Chemical and isotopic composition of interstitial waters and hydrocarbon composition at ODP Site 184-1146

Bottom-simulating reflectors were observed beneath the southeastern slope of the Dongsha Islands in the South China Sea, raising the potential for the presence of gas hydrate in the area. We have analyzed the chemical and isotopic compositions of interstitial water, headspace gas, and authigenic siderite concretions from Site 1146. Geochemical anomalies, including a slight decrease of chlorine concentration in interstitial water, substantial increase of methane concentration in headspace gas, and 18O enrichment in the authigenic siderite concretion below 400 meters below seafloor are probably caused by the decomposition of gas hydrate. The low-chlorine pore fluids contain higher molecular-weight hydrocarbons and probably migrate to Site 1146 along faults or bedded planes.

Benthic foraminiferal faunas of Miocene to Holocene sediments of ODP Site 184-1143 from the southern South China Sea

Deep-sea benthic foraminiferal assemblages from Ocean Drilling Program (ODP) Site 1143 located in the southern South China Sea (SCS) were investigated to evaluate the relationship between faunal composition patterns and paleoceanographic changes during the last 6 million years (late Miocene to Holocene). We used multivariate statistics (correspondence analysis) to analyze carbon-flux-related changes in assemblage composition of benthic foraminifers. Additional proxies for carbon flux and deep-water ventilation include delta13C records of epifaunal Cibicidoides wuellerstorfi and infaunal Uvigerina peregrina var. dirupta and Melonis pompilioides, benthic foraminiferal accumulation rates (BFARs), diversity indices, and relative abundances of indicator species. We observe three significant benthic faunal changes in the southern South China Sea during the last 6 million years. Strong fluctuations in BFAR and relative abundance of productivity indicator species between glacial and interglacial stages after the mid-Pleistocene revolution (MPR) at approximately 0.9 Ma, indicating stronger seasonal carbon flux fluctuations, are accompanied by the extinction of such species as Stilostomella spp. Increases in carbon flux indicator species are coupled with an overall decrease in benthic foraminifer diversity around 3.0 Ma in the late Pliocene. This may indicate increasing carbon flux in a period of productivity maximum caused by enhanced offshore upwelling from intensified winter monsoon wind strength.

Selected age-diagnostic datums of ODP Site 184-1148

The 853 m thick sediment sequence recovered at ODP Site 1148 provides an unprecedented record of tectonic and paleoceanographic evolution in the South China Sea over the past 33 Ma. Litho-, bio-, and chemo-stratigraphic studies helped identify six periods of changes marking the major steps of the South China Sea geohistory. Rapid deposition with sedimentation rates of 60 m/Ma or more characterized the early Oligocene rifting. Several unconformities from the slumped unit between 457 and 495 mcd together erased about 3 Ma late Oligocene record, providing solid evidence of tectonic transition from rifting/slow spreading to rapid spreading in the South China Sea. Slow sedimentation of ~20-30 m/Ma signifies stable seafloor spreading in the early Miocene. Dissolution may have affected the completeness of Miocene-Pleistocene succession with short-term hiatuses beyond current biostratigraphical resolution. Five major dissolution events, D-1 to D-5, characterize the stepwise development of deep water masses in close association to post-Oligocene South China Sea basin transformation. The concurrence of local and global dissolution events in the Miocene and Pliocene suggests climatic forcing as the main mechanism causing deep water circulation changes concomitantly in world oceans and in marginal seas. A return of high sedimentation rate of 60 m/Ma to the late Pliocene and Pleistocene South China Sea was caused by intensified down-slope transport due to frequent sea level fluctuations and exposure of a large shelf area during sea level low-stands. The six paleoceanographic stages, respectively corresponding to rifting (~33-28.5 Ma), changing spreading southward (28.5-23 Ma), stable spreading to end of spreading (23-15 Ma), post-spreading balance (15-9 Ma), further modification and monsoon influence (9-5 Ma), and glacial prevalence (5-0 Ma), had transformed the South China Sea from a series of deep grabens to a rapidly expanding open gulf and finally to a semi-enclosed marginal sea in the past 33 Ma.

Mineral XRD peak areas and ratios of ODP Site 184-1146 clays, South China Sea

Site 1146 (19°27.40'N, 116°16.37'E) was drilled in ~2092 m water depth in a rift basin on the continental slope of the South China Sea. A total of 607 m of sediment was cored in Hole 1146A, and a composite section from three holes extends down to 640 meters composite depth (mcd). Three stratigraphic sedimentary units were recognized at this site: late Pliocene to Pleistocene nannofossil clay (Unit I), middle Miocene to late Pliocene foraminifer and nannofossil clay mixed sediment (Unit II), and early to middle Miocene nannofossil clay (Unit III). This study reports the mineralogy from the late Miocene through early Pleistocene, 150-440 mcd.

Pollen ages and depths in ODP Site 184-1144

A high-resolution pollen record (sampling interval averages 820 years) has been obtained from ODP Site 1144 (water depth 2037 m), northern South China Sea. The 504-m sequence (in composition length) covers the last 1.03 million years according to micropaleontological and isotopic stratigraphy. The pollen assemblages are characterized by high proportions of Pinus and herb pollen, and by their frequent alternations. Based on these alternations, 29 pollen zones have been recognized that are closely correlated to the Marine Oxygen Isotope Stages (MIS) 1-29. Pinus- dominant pollen zones correspond to interglacial periods with lighter delta18O values, while herb-marked ones relate to the heavier delta18O stages assigned to glacials. Judging from the pollen data, the exposed northern continental shelf of the South China Sea during the glacials was covered by grassland, and the extensive northern shelf has formed only since MIS 6 (ca. 150 ka), probably as a result of tectonic subsidence. Tree pollen influx values are indicative of winter monsoon which began to intensify 600 ka ago. The summer monsoon variations can be approximated by the fern percentage within the total pollen and spore abundance, and the result shows high values in general occurring at interglacials, with the maxima at MIS 15, 5e and 1. The relatively high fern percentage with smaller amplitude in variations before 600 ka might suggest more stable humid conditions before the intensification of winter monsoon.