RESULTS

Oxygen and carbon isotope ratios were measured on benthic and planktonic foraminifers from a 190-m Pliocene-Pleistocene section of Site 1143 (Fig. F1, F2) at a sampling interval of ~10 cm (Tables T1, T2). This sample spacing corresponds to a temporal resolution of ~2.6 k.y. Based on biostratigraphic events and the comparison of the benthic ¹⁸O curve with that of a newly compiled 6-m.y. ¹⁸O curve by Shackleton (Shackleton, unpubl. data at delphi.esc.cam.ac.uk/coredata/v677846.html ), a total of 191 oxygen marine isotope stages were identified for the last ~5 m.y. (190.77 m) (Tian et al., 2002).

Three major intervals are distinguished in the ¹⁸O records, as suggested by changing trends in both planktonic and benthic ¹⁸O curves (Figs. F1, F2): (1) the last 0.9 m.y. shows 100-k.y. cycles and relatively stronger amplitude fluctuations, (2) the interval 3.3-0.9 Ma is characterized by a steplike increase in ¹⁸O values that is separated by three terraces, and (3) the period 5-3.3 Ma is marked by relatively low amplitude variation and a constant trend. In the second interval, the terrace at 3.1-2.5 Ma has the steepest trend. In the planktonic foraminiferal ¹⁸O record, the 3.3 to 0.9 Ma interval also shows three terraces. However, the earliest terrace at 3.3-2.5 Ma has a more constant trend than in the benthic record. The record indicates several paleoceanographic events: (1) a general cooling trend; (2) a decline of South China Sea deepwater temperature at 3.1-2.5 Ma, reflecting the increase in Northern Hemisphere glaciation; (3) two increases in surface water temperature at 4.8-4.5 and 4.4-3.5 Ma in the southern part of the South China Sea; and (4) a warm deepwater period at 5-3.3 Ma.