Using Stable Isotopes to Disentangle Marine Sedimentary Signals in Reactive Silicon Pools

Many studies use sedimentary biogenic silica (bSiO(2)) stable isotopes (e.g., delta Si-30) as paleoproxies but neglect signals from other sedimentary reactive SiO2 phases. We quantified delta Si-30 for multiple reactive Si pools in coastal river-plume sediments, revealing up to -5 difference between acid-leachable and alkaline-digestible amorphous SiO2. Thus, previous studies have missed valuable information on early diagenetic products and, in cases where sediments were not cleaned, potentially biased bSiO(2) delta Si-30 values. Acid-leachable delta Si-30, that is, from authigenic products, are the result of either multistep fractionation from a bSiO(2) source or an similar to 2 fractionation (consistent with metal hydroxide formation) from slowly dissolving lithogenic SiO2. This analysis also suggests that sedimentary diatom bSiO(2), which has increased regionally in the last half-century, is the critical substrate of early authigenic Si precipitates. Regional eutrophication, which has stimulated sedimentary bSiO(2) accumulation, may have facilitated additional sequestration of both sedimentary Si and cations from early diagenetic products.