Insights into silicon isotopic fractionation during reverse weathering from in vitro incubations of sediments from a temperate estuary

Authigenic precipitation of clay minerals in marine sediments, termed reverse weathering, is an important process in global biogeochemical cycles due to its role in regulating alkalinity and nutrient budgets. Dissolved silicon (silicic acid) is inherently linked with reverse weathering and is a key nutrient required by a major algal group, diatoms. Previous research has suggested that diatom biogenic silica (BSi) provides an important component for reverse weathering reactions. Stable silicon isotope measurements of extracted reactive sediment ‘pools’ reveal that there is a strong isotopic fractionation associated with this authigenic precipitation, and are a potentially useful tool for tracing these reactions. However, previous reverse weathering studies have largely focused on tropical and subtropical deltaic environments, with less attention paid to temperate estuaries. Here, we use a two-year sediment incubation experiment with samples collected from the Severn Estuary (UK) to investigate reverse weathering processes in temperate estuary sediments. We present the first experimental constraint on silicon isotopic fractionation during the initial stage of the precipitation of amorphous authigenic phases, with fractionation factors (30εRW_authi-solution) ranging from −2.20 ‰ ± 0.45 ‰ (1σ) to −4.61 ‰ ± 0.47 ‰ (1σ), which is larger than previous estimates. This high degree of fractionation may help explain the strongly fractionated silicon isotope compositions observed in both reactive solid phase pools and pore fluids where reverse weathering is active. Our comparison of sterilized and non-sterilized samples demonstrates that reverse weathering type reactions can occur abiotically. However, microbial activity and the sediment mineralogy can still affect the reverse weathering type reactions, by competing for key elements in the porewaters, modulating redox cycles or promoting the rapid neoformation of authigenic phases. The use of glass beads as a substrate demonstrates that the dissolution of amorphous silica, analogous to diatom BSi, can obscure the silicon isotope fractionation signal in the porewater. These findings highlight the significant influence that biological activity and sediment composition can have on reverse weathering, offering new insights into its dynamics in different sedimentary environments.