Cenozoic history of phosphogenesis recorded in the ferromanganese crusts of central and western Pacific seamounts: Implications for deepwater circulation and phosphorus budgets

Ages of seamount phosphatization episodes have been revised for 14 mid-plate seamounts in the western and central Pacific using the most up-to-date marine Sr isotope curve and 87Sr/86Sr data from the literature. The new ages provide a composite geologic history of phosphatization consistent with Cenozoic climate history. Phosphatization on mid-Pacific seamounts took place only between 36 and 12 Ma, and it peaked at the Eocene/Oligocene (E/O) transition (~ 34 Ma). The unprecedented appearance of prominent amounts of phosphorites at the E/O transition, and frequent recurrences of it afterwards, suggests that a new mechanism came into play at that time, namely the intensification of the advective flow of bottom water, which accelerated the phosphorus cycle. In contrast, the absence of phosphorites younger than 12 Ma suggests another fundamental change in process, associated with the shift to a dry and cold climate that probably resulted in a decrease in the riverine input of labile phosphorus. The drastic decrease in the frequency of hiatuses in ocean-floor depositional records also indicates weakened bottom currents as a possible explanation for the absence of seamount phosphorite since 12 Ma. A phosphorus flux is a critical variable in modeling the response of the Earth system to climate forcing in the geologic past. Thus, the changes in phosphorus flux across the E/O boundary and around 12 Ma justify further study.