The marine geochemistry of dissolved gallium: A comparison with dissolved aluminum

Abstract

Dissolved Ga concentrations in the Pacific Ocean range from 2 to 30 picomolar: they are low in surface waters (2–12 pM), with a subsurface maximum at 150–300 m (6–17 pM), a mid-depth minimum from 500 to 1000 m (4–10 pM) and increasing values with depth to a maximum in the bottom waters (12–30 pM). The highest concentrations are in the central gyre, with lower values toward the north and east where productivity and particle scavenging increase. Dissolved Ga concentrations in the surface waters of the northwest Atlantic are nearly an order of magnitude higher than in the central North Pacific, with higher values in the Gulf Stream than in the continental slope boundary region. Dissolved Al concentrations in the high latitude North Pacific are also lower than in the central gyre, and are the lowest yet reported for this element. The features are similar, however, to other dissolved Al profiles: a surface maximum (0.8 nmol/kg), a mid-depth minimum (≤0.06 nmol/kg) and increasing concentrations toward the sediment-water interface (0.4 nmol/kg). The vertical distributions and horizontal transects indicate three sources of dissolved Ga to the oceans. The surface distribution reflects an eolian source with no net fluvial input to the open ocean; the subsurface maximum (a feature not seen for North Pacific dissolved Al) is attributed to vertical exchange processes; the source for the deep waters of the North Pacific is from a sediment surface remineralization process or a pore water flux. Scavenging removal throughout the water column is evident in the vertical profiles for both dissolved Ga and Al, with intensified removal in the boundary regions where productivity and particle scavenging are at a maximum. Residence times of dissolved Ga in surface waters are nearly an order of magnitude longer than the corresponding values for Al.