Depth and ages of calcareous nannofossil events from ODP Leg 184 sites, South China Sea

Sites 1146 and 1148 of Ocean Drilling Program Leg 184, in the South China Sea (SCS), comprise long sediment sections with a time span from the early Oligocene to the Pleistocene. Calcareous nannofossils from these two sites were biostratigraphically studied. We recognized 53 early Oligocene to Pleistocene events that are commonly found in open sea areas and can therefore be correlated within a large geographic range. This study also revealed that a few conventionally used nannofossil events are not suitable for the SCS, and further evaluation is needed. The lower Oligocene to Pleistocene sequences recovered at Sites 1146 and 1148 were subdivided into the 4 Paleogene zones and 21 Neogene to Quaternary zones of Martini, in correlation with the Paleogene to Quaternary zones of Okada and Bukry. This provided a lower Oligocene through Pleistocene nannofossil biostratigraphic framework. A significant unconformity was recognized in the Oligocene-Miocene transition, in which the upper part of Oligocene Zone NP25 and lower part of Miocene Zone NN1 were missing. The time span of the unconformity was estimated to be ~1 m.y. Very high sedimentation rates were seen in the Oligocene, relative low values were seen in the Miocene, and the highest values were seen in the Pleistocene, which was believed to be the result of tectonic and sedimentation history of the SCS.

High-resolution isotope stratigraphy of ODP Site 184-1144

Narrow-spaced oxygen and carbon stable isotope records of the planktonic foraminifer Globigerinoides ruber (white) were obtained at Ocean Drilling Program Leg 184 Site 1144 to establish a first record of high-resolution Pleistocene monsoon variability on orbital to centennial timescales in the northern South China Sea. The new records extend from the Holocene back to marine isotope Stage (MIS) 34 (1.1 Ma). Sedimentation rates average 0.56 m/k.y. for the upper Matuyama and Brunhes Chrons and increase to 1.8 m/k.y. over the last 100 k.y. Stable isotope records thus reach an average time resolution of 270-500 yr for the last 375 k.y. and 570 yr further back to 700 ka. On the other hand, major stratigraphic gaps were identified for peak warm Stages 5.5, 7.5 (down to 8.4), 11.3, and 15.5. These gaps probably resulted from short-lasting events of contour current erosion induced by short-term enhanced incursions of Upper Pacific Deep Water near the end of glacial terminations. A further major hiatus extends from MIS 34 to MIS 73(?). The long-term variations in monsoon climate were largely dominated by the 100-k.y. eccentricity cycle. Planktonic delta13C values culminated near 30, 480, and 1035 ka and reflect an overlying 450-k.y. eccentricity cycle of minimum nutrient concentrations in the surface ocean. Superimposed on the orbital variations, millennial-scale cycles were prominent throughout the last 700 k.y., mainly controlled by short-term changes in monsoon-driven precipitation and freshwater input from mainland China. During the last 110 k.y. these short-lasting oscillations closely match the record of 1500-yr Dansgaard-Oeschger climate cycles in the Greenland ice core record.