Processes controlling trace metal and nutrient geochemistry in two southeast Alaskan fjords

Abstract

Environmental processes controlling trace metal and nutrient chemistry in sediment were investigated in Smeaton Bay and Boca de Quadra, two southeast Alaskan fjords. These pristine, non-glacial fjords (55°20'N) are located in a rugged mountainous region just north of the Alaska-British Columbia border. Close interval sampling and the remote location of the study allowed detailed examination of biogeochemical cycles in an unperturbed system. To do this, for a period of three years, spatial and temporal variations in watershed inputs, marine primary productivity, and sediment geochemistry were examined. Unlike temperate estuaries where the rivers are often significant sources of nutrients and trace metals, in the Wilson and Blossom Rivers (which drain into Smeaton Bay), the concentrations and export rates of nutrients and copper are low for most of the year. The maximum nutrient export from the Wilson-Blossom system appears to be closely tied to the annual salmon cycle. Iron and manganese export rates from the watersheds are much higher than those for copper, reflecting solubilization of iron and manganese under reducing conditions that develop in muskeg ponds during drought periods. The association of metals with organics allows transport of iron and possibly other metals throughout the fjord system, in contrast with the large-scale removal of metals in or near the river's mouth, observed elsewhere. Episodic physical mixing and the supply of reactive (autochthonous) organic matter are responsible for the temporal variations observed in interstitial water profiles. In the shallower areas of the fjords, non-linear log ^210Pb profiles and enhanced ^137Cs penetration depths suggest a substantial terrestrial contribution of sediment, and mixing coefficients ranging from >6.6 to >65 cm^2yr^-1. Small scale variability is considerable and interstitial waters are greatly undersaturated with respect to manganese- and iron- phosphates or carbonates. In the deep basin locations, sediment focusing and less mixing result in linear log ^210Pb distributions with apparent accumulation rates of 88 ± 15 mg cm^-2yr^-1, and reducing conditions near the sediment-water interface.