Leg 184, the first major Ocean Drilling Program campaign in the South China Sea, recovered a continuous sequence of deep-sea sediments that spans the past 32 m.y. The geographic distribution of drill sites allows a comparison for the last 10 m.y. between the northern and southern parts of the South China Sea. The water-depth distribution of sites on the northern continental slope will allow comparisons between records from above and below the modern Bashi Strait sill depth (~2600 m), which connects the Pacific Ocean with the South China Sea. On the upper slope, we also recovered a section with extremely high sedimentation rates of >800 m/m.y. for the late Pleistocene.
The South China Sea sediments recovered during Leg 184 represent the mixing of nannofossil ooze and detrital clays derived from the Asian continent on the northern and western margins and the island arcs on the southern and eastern margin. These fine-grained, hemipelagic sediments appear to represent deep-water deposition throughout the Neogene and back into the early Oligocene. The apparently monotonous lithology, however, does show significant changes in percentage of biogenic CaCO3 (from a few percent to nearly 80%), with cyclic fluctuations in the Pliocene-Pleistocene sections. Bioturbation is ubiquitous throughout the sequences, whereas slumping and sediment plastic and brittle deformation occur only at certain sections (such as the bottom of Site 1143, the Oligocene/Miocene boundary, and the bottom of Site 1148), indicating periods of tectonic activity.
All cores are rich in calcareous microfossils, dominantly nannoplankton and planktonic foraminifers (Fig. F21). The microfossils are well preserved in most of the cores. Dissolution effects are significant only for certain sections of deeper sites such as Sites 1145 and 1148, and the Oligocene section contains recrystallized foraminiferal tests. Despite the variations in preservation and the frequent reworking of nannofossils, the two microfossil groups yielded some 70 datum levels from the early Oligocene onward for the stratigraphic framework of the leg. All fossil assemblages are deep water in nature, and benthic foraminifers are rare aside from the Oligocene. Siliceous fossils (diatoms and radiolarians) are frequent in the uppermost part of the holes and in some lower sections at certain sites. Ostracods, bryozoans, pteropods, and other macrofossils have been encountered only in some parts of the sequences. The presence of small pieces of fossil wood, although rare, implies transport of plant debris from land. The high diversity and abundance of microfossils offer an ideal opportunity for paleoenvironmental reconstructions.
The volume of volcanic ash found in Leg 184 sediments is not large. All recovered ashes are thin, generally <5 cm, and are light colored in the Pleistocene, reflecting the dominant dacitic-rhyolitic composition of the arc's explosive fraction. Most of the ashes were deposited since 1 Ma on the northern margin and since 2 Ma in the south. The uphole increase in volcanic ash may reflect either more volcanic eruptions during the Pleistocene or the diagenetic alteration (loss) of chemically unstable volcanic glass in the older, deeper parts of the section. This latter explanation may account for much of the pattern on the northern margin because older ashes in this area tend to be devoid of glass and are simply composed of angular quartz, mica, and other accessory mineral grains. However, fresh glass is found in Miocene-age ash beds at several sites.
Green clay layers are a common, yet volumetrically small, part of the sequence at most of the Leg 184 core sites. They occur as discrete layers as thick as 3 cm and even more commonly as disrupted layers, patches, or mottles. The XRD analyses show that they are not composed of glauconite. No clear relationship is observed between green clay layers and depth of burial, although most of the layers are confined to the Pliocene-Pleistocene (Fig. F21) except for a lower Miocene set recovered at Site 1148. Their common association with burrows and patches caused by burrowing suggests that they may be linked to the former presence of organic matter. Certainly their green color is suggestive of reducing conditions, which are linked to organic matter alteration. They do not seem to be equivalent to green layers found by Lind et al. (1993) on the Ontong Java Plateau and by Gardner et al. (1986) from the Lord Howe Rise, which were interpreted as altered volcanic ash. In the case of the South China Sea, the green clay layers are interbedded with clear tephra-bearing unaltered volcanic glass. No appreciable change in the background sediment is noted over these intervals; thus, the diagenetic environment seems uniform between beds. Other diagenetic minerals noted in the Leg 184 sediments are "iron sulfide" minerals (well-crystallized golden-colored pyrite often present as nodules, concretions, and replacement burrows) and fine-grained black sulfide dispersed in the sediment. The latter style is described as "FeS" in the cores, but this is chemically unstable; therefore, this material must also be pyrite in mineralogy. The lack of a clear regional pattern either in depth or age in the distribution of pyrite or "FeS" suggests that the minerals' development reflects only local variations in sediment composition and burial.
The interstitial water profiles measured during Leg 184 reflected sulfate reduction and methanogenesis in the upper sediments. Below the zone of organic matter reduction, interstitial water profiles reflected alteration of volcanic ashes, diagenesis of clays, dissolution of silica, and dissolution/recrystallization of calcite at depth. These data revealed two clear trends in the SCS sediments:
We found that the sulfate gradient (i.e., the decrease in sulfate values from oceanic values, 28.9 mM, to the value of the sulfate plateau) increased linearly with sedimentation rate but not with the TOC concentration. The sulfate gradient is controlled both by the supply of organic matter to be consumed by sulfate reduction and by the extent to which seawater sulfate can continue to diffuse into interstitial water and replenish the sulfate removed by sulfate reduction. Hence, the linear relationship between sulfate gradient and sedimentation rate is established both by the correlation between high LSR and higher TOC flux to the seafloor and by the length of time that near-surface sediments continue to receive new sulfate from seawater. The extreme sulfate gradient observed at Site 1144 appears to be more a function of high sedimentation rates than the supply of organic matter because the LSR is so high (250-1000 m/m.y. in the Pleistocene) that sediments move out of the diffusional contact with seawater faster than organic matter can be depleted. This rapid removal of organic matter from the zone of sulfate reduction is consistent with the good preservation of organic matter with depth at this site. All other sites follow a more linear relationship between sulfate gradient and both LSR and TOC. Site 1143 has the lowest values observed and is the most distant from continental sources.
All the dissolved silica profiles showed an increase of similar magnitude between ~1 and 0.5 Ma, which corresponds to an increase in the abundance of biogenic silica in the sediments (Fig. F22). This increase in dissolved silica occurs at very different depths at different sites, suggesting that it is not related to diagenetic changes in silica. Instead, the increase in dissolved silica indicates that the higher biogenic silica observed in the sediments is a real change in silica flux to the sediments. These changes are coincident with increasing sedimentation and organic carbon accumulation rates, which could indicate increased preservation related to a higher LSR and higher overall productivity. In either case, the increase in silica appears to be a regional change, suggesting that it is caused by climatic or tectonic changes.
The downhole temperature measurements at Leg 184 sites revealed two distinct temperature gradients as a function of the water depth (Fig. F23). At Sites 1143 (2772 m), 1145 (3175 m), and 1148 (3294 m), the gradients were relatively high (83between the sites. These sites are all deep and relatively close to the boundary between continental and ocean crust. Preliminary calculations suggest that temperature gradients at these sites are close to predictions based on the pure shear stretching model of continental lithosphere, the corresponding water depth of the drill sites, and the assumed age of initial rifting (32 Ma). The two sites on the mid-continental slope (Sites 1144, 2037 m; and 1146, 2092 m) have smaller gradients (24from each other. The gradient at Site 1144 is substantially smaller than the predicted value based on water depth and spreading age, and a local process must be inferred to explain the low temperatures at this high sedimentation rate site.
Significant hydrocarbon concentrations were detected at three sites during Leg 184 (Fig. F24). Abundant methane of biogenic origin was inferred only from Site 1144. High TOC abundance and complete sulfate reduction in the upper few meters of this site (and the absence of significantly heavier hydrocarbons downhole) are characteristic of methanogenesis in immature sediments. In contrast, the downhole increase in hydrocarbon abundance at Sites 1146 and 1148 is characteristic of thermogenic generation with age and increasing temperatures with depth. These two deeper penetration sites on the continental margin have predicted bottom-hole temperatures of only 35but 71of Site 1148 are thought to be high enough to begin the process of thermal generation of hydrocarbons from the low amounts of organic carbon (0.3 to 0.5 wt%) and may well explain the observation of sparse, heavy hydrocarbons (C2+) at the site. The Leg 184 sites did not reveal conditions for source rock production (except Site 1148, where low TOC produced small amounts of C1-C5), and no gas hydrates were directly observed.
In summary, the sequence of hemipelagic sediments over 32 m.y. is rich in calcareous microfossils and yields almost continuous records of the environmental history of the South China Sea. The depositional history of the northern slope had three important stages: the Oligocene, with extremely high sedimentation rates; the Miocene and early Pliocene, with a low sedimentation rate and high carbonate content; and the last 3 m.y., with high clastic sediment accumulation rates. A different trend of depositional history is indicated at the southern Site 1143: the carbonate accumulation rate decreases from the late Miocene toward the late Pleistocene, and the noncarbonate rate rises again after 3 Ma. However, the upper Miocene sediments were similar in composition between the northern and southern sites, containing more than 50 wt% carbonate.
The lower and mid-Oligocene sediments (Site 1148; 32-27 Ma, 850-480 mcd) are composed of laminated claystone with brownish gray and greenish gray stringers (Figs. F20, F21) and have high LSRs comparable to those in the Pleistocene (Figs. F25, F26). The carbonate MAR from 480 to 700 mcd (23.7-31.0 Ma) averages 3.45 g/cm2/k.y. (Fig. F26), which is by far the highest observed over the past 32 m.y. at Site 1148. At the same time, the relatively high noncarbonate MAR implies a significant supply of terrigenous fine-grained clasts (Fig. F26). Because the ratio of carbonate to noncarbonate MAR is relatively constant, the high carbonate accumulation rate may reflect sediment focusing of upslope sediments as well as a higher surface productivity. The relatively high organic carbon content, the presence of siliceous microfossils, and the composition of benthic foraminiferal fauna also suggest higher productivity.
On the basis of magnetic anomaly (C11-C5c) patterns, the seafloor-spreading phase of the SCS took place between 32 and 16 Ma (Briais et al., 1993; Taylor and Hayes, 1983). Before 27 Ma, the newly opened deep-sea basin was oriented east to west and connected with the Pacific in the East. At that time, the South China Sea was a narrow basin between the Asian continent and a number of terranes that were eventually rifted to the southern part of the SCS (North Palawan and Reed Bank, among others). The high sedimentation and accumulation rates recorded at Site 1148 may be related to the incipient spreading phase, which might focus sediment input into the newly opened basin. Judging from the absence of marine sediments of this age on the northern shelf and upper slope, the ocean waters were restricted in the area. The proximity of continental runoff and sediment transport may be one of factors responsible for the higher rates of deposition in this interval. The poor core recovery in the upper part of this sequence (473-562 mcd) suggests some lithologic changes in the latter part of the time interval. The scarcity of core samples, however, precludes any conclusion about the nature of the possible events.
Lower Oligocene coal-bearing swamp and littoral plain sediments are known to occur in wells of this age farther north within the Pearl River Mouth Basin (e.g., Su et al., 1989). The lack of similar coarse clastic material suggests that the Pearl River Mouth Basin and shallower slope basins were acting as efficient sediment traps. In addition, the presence of a deep-water, bathyal facies during the initial seafloor spreading period does not readily fit with simple rift models of the South China Margin. These facies may indicate that the margin extension proceeded very rapidly during the initial rifting phase in the middle Eocene (e.g., Taylor and Hayes, 1980; P. Clift and J. Lin, pers. comm., 1999).
A distinct and regionally consistent seismic reflector was observed in the lower section of Site 1148. The JOIDES Resolution seismic reflection data indicated that the reflector was ~0.86 s below seafloor; the P-wave velocity data from downhole logging in Hole 1148A suggested that the reflector was ~800 mbsf. Although the drillers reported a distinct decrease in penetration rate at 800 mbsf, we did not recover any material that was obviously indicative of a strong reflector. All cores in this interval were greenish gray claystone. The lack of coarse sediments, a weathering zone, or other features indicative of erosion or shallower water signify that this site was relatively deep throughout the deposition of the Oligocene sequence. The true nature of the reflector is unclear.
The latest stage of the late Oligocene produced drastic changes in sediment deposition at Site 1148. Above a section of poor core recovery but with clear late Oligocene index species (P21b and NP24) below 477 mcd, carbonate content abruptly increased (~50-70 wt%) as chalk replaced clay over the depth interval of 454-477 mcd (Fig. F27). Almost all physical parameters reflect this rapid change, including sharp increases in bulk density, P-wave velocity, photoelectric effect, and CR parameters and decreases in porosity, NGR, and MS values (Figs. F20, F28). The microfossil assemblages at 454 m indicate an early Miocene age (N4, NN2). The chalk section is overlain by slump deposits of chalk and clay with a mixed nannofossil assemblage. The absence of nannofossil Zone NP25 indicates a deposition hiatus of ~1-3 m.y. at the end of the Oligocene (Figs. F20, F25, F26), and planktonic foraminifer markers between the lowermost Zone N4 and lowermost Zone P22 indicate a hiatus spanning from ~24 Ma to ~27.0-27.5 Ma.
The Oligocene-Miocene transition represents one of the most significant Cenozoic changes in the tectonic and environmental history of the South China Sea (Wang, 1990). Although this period is within the drift phase of SCS evolution, the sedimentary basins of the northern SCS shelf are thought to have experienced a transition from the rifting stage to one of broad subsidence (Ru et al., 1994). According to the age model for Site 1148 (Fig. F25), the Oligocene/Miocene boundary events started ~27 Ma, at the same time that the spreading ridge of the SCS basin is thought to have jumped southward (Briais et al., 1993). Because of its position close to the boundary between continental and ocean crust, Site 1148 should have been sensitive to any tectonic episode associated with changes in the spreading of the SCS basin. Postcruise research will show whether the drastic changes in deposition regime at the site is related to the spreading event.
The early Miocene (16.5-23.7 Ma) at Sites 1146 and 1148 is represented by a calcareous clay with an average carbonate content of ~35 wt% (Fig. F27). The total MARs for this interval average 1.13 g/cm2/k.y. at both Sites 1146 and 1148, or about three times lower than in the Oligocene (Fig. F26). These lower rates are more representative of pelagic sedimentation and may also imply that the previously high rates were affected by sediment focusing on the margin. During the second stage of seafloor spreading (27-16 Ma), the South China Sea basin became much broader than in the Oligocene (Briais et al., 1993; Lee and Lawver, 1994). This early Miocene interval was distinguished by an expansion of reef facies in the shallow waters of the western Pacific including the Pearl River Mouth Basin (Fulthorpe and Schlanger, 1989). The relatively low carbonate accumulation rate but high carbonate content may be attributed to the more pelagic environment of the larger SCS basin, the lack of sediment focusing, and the wide distribution of reef facies on the shelves.
On the northern shelf, the early Miocene was a time of marine transgression. The marine intercalations in nearshore facies are first observed in the upper Oligocene (NP25) on the upper slope and shelf break in industrial wells (BY7-1-1, at a water depth of 499 m; and PY33-1-1, at a water depth of 188 m; Huang, 1997), and in the lower Miocene on the shelf (Wang, 1990). Further studies are needed to determine whether the local rise in sea level resulted from global eustatic changes or from tectonic subsidence of the basins as well as how this shelf transgression phase affected sedimentation at the deeper water Sites 1146 and 1148.
The seafloor-spreading phase of the SCS basin stopped at magnetic Anomaly C5c, or ~16 Ma, which is close to the boundary between the early and middle Miocene (also close to the T4 reflector). The middle Miocene section (~16-11 Ma) from the northern continental margin (Sites 1146 and 1148) has relatively high carbonate content (>30 wt%), only slightly lower than the early Miocene but much higher than the modern values (Fig. F26). Carbonate increases at the shallower Site 1146 and remains high at Site 1148 despite its greater water depth, which is below the modern lysocline (~3000 m). Total accumulation rates during the early Miocene were 1.91 g/cm2/k.y. at Site 1148, slightly lower than in the early Miocene and much lower than at Site 1146 (~4.34 g/cm2/k.y.) (Fig. F27). Additional postcruise biostratigraphic control will be needed to establish whether the slower accumulation rates are related to a change in tectonics or to depositional hiatuses observed on the northern shelf.
Upper Miocene sediments were recovered in the northern SCS at Sites 1146 and 1148 and in the southern SCS at Site 1143. About half of the mass of sediments from both northern and southern sites above the modern lysocline (Sites 1146 and 1143) is composed of carbonate (Figs. F26A, F27). At Site 1143, the carbonate and noncarbonate accumulation remains high (Fig. F26A); Site 1146 shows a drastic increase in carbonate percentage (from ~40 wt% in the middle Miocene to ~55 wt% in the late Miocene), but the increase in carbonate accumulation rate was not significant (from 1.46 g/cm2/k.y. in the middle Miocene to 1.55 g/cm2/k.y. in the late Miocene) (Figs. F26A, F27). At Site 1148, the average carbonate percentage still exceeds 30 wt% (Fig. F27), but the poor preservation of planktonic foraminifers shows enhanced carbonate dissolution. During this late Miocene interval, differences begin to develop between the shallower Site 1146 and the deeper Site 1148. In the shallow site, NGR values are lower and decreasing whereas PEF values are increasing; in the deeper site, the NGR values are higher and much more variable, as are the PEF values. These changes suggest more clay carbonate contrasts in the deeper Site 1148 and are generally consistent with the higher carbonate in Site 1146.
Despite the similar carbonate concentrations in upper Miocene sediments, the accumulation rate at Site 1143 is about two times higher (~3 g/cm2/k.y.) than at Site 1146 (~1.5 g/cm2/k.y.) (Figs. F26, F27). The high carbonate accumulation at tropical Site 1143 might be related to the late Miocene to early Pliocene "biogenic bloom" in the equatorial Pacific (Berger et al., 1993; Farrell et al., 1995) but also seems partly related to redeposition of adjacent sediments as evidenced by the frequent turbidites and slumped sediments in the lower section. The preservation of siliceous microfossils in the lower section may indicate higher productivity at that time. The high carbonate percentages in the Miocene deposits from the northern sites imply a low supply of terrigenous material from the land that may be related to rising sea levels during this interval (Prell and Kutzbach, 1997). In general, the high carbonate sediments throughout the Miocene and the similarity between the northern and southern sites suggest a much more stable environment than during the Pliocene-Pleistocene.
Leg 184 recovered Pliocene deposits at four sites (1143, 1145, 1146, and 1148) and Pleistocene sediments at all six sites, although with substantially different accumulation rates (Fig. F26B). Despite the different LSRs, all sites exhibit a significant increase in silica at ~1 Ma (Fig. F22), which can be ascribed to increased productivity and/or increased preservation. Differences between the southern and northern SCS also begin to emerge during this interval.
At the southern Site 1143, both carbonate and noncarbonate accumulation rates decreased from the late Miocene to Pliocene (Fig. F26B): carbonate from 2 to 4 g/cm2/k.y. to ~1 g/cm2/k.y., and noncarbonate from 3 to 4 g/cm2/k.y. to ~2 g/cm2/k.y. The decreasing trend continues to the Pleistocene for carbonate, whereas the noncarbonate rate increases again after ~3 Ma, indicating some increased supply of terrigenous material. However, unlike the northern sites, the MAR at Site 1143 decreases toward the present despite the increase in LSR, a result of decreased bulk density within that most recent interval.
On the northern continental margin, Pliocene accumulation rates remain at the late Miocene level at Sites 1146 and 1148 but with slightly lower rates in the deeper Site 1148. Both sites exhibit the rapid increase in LSR and noncarbonate deposition that started at ~3 Ma. Site 1145 records only the past 3 m.y., but the noncarbonate accumulation increases after ~ 2.5 Ma. This apparently regional increase in noncarbonate accumulation may be evidence for an intensification of erosion that is related to climatic/sea-level and/or tectonic events. For example, Chinese geologists report evidence for significant uplift of the Tibetan Plateau at ~3 Ma (e.g., Li et al., 1996), and the widespread accumulation of loess in central China started at ~2.4 Ma. However, sea-level changes associated with increased global glaciation may have also contributed to transporting sediments to these continental margin sites.
In contrast to the southern site, all the northern sites show an increase in MAR in the late Pleistocene, especially the last ~0.25 m.y. (Fig. F26B). The higher MAR is mainly the result of increased supply of terrigenous material, as the carbonate contribution is insignificant for that time interval. The trend is most prominent for Sites 1144, 1145, and 1146. The cause of this most recent increase is not yet clear. The relatively high content of biogenic silica and organic carbon indicate enhanced productivity for the latter part of the Pleistocene.
The general decrease in carbonate concentration upsection, particularly since the mid-Pliocene (Figs. F21, F26, F27), could also reflect decreasing carbonate productivity as well as increasing clastic input, both of which might be expected from an intensification of glaciation at that time. Glaciation is known to have produced arid conditions in Asia and reduced runoff into the South China Sea, although windblown material such as loess may be more important at these times. However, glaciation in the Himalayas (and possibly Tibet) would be expected to produce more clastic debris that would then be available for redeposition when rainfall increased again at the start of deglaciation. The increasing strength of glacial cycles, especially since ~2.6 Ma, correlates well with the general increase in the detrital component since that time. The variation from darker to lighter sediment intervals noted at a number of the northern margin sites—more precisely represented as cyclic variations in core-logging data such as CR, NGR, and MS—most likely reflects such glacial-interglacial variation in clay and carbonate accumulation.