Description: A high resolution record (~1100 yr) of planktonic foraminifers from ODP Site 1144 in the northern South China Sea reveals rapid and strongly variable climatic changes during the mid-Pleistocene transition period. The abundance of warm water species decreased from an average of 60% in marine isotope stage (MIS) 29 to 〈40% at MIS 22, followed by a steady increase in cool water species toward younger intervals. Many deep dwelling, warm water species decreased to a minimum during MIS 22 and remained extremely rare or even became absent in younger glacial intervals, indicating stepwise sea surface cooling in the region. Estimated SSTs show large fluctuations mostly at glacial-interglacial transitions. A maximum winter temperature difference of 11 °C (17-28 °C) across MIS 23/22 boundary likely corresponded to a major growth of boreal ice sheets across the MPT center at 0.9 myr, coupled with a strengthened winter monsoon over East Asia . The MPT event not only led to a better correlation between changes in species abundances and glacial-interglacial cycles but also a more constrained thermocline that shoaled considerably during subsequent glacial periods. The oxygen isotope record and the abundance of shallow water species display power spectra closely in pace with the 41,000 and 100,000 years cyclicities. A lower coherence over these cyclicities between deep-water dwelling species and the planktonic delta18O, a shoaled thermocline, and more positive glacial delta18O together suggest disturbances of surface and subsurface waters by intensified winter monsoons over the last 1.1-0.5 myr in the South China Sea.

Description: Over 30 first and last occurrence (FO and LO, respectively) planktonic foraminifer datums were recognized from the Oligocene-Miocene section of Ocean Drilling Program (ODP) Site 1148. Most datum levels occur in similar order as, and are by correlation as probably synchronous with, their open-ocean records. Several datum levels represent local bioevents resulting from dissolution and Site 1148's unique paleoceanographic setting in the northern South China Sea. An age of 9.5-9.8 Ma is estimated for the local LO of Globoquadrina dehiscens (257 meters composite depth [mcd]), whereas the local LO of Globorotalia fohsi s.l. (301 mcd) is projected to be at ~13.0 Ma and the local FO of Globigerinatella insueta (367 mcd) is projected to be at ~18.0 Ma. The combined planktonic foraminifer and nannofossil results indicate that the Oligocene-Miocene section at Site 1148 is not complete. Unconformities up to 2-3 m.y. in duration, occurring at and before the Oligocene/Miocene boundary (OHS1, OHS2, OHS3, and OHS4 = MHS1), are associated with slump deposits between 457 and 495 mcd that signal tectonic instability during the transition from rifting to spreading in the South China Sea. Shorter unconformities of 〈0.5 m.y. duration that truncate the Miocene section were more likely to have been caused by sea-bottom erosion as well as dissolution. A total of 12 Miocene unconformities, MHS1 through MHS12, are mainly affected by dissolution and an elevated carbonate compensation depth (CCD) during Miocene third-order glaciations recorded in deep-sea positive oxygen isotope Mi glaciation events. Respectively, they fall at ~457 mcd (MHS1 = Mi-1), 407 mcd (MHS2 = Mi-1a), 385 mcd (MHS3 = Mi-1aa), 366 mcd (MHS4 = Mi-1b), 358 mcd (MHS5 = MLi-1), 333 mcd (MHS6 = Mi-2), 318 mcd (MHS7 = MSi-1), 308 mcd (MHS8 = Mi-3), 295 mcd (MHS9 = Mi-4), 288 mcd (MHS10 = Mi-5), 256 mcd (MHS11 = Mi-6), and 250 mcd (MHS12 = Mi-7). The correlation of these unconformities with Mi events indicates that some related driving mechanisms have been operating, causing deepwater circulation changes concomitantly in world oceans and in the marginal South China Sea.