Site 1143 (proposed site SCS-9) is located 9o21.72就, 113o17.11媧, at a water depth of 2772 m (Table T1), within a basin on the southern continental margin of the South China Sea (Figs. F8, F10). On the Admiralty charts, the site lies within the Nansha or Dangerous Grounds area, which is riddled with reefs, shoals, and small islands, some of which are within 20 to 30 miles of the site. Site 1143 was located in the southern SCS in order to provide a Neogene paleoceanographic record within the Western Pacific Warm Pool. The sediment record of high and relatively stable sea-surface temperature will allow the reconstruction of SST gradients (with the northern sites) across the SCS and should reflect the development of climatic seasonality in this region. Climate records from Site 1143 will provide the basic data to identify and interpret the evolution of summer and winter monsoon circulation in the South China Sea. Also, the southern location of Site 1143 was chosen to capture the long-term records of sediment accumulation rates and lithologic variability associated with the Mekong and Sunda River systems that might be related to uplift and denudation of Tibetan and East Asian tectonic systems.
We cored three APC/XCB holes at Site 1143. Hole 1143A reached 400 mbsf with 95% recovery, and Hole 1143B reached 258 mbsf with 95% recovery. Hole 1143C (following PPSP approval to deepen the hole) reached 500 mbsf with 96% recovery and a basal age of ~9.9 Ma (Table T1). We requested approval to deepen Hole 1143C beyond the originally approved 400 m penetration to extend the paleoenvironmental record in time, after the sediment age turned out to be younger than expected. We had confirmed that no significant hydrocarbon concentrations occurred in the entire interval recovered in Hole 1143A. Wireline logging was accomplished in Hole 1143A with the triple combo tool suite (86-400 mbsf) and the FMS-sonic tool combination (158-380 mbsf). Hole conditions did not allow deployment of the GHMT string.
The sediments at Site 1143 represent continuous hemipelagic sedimentation of fine-grained terrigenous material and pelagic carbonate from the late Miocene (~10 Ma) to present (Fig. F11). The three holes at Site 1143 were combined into a composite (spliced) stratigraphic section that is continuous in mcd scale from 0 to 190.85 mcd, the interval of APC coring. Incomplete core recovery and decreased core quality precluded splice construction below this interval, but intervals of the cored section can be correlated over the 190-400 mcd interval. Overall, most of the Pliocene/Pleistocene interval has a continuous and reliable spliced record. In general, the long-term decrease in sedimentation rates observed at Site 1143 from the upper Miocene to the Pleistocene is caused by declining accumulation rates of both carbonate and noncarbonate components. A temperature gradient of ~86mined from five advanced hydraulic piston corer temperature tool (APCT) downhole measurements.
The Pleistocene-age sediments at Site 1143 consist mostly of olive, greenish, and light gray-green and greenish gray clayey nannofossil mixed sediment, clay with nannofossils, and clay. In general, bedding is not evident, and compositional changes are gradual throughout the site. Minor lithologies vary with depth and include ash layers, turbidites, and green clay layers. Carbonate content of this interval is variable but averages ~18 wt%, with both calcareous nannofossils and planktonic foraminifers abundant and well preserved. Benthic foraminifers are generally rare throughout the site. Changes in color define lithologic subunits, and these changes are mainly controlled by carbonate content. The CR measurements (lightness parameter L*) vary with the carbonate data and increase downcore. Core-logging data, especially CR, MS, and NGR, show an increasing trend downcore within the Pleistocene with superposed patterns of orbital-scale cyclicity, many of which can be correlated with glacial-interglacial cycles and marine oxygen isotope stages. Magnetic susceptibility data show a number of significant spikes that correspond to observed volcanic ash layers and are particularly abundant in the intervals 20-30 and 70-100 mcd. The interstitial waters are characterized by sulfate reduction downhole to ~200 mbsf. Organic carbon content decreases from ~0.9 wt% at core top to 0.2 wt% at the base of the Pleistocene section and remains low throughout the core. Methane concentration in the sediment is <10 ppmv over the entire interval to 500 mbsf. The extended interval of sulfate reduction appears consistent with the low organic carbon and methane content in the sediments. The Brunhes/Matuyama polarity reversal was observed at ~42.5-43.8 mcd, and the Pleistocene/Pliocene boundary is located between 93.5 and 94.3 mcd. Over the Pleistocene interval, the linear sedimentation rate (LSR) averages 50 m/m.y., and the total and carbonate mass accumulation rates (MAR) (g/cm2/k.y.) are 3.6 and 0.6, respectively.
The Pliocene-age sediments at Site 1143 are characterized by steadily increasing carbonate content, ranging from 20 to 40 wt% and averaging 28 wt% (Fig. F11). Calcareous nannofossils and planktonic foraminifers are abundant and generally well preserved. This interval exhibits a continued increase in the L* value and grain density, reflecting the increase in carbonate. The MS and NGR values reach a plateau in the mid-Pliocene and rapidly decrease in the lowermost Pliocene. Magnetic susceptibility spikes related to volcanic ashes occur between 120 and 190 mcd. Cyclicity of most properties continues as in the Pleistocene section but with reduced amplitude. Near the base of the Pliocene (~200 mbsf), increases in the concentrations of dissolved silica, strontium, and lithium as well as alkalinity are consistent with a lithology change observed at that depth. Change from APC to XCB coring in the lower Pliocene significantly affected the core-logging data, particularly the NGR and MS signals. Their values decreased by half across the APC/XCB coring transition, although this also partly reflects the change in lithology at that depth. NGR data from wireline logging confirm the change in lithology. The Pliocene/Miocene boundary is located between 213 and 200 mcd. Over the Pliocene interval, the average LSR is 36 m/m.y., and the total and carbonate MAR (g/cm2/k.y.) are 3.7 and 1.0, respectively.
The Miocene-age sediments at Site 1143 are distinguished by high carbonate content (averaging 47 wt%) (Fig. F11), which is the primary criterion for identifying lithologic Units I and II. The green clay layers are less frequent, and turbidite layers are more frequent in the Miocene sediments. Sediments of this interval also exhibit higher CR and bulk density but lower MS and NGR. These variations probably reflect changes in clay content or abundance of foraminifer turbidite layers. Calcareous nannofossils are abundant, but their preservation deteriorates through the Miocene, whereas planktonic foraminifers are abundant and have good preservation. Over the upper Miocene interval, the average LSR is 114 m/m.y., and the total and carbonate MAR (g/cm2/k.y.) are 14.0 and 6.6, respectively. These higher sedimentation rates are caused by frequent turbidites that were observed in the cores and clearly distinguishable both in logging data and FMS images. The sand-rich base of the turbidites is characterized by lower gamma-ray, density, and resistivity and higher porosity and P-wave velocity values. The opposite is true for the top clay-rich sediments.
Overall, Site 1143 provides an excellent continuous record with moderate accumulation rates, especially over the past 6 m.y. The site should enable researchers to develop high-resolution orbital-scale time series of paleoceanographic proxies and to reconstruct the record of seasonality in the South China Sea.
Site 1144 (proposed site SCS-1) is located 20o3.18就, 117o25.14媧, at a water depth of 2037 m (Table T1), on a sediment drift on the northern margin of the South China Sea (Figs. F9, F12). Site 1144 was chosen to take advantage of extremely high sedimentation rates to recover a continuous sequence of hemipelagic sediments that will yield reconstructions of paleomonsoon history on millennial, centennial, or higher resolution time scales for the mid- to late Pleistocene (~1 Ma). Site 1144 data will allow comparison of SCS records with orbital-scale and higher frequency records from ice cores, marginal seas, and terrestrial deposits. Situated along the northern margin of the SCS, Site 1144 is ideally located to reconstruct the sedimentologic, isotopic, and faunal/SST changes associated with development of the winter monsoon. This site will provide the northern constraints for reconstruction of SST gradients in the South China Sea.
We cored three APC/XCB holes at Site 1144. Hole 1144A reached the target depth (453 mbsf) with an average recovery of 104%, Hole 1144B reached 452 mbsf with 99% recovery, and Hole 1144C was APC cored to the target depth of 204 mbsf with an average recovery of 100%. The total recovery for Site 1144 was 1113 m, representing 101% of the cored interval (Table T1). Following completion of Hole 1144A, wireline logs were successfully acquired at depths from 87 to 452 mbsf for all three tool suites (triple combo, FMS-sonic, and GHMT). The temperature gradient within the sediment column at Site 1144 is anomalously low (24include the rapid sedimentation rate or subsurface water circulation.
Coring at Site 1144 recovered a mid- to upper Pleistocene and Holocene sequence of rapidly accumulating, hemipelagic clays with a basal age of ~1.1 Ma (Fig. F13). The recovered sequence spans 452 mbsf (519 mcd). The continuous spliced record (mcd scale) extends from 0 to 235.41 mcd. Splice construction below this interval was precluded by incomplete core recovery and alignment of coring gaps. However, a discontinuous ("floating") mcd depth scale was constructed from 235 to 519 mcd based on correlations among cores from the three holes.
The Pleistocene-age sediments at Site 1144 are notable for their high sedimentation rates and organic carbon content, the cyclicity of their physical properties, and the variations of "iron sulfide" and pyrite. The sediments represent a rapidly accumulating drift deposit with hemipelagic sedimentation of clay with quartz silt and nannofossils, completely homogenized by bioturbation. Definitions of the lithologic subunits reflect the abundance of "iron sulfides" (which mainly occur in the uppermost interval, 0-283 mcd), siliceous biota (highest proportion in middle interval, 283-404 mcd), and pyrite (which mainly occurs in the lowermost interval, 404-518 mcd). Minor lithologies (5%-10% of the sediment) include clay with quartz silt and sponge spicules, clay with quartz silt and diatoms, clay with silt, silty clay, and clay. Ash layers and green clay layers are rare throughout, but a number of rather indistinct green clay layers and patches occur over the same intervals that contain more discrete layers at Site 1143. Downslope redeposition is suggested by thin layers of foraminifer ooze with small amounts of pyrite and a variety of macrofossils, including gastropods, scaphopods, pteropods, fragments of echinoderms, and poorly defined shell debris. Wood debris as long as 4 cm was also observed. Bulk X-ray diffraction (XRD) analysis revealed that the mineralogy mimics the overall visual homogeneity of the sediment.
Carbonate content of this Pleistocene section is low and ranges from 10 to 20 wt%. Unlike at other sites, CR data did not correlate well with shipboard carbonate measurements taken at much lower sampling resolutions. However, the CR data do show detailed, high-resolution variations with patterns similar to recognized glacial-interglacial scale variations. Over the upper section of Site 1144 (0-100 mcd), increases in bulk density and NGR and decreases in porosity reflect rapid compaction, with superimposed fluctuations of glacial-interglacial and higher frequency compositional variation. Both core data and downhole logging show a markedly decreased rate of compaction over the next 320-m interval (100-420 mcd), and the superimposed signal variability is of larger amplitude. Magnetic susceptibility is low and rather featureless through these two upper intervals, apart from some spikes representing ash layers. However, an abrupt, threefold increase in MS takes place at ~420 mcd and is associated with a decrease in porosity from 55% to 50% as well as increased downhole sonic velocities. These changes correspond to the depth of a prominent seismic reflector. Because of the scarcity of biostratigraphic markers in this section of high sedimentation rates, we have not yet determined if a hiatus exists at that depth and/or if the section may reflect past mass wasting events in the sediment drift.
Organic carbon is relatively high at Site 1144 and decreases from ~1.5 wt% near the core top to 0.3 wt% at 519 mcd. Methane measured in headspace and void spaces ranged from 0.5% to > 60% by volume, respectively, and was biogenic in origin. No heavier hydrocarbons were observed. The interstitial waters at Site 1144 indicated sulfate reduction in the upper few meters and ammonia production throughout the remaining core. Dissolution of biogenic phases and alteration of volcanic material and clay accounted for a number of the interstitial water profiles. A few profiles indicated a change at ~420 mcd, where physical properties and seismic records also detect a boundary.
The chronostratigraphy of Site 1144 is primarily derived from the calcareous nannofossil and planktonic foraminiferal zones and events and is aided by the abundance patterns of several planktonic foraminifers, siliceous microfossils, benthic foraminifers, and pteropods. Because of the extremely high sedimentation rates at this location, only eight of the standard biostratigraphic zones and markers could be identified over the past 1.1 m.y. Unfortunately, the magnetopolarity stratigraphy was also limited by the high sedimentation rates and poor magnetization. Only the Laschamp Event (0.04 Ma) is tentatively identified at 23.5-25.5 mcd. The temporal pattern of magnetic intensity apparently correlates well with other intensity records and is consistent with the biostratigraphic age model. Sedimentation rates varied within the section but tend to decrease downhole, with the exception of the lowermost 100 m. Over the upper half of the site (0-250 mcd, 0-0.31 Ma), the linear sedimentation rates average 870 m/m.y., and the total and carbonate MARs (g/cm2/k.y.) average 85 and 11, respectively. In the lower half of Site 1144 (250-519 mcd, 0.31-1.03 Ma), the LSR averages 370 m/m.y., and the total and carbonate MAR (g/cm2/k.y.) average 64.0 and 9.6, respectively.
In summary, Site 1144 offers an exceptionally high sedimentation-rate section with well-defined variability of many properties for the study of climate-ocean response on the orbital, millennial, centennial, and higher resolution time scales for the mid- to late Pleistocene (~1 Ma). The data from Site 1144 will be among the highest resolution marine records and should enable direct comparison of the South China Sea climates with records from ice cores, laminated sequences, and terrestrial deposits.
Site 1145 (proposed site SCS-2) is located 19o35.04就, 117o37.86媧, at a water depth of 3175 m (Table T1), near the base of the northern continental margin of the South China Sea (Figs. F9, F14). The location of Site 1145 was selected to provide a deep-water contrast to Site 1144 (i.e., below the depth of the Bashi Strait, 2600 m) so that the water-mass depth gradients and ventilation history of the South China Sea could be reconstructed for the Quaternary. Site 1145 was also envisioned as a mid-Pliocene through Pleistocene record of the intensification of the winter monsoon.
We cored three APC/XCB holes at Site 1145. Hole 1145A reached the target depth of 200 mbsf with average recovery of 93%, Hole 1145B reached 200 mbsf with an average recovery of 90%, and Hole 1145C reached 198 mbsf with an average recovery of 96%. The total core recovered at this site was 555 m, representing 93% recovery (Table T1). Downhole and bottom-water temperature measurements revealed a thermal gradient of 90of the site.
Site 1145 recovered a continuous sequence of hemipelagic clays of late Pliocene to Holocene age with a basal age of ~3.3 Ma (Fig. F15). A complete composite (spliced) section was constructed over the entire depth of 213 mcd. The site is noteworthy for its excellent paleomagnetic stratigraphy and distinct patterns of cyclicity.
The Pleistocene-age sediments at Site 1145 mainly consist of intensely bioturbated clay with distinct light carbonate-rich clay layers of ~0.5 to 4.0 m thickness. Although average carbonate content was low during the Pleistocene (averaging ~10 wt%), the light carbonate-rich layers were clearly recorded in the CR data, especially the lightness parameter (L*). Other core-logging data (GRA bulk density, NGR, and MS) are also characterized by high-amplitude cyclical fluctuations over the entire section. These cycles are best defined in the NGR data from the APC interval (above ~133 mcd), where the dominant wavelength changes at ~80 mcd from ~10-15 m in the upper interval to ~2-5 m in the lower interval. The first five major CR and NGR intervals are easily correlated with the marine oxygen isotope interglacial Stages 5, 7, 9, 11, and 13. Identification of light layers became more problematic with depth as the average sediment composition became more carbonate rich and the contrast between dark and light layers decreased. In the lower half of the Pleistocene, below ~80 mcd, the average values of GRA, MS, and NGR increase. The abrupt increase in these values is accompanied by a sudden decrease in porosity from ~70% to 55%. Porosity clearly diverges from a linear trend between 80 and 170 mcd. This porosity decrease alone cannot account for the changes observed in MS and NGR over the interval from 80 to 170 mcd, and the cause of these changes remains enigmatic. The sharp offset in GRA bulk density at ~133 mcd is caused by the change from APC to XCB coring. The XCB cores are moderately disturbed by partial remolding and incorporation of drilling slurry.
The upper part of the section (0-86 mcd) is characterized by small amounts (<10%) of biogenic silica, mainly in the form of radiolarians, diatoms, sponge spicules, and silicoflagellates. Green layers (typically 1-3 cm thick) and less distinct green mottles, as well as slightly yellowish gray patches that probably represent traces of bioturbation, appear frequently throughout the Pleistocene section. The occurrence of complete pteropod tests in the upper section (0-~70 mcd) along with abundant calcareous nannofossils and planktonic foraminifers reflects the good carbonate preservation in the upper Pleistocene. Evidence of downslope transport at Site 1145 includes several complete echinoderms, echinoderm fragments remineralized by pyrite, and several wood fragments. Although a few cores contain fresh angular volcanic glass shards within burrows and dispersed ash in low concentrations, volcanic ash represents a very minor component of the total sequence.
Total organic carbon (TOC) declined steadily from more than 1 wt% at the top of the section to much lower amounts (~0.2 wt%) at the base, consisting of mostly marine organic matter. Only trace amounts of methane (<16 ppmv) and no other hydrocarbon gases were detected in sediments at Site 1145. Interstitial water profiles are characterized by relatively constant chloride values close to those of the seawater value. In the upper part of the sedimentary column (0-~100 mcd), the sulfate, alkalinity, ammonium, and phosphate profiles show significant changes, which are caused by the diagenesis of organic matter via sulfate reduction. Sulfate reduction is incomplete, and methanogenesis is a minor process in these sediments. A large decrease in dissolved interstitial water silica occurs between ~80 and ~110 mcd (~0.5-1.0 Ma), which corresponds to a decrease in biogenic silica. This shift was also observed at ~420 mcd at Site 1144, where the shift is ~0.79 Ma.
The paleomagnetic records at Site 1145 clearly established the Brunhes/Matuyama transition at 93 mcd, the upper Jaramillo transition at 110 mcd, and the lower Jaramillo transition at ~116 mcd. Farther downcore the overprint resulting from the coring process increases, but long core measurements identify the Gauss/Matuyama boundary at 190 mcd as a jump of the inclination from ~25obtained from XCB cores). Demagnetization of the discrete samples proved to be more efficient in removing the overprint, and the Olduvai Event was clearly revealed from 155 to 165 mcd. The inclinations around the Gauss/Matuyama boundary are closer to their expected values. From 190 mcd to the bottom of Hole 1145A, two samples yield unequivocal reverse inclinations. This may indicate that the upper Kaena reversal (3.04 Ma) had been reached in this hole, although this conclusion conflicts with the biostratigraphic age for the bottom of the hole and must be confirmed with shore-based analysis. Based on paleomagnetic and biostratigraphic control, the Pleistocene/Pliocene boundary can be placed at ~160 mcd. At Site 1145, the upper Pleistocene interval (0-70 mcd) had a linear sedimentation rate of 210 m/m.y. and total and carbonate MARs of 14 and 1.4 g/cm2/k.y., respectively. The lower Pleistocene interval had lower rates, with an average LSR of 60 m/m.y. and total and carbonate MARs of 6.2 and 0.7 g/cm2/k.y., respectively.
In the Pliocene-age section at Site 1145, average carbonate contents (17 wt%) and variability (10-30 wt%) increase (Fig. F15). Although planktonic foraminifers and calcareous nannofossils remain common to abundant, planktonic foraminifer preservation degraded from good to poor downsection, and the nannofossils were occasionally to commonly reworked. Benthic foraminifers are generally few. The CR and bulk data both increase in the Pliocene section and reflect the increased carbonate content. Other properties display cyclic variability but show no longer term trends. The biostratigraphic age of the oldest sediments recovered at Site 1145 is estimated at 3.12-3.35 Ma. The Pliocene section has the lowest rates observed at Site 1145, with LSRs of 38 m/m.y. and total and carbonate MARs of 4.5 and 0.8 g/cm2/k.y., respectively (Fig. F15).
In general, both carbonate and noncarbonate accumulations decrease downhole at Site 1145, with a small increase in the lower Pleistocene. Overall, the variations in the noncarbonate components dominate the accumulation rate pattern as at Site 1144. Site 1145 provides a high-quality section with excellent paleomagnetic stratigraphy for the resolution of orbital-scale variations of climate changes that may be compared with continental records of extensive loess deposition in China from ~2.4 Ma.
Site 1146 (proposed site SCS-4) is located 19o27.40就, 116o16.37媧, at a water depth of 2092 m (Table T1), within a small rift basin on the mid-continental slope of the northern South China Sea (Figs. F9, F16). Site 1146 was intended to take advantage of the relatively shallow slope basin with moderate sedimentation rates to recover a continuous sequence of hemipelagic sediments that would yield reconstructions of East Asian monsoon history from the middle to upper Miocene (~10 Ma). Such a long-term record at orbital-scale resolution (2 k.y.) would allow comparisons of East Asian monsoon variability with orbital forcing, glacial forcing, and internal feedbacks within the climate system and provide a new set of constraints on the possible relationship between Tibetan Plateau uplift, monsoon evolution, and global cooling. Specifically, we seek to determine whether monsoonal indices intensify or weaken during the late Miocene and whether the Miocene-Pliocene pattern of accumulation rates is consistent with models of Himalayan-Tibetan uplift, monsoon intensification, and sea-level changes. Lastly, we envisioned that the Site 1146 record of the East Asian monsoon would provide an appropriate counterpart to Site 1143 in the southern SCS and Site 722 in the Arabian Sea and that their comparison could serve to identify potential sources of common causality.
We cored three APC/XCB holes at Site 1146. Hole 1146A reached the target depth of 607 mbsf with an average recovery of 100%, Hole 1146B reached 245 mbsf with an average recovery of 99%, and Hole 1146C reached 604 mbsf with an average recovery of 101% (Table T1). We requested approval to deepen Site 1146 beyond the originally approved 520 m penetration to extend the paleoenvironmental record in time, after the sediment age turned out to be younger than expected and after we had confirmed that no significant hydrocarbon concentrations occurred in the entire interval recovered. Following completion of Hole 1146A, three successful downhole logs were acquired with the triple combo (85-600 mbsf), FMS-sonic (239-600 mbsf), and GHMT (239-600 mbsf) tool strings. Downhole and bottom-water temperature measurements yielded a thermal gradient of 59
Coring at Site 1146 recovered a lower Miocene through Pleistocene section of relatively carbonate-rich, hemipelagic nannofossil clays with a basal age of ~19 Ma (Fig. F17). The core-logging data enabled construction of a continuous mcd scale and a continuous spliced record from 0 to 266.7 mcd. A discontinuous ("floating") mcd scale and splice were also developed for the interval spanning 266.70-642.31 mcd, which is the bottom of the cored sequence. We expect that postcruise correlation with downhole logging data will allow construction of a complete, continuous section to 640 mcd.
The Pleistocene-age sediments are composed of greenish gray nannofossil clay that is relatively enriched with quartz, plagioclase, and chlorite relative to lower sections. This unit grades downhole into clayey nannofossil ooze within the Pliocene section. The Pleistocene sediments average ~21 wt% carbonate and are characterized by slightly lighter intervals with higher carbonate content (Fig. F17). Planktonic foraminifers are abundant and have good preservation for the site's entire interval. The upper part of the section (0-110 mcd) shows constant or slightly increasing values in CR (L* parameter), MS, and NGR, a high scatter in grain densities, and normal compaction-related increase in bulk density and P-wave velocity. This interval is characterized by high-amplitude variations representing orbital-scale cyclicity. Minor sediment components include large pyrite nodules that preserve organic structures (burrow-fill and Xenophyophorians) and thin, light-gray ash layers, often dispersed by bioturbation, as well as isolated pumice clasts. Diatoms, silicoflagellates, radiolarians, sponge spicules, and large (>1 cm in diameter) pteropods are common.
Total organic carbon decreases systematically from the Pleistocene sediments (1 wt% at the top of the section) to trace abundance (<0.2 wt%) below the mid-Pliocene (~225 mcd). Interstitial water profiles at Site 1146 were primarily driven by the removal and release of elements in the process of organic matter reduction, with sulfate reduction the dominant process above 68 mcd and methanogenesis dominant below. Depletion of methanogenesis products such as alkalinity, phosphate, and ammonium with depth suggests that methanogenesis is active only in the upper sediments. This agrees with the interpretation of higher methane levels and higher order hydrocarbons in headspace gas samples below 200 mcd as gases that have migrated into this site, either laterally or from depth. Dissolved silica and strontium correlate with the amount of biogenic silica and carbonate in the sediments, respectively. A major decrease in the dissolved silica concentration is observed between 109 and 140 mcd (~0.7-1.1 Ma) and accompanies a decrease in the amount of biogenic silica to near-zero values. A similar decrease was observed at Sites 1144 and 1145 at an age of ~0.8 Ma.
Paleomagnetic measurements reveal the Brunhes-Matuyama transition at 115 mcd, the upper Jaramillo transition at 133 mcd, and the lower Jaramillo transition at 138 mcd. Between 99.7 and 101.3 mcd, a swing in declination with correlative very low inclinations could document the Big Lost (geomagnetic) Event (dated at 510 to 650 ka). A transition from reverse declinations to normal occurs between 160.5 and 165.8 mcd, possibly marking the Olduvai Event. The biostratigraphy places the Pleistocene/Pliocene boundary between 185.5 and 195.1 mcd. At Site 1146, the sedimentation rates decrease downcore; the Pleistocene section has the highest rates, with an average linear sedimentation rate of 150 m/m.y. and total and carbonate MARs of 11.5 and 2.3 g/cm2/k.y., respectively (Fig. F17).
The Pliocene-age sediments at Site 1146 (~190-310 mbsf) are distinguished by significantly higher carbonate content (an average of 47 wt% compared to 21 wt% in the Pleistocene) (Fig. F17). The transition to higher carbonate occurs in the upper Pliocene and is accompanied by an increase in the CR L* values and a decrease in the NGR and MS. This transition interval at ~235 mcd is also characterized by a very pronounced decrease in porosity and an increase in bulk density (Fig. F17). This corresponds to a general downhole decrease in accumulation rate at that depth. The sediments grade from the overlying greenish gray nannofossil clay to homogeneous/rarely mottled, light brownish gray foraminifer and nannofossil clay mixed sediment. A small number of thin (<1-2 cm), dark ash layers, containing large volcanic glass shards as long as 1 cm, are present in the lower part of the interval. A major increase in dissolved strontium in the Pliocene/Pleistocene section between 109 and 350 mcd corresponds to the increase in the weight percent carbonate in that interval. In both cases, the availability of dissolvable biogenic sediments appears to be a strong control on dissolved concentrations of silica and strontium at Site 1146. The Pliocene/Miocene boundary at Site 1146 is between 308.42 and 317.99 mcd. The Pliocene section has an average LSR of 39 m/m.y. and total and carbonate MARs of 4.3 and 1.9 g/cm2/k.y., respectively.
The Miocene-age sediments at Site 1146 (310-642 mbsf) grade from light brownish gray foraminifers and nannofossil clay mixed sediment of the late Miocene to the green nannofossil clay of the middle to early Miocene. This transition is marked by a progressive change in the sediment color from brownish gray to a distinct greenish gray. The late Miocene to early Pliocene-age interval is slightly more carbonate rich than the middle Miocene sediments, in which kaolinite and quartz become significant contributors to the mineral composition. In this lower interval, characteristic bluish green nannofossil clay appears, which contains large amounts of pyrite as nodular irregular layers or as finely disseminated particles. The carbonate content declines throughout the Miocene: 53 wt% in the upper Miocene, 35 wt% in the middle Miocene, and 30 wt% in the lower Miocene (Fig. F17). Over much of the Miocene-Pliocene interval, the CR L* value correlates well with the carbonate pattern but NGR and MS signals are depressed, which implies a carbonate dilution effect. Negative excursions in the chromaticity ratio a*/b* at 325, 355, and 418 mcd correspond to distinct green intervals associated with foraminifer turbidites observed in the cores. The interval from 420 to 550 mcd is marked at the top by a pronounced downhole increase in NGR and MS, which is most likely the result of the lower carbonate content and associated L* reflectance below that depth. The top of the interval from 550 to 642 mcd is defined by a sharp decrease in the chromaticity ratio a*/b*, corresponding to a color change in the cores. The MS values also drop at this depth, affirming that the drop in chromaticity is accompanied by a change in magnetic mineralogy. Overall, the sediment character is typical of deposition at bathyal depths on a continental slope, with oxygenated bottom water implying water depths exceeding the oxygen minimum zone (~600 m). However, the bulk mineralogy suggests that either a change in the source of the terrigenous material or a change in the weathering regime of the source region took place over time.
The concentration of methane (headspace analysis) increased downhole from <10 ppmv at the top to a maximum of 85,000 ppmv at 599 mcd. Ethane (C2H6) and propane (C3H8) initially appeared at 536 mcd and peaked at 608 mcd with concentrations of 155.0 and 7.3 ppmv, respectively. The C1/C2 ratio reached a minimum of 345 at the bottom of the hole. A major decrease in the interstitial water salinity and chloride content occurs below 500 mcd, corresponding to the top of the interval of highest methane values, the appearance of higher order hydrocarbons, changes in sediment color, and changes in physical properties. All of these changes appear to correlate with seismic Reflector T4, which can be traced back to a possible fault ~1 nmi from the site (see "Site 1146 [SCS-4]" in "Seismic Stratigraphy of Leg 184 South China Seas Sites" in the "Seismic Stratigraphy" chapter). This suggests that hydrocarbon and freshwater signals may have migrated laterally and that the other sedimentary changes are a diagenetic response to this change in environment.
At Site 1146 calcareous nannofossils are abundant and well preserved, although nannofossil preservation deteriorates below ~530 mcd. Benthic foraminifers are generally few but become more abundant in the lower part of the section. However, we found no clear evidence for reworked benthic foraminifers from the shelf and upper slope. Planktonic foraminifers are abundant and well preserved. The age at the bottom of the section is ~19 Ma. The Miocene section has relatively constant sedimentation rates. The upper Miocene has an average LSR of 28 m/m.y. and total and carbonate MARs of 3.7 and 2.0 g/cm2/k.y., respectively; the middle Miocene has an average LSR of 28 m/m.y. and total and carbonate MARs of 4.2 and 1.4 g/cm2/k.y., respectively; and the lower Miocene has an average LSR of 31 m/m.y. and total and carbonate MARs of 4.9 and 1.4 g/cm2/k.y., respectively.
Overall, Site 1146 provides one of the most continuous Neogene sections ever recovered by the Ocean Drilling Program. The sediments are relatively high in carbonate and have rates of 30-150 m/m.y., which will allow construction of an orbital-scale stratigraphy back to the middle Miocene. This site will also allow the reconstruction of monsoon proxies that can be used to test hypotheses about the late Miocene intensification of the Asian summer monsoon and its relationship to tectonic events.
Sites 1147 and 1148 (proposed site SCS-5C) are located on the lowermost continental slope off southern China, near the continent/ocean crust boundary, and are the most offshore sites drilled during Leg 184 (Figs. F9, F18). Site 1147 (18o50.11就, 116o33.28媧, at a water depth of 3246 m) recovered the uppermost section that appeared to be missing on seismic profiles from Site 1148 (18o50.17就, 116o33.94媧, at a water depth of 3294 m) (see "Sites 1147 and 1148 [SCS-5C]" in "Seismic Stratigraphy of Leg 184 South China Seas Sites" in the "Seismic Stratigraphy" chapter) (Table T1). The greater water depth and distance from terrigenous sources common to these sites combine to produce lower sedimentation rates at this location, at least within the late Neogene. Site 1148 was located on the lower continental slope to take advantage of the thinner sediment thickness in order to recover the Oligocene and Miocene hemipelagic sediments that record the evolution and early paleoclimate history of the South China Sea. Elsewhere on the Chinese margin, the Oligocene sediments are too deep or are hydrocarbon bearing and thus cannot be cored to establish onset of monsoonal climates and variability in the SCS. We hoped that the combination of Sites 1148 and 1146 would provide a continuous history of accumulation rates that could be used to evaluate models of the SCS continental margin evolution, sea-level influence on deposition, and the impact of Himalayan-Tibetan uplift on monsoon onset and intensification. Site 1147 is located upslope ~0.45 nmi west of Site 1148 and was proposed during Leg 184 when Site 1148 was moved to a location within a surface slump scar (see "Sites 1147 and 1148 [SCS-5C]" in "Seismic Stratigraphy of Leg 184 South China Seas Sites" in the "Seismic Stratigraphy" chapter). Hence, Site 1147 was designed to recover the continuous sequence of the uppermost hemipelagic sediments thought to be lost to slumping or channeling at Site 1148.
We cored three APC holes at Site 1147. Holes 1147A, 1147B, and 1147C were cored to depths of 81, 86, and 79 mbsf, respectively, with an average recovery of 99% (Table T1). At Site 1148, we cored two APC/XCB holes. Hole 1148A was continuously cored to 704 mbsf and then wireline logged with the full complement of logging tools: triple combo (111-711 mbsf), FMS-sonic (201-711 mbsf), and GHMT (201-711 mbsf). During logging operations, after we had confirmed that no significant hydrocarbons occurred over the entire interval recovered in Hole 1148A, we requested approval to deepen Hole 1148B beyond the originally approved 700 m penetration to extend the paleoenvironmental and tectonic record. Hole 1148B was APC cored to 145 mbsf; then, due to time constraints, drilled down from 145 to 440 mbsf, XCB cored to 646 mbsf, drilled down from 646 to 700 mbsf, and cored to 853 mbsf. The drilled-down intervals had excellent recovery in Hole 1148A. Downhole and bottom-water temperature measurements at Site 1148 yielded a thermal gradient of 83cation and water depth.
The sediments at Sites 1147 and 1148 reflect a complex sequence of hemipelagic deposition, the beginning of which is coeval with the initiation of seafloor spreading in the South China Sea at ~32 Ma (Figs. F19, F20). The dominant lithologies are grayish green clay with quartz and nannofossils, olive-gray and reddish brown clay with nannofossils, light grayish green clayey nannofossil ooze, brown nannofossil clay, and greenish gray nannofossil clay mixed sediment. Lower sections have the same composition but contain slumped and faulted intervals. The mcd scale and splice at Sites 1147 and 1148 are based on the stratigraphic correlation of whole-core multisensor track and split-core CR data (lightness, L*) collected at 5- and 4-cm intervals, respectively. Magnetic susceptibility data were the most useful stratigraphic tool for correlation at these sites. Natural gamma radiation and CR data were helpful in intervals where structure in the MS profile was ambiguous. At Site 1147, a composite spliced section was constructed from the three holes that spans the entire interval from 0 to 91.7 mcd. At Site 1148, an mcd scale was constructed over the entire cored sequence, 0 to 860 mcd. However, because of core recovery gaps throughout the sequence, the scale is discontinuous ("floating") rather than linked to the sediment/water interface. A floating splice that extends from 46.57 to 155.34 mcd can be combined with data from nearby Site 1147 to construct a continuous mcd and splice, extending from 0 to 155.34 mcd.
Sediments of Pliocene-Pleistocene age at Sites 1147 and 1148 (0 to ~190 mcd) are composed of intensely bioturbated clay with quartz and nannofossils; the upper part is more clay rich and the lower part more nannofossil rich. Green clay layers and irregular green clay mottles decrease downsection, whereas lighter intervals with increased nannofossil content increase slightly downhole as do L* value, MS, and bulk density. Siliceous microfossils are abundant in the upper Pleistocene and decrease rapidly to zero in the Pliocene. The core and wireline logging data show that the Pleistocene part of the section (to ~160 mcd) is characterized by a downhole increase in MS, which decreases during the Pliocene interval. A number of properties, including bulk density, porosity, and P-wave velocity, show typical downhole patterns related to compaction and dewatering. Natural gamma radiation and CR display more complex patterns related in part to the carbonate content. Almost all properties show the cyclic fluctuations that are associated with orbital-scale climate changes. The Miocene/Pliocene boundary is marked by an increase in the light carbonate-rich nannofossil clay layers and the disappearance of pyrite concretions. Total organic carbon decreases systematically from a maximum of 0.8 wt% at the top of the hole to <0.2 wt% by 130 mcd and remains at this level throughout the Miocene (to ~485 mcd). Based on C/N values, a purely marine organic source for organic matter is suggested for the upper 130 m of Site 1148. The variation in sulfur abundance follows that of TOC in the top 130 m of the hole, decreasing slowly with depth but exhibiting a normal marine S/C ratio (0.4). Interstitial water profiles at Site 1148 are dominated by sediment/water exchanges driven by sulfate reduction in the upper 110 mcd. Sulfate values never reach zero, indicating that sulfate reduction is incomplete and methanogenesis is limited. The upper Pleistocene interval has high dissolved silica, which correlates with the higher number of siliceous organisms in the sediment.
Calcareous nannofossils and planktonic foraminifers are abundant and well preserved in Pliocene/Pleistocene sediments. At Site 1148 (1147), the Brunhes/Matuyama boundary can be tentatively placed at 55.2 (58) mcd, the upper Jaramillo Subchron at 69.1 (71) mcd, the lower Jaramillo at 73 (76) mcd, the upper Olduvai Event at 111.4 mcd, and the lower Olduvai (tentatively) at 118.5 mcd. The age of the oldest sediments recovered at Site 1147 is estimated at 1.22-1.47 Ma. The combined biostratigraphy placed the Pleistocene/Pliocene boundary between 125.8 and 135.5 mcd and the Pliocene/Miocene boundary between 184.5 and 193.8 mcd.
The Miocene-age sediments at Site 1148 (~190-475 mcd) are a mixture of olive-gray and reddish brown clay with nannofossils, light grayish green clayey nannofossil ooze, brown nannofossil clay with intervals and patches of reduced green ooze, and greenish gray nannofossil clay mixed sediment and nannofossil clay. The wireline logs of this interval reveal downhole increases in bulk density, electric resistivity, P-wave velocity, and photoelectric effect (PEF) but decreases in neutron porosity (Fig. F20). Similar to the core-log data, MS has several local maxima but decreases downhole. Natural gamma radiation is variable but has no long-term trend until the lowermost Miocene, when it rapidly decreases at the slumped section. None of the units above the slumped section shows any evidence of sediment redeposition; they are representative of continuous hemipelagic sedimentation. These lithologic changes are reflected in the carbonate content and especially the CR a* and L* variations. The lowermost sediments have relatively high concentrations of diagenetically precipitated "iron sulfide," seen as black, fine-grained material. Trace fossils are common, most notably Zoophycos and Chondrites, both characteristic deep-water (bathyal) forms. Evidence for redeposition in the lower Miocene sediments is sparse, although a few thin carbonate sand turbidites do occur.
Interstitial water profiles in the Miocene and Oligocene interval are dominated by sediment/water exchanges driven by volcanic alteration, clay mineral diagenesis, and calcite recrystallization at depth. Sulfate values never reach zero, indicating that sulfate reduction is incomplete and methanogenesis is not an important process in these sediments. As a result, the higher methane values at depth are related to thermogenic production of hydrocarbons.
The Oligocene-age sediments at Site 1148 (475-852 [mcd]) represent a major change in deposition. The uppermost Oligocene sediments are light tan in color, which reflects a distinct increase in carbonate content and the associated CR a* and L* values (Fig. F20). This interval is also marked by a sharp increase in P-wave velocity, PEF, bulk density, and low porosity. These properties most likely reflect the increased carbonate content in this interval (50-75 wt%) and are probably responsible for prominent double reflectors seen in seismic reflection profiles. The sonic P-wave velocity of this interval is 2.3 km/s, which is substantially greater than the value of 2.1 and 1.9 km/s at the top and bottom of this interval, respectively. Although similar in composition to overlying sediments (i.e., dominantly clay nannofossil mixed sediments and nannofossil clays), this layer represents gravitational redeposition by mass flows and slumping as evidenced by convolute bedding, soft-sediment plastic deformation, and the presence of light-colored carbonate mud clasts within a massive bed of light gray to grayish brown nannofossil clay. These sediments also show clear evidence of brittle faulting in the form of small normal microfaults and thus are likely related to tectonic activity in the formation of the South China Margin. However, the matrix sediments often contain a deep-water trace fossil assemblage of Zoophycos and Chondrites and provide no evidence that water depths differed significantly from the overlying Miocene sediments. "Iron sulfides," pyrite concretions, and green clay layers are rarely observed. A sudden increase in TOC is noted below 485 mcd (>0.4 wt%), and the concentration of TOC remains in this range downhole (0.2-0.5 wt%). The higher C/N values in the lower section may indicate significant terrestrial input. Just below the slumped interval, salinity and chlorinity values become more variable, and ammonium and silica values increase. Changes in ammonium (as well as chloride and salinity) below 470 mcd may also be related to the dehydration reaction of clay minerals. In this interval, XRD data show that below 470 mcd, smectite, illite, and kaolinite are absent and an unidentified mixed-layer clay appears. The slump and the underlying chalk layer may act as a barrier to diffusion of gas and possibly to some elements dissolved in interstitial waters. The microenvironments within fractures may also lead to variable interstitial water concentrations in this lower interval.
The bulk of the Oligocene sediment is an intensely bioturbated sequence of quartz-rich, grayish olive-green nannofossil clay. The whole sequence is extremely monotonous, with little lithologic variation, and is characterized by low values of MS, bulk density (although disturbed by the XCB coring), NGR, and PEF and by decreased L* but increasing a*/b* parameter of CR. These trends generally reflect the decreased carbonate and increased clay content in the rapidly accumulating Oligocene section. The abundant bioturbation traces are strongly compacted and give the sediment a laminated appearance. A higher interstitial water dissolved silica concentration in the mid-Oligocene interval is associated with intervals of higher biogenic silica content. Toward the base of the section, evidence of current activity is found in the form of occasional flaser sandstone laminae that are dominated by quartz and lithic fragments as well as mica, glauconite, and foraminifer fragments. As with younger sediments, little evidence suggests that these early Oligocene sediments were deposited in substantially shallower water.
In the Oligocene sediments, total sulfur concentration increases, following TOC. However, the S/C ratio is anomalously high for normal marine sediments (>1), suggesting the addition of sulfur from another source. An increase in methane to the bottom of the hole (711 mcd) is accompanied by the presence of ethane and propane as well as heavier hydrocarbons downhole. Maximum methane and ethane concentrations were detected at 593 mcd (569 and 25 ppmv, respectively). From the first detection of ethane at 480 mcd, the C1/C2 ratio declined rapidly from 99 to approach a minimum of 15 at the bottom of Hole 1148A. Between 715 and 851 mcd, methane concentrations remained low (<200 ppmv) and decreased with depth downhole. The C1/C2 ratio decreased to as low as 4; this was expected, however, because of the small amounts of organic matter in these poor source rocks as they entered the zone of petroleum maturation. As much as 50 ppmv of C5 and lesser amounts of other light hydrocarbons were detected.
Thirty-nine nannofossil and 29 planktonic foraminifer biostratigraphic datums were recognized from the lower Oligocene to Pleistocene sediments at Site 1148. A gap in the nannofossil and foraminifer datums indicates that sediments between the lowermost part of Zone NP25/Zone P22 to Zone NN2/Zone N4 are missing and that the base of Hole 1148B is still within Zone NP23/P19 (<32.3 Ma). Site 1148 yields few to abundant deep-sea benthic foraminifers, and the ratio of benthic to planktonic foraminifers is high in the Oligocene. The benthic foraminifers (e.g., Heterolepa, Gavelinopsis, Globocassidulina, Martinottiela, Sigmoilopsis, Textularia, and Uvigerina) in the lower part of Hole 1148A (>~510 mcd) are comparable to those observed at 1000-2000 m in the modern South China Sea. An increase in the abundance of Globobulimina and Chilostomella (indicative of high productivity) was observed in the upper (above ~50 mcd) and lower (below ~500 mcd) sections of Hole 1148A. This corresponds to the higher organic carbon content and abundant siliceous fossil content (radiolarians and diatoms) found in the two intervals. Down to the middle Miocene, nannofossils are moderately to well preserved, and planktonic foraminifers are poorly to moderately preserved; in the lower section (Oligocene), foraminiferal tests are recrystallized, and some nannofossils show postburial overgrowth.
The stratigraphy at Site 1148 spans most of the postrift history of the South China Sea, including the entire duration of active seafloor spreading (Briais et al., 1993). Despite this, apart from a series of sharp color changes and associated differences in physical properties and carbonate content, very little lithologic variation has occurred since the early Oligocene. Lithologically, we found no apparent deepening or shallowing of the water depth of sedimentation, remaining hemipelagic and probably bathyal throughout. The most noteworthy sedimentary feature at Site 1148 is the mass-flow sequence, which is also responsible for the prominent reflector at the base of the Miocene section. Ironically, the strong basement reflector at ~800 mbsf is within the gray-green Oligocene clays and does not show any distinct lithologic change (although recovery in this section was poor).