The mantle plume process is thought to be the prevailing dynamic mechanism for the South China Sea opening, but controversy persists due to the lack of critical evidence of magma in the initial ...seafloor spreading. International Ocean Discovery Program (IODP) Expedition 367 successfully recovered at Site U1500 the mid-ocean ridge basalt (MORB) representing the magma activity of the initial spreading of the South China Sea during the earliest Oligocene. Here we present the whole-rock and olivine phenocryst geochemistry of the basalts to constrain the potential influence of the Hainan mantle plume on the evolution of the South China Sea. Major and trace elemental compositions indicate that the basalts were mainly influenced by fractional crystallization of olivine and formed by melting of a spinel peridotite source without any pyroxenite in mantle source. The calculated mantle potential temperature of those most primitive basalts is much lower than plume-related MORB of Iceland, but similar to normal MORB elsewhere. Both lithological composition and mantle potential temperature clearly contradict with the mantle plume model, signifying that the mantle plume didn't exist at the earliest Oligocene. Therefore, the initial spreading of the South China Sea should be caused by non-plume processes, most likely by the westward subduction of the Pacific Plate.
Kerogen exhumed on Earth's surface is subject to oxidation and microbial utilization and is also remobilized and reburied into new geological strata. In the former case, carbon is introduced to the ...atmosphere and biosphere, while in the latter, neither is carbon released, nor is oxygen consumed. Therefore, kerogen exhumation and its entailing fate contributes to the balance of atmospheric CO2 and O2 over geological timescales as well as the overall geochemical fingerprint of sedimentary organic matter containing both biospheric and reburied kerogen components. Until now, quantitative constraints on kerogen reburial have been largely limited to the Holocene. Here, we propose the usage of illite and chlorite abundance for quantitatively constraining kerogen reburial in the geologic past. Orogenic source terrains typically shed this lithoclast assemblage, which shows a strongly linear relationship between kerogen and illite+chlorite content in marine foreland basin sediments as observed for the Taiwan Orogeny. By applying this relationship to sedimentary archives of orogenic foreland basins, we envision that quantitative changes in organic carbon cycling can be inferred from the clay mineral record.
Siliciclastic grain size, clay mineralogy, and major element geochemistry of Core MD05–2893 from the lower continental slope off the Sunda Shelf in the southern South China Sea were investigated to ...assess the response of terrigenous sediment input to sea-level and climatic changes since the last deglaciation. The chronology is based on foraminiferal AMS 14C dates and oxygen isotope records combined with carbonate stratigraphy. In this study, we reconstruct variations in terrigenous sediment input using clay/silt, TiO2/CaO, SiO2/Al2O3, Al2O3/K2O, and smectite/(illite + chlorite) proxies. Clay/silt, TiO2/CaO, and SiO2/Al2O3 ratios are clearly correlated to sea-level change, showing high values during the early phase of deglacial sea-level rise (17.2–14.5 cal ka BP, Stage I) and rapidly decreasing values during the meltwater pulse 1A and further sea-level rise (14.5–11.1 cal ka BP, Stage II). Meanwhile, variations in Al2O3/K2O and smectite/(illite + chlorite) correspond well to monsoon rainfall variability during the late phase of deglacial sea-level rise and Holocene sea-level highstand (11.1–1.5 cal ka BP, Stage III). Based on these records, we recognize three stages of evolution of terrigenous sediment input to the southern South China Sea continental slope. Dominance of terrigenous sediment input occurred during Stage I, when most part of the Sunda Shelf was still exposed and drained by several rivers. The stepwise elevated sea level by meltwater pulse 1A and subsequent rising sea-level after 14.5 cal ka BP could have led to a drop in terrigenous sediment input to this area due to regression of the paleo-Sunda river systems (Stage II). Thereafter during last phase of sea-level rise and sea-level highstand with the evolution of the modern coast, sediments have a clear fingerprint of chemical weathering induced by a strengthening of the East Asian summer monsoon (Stage III). Our findings highlight that the variability of terrigenous sediment input to the lower continental slope in the southern South China Sea was first in the late Pleistocene mainly driven by sea-level change and later in the Holocene by a strengthened East Asian summer monsoon.
•Multiple sources have contributed terrigenous sediments to the southern South China Sea.•Higher terrigenous sediment input to the South China Sea occurred during the sea-level lowstand.•Terrigenous input has been controlled by sea-level change and East Asian monsoon evolution.
To obtain the optimal processing parameters of ultrasonic roll extrusion, 42CrMo bearing steel was taken as the research object, and the orthogonal test method was used to design an ultrasonic roll ...extrusion experiment with spindle speed, feed speed, static pressure and amplitude as parameters. Based on the orthogonal test data, the prediction models of surface roughness, surface residual stress and surface hardness were established by a multiple regression method, and the reliability of the model was verified. An algorithm combining SPEA2 and the shift density estimation strategy (SPEA2SDE) was introduced. The performance of the SPEA2SDE algorithm, NSGA II algorithm and SPEA2 algorithm is tested and compared on a three-dimensional test function set to verify its effectiveness. The SPEA2SDE algorithm are used to solve the multi-objective optimization model to obtain the optimal combination of processing parameters, and the ultrasonic roll extrusion experiment is carried out. The research results show that the surface roughness, surface residual stress and surface hardness optimized by the SPEA2SDE algorithm are in good agreement with the experimental values, and the average error is controlled within 10%, which shows that the algorithm can achieve high precision. It can effectively solve the multi-objective optimization problem of ultrasonic roll extrusion process parameters and can be used to guide actual production machining.
The detrital component of marine sediment is a powerful recorder of paleoenvironmental changes in a marginal sea such as the South China Sea. This is in particular valid for the magnetic fraction ...that is one of the key parameters for paleoenvironmental studies in the South China Sea, although poorly used so far. We report here on the analysis of the magnetic properties of a 50 m-long sedimentary sequence retrieved from the northern South China Sea, on the continental slope off the Pearl River mouth. Magnetic minerals with different coercivities (magnetite, pyrrhotite, and hematite) are mixed. The variations in relative content of these magnetic minerals illustrate influences of various external forcing mechanisms at different timescales. The pyrrhotite content exhibits a long-term increase, which is also observed in illite + chlorite content, indicating a continuous enhancement of supply from Taiwan most likely related to active Taiwan orogeny. Glacial–interglacial fluctuations are characterized by more magnetite and pyrrhotite with coarser silt and magnetic grains during glacials than interglacials. This is attributed to sea-level changes with the enormous continental shelf exposed during glacials, in turn affecting the sediment transport distance and pathway. On a shorter timescale, larger hematite inputs in fine-grained sediments coincide with precession minima. We suggest that this periodic hematite supply change is related to the eolian dust deposited at the studied site in addition to the fluvial and oceanic transported materials.
•Mixed magnetic mineralogy at the studied site from northern South China Sea.•Supply from Taiwan increased over the last 400 ka most likely due to active orogeny.•Sea-level changes control the terrigenous flux on glacial–interglacial scales.•Higher hematite content illustrates weak East Asian summer monsoon on precession band.
The surface uplift history of the Tibetan Plateau and Himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during Cenozoic time, its ...influence on monsoon intensity, and its reflection of the dynamics of continental plateaus. Models of plateau growth vary in time, from pre-India-Asia collision (e.g., almost equal to100 Ma ago) to gradual uplift after the India-Asia collision (e.g., almost equal to55 Ma ago) and to more recent abrupt uplift (<7 Ma ago), and vary in space, from northward stepwise growth of topography to simultaneous surface uplift across the plateau. Here, we improve that understanding by presenting geologic and geophysical data from north-central Tibet, including magnetostratigraphy, sedimentology, paleocurrent measurements, and ⁴⁰Ar/³⁹Ar and fission-track studies, to show that the central plateau was elevated by 40 Ma ago. Regions south and north of the central plateau gained elevation significantly later. During Eocene time, the northern boundary of the protoplateau was in the region of the Tanggula Shan. Elevation gain started in pre-Eocene time in the Lhasa and Qiangtang terranes and expanded throughout the Neogene toward its present southern and northern margins in the Himalaya and Qilian Shan.
IODP Hole U1502B penetrates >180 m into the crystalline basement generated at a rifting margin of the South China Sea (SCS), which is the first confirmed intermediate-type margin between the ...magma-rich and -poor endmembers. The recovered lavas show petrographic characteristics of mid-ocean ridge basalt (MORB), but suffered pervasively from hydrothermal alteration. This sequence represents the oldest SCS oceanic crust ever drilled in-situ, and offers a globally unique window to explore the hydrothermal processes during continental breakup. Here, 50 whole-rock samples representative of Hole U1502B were analyzed for major and trace elements and Sr-Nd isotopes, presenting the first report of hydrothermally altered basalts for the SCS. The protolith appears to be tholeiite, enriched MORB, and little affected by crustal contamination due to the constant mantle-values of εNd. However, the altered rocks are characterized by significant Ca depletion and 87Sr/86Sr modification. Major processes are identified to be tightly involved with plagioclase: chloritization and albitization. Both reactions are responsible for the Ca-loss and Sr-mobility, and for the resultant Mg- and Na-uptakes, respectively. Environments varying from the peripheral to the deep parts within a discharge zone are evidenced by the co-existence of hydro-fracturing brecciation, enrichments of base metals (Zn, Cu), high water/rock ratios (~1–25), and lower greenschist facies alteration (albite + chlorite + epidote ± quartz ± pyrite). This variability can be attributed to detachment-related faulting, allowing deeply channeled pathways of fluids. Besides, such tectonic effects permit a penetration of Hole U1502B into the porosity boundary of seismic layer 2A/2B, corresponding to either 1) the lithologic lava–dike transition and/or 2) an alteration front of hydrothermal circulation. In any case(s), our results imply a more complex and longer SCS rifting than previously thought.
High-resolution major element geochemistry of Core MD05–2892 from the lower Sunda Slope was analyzed to investigate the variation of chemical weathering records and its controlling factors in the ...southern South China Sea since the last glaciation. Chemical index of alteration (CIA) and SiO2/Na2O ratio were selected as chemical weathering proxies. The values of the proxies are higher during the last glaciation than during the Holocene. We attribute this to the provenance shift caused by sea level change. Provenance analysis indicates that the Malay Peninsula and Sumatra were the major sediment sources for the study region during the last glaciation, while the Indochina Peninsula has been the major source since the early Holocene. Accordingly, the weathering evolution analysis reveals increased physical erosion in the Malay Peninsula and Sumatra during enhanced East Asian summer monsoon rainfall, while increased chemical weathering in the Indochina Peninsula during the intensified monsoon rainfall. Therefore, the chemical weathering records of the deep-sea sediments in the southern South China Sea were regulated by monsoon rainfall-driven physical erosion in the Malay Peninsula and Sumatra during the last glaciation, while the records have been influenced by monsoon rainfall-induced chemical weathering in the Indochina Peninsula since the early Holocene. This study emphasizes that higher weathering values of the deep-sea sediments during the last glaciation could be related to the provenance variation driven by sea level change, rather than to enhanced chemical weathering in the same provenance.
•Provenance of sediments off the Sunda Shelf has changed from glaciation to Holocene.•Intensive chemical weathering occurred on Malay Peninsula/Sumatra during glaciation.•Sea level and East Asian monsoon have jointly affected deep-sea weathering records.
Low-level viremia generally refers to detectable HBV DNA levels lower than 2000 IU/mL. Studies show that low-level viremia is a risk factor for hepatocellular carcinoma. The aim of this study was to ...explore the characteristics of low-level viremia patients with hepatitis B-related hepatocellular carcinoma and identify prognostic factors after curative hepatectomy.
Data from chronic hepatitis B patients with hepatocellular carcinoma receiving curative hepatectomy for the first time in the first hospital of China Medical University were studied. Patients were divided into two groups based on preoperative HBV DNA levels: group 1 (low-level viremia group, HBV DNA < 2000 IU/mL) and group 2 (HBV DNA ≥ 2000 IU/mL).
Of the 212 patients, 104 patients were in group 1 and 108 patients were in group 2. There was a lower proportion of patients with HBsAg levels > 250 IU/mL (the upper limit of detection in our laboratory) in group 1 than in group 2 (71.2% vs. 86.1%, P < 0.01). The percentage of patients with a tumor diameter < 5 cm was 67.3% in group 1 and 37.0% in group 2 (P < 0.000). The percentage of tumor recurrence was 40.4% (42) in group 1 and 54.6% (59) in group 2 (P < 0.05). Median recurrence-free survival was 30.1 months in group 1 and 17.6 months in group 2 (P < 0.01). Multivariate analysis showed that a tumor diameter ≥ 5 cm (hazard ratio HR = 1.819, 95% confidence interval CI 1.193-2.775, P = 0.005), intrahepatic metastasis (HR = 1.916, 95% CI 1.077-3.407, P = 0.027), and an HBV DNA level ≥ 100 IU/mL (the lower limit of detection in our laboratory, HR = 2.943, 95% CI 1.916-4.520, P < 0.000) were independent prognostic factors associated with an increased risk of hepatocellular carcinoma recurrence.
Preoperative low-level viremia was related with a long tumor recurrence interval and complete virologic response after curative hepatectomy was associated with a lower risk of hepatocellular carcinoma recurrence.
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