The silicon isotope record of early silica diagenesis

Abstract

The heavy isotopes of silicon are strongly enriched in some of the youngest, early diagenetically formed porcellanite layers from the Southwest Indian Ridge (Pleistocene) and the Maud Rise (Pliocene). These porcellanite layers are composed of opal-CT and were formed by the conversion of amorphous silica (opal-A) from siliceous sediment via dissolution–reprecipitation. Their bulk δSi30 values range between 1.7 and 2.3‰. Detritus-poor siliceous sediment surrounding these layers is significantly lower at −0.3 to 1.5‰. Sequential chemical extractions of bulk siliceous sediment show (i) preferential dissolution of diatoms featuring higher δSi30 than radiolaria and Al–Si components. The detailed investigation of porcellanite layers by micro-scale Si isotope and Al/Si analyses using UV femtosecond laser ablation ICP mass spectrometry show that (ii) precipitation of authigenic aluminum silicates enriched in light Si isotopes drives pore waters to even higher δSi30. We suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert. We infer that past environmental conditions can be reconstructed with high fidelity from the stable Si isotope composition of chert when initial seawater Si concentrations were high (such as in the Precambrian). Exchange of Si between layers during phase transformation (from opal-A to opal-CT and from opal-CT to quartz) is impeded when variable amounts of detrital minerals are present, because they control rates of silica phase transformation and hence the timing of dissolution–reprecipitation during burial.