Evolution of the Central Northwest Pacific paleoceanography over the past 10 million years focusing on the Late Miocene Global Cooling (ODP Site 1208)

Since the middle Miocene climatic transition, Earth’s climate has steadily cooled. The Late Miocene Global Cooling (LMGC) and the Northern Hemisphere Glaciation (NHG) were key cooling events. I analyzed changes of radiolarian microfossil assemblages to try to reconstruct the paleoceanographic changes during the last 10 million years at Ocean Drilling Program (ODP) Site 1208 to better understand the climate-cooling mechanism. I reconstructed sea surface temperatures (SSTs) based on extant radiolarian species from 0 to 10 million years ago to verify the suitability of radiolarian-based SSTs. A comparison with previously published alkenone-based SSTs at Site 1208 indicated that radiolarian-based SSTs for the Miocene based on only extant species are satisfactory. However, large discrepancies were observed between radiolarian-based and alkenone-based SSTs during the LMGC and NHG. I attributed these discrepancies to a sustained influence of subsurface water (~50 to 100 m) on assemblages of radiolarians during extreme cooling events. Relative abundances of other radiolarian groups indicated that during the LMGC there was a reorganization of regional paleoceanography that probably weakened the Pacific Meridional Overturning Circulation, increased meridional temperature gradient, and caused a southward migration of the subtropical front.  It is probable that North Pacific Intermediate Water expanded southeastward during the NHG.