Resolving Stable Silicon Isotope Signatures in Reactive Silicon Pools of Coastal Marine Sediments

Silicon isotope ratios (δ³⁰Si) in sediment biogenic opal (bSi) are widely used as a paleoproxy for marine silicic acid (Si(OH)₄) usage and productivity by pelagic diatoms. Despite the growing body of work that uses δ³⁰Si signals, we lack δ³⁰Si data for other operationally defined reactive Si pools (e.g. acid leaches), raising the possibility that information about potential diagenetic processes involving these reactive pools in coastal systems may have been overlooked. We quantified δ³⁰Si values for the operationally defined reactive Si pools in Mississippi River plume sediments, describing previously hidden δ³⁰Si values for the first time. The isotopic shift between pools is drastic, as the δ³⁰Si of sedimentary Si liberated in the acid pre-leach is 4.5–5 ‰ lower than that of both traditional and sequential sodium-carbonate digestions (measured in separate analyses), and is ~2 ‰ lower than that of sodium-hydroxide digested material (a blend of lithogenic silica and bSi from sponge spicules, Rhizaria). These data offer independent support for the existence of marine authigenic products. Here we aim to resolve the isotopicdifferences between reactive Si pools by examining organic matter, metal-hydroxide formation and a suite of metal ion concentrations to separate the components of sedimentary amorphous Si.