Among the largest marginal seas, the northern South China Sea's (SCS) climate is greatly affected by the Asian monsoon system (Wang and Wang, 1990; Miao et al., 1994; Shipboard Scientific Party, 2000). In the present regime (Fig. F1), the winter monsoon brings cold winds and water from the north to cool the South China Sea and create a north–south surface temperature gradient (Wyrtki, 1961; Chen et al., 1985; Levitus et al., 1994; Huang et al., 1997a, 1997b). The summer monsoon from the southwest pushes warm air and water into the sea causing warmer and more homogeneous surface temperatures throughout the SCS (Wyrtki, 1961; Chen et al., 1985; Levitus et al., 1994; Huang et al., 1997a, 1997b). The changes of the monsoon system and how it affects the northern SCS during glacial times have been addressed in several studies (Wang and Wang, 1990; Huang et al., 1997a, 1997b; Chen and Huang, 1998; Pelejero et al., 1999a, 1999b). Much lower sea-surface temperatures (SSTs) (by 4°C) and larger seasonality during the last glacial maximum have been attributed to a strengthened winter monsoon, Polar Front shift, and Kurishio Current migration eastward (Huang et al., 1997b; Shipboard Scientific Party, 2000). However, few detailed analyses of the early development of the strengthening of the Asian monsoons have been completed (Shipboard Scientific Party, 2000).
Current models suggest that the Tibetan uplift affected the climates of the Western Pacific marginal seas and China greatly by triggering first the summer monsoon and later the winter monsoon (Liu and Ding, 1982; Wang, 1990, 1997; Shipboard Scientific Party, 2000). The models show an absent monsoon before the uplift (Paleocene and early Eocene) and an unstable monsoon system at the time of the collision between India and Asia (middle Eocene–Oligocene). It is thought that a summer monsoon developed when the initial Himalayan uplift occurred (Miocene–Pliocene), and then both the summer and winter monsoons developed because of intensive uplift during the late Pliocene and into the Pleistocene (Liu and Ding, 1982; Wang, 1990, 1997; Shipboard Scientific Party, 2000). Further, many environmental changes took place on the Asian continent between 8 and 2.6 Ma, including an increase in eolian dust, a shift from C3 to C4 plants in Pakistan, and a shift from forested land to grasslands on the Tibetan plateau (Cerling, 1997; Ma et al., 1998; Rea et al., 1998; An et al., 2001). These all imply increased seasonality and have been interpreted as an environmental response to uplift that occurred at 9–8 Ma (An et al., 2001). Even further, An et. al (2001) have identified three periods of monsoon strength variations between 8 and 2.6 Ma using loess-paleosol sequences. At 6–3.6 Ma, the winter and summer monsoon strengths were variable. From 3.6 to 2.6 Ma, winter and summer monsoons were intensified and from 2.6 Ma (the onset of glaciation), there has been an intensified winter monsoon and a weaker summer monsoon (An et al., 2001).
Uk´37 reconstruction of paleo-SSTs may provide some marine evidence to support the Asian monsoon development models and give us some insight into the environmental response to monsoon development in the northern SCS. The long-chain unsaturated ketones (alkenones), which are produced by Haptophyceae, have been shown to resist biodegradation and diagenesis in marine sediments as old as Eocene (Marlowe et al., 1984; Sun and Wakeham, 1994). The purpose of this study was to reconnoiter a 35-m.y. stratigraphic record of the northern SCS for di- and triunsaturated ketones and evaluate the Uk´37 record as it is related to the initial strengthening of the monsoon system. Samples from Sites 1147 (18°50.11´N, 116°33.28´E) and 1148 (18°50.17´N, 116°33.94´E) from Ocean Drilling Program (ODP) Leg 184 were used because they provide a long-term sedimentary record of the SCS dating back to ~35 m.y. ago. These sites are situated on the lowermost continental slope off southern China (Fig. F1) and were the most offshore drilling sites during ODP Leg 184 (Shipboard Scientific Party, 2000).
The Uk´37 method has been applied widely to reconstruct SST. It was originally developed using the di-, tri-, and tetra-unsaturated ketones (C37:2, C37:3, and C37:4), which are produced by Haptophyceae (mostly Emiliania huxleyi) in the photic zone (Marlowe et al., 1984; Brassell et al., 1986). Later, it was modified to make use of only the di- and tri-unsaturated ketones, thus called Uk´37 (Prahl and Wakeham, 1987). The number of double bonds in the alkenones varies between two and four, and as water temperature decreases, unsaturation in the compounds increases (Brassell et al., 1986; Prahl and Wakeham, 1987). The Uk´37 values were found to be linearly related to temperatures between 8° and 25°C through studies of laboratory-cultured coccolithophores, particulate matter, and core-top sediments (Prahl and Wakeham, 1987; Prahl et al., 1988, 1993; Sikes et al., 1991; Kennedy and Brassell, 1992; Conte et al., 1992; Freeman and Wakeham, 1992; Conte and Eglinton, 1993; Sikes and Volkman, 1993; Rosell-Melé et al., 1995; Madureira et al., 1995; Ternois et al., 1997; Sonzogni et al., 1997). A study by Pelejero and Grimalt (1997) addressed the applicability of the Uk´37 method in the warm SCS where temperatures range between 23° and 29°C. The relationship between Uk´37 and temperature was maintained at the upper limits, allowing for application of the Uk´37 method in warmer ocean waters (Pelejero and Grimalt, 1997). The Uk´37 method has been applied to the SCS for the past 220 k.y. by Pelejero et al. (1999a, 1999b) where the temperature record followed glacial–interglacial stages. Additionally, high-resolution Uk´37 records have been completed by Huang et al. for the last 25 k.y. (1997a, 1997b).