Origin of the rhythmic reddish-brown and greenish-gray sediments in the abyssal South China Sea: Implications for oceanic circulation in the late Miocene

During International Ocean Discovery Program (IODP) Expedition 368, a series of drillings were conducted in the northern South China Sea (SCS). At the abyssal drilling Site 1502 Hole A (3764 m below sea level), successive core sections were recovered from 442 to 497 m below sea floor. These cores were placed into the late Miocene period, and the most remarkable feature of these ~50-m-long cores were the consecutive occurrences of the rhythmic reddish-brown and greenish-gray sediments. Such color transitions during the late Miocene period were not remarkable for the cores at the adjacent shallower IODP 368 Sites 1501 and 1505. In this study, we aimed to determine the origin of these cyclicity-like color transitions at Site 1502 in the abyssal SCS. We conducted experiments dealing with sediment color reflectance, nannofossil, key element content, and clay minerals. We inferred that the in-situ formed amorphous hematite caused the coloration of the reddish sediments, in which ferrum and manganese contents were high and carbonate was not poorly preserved. We suggest that the occurrence of the cyclicity-like color transitions could result from the extent of deep-water ventilation in the abyssal SCS. Such changes in deep-water ventilation were possibly related to an orbital-forced rearrangement of deep oceanic circulation in the Pacific Ocean.