Coupled Si–Al Biogeochemistry: Occurrence of Aluminum in Diatom‐Derived Biogenic Silica

Abstract

Diatoms play an important role in the biogeochemical cycling of aluminum (Al) in oceans. This occurs via the uptake of biological Al (Albio), which is incorporated into the structure of diatom‐derived biogenic silica (DBSi) and the formation of adsorbed Al (Alads) on the DBSi surface of post‐mortem diatoms. Al occurrence influences DBSi dissolution and thus diatom‐driven carbon sequestration. However, the mechanism of Al occurrence in DBSi remains unclear. In this study, Albio and Alads of DBSi from various diatom fossils in marine diatomaceous sediments were identified and quantified by combined focused ion beam thinning, elemental analysis, and the Al K‐edge X‐ray absorption near edge structure. Results showed the coexistence of Albio and Alads in all diatomaceous sediments and Al‐bearing DBSi thus constitutes a biological Al pool. Albio and Alads were mainly fourfold‐ and sixfold‐coordinated, respectively and Alads was much more abundant than Albio. Moreover, even at low concentrations (with an Al/Si atomic ratio of 0.0031), Albio can inhibit DBSi dissolution, effectively decreasing the extent of DBSi dissolution by ∼14%. Albio also significantly increased the mechanical strength of DBSi. The average Young's modulus (a measure of the stiffness of a material) of cribrum layers in Al‐incorporated DBSi was ∼1.4 times higher than that of Al‐free DBSi. Our results further demonstrate that diatoms play a dominant role in the biogeochemical cycling of Al in oceans, and the Al of DBSi participates in diatom‐driven Si and C coupled cycles in oceans, influencing the effectiveness of diatom‐driven carbon export by regulating the dissolution and mechanical strength of DBSi.