Rapid transformation of biogenic silica to authigenic clay during reverse weathering

Formation of authigenic clay minerals during early diagenesis of marine sediments, termed “reverse weathering” (RW), is an important process for regulating ocean pH and atmospheric CO2 over geologic time scales. It is also considered a“missing sink” of the modern marine Si cycle. Although the importance of RW has been increasingly recognized, the rate and mechanisms of this process remain elusive. This study investigated the mechanisms, kinetics, and mineral products derived from diatom biogenic silica in RW. We show the formation of annite (Fe(II)-rich mica) in 40 days, the most rapid process observed to date and the first specific mineral phase identified for RW. Unraveling the kinetics and mechanisms of secondary phase formation suggests an up to one order of magnitude upward revision of potential RW reaction rates in marine sediments. This implies that RW is far more responsive to changes in ocean chemistry and climate state than previously envisioned, suggesting a major role for RW in regulating Earth’s carbon and alkalinity cycles in the modern ocean, through Earth’s history.