Long eccentricity control on the clay minerals deposition in the northwestern Philippine Sea during the Miocene Climate Optimum

The Miocene Climate Optimum (MCO, spanning 17–14 Ma) represents a notable warming phase within the overarching Cenozoic cooling trend. Nevertheless, the response of the western Pacific to this distinct climate change remains unclear. In this research, we analyzed clay minerals in sediment samples dating from 17.5 to 12.2 Ma, collected from Site U1438 in the Amami Sankaku Basin (ASB) during the Expedition 351 of the International Ocean Discovery Program (IODP). The finding indicated that during the MCO, smectite was the dominant clay mineral, accounting for approximately 88 %, followed by illite at around 10 %, with chlorite and kaolinite present in minor quantities. After the MCO, the proportion of smectite decreased to approximately 67 %, while those of illite (around 24 %), chlorite (around 5 %) and kaolinite (around 4 %) increased. Provenance analysis indicated that during the MCO, smectite primarily originated from the volcanic island arc in the northwestern Philippine Sea (NWPS), while illite and chlorite were derived from the Asian continent. The smectite/(illite+chlorite) (S/IC) ratio increased significantly during the MCO. This increase was consistent with the rise in low-magnetic susceptibility (χ_lf) in the Asian continent, as well as the increase in the smectite/illite ratio and the chlorite/(chlorite + hematite + goethite) (C_RAT) ratio in the South China Sea. These correlations imply that S/IC ratio is a sensitive indicator of the warm and humid climate during the MCO. Spectral and wavelet analyses further revealed a prominent 405-kyr cycle in the S/IC ratio during the MCO. The peak of this 405-kyr cycle in the S/IC ratio corresponded to the minimum long eccentricity and insolation, the maximum χ_lf and χ_fd/HIRM in inland Asia, and a positive deviation in deep-sea carbon and oxygen isotopes. During periods of long eccentricity minima, the cold climate, on one hand, caused the northern hemisphere westerly jet to shift southward. The westerly circulation then carried more moisture to the Asian continent, reducing the input of illite and chlorite. On the other hand, the cooler climate promoted the northward flow of the Circumpolar Deep Water (CDW). This brought more heat and dissolved matter to the deep North Pacific and enhanced the exchange of dissolved substances (such as oxygen and carbon dioxide) between the deep and surface waters of the Pacific Ocean. These processes strengthened deep-water ventilation in the NWPS, intensifying submarine weathering and increasing the supply of smectite. This study thus demonstrates the regulatory influence of long eccentricity on the climate and sediment supply from the mid-latitude Asian continent to the western Pacific during the MCO. These findings indicate that clay minerals can be used for astronomical calibration across geological periods.