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

Rebecca A Pickering1,2, Lucie Cassarino3, Katharine R Hendry3, Xiangli L Wang1,2, Kanchan Maiti4 and Jeffrey W Krause1,2, (1)University of South Alabama, Marine Sciences, Mobile, AL, United States, (2)Dauphin Island Sea Lab, Dauphin Island, AL, United States, (3)University of Bristol, School of Earth Sciences, Bristol, United Kingdom, (4)Louisiana State University, Department of Oceanography and Coastal Sciences, Baton Rouge, LA, United States
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.