•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.

Phytoplankton biomarkers were analyzed using suspended particles collected from the northern South China Sea (SCS) during the summer cruise of 2008, with the goal of understanding the algal community structure and biomass distribution pattern in the summer season. The results indicated that the distribution of algal biomarkers in surface water of SCS was impacted and constrained by the local hydrological settings: the high biological community generally appeared in Pearl River estuaries, southwestern off Taiwan island and southeastern off Hainan Island, while the relatively lower biomass was found in the deep basin and strait areas. Diatoms were the dominant phytoplankton species, which were followed by dinoflagellates. Coccolithophorid biomass gradually increased toward the open ocean. The present work indicated that the algal biomarkers effectively documented the variability of the phytoplankton biomass and community structure as well as their linkage with the oceanic dynamics in SCS during summer 2008. This research provided not only the foundation for the application of algal lipids in the modern ocean ecosystem, but also the basis for the reconstruction of the past oceanic algal community structures.

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.

•Alkane chain length record indicates hydroclimate changes in the western SCS.•Wetter conditions prevailed 420 ka to 270 ka before shifting to drier conditions.•Extra-glacial cycle dynamics may involve long term El Niño/La Niña variations.The inert leaf wax deposited in deep sea sediment preserves clues of the past climate change. In particular, the n-alkane distribution patterns characterized by the n-alkane average chain length index (ACL) provide useful information on the hydrological changes, as longer chain alkanes are more frequently biosynthesized under relatively arid condition to prevent water loss. Our data from the western South China Sea reveal the superimposition of a systematic regime shift in the regional mean precipitation at ca. 420–270 ka on the glacial–interglacial changes in the Quaternary. The dominance of shorter n-alkane chain length between 420 ka and 270 ka is interpreted as overall wetter conditions during this period compared to the older or younger intervals. A similar timing of hydrological regime shifts has also been observed in other paleoclimate records, including major changes in productivity in the eastern Pacific and pollen records from the western Pacific. We suggest that the changes in the regional mean hydroclimate states near 420 ka and 270 ka are most likely related to a regime change in the frequency and/or intensity of the El Niño-Southern Oscillation, with the overall wetter hydroclimate prior to 270 ka resulting in a more prevalent La Niña-like condition in the Pacific.