•The littoral sandstone sequence from IODP Hole U1435A formed in the middle to late Eocene•Mesozoic felsic rocks as the main source to deposition along the South China margin•Intensive rifting centered around 42 Ma before the breakup of the South China SeaThe pre-Oligocene sediment succession including shipboard lithological Units II (77.65–275.54 mbsf) and III (275.54–300 mbsf) from International Ocean Discovery Program (IODP) Hole U1435A in the northern South China Sea is characterized by greenish to dark gray sandstone and siltstone of coastal marine facies with age previously undetermined due to a lack of age-diagnostic fossils. Detrital zircon UPb ages (9 samples) and thin sections (76 samples) from these two units, together with geochemical elements (197 samples) from Units I to III were analyzed to distinguish provenance, tectonic setting, and depositional age and environment. Petrographic study reveals that most samples are fine sandstones, mainly composed of subangular quartz (70–80%) and alkaline feldspar (10–15%; mostly K-feldspar), indicating a near proximal provenance. Discrimination diagrams, element ratios and chondrite-normalized rare earth element patterns suggest a relatively stable source of felsic rocks. Detrital zircon UPb dating results reveal a dominance of Mesozoic ages with a pronounced Early Cretaceous peak at ~ 110 Ma. Fourteen zircon grains from 7 samples yield much younger ages between ~ 65 and 38 Ma, indicating middle to late Eocene deposition, broadly corresponding to the lacustrine-shallow neritic Enping-Wenchang Formations of the Pearl River Mouth Basin. Comparison among various elemental proxies from the Eocene sediments in Hole U1435A and in offshore industrial wells confirms a continental island arc setting in the Cretaceous when the zircons were produced. In the middle and late Eocene, these Cretaceous igneous rocks were likely the source of the sediment deposited in a coastal marine environment. The relatively minor variations in elemental distribution and the concentration of Paleogene zircons (~ 50–38 Ma) further imply a consistent supply of weathered material from this magmatic landmass during an active rifting period before the opening of the South China Sea.

•Strong marine signal influence on br GDGT based proxies during interglacials.•CBT-derived pH during glacial periods reflected surrounding precipitation.•Connection between Antarctic and tropic oceans on different timescales observed.Two proxies derived from branched glycerol dialkyl glycerol tetraethers (br GDGTs), the methylation index of branched tetraethers (MBT) and the cyclization ratio of branched tetraethers (CBT), are often used to reconstruct paleo mean annual air temperature (MAAT) and soil pH based on the premise that br GDGTs in the marine environment are mainly of terrigenous origin. However, mounting evidence indicates that br GDGTs can be produced in situ in oceanic settings, which may affect MAAT reconstruction and the use of other related paleoenvironmental proxies. We have determined br GDGT distributions in a sedimentary core (MD05-2896/7) from the southern South China Sea, which provided high resolution profiles of MBT and CBT indices as well as the branched and isoprenoid tetraether (BIT) index. BIT varied systematically with glacial–interglacial cycles, reaching much lower (< 0.1) values during the interglacial periods (MIS 1 and MIS 5) than during the glacial periods (MIS 2, MIS 3, MIS 4 and MIS 6). MBT/CBT-derived temperature showed, on the other hand, lower values during the interglacial periods but higher values during glacial periods. We hypothesize that the lower MBT/CBT-derived temperature during interglacial periods reflects bottom water temperature registered via br GDGTs produced under marine conditions, whereas the higher MBT/CBT derived temperature during glacial periods reflects terrestrial MAAT because of the overwhelming input of br GDGTs from land when the sea level was low. Similarly, the CBT-derived soil pH appeared to have been overprinted by marine br GDGT production during interglacial periods but responded to precipitation on land during glacial periods, showing patterns similar to, or as a positive response to, the southern hemispheric climate oscillation due to teleconnection. Our results demonstrate an unprecedented pattern of MBT/CBT variation constrained by glacial–interglacial cycles in the South China Sea. Under this constraint, MBT/CBT revealed deep water production of br GDGTs during interglacial periods and recorded changes in paleohydrology on land during glacial periods, providing a new perspective for paleoclimate studies using organic proxies.

We have analyzed core MD01-2392, ∼ 360 km east of the Mekong River mouth in the South China Sea (SCS). Over the past 500 ka, planktonic foraminiferal oxygen-isotopic values are consistently lighter than northern SCS and open-ocean records by up to 0.5‰, indicating the influence of run-off from the Mekong River during both glacial and interglacial periods. Carbonate content is higher during interglacials; sedimentation rates were higher during glacials. Increased sedimentation rates since 30 ka imply increased run-off during the last glacial maximum and Holocene Period. Contrary to general experience, in which it is classed as a warm species for temperature estimates, the thermocline-dwelling species Pulleniatina obliquiloculata increased its numbers during glacial periods. This implies an estuarine circulation and even brackish-water caps during glacial periods, reinforcing the sense of strong run-off. In an overall decline of warm water, the thermocline shoaled stepwise, with rapid rises across the glacial terminations. We infer that the southern SCS was opened to an influx of Indian Ocean waters through southern passages at those times of rising sea levels.

Based on core-top calibration, the TEX 86H-derived temperature has been considered as representing subsurface sea temperature (SSST), and the difference between the U37K′-derived sea-surface temperature (SST) and the TEX 86H-derived SSST can be used to reflect the depth of thermocline (DOT) in the South China Sea region (Jia et al., 2012). We evaluated the DOT dynamics in late Quaternary records using this approach on paired analysis of samples from core MD05-2896/7 in the southern South China Sea. The reconstructed DOT over the last 180,000 yr (180 ka) displays a shoaling trend in glacial periods, which may be attributed to the strengthened cyclonic gyre by the enhanced East Asian winter monsoon and Walker circulation with prominent La Niña-like state, and vice versa in interglacial periods corresponding to reduced winter monsoon with enhanced El Niño-like state. These upper-water thermal variations testify that enhanced winter monsoon was the direct cause of an uplifted local thermocline during glacial or La Niña-like states with strengthened cyclonic gyre due to positive wind stress curl in the southern South China Sea. Our results provide insights into the relationship between monsoon and ENSO on both glacial and millennial time scales.

Four algal biomarkers, brassicasterol, C30-diol/keto-ol, dinosterol and C37-alkenone, representing diatoms, estigmatophytes, dinoflagellates and coccolithophorids, respectively, were detected in samples collected during two South China Sea cruises to study the modern phytoplankton community structure in the region. For the first time, the basin-wide distribution of these phytoplankton algal biomarkers in the sea surface water during two intermonsoon seasons (spring 2010 and autumn 2011) is documented and charted. Generally, the abundance of the biomass is higher in spring than autumn, with high productivity mostly in the regions of Pearl River estuary, off Palawan and around the Luzon Strait, showing the abundance order: diatoms>estigmatophytes>dinoflagellates>coccolithophorids. We run both redundancy analysis (RDA) and SPSS correlation analysis to interpret the relationship between individual groups and environmental variables. The results indicate that temperature and salinity play a dominant role in controlling the distribution of phytoplankton in these intermonsoon seasons, followed by nitrate playing a secondary role. Our biomarker survey provides important reference data for interpreting paleo-productivity in the geological records in the SCS.

Marine algal-derived lipid biomarkers (alkenones, brassicasterol, dinosterol, and long-chain diol/keto-ol representing haptophytes, diatoms, dinoflagellates, and eustigmatophytes, respectively) were used to evaluate the phytoplankton productivity and community structure changes in core MD05-2901 from the western South China Sea, which features distinct summer upwelling induced by southwest Asian monsoon. The results revealed substantial differences in the distribution patterns between the four major marine primary producers. Diatom and dinoflagellate biomarkers displayed slightly higher abundances, mostly in interglacials especially after MIS 8, while alkenones exhibited lower values in MIS 12 and MIS 1, with higher values in between especially in the middle of MIS 7, but eustigmatophytes increased in most glaciations, indicating complex responses of different phytoplanktons to paleoclimatic and paleoenvironmental changes over the past 450 ka. The sum of the four phytoplanktons shows subtle glacial–interglacial patterns, probably reflecting the combined hydrological dynamics driven by enhanced summer monsoon during summer/interglacials and enhanced winter monsoon during winter/glacials in the region. The biomarker-based community structure showed relative high contribution from diatoms and dinoflagellates during interglacials, high contribution in the middle part of the section centered at ~210 ka from the coccolithophorids, but varying levels from the eustigmatophytes with high percentages in most glacials. Diatoms show strong nutrient sensitivity and positive relation with other paleo-proxies, and their enrichments during interglacials can be attributed to enhanced nutrient level induced by the East Asian summer monsoon, which could have been coupled with the influence of the global ice volume, the summer insolation and the Southern Hemisphere latent heat.

•High resolution record (∼3 ka) of the terrestrial plant wax alkanes over 5 Ma.•The tightly relationship between alkane chain length and regional wet condition.•New evidence for hydro-climate change in SE Asian.•Implications of the low latitude hydrological fluctuation on global climate change.High resolution records of long chain n-alkane biomarkers from the southern South China Sea reveal tightly response of n-alkane distribution to hydro-climate changes over the past 5 Ma, with increasing longer chain n-C31 alkanes indicating a correlation with drier conditions and increasing shorter chain n-C27 alkanes with wetter conditions. The variations of the C31/C27 ratio, or the alkane chain length index, imply humid conditions before 2.9 Ma, progressively reduced moisture since then and to bigger fluctuations between wet and dry conditions since 1.2 Ma. This long term hydro-climate trend is superimposed by glacial dry and interglacial wet patterns over the Plio-Pleistocene glacial cycles. Combined with other proxy records, our results indicate that precipitation over the tropical Asia-Pacific strengthened before the onset of the northern hemisphere glaciation and the mid-Pleistocene climate transition at about 1.2 Ma. These dramatic humidity changes over major climate transitions imply a crucial role of tropical hydrology dynamics on global climate change in the late Cenozoic.

•Carbonate platform first established in the early Miocene•Active platform growth in the Middle Miocene but severe reduction afterwards•Regional subsidence and cooling affected Late Miocene and younger reef buildupsThe variations of lithostratigraphy, foraminifera, geochemical elements, and organic biomarkers in well XK1 from the Xisha (Paracel) Islands reveal the history of carbonate platform development since the early Miocene in the South China Sea. Our results indicate that reef growth was more active in the middle Miocene than in any earlier periods, decreased in the late Miocene, revitalized in the Pliocene, and active again in the Pleistocene interglacial periods. Unlike those limestone-rich intervals, the late Miocene succession in well XK1 is characterized by dolomitized limestones with a low deposition rate (30–60 m/Ma post-compaction). These late Miocene rocks also contain relative high MgO (~ 20%), more ∑ REE, abundant algae and some small sized foraminifera, but rare larger benthic foraminifera, very low land-sourced brGDGTs (0–0.5 ng/20 g), low BIT index (0.1–0.5), and low TEX86-derived sea temperature (~ 20–25 °C), indicating limited active reef growth in relative cooler mesophotic environments at > 30 m water depths. Similar dolomitized limestones of late Miocene age also occur in other carbonate platform areas including the Nansha (Spratly) Islands in the southern South China Sea. Together, these South China Sea records provide regional evidence of a recession in global carbonate platform development starting from the late Miocene. While carbonate platform reduction in other regions may have been triggered by global cooling and changing oceanic chemistry, accelerated basin subsidence coupled with monsoon cooling are considered as the main factors causing the reef decline in the late Miocene South China Sea.