Terrigenous provenance of late Oligocene–Miocene sediments in the central basin of the South China Sea and its implications for chemical weathering and climate change

Abstract

The oceanic red beds (ORB) are unique pelagic sediments in the global ocean, providing excellent archives for understanding the chemical weathering and relevant climate change in their source areas. Clay minerals, Nd and Sr isotopes, and major elements of terrigenous components were investigated on the ORB (27.0–10.6 Ma) recovered at International Ocean Discovery Program (IODP) Expedition 367 Site U1499 in the central basin of the South China Sea, to determine the terrigenous provenance and constrain the main factors controlling terrestrial chemical weathering during the late Oligocene–Miocene. The clay mineral assemblage of the ORB is characterized by dominant smectite (52–95%), moderate illite (4–23%), and minor kaolinite (0–19%) and chlorite (0–7%). Nd isotope value (εNd) and 87Sr/86Sr ratio vary from −12.1 to −6.4 and from 0.7122 to 0.7275, respectively. These results display a three-stage temporal variation pattern with the most significant shift in the sedimentary sources at ∼23 Ma. Stage I (27.0–23.0 Ma) sediments were mainly derived from Palawan, whereas Stages II (23.0–14.0 Ma) and III (14.0–10.0 Ma) sediments were derived from South China and Luzon. Simple NdSr isotope mixing model result shows the greater terrigenous contribution of South China from Stage II (49–60%) to Stage III (64–80%), consistent with an increase in illite, chlorite, and kaolinite contents. Chemical index of alteration, K2O/Na2O ratio, Al2O3/TiO2 ratio, smectite/(illite + chlorite) ratio, and illite crystallinity indicate moderate chemical weathering intensity through the late Oligocene to the Miocene. The weathering was influenced by the lithology of parent rocks in Stage I and primarily by the climatic evolution during Stages II and III. The synchronous reduction of various indicators from the ORB reveals the weakening of terrestrial weathering around the South China Sea at ∼14 Ma, induced by the global cooling at the Middle Miocene Climate Transition.