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

Abstract

AbstractMany studies use sedimentary biogenic silica (bSiO2) stable isotopes (e.g., δ30Si) as paleoproxies but neglect signals from other sedimentary reactive SiO2 phases. We quantified δ30Si 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 bSiO2 δ30Si values. Acid‐leachable δ30Si, that is, from authigenic products, are the result of either multistep fractionation from a bSiO2 source or an ~2‰ fractionation (consistent with metal hydroxide formation) from slowly dissolving lithogenic SiO2. This analysis also suggests that sedimentary diatom bSiO2, which has increased regionally in the last half‐century, is the critical substrate of early authigenic Si precipitates. Regional eutrophication, which has stimulated sedimentary bSiO2 accumulation, may have facilitated additional sequestration of both sedimentary Si and cations from early diagenetic products.