Distribution of dissolved aluminium in the high atmospheric input region of the subtropical waters of the North Atlantic Ocean

Concentrations of dissolved Al (0.2 µm filtered) have been determined in the Canary basin and on a transect towards the Strait of Gibraltar, in order to allow sampling across the Saharan dust plume. The highest surface water concentrations of up to 25 nM were observed in the northwest part of the studied region, indicating substantial atmospheric input from the Sahara desert. This observation was supported by relatively pronounced Al surface maxima of the shallow casts performed in this region. The positive covariance between dissolved Al and dissolved Fe concentrations on the transect from the Canary Islands towards the Strait of Gibraltar suggests a common atmospheric source of the two metals. Outside the Canary basin, the Al concentrations rapidly dropped to values below 10 nM, coherent with lower aeolian inputs. A good correlation has been found between dissolved Al surface seawater concentrations and dissolved orthosilicic acid (Si(OH)4), which can most likely be attributed to conservative mixing of warm, more saline Al- and Si-rich water masses with colder waters of lower salinity which are more depleted in both elements. In spite of the oligotrophic waters that were met most of the cruise, a strong minimum in dissolved Al was observed at the chlorophyll a maximum at a few stations, suggesting rapid Al scavenging or biological uptake in this region.