Rujian Wang

Foraminiferal responses to major Pleistocene paleoceanographic changes in the southern South China Sea

A detailed age model for core 17957-2 of the southern South China Sea was developed based on delta(18)O, coarse fraction, magnetostratigraphy, and biostratigraphy for the last 1500 kyr. The delta(18)O record has clear similar to 100-kyr cycles after the Mid-Pleistocene Revolution (MPR) at the entrance of marine isotopic stage (MIS) 22. Planktonic foraminifera responded to the MPR immediately, showing the increased sea surface temperature (SST) and dissolution after the MPR. Benthic foraminifera did not respond to it until the Brunhes/Matuyama boundary. Since the MPR, the depth of thermocline gradually became shallower until MISs 6-5. This major change within MISs 6-5 was also reflected in the decreased SSTs and increased productivity and Deep Water Mass. Thus two major Pleistocene paleoceanographic changes were found: One was around the MPR; the other occurred within MISs 6-5, which speculatively might be ascribed to the reorganization of surface and deep circulation, possibly induced by tectonic forces.

Development of east Asian summer monsoon environments in the late Miocene: radiolarian evidence from Site 1143 of ODP Leg 184

Abundant radiolarians are preserved in the relatively complete upper Miocene section of ODP Site 1143. Three radiolarian zones, RN6, RN7 and RN8, are recognized on the basis of Diartus petterssoni (total range in RN6, 6.7 Ma). Variations in the abundance of radiolarians, especially Pyloniid forms, radiolarian flux and species diversity are good proxies of upwelling which, similar to todays, was likely driven by summer monsoons. These radiolarian paleomonsoon proxies indicate that the east Asian summer monsoon first initiated close to the middle/late Miocene boundary at similar to 12-11 Ma and reached a maximum strength at similar to 8.24 Ma. Therefore, the initiation of the east Asian summer monsoon was likely earlier than the first Indian monsoon, which appeared similar to 8 myr ago

Radiolarian responses to paleoceanographic events of the southern South China Sea during the Pleistocene

Abstract A quantitative radiolarian study at Core 17957-2 from the South China Sea (SCS) allows reconstructing two paleoceanographic events occurring during the last 1500 ka. Based on the abundance variations of siliceous microfossils (radiolarian, diatom and sponge spicule), the definitions of the productivity and thermocline indexes (DAR, diatom accumulation rate, and TSR, thermocline surface radiolarian indexes) as indicators for productivity and thermocline changes as well as changes in the species composition two distinct changes at ∼900 ka and ∼600 ka could be detected being related to the multiple transition phenomena of the mid-Pleistocene revolution (MPR). The change in the species composition at ∼900 ka indicated by an increase in subtropical radiolarians and a decrease in tropical radiolarians exhibits a lowering of the sea surface temperatures in the SCS that can be related to a southward shift of the North Equatorial Current induced by variations in the northern trade wind system. Increasing subsurface and intermediate species, and DAR and TSR indexes at 700–600 ka may reflect higher sea surface productivity and shallowing thermocline caused by stronger upwelling and nutrient supply. The comparison of the radiolarian signal recovered from Core 17957-2 with the magnetic susceptibility signal obtained from the Chinese Loess Plateau reveals the close connection of the climatic transitions at 700–600 ka in the southern SCS to strengthening of the East Asian summer monsoon circulation. The spectral analyses indicate that ice mass fluctuations (oxygen isotope record) have clear ∼100-ka eccentricity-dominated and processional signals after the MPR, but with a more significant obliquity-related response before the MPR. The surface productivity (e.g. DAR) and thermocline depth (e.g. TSR index) show the ∼100-ka dominant cyclicity before the MPR. The ∼32-ka significant regional cyclicity for the last 1500 ka responds to the surface productivity (e.g. DAR) and thermocline depth (e.g. TSR index) signals.

Quaternary high-resolution opal record and its paleoproductivity implication at ODP Site 1143, southern South China Sea

The correlation of opal content and MAR with oxygen isotopic records of benthonic foraminifera at Site 1143, southern South China Sea indicates that, since about 900 ka, the increasing opal content and MAR during the interglacial periods is inferred to reflect the higher surface productivity, for the intensified summer monsoon during the interglacial periods would result in the enhanced upwelling and nutrient supply. Time-sequence spectral analyses of oxygen isotopic record, opal content and MAR at intervals of 0–900 ka reveal that the changes of surface productivity were dominantly forced by the variations of the earth orbital cycles.

Siliceous microplankton fluxes and seasonal variations in the central South China Sea during 1993–1995: monsoon climate and El Niño responses

Seasonal variations of radiolarian and diatom fluxes in the central South China Sea during 1993–1995 were overwhelmingly controlled by monsoon climate. Radiolarian and diatom increased obviously during the Northeast (from November to February) and Southwest (from June to September) monsoons and decreased during the periods between the monsoons. The change of circulation driven by the monsoons improved water exchange in the different areas that brought rich nutrient materials for the surface microplankton, thereby enhancing radiolarian and diatom fluxes. Variation of radiolarian flux coincided with organic carbon flux, surface primary and export productivities. High radiolarian flux corresponded to high surface primary productivity. Radiolarian and diatom fluxes raised abnormally during 1994–1995 could be attributed to the El Nino event during the period.

Abrupt variations of the radiolarian fauna at Mid-Pleistocene climate transition in the South China Sea

Based on a detailed study of the radiolarian fauna, the abundance pattern of planktic foraminifera as well as on isotope and sedimentological records, the Mid-Pleistocene climate transition as a multiple transition phenomenon could be recognized at Core 17957-2 from the South China Sea. Distinct changes in the radiolarian/foraminfera ratio, the coarse fraction and the radiolarian assemblages can be related to the global climate cooling observed at the Mid-Pleistocene revolution (MPR) around 900 ka. A pronounced southward shift of the North Equatorial Current that leads to lower sea-surface temperatures in the South China Sea is documented by the shift of tropical to subtropical radiolarian assemblages at 900 ka. Increasing radiolarian abundance after the MPR can be interpreted as a result of stronger upwelling and nutrient supply. These abrupt variations could result from the northern trade wind system and East Asian monsoon circulation.

Ice rafting history and paleoceanographic reconstructions of Core 08P23 from southern Chukchi Plateau, western Arctic Ocean since Marine Isotope Stage 3

Multiproxy investigations have been performed on Core 08P23 collected from the Chukchi Plateau, the western Arctic Ocean, during the Third Chinese National Arctic Expedition. The core was dated back to Marine Isotope Stage (MIS) 3 by a combination of Accelerator Mass Spectrometric (AMS) carbon-14 dating and regional core correlation. A total of five prominent ice-rafted detritus (IRD) events were recognized in MIS 2 and MIS 3. The IRD sources in MIS 3 are originated from vast carbonate rock outcrops of the Canadian Arctic Archipelago and clastic quartz in MIS 2 may have a Eurasian origin. Most 18O and 13C values of Neogloboquadrina pachyderma (sinistral) (Nps) in Core 08P23 are lighter than the average values of surface sediments. The lighter 18O and 13C values of Nps in the two brown layers in MIS 1 and MIS 3 were resulted from meltwater events; and those in the gray layers in MIS 3 were caused by the enhanced sea ice formation. The 18O values varied inversely with 13C in MIS 2 indicate that the study area was covered by thick sea ice or ice sheet with low temperature and little meltwater, which prevented the biological productivity and sea-atmosphere exchange, as well as water mass ventilation. The covaried light values of 18O and 13C in MIS 1 and MIS 3 were resulted from meltwater and/or brine injection.

Possible conditions and the formation time of the Chukchi Plateau pockmarks

The study of the bottom sediments accumulated during the last 60–65 ka in the pockmark craters of the Chukchi Plateau in the Arctic Ocean showed that their composition and lithostratigraphy in general are similar to those of the background areas. A specific feature is the presence of sedimentary interlayers and horizons with signs of gravitational stirring, carbonate mineralization, and accumulation under H2S contamination conditions. The micropaleontological remains found in the pockmark sediments accumulated during the glacial periods are represented by redeposited species from Paleozoic, Mesozoic, and Cenozoic deposits, which are locally exposed in the pockmark walls. It was concluded that the pockmarks were formed under the influence of pulsed fluid flows. The present-day topography of the pockmarks was formed at the last stage of active defluidization (35−20 ka ago).

Paleoenvironmental implications of Holocene long-chain n -alkanes on the northern Bering Sea Slope

The records of high-resolution terrestrial biological markers (biomarkers) from Core B2-9 from the northern Bering Sea Slope over the last 9.6 ka BP were presented in the study. Variations in input of terrestrial long-chain nalkanes (referred to as n-alkanes) and vegetation structure in their source regions were investigated. The results show that the nC27 is the main carbon peak and has the greatest contribution rate of the total n-alkane content; this might be related to the abundance of woody plants and their spatial distribution in the source region. nC23 is another n-alkane having a relatively high content; this was mainly derived from submerged plants widespread along the coastal areas in the northern hemisphere. Total n-alkane content dropped quickly at ca. 7.8 ka BP, ca. 6.7 ka BP and ca. 5.4 ka BP, and was followed by four relatively stable stages mostly controlled by sea-level rise, climate change and vegetation distribution in the source region. Variation in carbon preference index (CPI) indicates that the n-alkanes primarily originated from higher land plants, and the average chain length (ACL) and nC31/nC27 ratio reveal the relatively stable presence of woody/herbaceous plants during the Holocene, and dominate woody plants in most of the time. Simultaneous variation in total n-alkane content, nC27 content and its contribution, CPI, ACL and nC31/nC27 ratio over several short periods suggest that the growth rate of the woody plant n-alkane contribution was lower than that of herbaceous plants and fossil n-alkanes under the particular climatic conditions of the source region.

Late Quaternary Deep Stratification‐Climate Coupling in the Southern Ocean: Implications for Changes in Abyssal Carbon Storage

The Southern Ocean plays an important role in modulating Pleistocene atmospheric CO2 concentrations, but the underlying mechanisms are not yet fully understood. Here, we report the laser grain-size distribution and Mn geochemical data of a 523 kyr-long sediment record (core ANT30/P1-02 off Prydz Bay; East Antarctica) to trace past physical changes in the deep Southern Ocean. The core sediments are predominantly composed of clay and silt-sized material. Three grain size end-members (EM) as well as three sensitive grain size classes (SC) were discerned, interpreted as Ice Rafted Debris (EM1 and SC1), and coarse (EM2 and SC2) and fine (EM3, SC3) materials deposited from bottom nepheloid layers, respectively. Ratios of EM2/(EM2+EM3) and SC2/SC3 reveal changes in the local bottom current strength, which is related to the deep ocean diapycnal mixing rate, showing higher values during interglacial periods and lower values during glacial periods. MnO was enriched at each glacial termination, probably caused by abrupt elevations in Antarctic bottom water (AABW) formation rate. Lower AABW formation rate and reduced deep diapycnal mixing during glacial periods enhanced deep Southern Ocean stratification, contributing to glacial atmospheric CO2 drawdown. The elevated AABW formation and enhanced deep diapycnal mixing during glacial terminations alleviated such deep stratification, promoting deeply sequestered CO2 to outgas.