Sedimentary Trace Metal and Stable Isotope Trends in Fjord & Shelf Environments along the Southeast Alaska Margin

Fjords are important sedimentary depocenters and can serve as valuable repositories for preserving high-resolution records of environmental change. Quaternary glacial activity along the Gulf of Alaska margin has carved hundreds of fjords, many with shallow sills that restrict basin water exchange. In this study, we examine the relationship between sedimentary trace metal concentrations as a function of bathymetric geometry (e.g. single-basin fjords, multiple-basin fjords, & open shelf) along the Gulf of Alaska margin. To elucidate environmental conditions that can lead to the accumulation of redox-sensitive elements, these concentration data are compared with bulk sedimentary δ¹³C & δ¹⁵N isotopic ratios and total organic carbon (TOC) content. Preliminary results indicate that the fjords of southeast Alaska accumulate a suite of elements in concentrations not observed in adjacent open shelf sites, including TOC, Fe, S, V, As, Mo, and U. Because these elements are sensitive to the concentration of dissolved oxygen, high concentrations of these elements suggest suboxic or anoxic conditions at the sediment-water interface. Ternary diagrams of Fe-S-TOC show that sediments from both fjords and the shelf plot along a line with a S/TOC ratio of 0.40, similar to that of normal Holocene anoxic marine sediments indicating that sulfate-reduction processes are equally vigorous along the shelf and within fjords. Binary plots of δ¹³C & δ¹⁵N demonstrate that fjord sites are dominated by marine C sources (δ¹³C = -20 to -22 per mil) under N-limiting conditions (δ¹⁵N values ≥ 8 per mil) due to enhanced primary productivity. In contrast, shelf sites tend towards greater contributions of terrestrial-derived organic matter sources. Collectively these data indicate that productivity-driven anoxia is a key process within the fjords of the Gulf of Alaska, and is thus an important boundary condition within which to consider the behavior of redox-sensitive elements along this margin over geologic time.