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

Measurements of natural cosmogenic Si-32 (t(1/2)similar to 140years) in tropical deltaic sediments demonstrate for the first time that most Si-32 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 Si-32, 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 (similar to 900 mu mol/g versus similar to 250 mu mol/g). Measurements of natural Si-32 inventories in sediments and initial specific activities in biogenic silica provide a means to independently constrain the marine Si cycle.