAl concentrations were observed near the
seafloor (station 2: up to 21 nM at a depth of 140 m; station 4: up to
27 nM at a depth of 800 m), associated with enhanced pAl concentrations of
up to 1.5
Enhanced dAl concentrations in the bottom layers of several basins (Iceland, Irminger, Labrador) accompanied by enhanced attenuation signals from the beam transmissometer were indicative of the presence of benthic nepheloid layers which are typically caused by strong bottom currents (e.g. ISOW and DSOW; Eittreim et al., 1976). Figure 9 shows dAl and transmissometry profiles for stations 26, 42, 69, and 77 (Fig. S7 shows pAl for the same stations). Enhanced dAl concentrations near the seafloor coincided with enhanced pAl (Gourain et al., 2018) and a beam attenuation signal. In contrast, at station 42, dAl concentrations decreased near the seafloor. Based on the eOMP analysis (Garcia Ibanez et al., 2018), the waters at 2900 m depth (dAl 25.1 nM) had an ISOW contribution of 66 % and a residual contribution of DSOW of 2 %. In contrast, near the seafloor, the contribution of DSOW increased to 91 % with a residual ISOW contribution of 6 %. Thus, the low dAl concentrations near the seafloor were probably related to low dAl DSOW in comparison with overlaying dAl-rich ISOW as reported by Middag et al. (2015b). Enhanced dAl concentrations with no concomitant decrease in transmissometry (station 77) could indicate that dAl was released from pore waters.
Dissolved Al (nM) and beam transmissometer (%) profiles for stations 26, 42, 69, and 77.
Overall, the observed enhanced Al concentrations suggest that along the GEOVIDE section, continental shelves and margins acted as a source of Al to adjacent waters. However, dAl concentrations did not always increase when enhanced pAl concentrations were present which implies that the release or sorption mechanisms from shelf and deep sea sediments may be different. These results suggest that, occasionally, scavenging of dAl onto resuspended particles may be a dominant process rather than partial dissolution of Al from resuspended sediments. However, the mechanisms controlling either a net dissolution or scavenging of Al from or by resuspended particles remain unclear. Moreover though, the general increase in Al concentrations in bottom layers suggests that these areas act as a potential source of Al to the North Atlantic Deep Water as observed in previous studies (Measures et al., 2015; Middag et al., 2015b; Moran and Moore, 1991).
Hydrothermal activity was assessed at station 38 over the Mid-Atlantic Ridge. Hydrothermal activity has been reported as a source of dAl to the deep ocean in the Pacific and Atlantic (Measures et al., 2015; Resing et al., 2015). No enhanced dAl (Fig. 6a) or dFe (Tonnard et al., 2018a) concentrations were evident, although Achterberg et al. (2018) observed enhanced dFe over the Reykjanes ridge and attributed this to a possible combination of hydrothermal sources and sediment resuspension. However, enhanced concentrations in particulate Al (up to 28 nM, Fig. 7), Fe, Ti, and Mn and a pFe to pAl ratio similar to the ratio in fresh mid-ocean ridge basalts were observed at station 38 (Gourain et al., 2018). Therefore, the minor pAl signature observed at station 38 could be partly related to hydrothermal inputs and resuspension of newly formed oceanic crust.
The dAl distribution in seawater is controlled by the relative strength of its sources and removal processes. At large basin scales, along the GEOVIDE section, the dAl distribution was mostly determined by low aerosol depositions, removal onto biogenic particles, and the remineralization of biogenic particles at depth. Yet we show that at smaller regional scales, local sources such as rivers and glacial runoff controlled the dAl signatures. Additionally, sediment resuspension events and the processes of sorption or desorption of dAl onto or from particles were important mechanisms determining the dAl concentrations at the sediment–water interface. Our results highlight the importance of phytoplankton (particularly diatom) abundance and dynamics for determining the interaction between dissolved and particulate Al phases in surface waters.
Overall, the Al distribution along the GEOVIDE section, in addition to other recent discoveries from the GEOTRACES programme, highlights the complex nature of dAl biogeochemical cycling in seawater as it can resemble a scavenged-type or a nutrient-type element. The large sets of parameters measured on each GEOTRACES cruise will allow us, in the coming years, to examine the global ocean dAl cycling from a holistic perspective.
The 2014 GEOTRACES GA01 section data for dAl are available
at the Intermediate Data Product 2017 (Schlitzer et al., 2018) and can be
downloaded from the GEOTRACES website
(
The supplement related to this article is available online at:
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.
We are greatly thankful to the captain, Gilles Ferrand, and crew of the N/O