The biogeochemistry of aluminum in the Pacific Ocean

Abstract

Vertical distributions and horizontal surface transects of dissolved and particulate Al at several locations in the Pacific Ocean are presented here. Concentrations of dissolved Al in the Pacific range from 0.3 to 5 nmol/kg in the surface waters, from 0.1 to 1 nmol/kg in mid-depths, and from 0.5 to 2 nmol/kg in the deep waters. From these data and published data from other regions, it is evident that Al has a unique oceanic chemistry relative to that of other metals. External sources and rapid particle scavenging throughout the water column are the major controls on dissolved Al distributions in the oceans. Vertical profiles generally exhibit a surface maximum, a mid-depth minimum, and increasing concentrations toward the sediment-water interface. This suggests two sources of dissolved Al, one to the surface waters and one to the deep waters. Concentrations of dissolved Al in surface waters are lowest within the eutrophic, low-salinity California Current (0.3–1 nmol/kg) and increase westwardly into the subtropical gyre of the North Pacific (∼ 5 nmol/kg). Values in the South Pacific gyre are fivefold lower. These horizontal trends indicate that the primary source of dissolved Al to the surface waters of the central Pacific is from eolian input. Dissolved Al supplied by rivers to the eastern North Pacific appears to be rapidly scavenged in estuaries and/or the highly productive coastal region. The source of dissolved Al to the deep waters is not known, but could be due to a flux out of the sediments or a remineralization process. Suspended particulate Al concentrations increase with depth at all stations and decrease with distance from the ocean boundary. Surface values range from ∼ 1.5 nmol/kg near shore to ∼ 0.3 nmol/kg in the central gyres of the Pacific. Deep water concentrations range from ∼ 22 nmol/kg near shore to ∼ 4 nmol/kg in the North Pacific central gyre, and decrease to ∼ 1.3 nmol/kg in the South Pacific central gyre. Observed distributions indicate that suspended particulate Al varies with the intensity of bottom sediment resuspension and the horizontal mixing of the resulting “nepheloid layer” out from ocean boundary regions. Particle residence times estimated are consistent with other reported values.