PHOSPHOGENSIS RECORDED IN CO-RICH CRUSTS OF NORTHWEST PACIFIC SEAMOUNTS

Abstract

To investigate Cenozoic episodes of phosphogenesis and its possible trigger mechanism, seamount phosphorites in the northwest Pacific was dated using Sr isotope ratios. In addition, published Pacific seamount phosporite Sr isotope data were compiled and new ages were assigned to them with a newly published 87Sr/86Sr-age curve (LOWESS v.3) to minimize biases due to inconsistent adaptation of different seawater Sr-age curve model. Newly assigned ages were as much as 5 million years younger than the published ages. This study identified two major episode of phosphatization at 21-23 Ma (n=10) and 33-36 Ma (n=21) with two additional minor events at 17 Ma (n=2) and 25 Ma (n=3) during Cenozoic. The two major phosphatization episodes corresponded to Mi-1 and Oi-1 glaciation that occurred at Oligocene/Miocene and Eocene/Oligocene epoch boundaries, respectively. Minor two events were also occurred in a series of smaller scale glaciations followed by Mi-1 and Oi-1. Mi-1 and Oi-1 were accompanied with reorganization of the climate/ocean system as evidenced by positive oxygen and carbon isotope excursions, major drop in the carbonate compensation depth, and global wide shifts in biological community. This climatic transition likely intensified the ocean circulation and accelerated rate of turnover, which supplied dissolved phosphorus for the formation of seamount phosphorites.