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.

Controls on Dissolved Barium and Radium‐226 Distributions in the Pacific Ocean Along GEOTRACES GP15

“…This recirculation allows deep waters to accumulate 226 Ra and Ba from the seafloor. This accumulation is consistent with the higher copper and neodymium concentrations (Bian et al, 2022a(Bian et al, , 2022bShiller, 2021bShiller, , 2021c and the high benthic Ba and 228 Ra fluxes reported in this region (Moore et al, 2024;Steiner et al, 2023).…”

“…Given the results of Steiner et al. (2023), who observed benthic Ba fluxes of up to 95 μmol m −2 y −1 in the North Pacific, our data indicate that between 25% and 100% of the observed Ba flux originates from sediments with the balance from the water column dissolution. Variations in 226 Ra and Ba fluxes released from the bottom may be due to differences in sedimentary 230 Th activity, bioturbation rates, sediment type and composition, and may also vary as a function of surface productivity, particle export and burial.…”

“…Bottom fluxes for Ba of +95 ± 61 μmol m −2 y −1 would be required to explain the Ba anomalies observed along GP15 (Figure 7). These fluxes are on the high end of Ba sedimentary fluxes reported in different ocean basins, which range from 0.21 to 400 μmol m −2 y −1 (Chan et al., 1976, 1977; Falkner et al., 1993; McManus et al., 1999; Paytan & Kastner, 1996; Steiner et al., 2023). While our estimated benthic fluxes are toward the higher end of published values, it is important to keep in mind that our flux calculation considers water column and sedimentary BaSO 4 dissolution as a single benthic flux.…”

“…While 226 Ra benthic flux is mostly derived from 230 Th decay, Ba benthic flux is mostly due to barite dissolution. The global Ba dissolution fluxes have been estimated to be +176 ± 243 μmol m −2 y −1 (Carter et al., 2020; Dehairs et al., 1980; Steiner et al., 2023). The most significant barite dissolution is expected to occur below 4,000 m, as Pacific seawater is saturated for barite above that depth (Mete et al., 2023).…”

“…Th 1,449 ± 502 176 ± 243 (Carter et al, 2020;Cochran, 1980;Dehairs et al, 1980;Kemnitz et al, 2023;Steiner et al, 2023) Burial activity also introduces significant quantities of 3 He-rich into the deep ocean basins, which can be used to trace the hydrothermal buoyant plumes Lupton, 1998).…”