Zonal distribution of dissolved aluminium in the Mediterranean Sea

Dissolved aluminium (Al) is an important tracer of atmospheric dust input to the oceans. The GEOTRACES expedition to the highly dust impacted Mediterranean Sea afforded the opportunity to study the distribution of dissolved Al in the Mediterranean Sea in detail. Interestingly, the elevated concentration of dissolved Al observed in Mediterranean surface waters (up to 80 nmol kg− 1) is strongly correlated with salinity, both showing an increase from west to east due to mixing of low Al, low salinity Atlantic surface waters with high Al, high salinity Mediterranean surface and intermediate waters. At intermediate depths (100-1250 m), a strong correlation between dissolved Al and silicic acid (Si) was observed. Vertical mixing between surface, intermediate and old uplifted deep water between depths of 0 and 1300 m is primarily responsible for the long recognized Al:Si relationship at intermediate depths. However, since the subsurface waters have a surface water origin, vertical transport of Al and Si is required to maintain the high concentrations of dissolved Al and Si in Mediterranean deep waters relative to surface waters. The most likely vertical transport mechanism is suggested to be biogenic particles which would thus be ultimately responsible for the Al:Si relationship at intermediate depths. Elevated concentrations of dissolved Al relative to Si were observed in some Mediterranean deep waters with sediment resuspension during episodes of deep water formation as the most likely source of the additional dissolved aluminium.