Pyritization of iron and trace metals in anoxic fjord sediments (Nordåsvannet fjord, western Norway)

Abstract

The accumulation and storage of trace metals in coastal sediments is an environmental concern. It is, therefore, important to understand better how these metals are bound or released under different redox conditions. This study of Fe and trace metal fixation under continuously anoxic conditions in the bottom sediments and the lower water column of the Nordåsvannet fjord in western Norway contributes further to such understanding. It allows investigation of both an end member redox state and one important mechanism of Fe and trace metal accumulation in sediments, the pyritization of Fe and trace metals. Pyrite formation occurs both in the water column and in the sediments of the Nordåsvannet fjord and favours the fixing of Fe and trace metals in the bottom sediments of the fjord. Thus, these sediments act as a continuous sink for Fe and trace metals. The DOP, and the degrees of trace metal pyritization for Mo, Ni and Cr correlate with organic matter content. While it is generally thought that Fe is the factor limiting pyrite formation in anoxic environments, this study found that degrees of pyritization of Fe (DOP) are clearly below 100%, and the availability of metabolizable organic matter is limiting pyrite formation. This is an important finding, because it indicates that increased supply of organic and mineral matter by higher runoff from land would further enhance the fixation of these metals in the fjord sediments, as would higher organic matter availability from increased productivity due to higher nutrient supply. The metals stored in the bottom sediments could be released into the biogeochemical cycle if redox conditions were to change from anoxic to suboxic or oxic. The fjord would then become a source rather than a sink for these metals.