Deep Water Ventilation and Stratification in the Neogene South China Sea

Combined data of physical property, benthic foraminifera and stable isotopes from ODP Sites 1148, 1146 and 1143 are used to discuss deep water evolution in the South China Sea (SCS) since the Early Miocene. The results indicate that 3 lithostratigraphic units respectively corresponding to 21-17 Ma, 15-10 Ma and 10-5 Ma with positive red parameter (a^*) marking the red brown sediment color represent 3 periods of deep water ventilation. The first 2 periods show a closer link to contemporary production of the Antarctic Bottom Water (ABW) and Northern Component Water (NCW), indicating a free connection of deep waters between the SCS and the open ocean before 10 Ma. After 10 Ma, red parameter dropped but stayed higher than the modern value (a^*=0), the CaCO₃ percentage difference between Site 1148 from a lower deepwater setting and Site 1146 from an upper deepwater setting enlarged significantly, and benthic species which prefer oxygen-rich bottom conditions dramatically decreased. Coupled with a major negative excursion of benthic δ¹³C at 10 Ma, these parameters may denote a weakening in the control of the SCS deep water by the open ocean. Probably they mark the birth of a local deep water due to shallow waterways or rise of sill depths during the course of sea basin closing after the end of SCS seafloor spreading at 16-15 Ma. Several Pacific Bottom Water (PBW) and Pacific Deep Water (PDW) marker species rapidly increased since 6 Ma, and from 5 Ma to 3 Ma the local deepwater became strongly stratified, as indicated by up to 40% CaCO₃ difference between Sites 1148 and 1146. Apart from a strengthening PDW due to global cooling and ice cap buildup on northern high latitudes, a deepening sea basin due to stronger subduction eastward may also have triggered the influx of more corrosive waters from the deep western Pacific. Since 3 Ma, the evolution of the SCS deep water entered a modern phase, as characterized by relative stable 10% CaCO₃ difference between the two sites and increase in infaunal benthic species which prefer a low oxygenated environment. The subsequent reduction of PBW and PDW marker species at about 1.2 Ma and 0.9 Ma and another significant negative excursion of benthic δ¹³C to a Neogene minimum at 0.9 Ma together convey a clear message that the PBW largely disappeared and the PDW considerably weakened in the mid-Pleistocene SCS. Therefore, the true modern mode SCS deep water started to form only during the "mid-Pleistocene climatic transition" probably due to the rise of sill depths under the Bashi Strait.