Trace Element Geochemistry in North Pacific Red Clay Sediment Porewaters and Implications for Water-Column Studies

Abstract

Geochemical analyses of trace elements in the ocean water column have suggested that pelagic clay-rich sediments are a major source of various elements to bottom-waters. However, corresponding high-quality measurements of trace element concentrations in porewaters of pelagic clay-rich sediments are scarce, making it difficult to evaluate the contributions from benthic processes to global oceanic cycles of trace elements. To bridge this gap, we analyzed porewater and bulk sediment concentrations of vanadium, chromium, cobalt, nickel, copper, arsenic, molybdenum, barium and uranium, as well as concentrations of the major oxidants nitrate, manganese, iron, and sulfate in the top 30 cm of cores collected along a transect from Hawaii to Alaska. The data show large increases in porewater concentrations of vanadium, manganese, cobalt, nickel, copper, and arsenic within the top cm of the sediment, consistent with the release of these elements from remineralized organic matter. The sediments are a sink for sulfate, uranium, and molybdenum, even though conditions within the sampled top 30 cm remain aerobic. Porewater chromium concentrations generally increase with depth due to release from sediment particles. Extrapolated to the global aerial extent of pelagic clay sediment, the benthic fluxes in mol yr⁻¹ are Ba 3.9 ± 3.6 × 10⁹, Mn 3.4 ± 3.5 × 10⁸, Co 2.6 ± 1.3 × 10⁷, Ni 9.6 ± 8.6 × 10⁸, Cu 4.6 ± 2.4 × 10⁹, Cr 1.7 ± 1.1 × 10⁸, As 6.1 ± 7.0 × 10⁸, V 6.0 ± 2.5 × 10⁹. With the exception of vanadium, calculated fluxes across the sediment–water interface are consistent with the variability in bottom-water concentrations and ocean residence time of the studied elements.