Serpentine alteration as source of high dissolved silicon and elevated \u03b430Si values to the marine Si cycle

Sonja Geilert,Catriona D Menzies,Volker Liebetrau,Klaus Wallmann,Patricia Grasse

doi:10.1038/s41467-020-18804-y

Abstract

Serpentine alteration is recognized as an important process for element cycling, however, related silicon fluxes are unknown. Pore fluids from serpentinite seamounts sampled in the Mariana forearc region during IODP Expedition 366 were investigated for their Si, B, and Sr isotope signatures (δ30Si, δ11B, and 87Sr/86Sr, respectively) to study serpentinization in the mantle wedge and shallow serpentine alteration to authigenic clays by seawater. While serpentinization in the mantle wedge caused no significant Si isotope fractionation, implying closed system conditions, serpentine alteration by seawater led to the formation of authigenic phyllosilicates, causing the highest natural fluid δ30Si values measured to date (up to +5.2 ± 0.2‰). Here we show that seafloor alteration of serpentinites is a source of Si to the ocean with extremely high fluid δ30Si values, which can explain anomalies in the marine Si budget like in the Cascadia Basin and which has to be considered in future investigations of the global marine Si cycle.

Highlights

Serpentine alteration is recognized as an important process for element cycling, related silicon fluxes are unknown
Our findings show that serpentine dissolution and the subsequent formation of authigenic minerals is a source of Si and B to the ocean with very high δ30Si and δ11B values
With regard to Si cycling, seafloor alteration reactions result in the release of isotopically heavy Si, which likely impacts marine water mass isotope signatures and may explain local anomalies in the marine Si cycle, for example in the Cascadia Basin

Summary

Introduction

Serpentine alteration is recognized as an important process for element cycling, related silicon fluxes are unknown. During IODP Expedition 366, pore fluids from serpentinite seamounts in the Mariana and Izu-Bonin arc region were sampled, in order to study Si isotope fractionation during serpentinite-seawater alteration reactions.

Results

Conclusion