Dissolved aluminum, particulate aluminum, and silicic acid in northern Gulf of Alaska coastal waters: Glacial/riverine inputs and extreme reactivity

Abstract

The coastal northern Gulf of Alaska receives significant fluvial inputs from the numerous glaciers and rivers that border the region. The distribution of dissolved (soluble and total) and particulate aluminum (leachable and total) was examined in coastal surface water transects and vertical profile samples from shelf stations in the northern Gulf of Alaska in August and September 2007. Both dissolved and particulate aluminum concentrations were dramatically increased in low-salinity plumes, indicative of the significant riverine/glacial input to coastal waters in the region. The percent-leachable particulate Al fraction in these low-salinity plumes was quite low (∼ 7%), indicative of the large refractory nature of the lithogenic material being delivered to the coastal waters. A consistency in Si:Al ratios from different time periods in the freshwater endmembers that mix to form the low-salinity plumes is discussed in terms of the weathering of biotite in this region. The dissolved Al in these coastal waters appears entirely in the soluble (< 0.03 µm) fraction, likely a consequence of the freshwater sources to the region being very low in dissolved organic carbon and organic colloidal complexes. An extreme decreasing gradient of dissolved Al in surface waters was observed moving offshelf and into the waters of the Alaskan subarctic gyre where some of the lowest dissolved Al concentrations reported in the world ocean were observed. A high-degree of particle scavenging of dissolved Al in the coastal waters is discussed with a residence time of dissolved Al in coastal shelf waters estimated to be ∼ 10 days.