Abundance of palynomorphs and dinoflagellate stratigraphy of ODP Hole 184-1148A, South China Sea

Dinoflagellate stratigraphy is described for the section from 364.75 to 843.85 meters below seafloor (mbsf) at Site 1148 (Sections 184-1148A-40X-1 through 76X-6 and 184-1148B-39X-CC through 56X-1) in the South China Sea. Two assemblage zones and two subzones are defined, based on characteristics of the assemblages and lowest/highest occurrences of some key species. These are the Cleistosphaeridium diversispinosum Assemblage Zone (Zone A; Oligocene), with the Enneadocysta pectiniformis Subzone (Subzone A-1) and the Cordosphaeridium gracile Subzone (Subzone A-2), and the Polysphaeridium zoharyi Assemblage Zone (Zone B; early Miocene). The highest concurrent occurrence of Enneadocysta arcuata, Eneadocysta multicornuta, Homotryblium plectilum, and Homotryblium tenuispinosum delineates the upper boundary of Zone A, which appears to mark a hiatus. Subzone A-1 is of early Oligocene age, as evidenced by the highest occurrences of E. pectiniformis and Phthanoperidinium amoenum at the upper boundary of the subzone. Subzone A-2 is of late Oligocene age based on the highest occurrences of C. gracile and Wetzeliella gochtii close to the upper boundary of the subzone and the occurrence of Distatodinium ellipticum and Membranophoridium aspinatum within the subzone. Zone B is dated as early Miocene based on the lowest occurrences of Cerebrocysta satchelliae, Hystrichosphaeropsis obscura, Melitasphaeridium choanophorum, Membranilarnacia? picena, and Tuberculodinium vancampoae within the zone. The present assemblage zones/subzones are correlative to various degrees with coeval zones/assemblages from areas of high to low latitudes in terms of common key species. We have compared the species content of the assemblage Zones A and B, and the subzones A-1 and A-2, with coeval assemblage(s)/zone(s) described from many, often widely distant, high- and low-latitude regions of the world. These comparisons show that, to various degrees and aside from a number of key species, the coordinated presence of certain important species may also help to assign an age to a given assemblage.

Age model of ODP Site 184-1146 (Table 1)

Clay mineral assemblages at ODP Site 1146 in the northern South China Sea are used to investigate sediment source and transport processes and to evaluate the evolution of the East Asian monsoon over the past 2 Myr. Clay minerals consist mainly of illite (22-43%) and smectite (12-48%), with associated chlorite (10-30%), kaolinite (2-18%), and random mixed-layer clays (5-22%). Hydrodynamic and mineralogical studies indicate that illite and chlorite sources include Taiwan and the Yangtze River, that smectite and mixed-layer clays originate predominantly from Luzon and Indonesia, and that kaolinite is primarily derived from the Pearl River. Mineral assemblages indicate strong glacial-interglacial cyclicity, with high illite, chlorite, and kaolinite content during glacials and high smectite and mixed-layer clay content during interglacials. During interglacials, summer enhanced monsoon (southwesterly) currents transport more smectite and mixed-layer clays to Site 1146 whereas during glacials, enhanced winter monsoon (northerly) currents transport more illite and chlorite from Taiwan and the Yangtze River. The ratio (smectite+mixed layers)/(illite+chlorite) was adopted as a proxy for East Asian monsoon variability. Higher ratios indicate strengthened summer-monsoon winds and weakened winter-monsoon winds during interglacials. In contrast, lower ratios indicate a strongly intensified winter monsoon and weakened summer monsoon during glacials. Spectral analysis indicates the mineral ratio was dominantly forced by monsoon variability prior to the development of large-scale glaciation at 1.2 Myr and by both monsoon variability and the effects of changing sea level in the interval 1.2 Myr to present.

(Table 2) Age model of ODP Site 184-1146

Orbital tuning of benthic d18O is a common approach for assigning ages to ocean sediment records. Similar environmental forcing of the northern South China Sea and the southeast Asian cave regions allows for transfer of the speleothem d18O radiometric chronology to the planktonic and benthic d18O records from Ocean Drilling Program Site 1146, yielding a new chronology with 41 radiometrically calibrated datums, spanning the past 350 kyr. This approach also provides for an independent assessment of the accuracy of the orbitally tuned benthic d18O chronology for the last 350 kyr. The largest differences relative to the latest chronology occur in marine isotope stages (MIS) 5.4, 5.5, 6, 7, and 9.3. Prominent suborbital-scale structure believed to be global in nature is identified within MIS 5.4 and MIS 7.2. On the basis of the radiometrically calibrated chronology, the time constant of the ice sheet is found to be 5.4 kyr at the precession band (light d18O lags precession minima by -55.4°) and 10.4 kyr at the obliquity band (light d18O lags obliquity maxima by 57.4°). These values are significantly shorter than the single 17 kyr time constant originally estimated by Imbrie et al. (1984), based primarily on the timing of terminations I and II and the 15 kyr time constant used by Lisiecki and Raymo (2005, doi:10.1029/2004PA001071).

(Table 1) Element concentrations in detrital sediments of ODP Site 184-1148

The chemical index of alteration (CIA) and elemental ratios that are sensitive to chemical weathering, such as Ca/Ti, Na/Ti, Al/Ti, Al/Na, Al/K, and La/Sm, were analyzed for detrital sediments at Ocean Drilling Program Site 1148 from the northern South China Sea to reveal information of chemical weathering in the source regions during the early Miocene. High CIA values of ~80, coupled with high Al/Ti and Al/Na and low Na/Ti and Ca/Ti, are observed for the sediments at ~23 Ma, indicating a high chemical weathering intensity in the north source region, i.e., south China. This was followed by gradual decreases in Al/Ti, Al/Na, La/Sm, and Al/K ratios, as well as the CIA values, and increases in Ca/Ti and Na/Ti ratios. These records together with other paleoclimate proxies, such as black carbon d13C and benthic foraminifer d18O, give reliable information on the climate changes in south China. Our results show that the climate in south China was warm and humid in the early Miocene (~23 Ma) according to the chemical weathering records. The humidity in south China decreased from the early Miocene to Present with several fluctuations centering at approximately 15.7 Ma, 8.4 Ma, and 2.5 Ma, coincident with the global cooling since the middle Miocene. These climate changes implied that the summer east Asian monsoon has dramatically affected south China in the early Miocene, whereas the influence of the summer monsoon on this region has decreased continuously since that time, probably because of the intensification of the winter monsoon. Such an evolution for the east Asian monsoon is different from that for the Indian monsoon.

(Table T1) N-alkane, n-alcohol and n-fatty acid concentrations in ODP Site 184-1147 and 184-1148 sediments

Lipid compositions of sediments recovered during Ocean Drilling Program Leg 184 in the South China Sea have been identified and quantified. The identified lipids can be ascribed to terrigenous and marine sources. Terrigenous lipids are mainly C27, C29, C31 n-alkanes, C26, C28, C30 n-fatty acids, and n-alcohols, which were derived from leaf waxes of higher land plants and transported to the sea by airborne dust or fresh water. Marine lipids, mainly C37 and C38 alkenones, C30 diol, and C30 and C32 keto-ols, were from microalgae, notably haptophytes and eustigmatophytes. Elevated concentrations and accumulation rates of both terrigenous and marine lipids in the interval 202-245 meters composite depth (mcd) and 0-166 mcd were postulated to be related to the development of the East Asian monsoon at 6-8 Ma and enhanced variations of the developed East Asian monsoon after 3.2 Ma, respectively. The pronounced late Oligocene input of terrigenous lipids reflects the paleoenvironment of a newly opened, narrow basin, with restricted ocean waters and the proximity of continental runoff.

Neodymium isotopic ratios and clay mineral composition from ODP Hole 1147A and Site 184-1148

Ocean Drilling Program sampling of the distal passive margin of South China at Sites 1147 and 1148 has yielded clay-rich hemipelagic sediments dating to 32 Ma (Oligocene), just prior to the onset of seafloor spreading in the South China Sea. The location of the drill sites offshore the Pearl River suggests that this river, or its predecessor, may have been the source of the sediment in the basin, which accounts for only not, vert, similar ~1.8% of the total Neogene sediment in the Asian marginal seas. A mean erosion depth of not, vert, similar ~1 km over the current Pearl River drainage basin is sufficient to account for the sediment volume on the margin. Two-dimensional backstripping of across-margin seismic profiles shows that sedimentation rates peaked during the middle Miocene (11-16 Ma) and the Pleistocene (since 1.8 Ma). Nd isotopic analysis of clays yielded epsilonNd values of -7.7 to -11.0, consistent with the South China Block being the major source of sediment. More positive epsilonNd values during and shortly after rifting compared to later sedimentation reflect preferential erosion at that time of more juvenile continental arc rocks exposed along the margin. As the drainage basin developed and erosion shifted from within the rift to the continental interior epsilonNd values became more negative. A rapid change in the clay mineralogy from smectite-dominated to illite dominated at not, vert, similar 15.5 Ma, synchronous with middle Miocene rapid sedimentation, mostly reflects a change to a wetter, more erosive climate. Evidence that the elevation of the Tibetan Plateau and erosion in the western Himalaya both peaked close to this time supports the suggestion that the Asian monsoon became much more intense at that time, much earlier than the 8.5 Ma age commonly accepted.

(Table 1) Radiolarian accumulation rates and geochemistry of ODP Site 184-1143

Late Miocene-Recent micropaleontological and geochemical records from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea (SCS) indicate that increase and decrease in abundance of siliceous plankton may be controlled mainly by the input of nutrients derived from land and provided by upwelling. A high export production event - a "biogenic bloom" event - occurred in the southern SCS between 12 and 6 Ma. During this period, high ratios of smectite/(illite + chlorite), smectite/quartz and Al/K indicate a high weathering intensity of the Asian continent, possibly due to the intensification of the East Asian Summer Monsoon (EASM), which may have increased the net flux of nutrients to the ocean, both directly through terrestrial input and indirectly through upwelling activity. A drop in Ba/Ti, Al/Ti and Ca/Ti values around 6 Ma may indicate a lowering of productivity, possibly due to the large consumption of sea surface nutrients by the "biogenic bloom". Alternatively, it may indicate a shift in terrigenous input source area. At about 5.4 Ma, a decrease in weathering intensity, as indicated by a sudden decrease in the values of smectite/(illite + chlorite), smectite/quartz and Al/K, might have led to a sudden decrease of terrestrial nutrient input to the SCS. We suggest that the biogenic bloom ended when nutrients in surface waters were exhausted, because of a decrease in supply as well as a decrease in upwelling intensity due to weakening of the EASM. As a result, radiolarians were absent in the studied area between ~6 and 3.2 Ma. At ~3.2 Ma, radiolarians began to recover, possibly because the start of Northern Hemispheric glaciation and the rapid uplift of the Tibet Plateau led to intensification of the East Asian monsoon. After the Mid-Pleistocene Climate Transition at 0.9 Ma, the abundance and mass accumulation rates of radiolarians increased, probably as a result of increased upwelling activity driven by the increasing intensity of the summer monsoon.

Granulometry, mineralogy and geochemistry of ODP Holes 184-1147A and 184-1148A, South China Sea

Green clay layers are reported from the Pliocene-Holocene intervals in five of the six sites drilled in the South China Sea (SCS) during Leg 184. Centimeter-scale discrete, discontinuous, and bioturbated layers, constituted by stiff and porous green clays, were observed, sometimes associated with iron sulfides and pyrite. Detailed mineralogical and geochemical analyses indicate that they differentiate from the host sediments in their higher content of iron, smectite, and mixed-layered clays and lower amounts of calcite, authigenic phosphorus, quartz, and organic matter. Although no glauconite was observed, the mineralogy and geochemistry of green clay layers, along with their geometrical relation to background sediments, suggest that they most likely represent the result of the first steps of glauconitization. Correlation between green layers and volcanic ash layers was suggested for green laminae observed elsewhere in Pacific sediments but was not confirmed at SCS sites. Statistical analysis of the temporal distribution of green layers in the records of the last million years suggests that green clay layers have become more frequent since 600 ka. Only at Site 1148 does the green layer record show a statistically significant cyclicity which may be related to orbital eccentricity. A possible influence of sea level variations, related both to climatic changes and tectonism, is postulated.

Stable isotopes of benthic foraminifera from Site ODP 184-1146

We generated a high resolution (~8 ky) benthic record from a West Pacific marginal basin to investigate the detailed structure and spectral characteristics of deep water isotope fluctuations during the middle Miocene. The benthic record from ODP Site 1146 allows unprecedented resolution of the structure of the middle Miocene delta13C excursion, as well as tighter control on the chronology of climatic events. Spectral analysis of the variance in the delta18O and delta13C records from ODP Site 1146 reveals spectral power concentrated in the eccentricity band (400-, ~100-ky) over the time interval between 13 and 17 Ma. The amplitude evolution in the 400-ky band is strikingly similar to that of the long eccentricity in Laskar's solution. There is an abrupt switch to the obliquity band in the delta18O record at -14.9 Ma, suggesting a shift in the ocean/climate response to orbital forcing (from low latitude eccentricity to high latitude obliquity forcing). The obliquity signal is pervasive in the delta18O record until -13.9 Ma, when a sharp increase in delta18O values indicates a major climatic transition. Comparison of delta18O and delta13C profiles from DSDP Site 588 (SW Pacific Ocean), ODP Site 761 (E Indian Ocean) and ODP Site 1146 (South China Sea) reveals significantly cooler deep water in the NE Indian Ocean throughout the middle Miocene and a restricted deep water exchange between the Pacific Ocean and Indian Ocean.