Aluminium in the North Atlantic Ocean and the Labrador Sea (GEOTRACES GA01 section): roles of continental inputs and biogenic particle removal

Abstract

Abstract. 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 a and biogenic silica (opal) concentrations. The abundance of diatoms exerted a significant (p<0.01) control on the surface particulate Al (pAl) to dAl ratios by decreasing dAl levels and increasing pAl levels. Dissolved Al concentrations generally increased with depth and correlated strongly with silicic acid (R2>0.76) west of the Iberian Basin, suggesting net release of dAl at depth during remineralization of sinking opal-containing particles. Enrichment of dAl at near-bottom depths was observed due to the resuspension of sediments. The highest dAl concentrations (up to 38.7 nM) were observed in Mediterranean Outflow Waters, which act as a major source of dAl to mid-depth waters of the eastern North Atlantic. This study clearly shows that the vertical and lateral distributions of dAl in the North Atlantic differ when compared to other regions of the Atlantic and global oceans. Responsible for these large inter- and intra-basin differences are the large spatial variabilities in the main Al source, atmospheric deposition, and the main Al sink, particle scavenging by biogenic particles.

Introduction to the French GEOTRACES North Atlantic Transect (GA01): GEOVIDE cruise

“…The Mediterranean Water (MW), meanwhile, was characterized by higher concentrations of trace metals, such as Pb, Hg, and Al (Cossa et al, 2018a;Menzel Barraqueta et al, 2018a;Zurbrick et al, 2018). It reflects the importance of Saharan and anthropogenic atmospheric inputs to the Mediterranean region, which are much higher than in our studied area (see below), as well as enhanced remineralization as indicated by the correlation between total mercury concentrations and the apparent oxygen utilization (Cossa et al, 2018a).…”

“…Almost all the stations displayed dFe minima in surface waters, in association with the chlorophyll maxima. The abundance of diatoms exerted a significant control on the surface concentrations of Fe and Al (Menzel Barraqueta et al, 2018b;Tonnard et al, 2018). Remineralization processes were also highlighted for some TEIs (see also Sect.…”

“…Some other TEIs were also strongly linked to water mass distribution: within GEOVIDE, silicon isotopes , lead (Pb) , mercury (Hg) (Cossa et al, 2018a), and particulate and dissolved Fe and Al Menzel Barraqueta et al, 2018a;Tonnard et al, 2018), for example. For instance, the Labrador Sea Water (LSW) is characterized by a relatively high silicon stable isotope composition for dissolved silicon (δ 30 Si DSi ) whose signature can be seen not only in the region where it is formed, but also throughout the mid-depth zone of the North Atlantic Ocean .…”

“…Different methods and TEIs were used to estimate atmospheric input fluxes: aerosol concentrations in aerosol and rainwater samples were compared with estimates derived from the measurement of beryllium-7 ( 7 Be) in aerosols, rainwater, and seawater (Shelley et al, 2017). Taking a different approach, Menzel Barraqueta et al (2018b) used dAl in the surface waters to estimate the atmospheric input flux. All these methods allowed conclusion that the atmospheric inputs of total trace elements were low in our study area, and the soluble input was even lower, based on their fractional solubility .…”

Reply to reviewer #2

“…Some other TEIs were also strongly linked to water mass distribution: within GEOVIDE, silicon isotopes (Sutton et al, 2018), lead (Pb) (Zurbrick et al, 2018), mercury (Hg) (Cossa et al, 2018a), and particulate and dissolved Fe and Al (Gourain et al, 2018;Menzel Barraqueta et al, 2018a;Tonnard et al, 2018), for example. For instance, the Labrador Sea Water (LSW) is characterized by a relatively high silicon stable isotope composition for dissolved silicon (δ 30 Si DSi ) whose signature can be seen not only in the region where it is formed, but also throughout the mid-depth zone of the North Atlantic Ocean .…”