Seasonal euxinia in a coastal basin: Impact on sedimentary molybdenum enrichments and isotope signatures

Abstract

Eutrophication and global warming are leading to increased deoxygenation of coastal waters worldwide. To predict future trajectories of oxygen loss, insight into the intensity and temporal dynamics of past coastal deoxygenation is essential. Here, we assess the use of sedimentary molybdenum (Mo) enrichments and isotopic signatures (δ98Mo) as redox proxies in sediments of a seasonally euxinic coastal basin (Scharendijke, Lake Grevelingen, the Netherlands). Strong drawdown of dissolved Mo (from ~100 to ~60 nmol L−1) was observed in the euxinic bottom waters from July to September in 2021, coinciding with sedimentary enrichment of Mo (with peak values up to 19 ppm). A mass balance for Mo indicates that both deposition of Mo in the form of particles and diffusion of dissolved Mo into the sediment account for this Mo enrichment. The δ98Mo composition of suspended matter in the euxinic waters (0.30 to −1.95 ‰) is in line with isotopic fractionation during formation of thiomolybdates, and removal of Mo by sulfur- and/or iron-rich particles. The δ98Mo composition of the bottom water increases from 2.42 to 3.16 ‰ throughout the euxinic period, thereby further supporting preferential uptake of light Mo isotopes during sulfidation in the water column and near the sediment-water interface. The high sedimentation rate at this site (>13 cm yr−1) allows seasonal variations in sedimentary enrichments to be preserved in the sedimentary record. Notably, sediment δ98Mo values peak during the summer euxinic period, reaching 1.28 ‰ in 2021 and as high as 2.15 ‰ in 2020. The 2020 value is close to the mean ocean value of 2.3 ‰, implying that sediments overlain by seasonal and permanently euxinic bottom waters can record the same isotopic signatures for Mo. Further work is needed to determine the potential role of diagenesis in altering δ98Mo records on time scales of months to several years.