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To investigate the Cenozoic episodes of phosphogenesis and its possible triggering mechanism, seamount phosphorites in the northwest Pacific were dated using Sr isotope ratios. Sr isotope data of Pacific seamount phosphorite were also compiled and new ages were re-assigned on the basis of a newly pu...

To investigate the Cenozoic episodes of phosphogenesis and its possible triggering mechanism, seamount phosphorites in the northwest Pacific were dated using Sr isotope ratios. Sr isotope data of Pacific seamount phosphorite were also compiled and new ages were re-assigned on the basis of a newly published ⁸⁷Sr/⁸⁶Sr-age curve to minimize biases due to inconsistent application of different seawater Sr-age curve model to different data set. Newly assigned ages were as much as 5 million years younger than the published values. This study identified two major episodes of phosphatization at 34-33 Ma (n=18) and 24-21 Ma (n=14) with an additional minor event at 19-16 Ma (n=6) during the Cenozoic. These three phosphatization episodes had common characteristics of high sea level, drastic increases in carbon isotope ratios and intensified bottom water current. This indicates that the increased upwelling at the continental margins and along the equator in the Pacific caused the high biological productivity and carbon isotope ratio shifts during these episodes. It is known that topographic barriers to flows such as seamounts act as sites for upwelling of nutrient-rich deep water. Thus, intensified bottome current also likely caused increased upwelling around the seamounts. In conclusion, the phosphatization in the Pacific seamounts was triggered by intensified bottom current due to the changes in global ocean/climate system and resulting high ocean productivity and increassed suppy of dissolved phosphorous ion. The studied seamounts show no systematic changes in phosphatization ages depending on latitudinal locations, implying that equatorial high productivity zone is not responsible for the supply of phosphorous.

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