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

Burial of biogenic silica (bSi(total)) in high sedimentation rate continental margins remains highly uncertain. Cosmogenic Si-32(t1/2 similar to 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 bSitotal storage in coastal sediments, conventional operational leaches targeting bSiopal and bSialtered (including bSiclay) were modified for large-scale samples necessary for measurement of Si-32. The Si-32 activity was used to estimate total biogenic silica burial (bSitotal = 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, 32Si 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 (similar to bSi(altered)) consistently underestimate authigenic clay formation (bSi(clay)) and thus the magnitude of bSitotal burial in temperate coastal zones and subtropical deltas by 2-4-fold. In tropical deltas, Si-32 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.