Clay mineral assemblages of the oceanic red beds in the northern South China Sea and their responses to the Middle Miocene Climate Transition

The Middle Miocene Climate Transition (MMCT, ∼14 Ma) is the largest cooling event in the Cenozoic “Coolhouse”, which significantly impacts the global chemical weathering pattern. In this paper, the responses of the MMCT global cooling event in the deep South China Sea were studied by clay mineral assemblages analysis of the oceanic red beds (ORB) at International Ocean Discovery Program (IODP) Expedition 368 Site U1502. The results show that the clay mineral assemblages of the ORB at Site U1502 are mainly composed of smectite (56–88%), illite (7–29%), and kaolinite (6–20%), without chlorite. The contents of these clay minerals and illite crystallinity show a four-stage variation pattern during early-middle Miocene (22.8−10.8 Ma). Smectite decreased from average 81% during 22.8–16.2 Ma and 16.2–14.4 Ma to average 67% during 13.8–10.8 Ma, with a rapid decrease of ∼14% during 14.4–13.8 Ma. On the contrary, illite and kaolinite increased rapidly by ∼8% and ∼6%, respectively, during 14.4–13.8 Ma. Illite crystallinity increased from average 0.18°Δ2θ during 22.8–16.2 Ma to average 0.19°Δ2θ during 16.2–14.4 Ma, and then decreased rapidly by ∼0.02°Δ2θ during 14.4–13.8 Ma. The provenance analysis of clay minerals shows that illite and kaolinite mainly originated from South China landmass due to physical erosion, while smectite mainly came from the Luzon arc as the product of chemical weathering. Therefore, smectite/illite ratio and illite crystallinity are used as proxies of chemical weathering intensity in the early-middle Miocene. High values of the ratio and the crystallinity represent the enhanced chemical weathering, whereas low values indicate the weakened chemical weathering or the strengthened physical erosion process. The smectite/illite ratio and illite crystallinity both decreased rapidly during 14.4–13.8 Ma, indicating the chemical weathering in the surrounding area of the South China Sea weakened rapidly, which we believe is the result of the MMCT event forcing. In addition, their values increased slightly during 16.2–14.4 Ma, which is in response to the relatively enhanced chemical weathering during the Middle Miocene Climate Optimum (MMCO). The variation pattern of clay mineral assemblages of the early-middle Miocene ORB in the South China Sea and its rapid transformation during the MMCT reveal that the Cenozoic cooling played a specific role in controlling the chemical weathering of the Earth’s surface.