Dissolved aluminium in the ocean conveyor of the West Atlantic Ocean: Effects of the biological cycle, scavenging, sediment resuspension and hydrography

Abstract

a b s t r a c tThe concentrations of dissolved aluminium (dissolved Al) were studied along the West Atlantic GEOTRACES GA02 transect from 64°N to 50°S. Concentrations ranged from~0.5 nmol kg −1 in the high latitude surface waters to~48 nmol kg −1 in surface waters around 25°N. Elevated surface water concentrations due to atmospheric dust loading have little influence on the deep water distribution. However, just below the thermocline, both Northern and Southern Hemisphere Subtropical Mode Waters are elevated in Al, most likely related to atmospheric dust deposition in the respective source regions. In the deep ocean, high concentrations of up to 35 nmol kg −1 were observed in North Atlantic Deep Water as a result of Al input via sediment resuspension. Comparatively low deep water concentrations were associated with water masses of Antarctic origin. During water mass advection, Al loss by scavenging overrules input via remineralisation and sediment resuspension at the basin wide scale. Nevertheless, sediment resuspension is more important than previously realised for the deep ocean Al distribution and even more intensive sampling is needed in bottom waters to constrain the spatial heterogeneity in the global deep ocean. This thus far longest (17,500 km) full depth ocean section shows that the distribution of Al can be explained by its input sources and the combination of association with particles and release from those particles at depth, the latter most likely when the particles remineralise. The association of Al with particles can be due to incorporation of Al into biogenic silica or scavenging of Al onto biogenic particles. The interaction between Al and biogenic particles can lead to the coupled cycling of Al and silicate that is observed in some ocean regions. However, in other regions this coupling is not observed due to (i) advective processes bringing in older water masses that are depleted in Al, (ii) unfavourable scavenging conditions in the water column, (iii) low surface concentrations of Al or (iv) additional Al sources, notably sediment resuspension.

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

“…Our low surface dAl values agreed with literature values for the ENAB (Barrett et al, 2015;Kramer et al, 2004;Measures et al, 2008;Ussher et al, 2013) and IrB (Middag et al, 2015b), and coincided with low pAl 10 concentrations except over the Greenland shelf (See subsection 3.2.4) (Fig. 2).…”

“…The removal is reported to be caused by active scavenging onto opaline frustules of diatoms, as shown in laboratory experiments and supported by positive correlations between orthosilicic acid (Si(OH) 4 ) and Al in depth profiles upon the sinking and remineralization of diatomous material (Gehlen et al, 2002;Hydes, 1989;Middag et al, 2009;Middag et al, 2015b;Moran and Moore, 1988a). However, post-mortem incorporation of Al into the diatoms frustules has also been postulated as an Al removal process (Koning et al, 2007;Vrieling et al, 1999).…”

“…In the eastern North Atlantic dAl concentrations in surface waters range between 1-5 nM (Measures et al, 2008) and in contrast to the western North Atlantic, a dAl mid depth maximum (>30 nM) is observed associated with Mediterranean Overflow Waters (MOW) (Measures et al, 2015;Rolison et al, 2015). Globally, the highest dAl 10 concentrations have been measured in the Mediterranean Sea (up to 174 nM) (Chou and Wollast, 1997;Rolison et al, 2015) and the subtropical North Atlantic (up to 60 nM) (Schlosser et al, 2014), while the lowest concentrations (< 1nM) were found in the Southern Ocean (Middag et al, 2011), Pacific Ocean (Orians and Bruland, 1986) and high latitude North Atlantic (Middag et al, 2015b).…”

“…Sediment resuspension processes at ocean margins with strong boundary currents (Jeandel et al, 2011) 20 and in benthic nepheloid layers (BNLs) (Hesse et al, 1999;Middag et al, 2015b;Moran and Moore, 1991) represent important sources of Al to the deep ocean. 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.…”

“…In the western North Atlantic reported dAl concentrations range from 1 nM in surface waters up to 27 nM near the seafloor (Hall and Measures, 1998;Middag et al, 2015b). In the eastern North Atlantic dAl concentrations in surface waters range between 1-5 nM (Measures et al, 2008) and in contrast to the western North Atlantic, a dAl mid depth maximum (>30 nM) is observed associated with Mediterranean Overflow Waters (MOW) (Measures et al, 2015;Rolison et al, 2015).…”