The Missing Silica Sink: Revisiting the Marine Sedimentary Si Cycle Using Cosmogenic 32 Si

Burial of biogenic silica (bSi total ) in high sedimentation rate continental margins remains highly uncertain. Cosmogenic 32 Si (t 1/2 ~140 years) can be used to trace the fates of bSi total postdeposition, including as opal (bSi opal ) and diagenetically altered opal (bSi altered ), the latter dominantly authigenic clay (bSi clay ). To determine the magnitude and form of bSi total storage in coastal sediments, conventional operational leaches targeting bSi opal and bSi altered (including bSi clay ) were modified for large‐scale samples necessary for measurement of 32 Si. The 32 Si activity was used to estimate total biogenic silica burial (bSi total  = bSi opal  + bSi altered ) in several depositional settings: Gulf of Papua, Gulf of Mexico, Long Island Sound, and in the previously studied Amazon‐Guianas deltaic system. In subtropical and temperate regions, 32 Si was detected in both traditional biogenic silica leaches (bSi opal ) and residual authigenic clays. Traditional bSi opal and modified operational leaches designed to target the most reactive authigenic silicates (~bSi altered ) consistently underestimate authigenic clay formation (bSi clay ) and thus the magnitude of bSi total burial in temperate coastal zones and subtropical deltas by 2–4‐fold. In tropical deltas, 32 Si activities in the residual fraction after removal of bSi opal demonstrate rapid and almost complete alteration of initial bSi opal to new forms, most likely bSi clay . Globally, 4.5–4.9 Tmol/yr Si may be trapped in marine nearshore deposits as rapidly formed clay (bSi clay ), 100% of the “missing silica sink” in the marine silica budget.