Cenozoic phosphogenetic episodes recorded in central and western Pacific seamounts: insights into global phosphorus cycle

Abstract

Phosphorites occur in various forms in ferromanganese (Fe-Mn) crust deposits developed on flanks of central and western Pacific seamounts. Among them, void- and fracture-filled phosphorites were collected and analyzed for 87Sr/86Sr ratios to determine the timing of phosphogenesis and to understand its formation in view of global climate changes. It was found that phosphorites were formed in a limited geologic time interval from the late Eocene (~36 Ma) to mid-Miocene (~12 Ma) nevertheless of continuous growth of Fe-Mn crust for the last 60 million years. Besides about 40% of phosphorites (20 out of 50) were formed episodically at the Eocene/Oligocene (E/O) transition (36 – 33 Ma). Absence of phosphorite formation for other geologic time, prior to 36 Ma and after 12 Ma, suggests fundamental shifts in Earth system parameters that influence phosphorus cycling at these critical climate transition periods. The E/O transition marks one of the most prominent climate transition accompanied by the first appearance of a permanent continental ice sheet on Antarctica and intensification of atmospheric and oceanic circulation. Thus the precipitation of prominent amount of phosphorite at the E/O transition and frequent recurrences of it afterwards suggest that a new mechanism that accelerated phosphorous cycle came into play since E/O transition. On the other hand, global climate has permanent