Cosmogenic ³²Si as a tracer of biogenic silica burial and diagenesis: Major deltaic sinks in the silica cycle

Measurements of natural cosmogenic ³²Si (t_1/2 ~ 140 years) in tropical deltaic sediments demonstrate for the first time that most ³²Si is present in rapidly formed authigenic clays and not biogenic opaline silica (bSi). The burial of bSi in deltaic and continental margin sediments has likely been greatly underestimated because of diagenetic alteration of bSi to clay, little of which dissolves in the classically used operational bSi leach. Rapid reverse weathering reactions during early diagenesis must be considered as a significant pathway of reactive Si storage in deltaic deposits. Based on ³²Si, actual storage may be 2–3 times the best recent estimates extrapolated from diagenetic models or attempts to modify operational bSi methods to include authigenic clay (~900 µmol/g versus ~250 µmol/g). Measurements of natural ³²Si inventories in sediments and initial specific activities in biogenic silica provide a means to independently constrain the marine Si cycle.