Diagenetic effects on the molecular structure of biogenic silica: Implications for palaeoclimate research

Abstract

Biogenic silica is abundantly preserved in sediment and is found in diatoms and many higher plants. The isotopic composition (δ18Osilica) of biogenic silica is used widely in paleoclimate research to infer conditions in which the organism grew. However, previous studies show that secondary alterations within the matrix of biogenic silica complicate the structural and geochemical analysis of silica. This study investigates how the hydrated structure of silica changes at different temperature with time. A statistical model is constructed that quantifies the degree of silica hydration (Q4/Q3) by calibrating Fourier transform infrared (FTIR) measurements against nuclear magnetic resonance. This study also conducts an investigation of various oxidation techniques to find an optimal method to be used to remove organics within biogenic silica, although residual contamination proved challenging to remove in most cases. These contaminants hinder the quantification of silica hydration using FTIR which is also used in the statistical modelling of Q4/Q3 measurements. The experimental results showed a relationship between time and the relative increase of the relative hydroxylation/dehydroxylation of biogenic silica. However, silica condensation is not a linear reaction, but reversible, potentially implying that the structure of silica readily undergoes temporary absorption/desorption at its surface in the presence of water.