Xianghui Li

Upper Cretaceous carbon- and oxygen-isotope stratigraphy of hemipelagic carbonate facies from southern Tibet, China

A high-resolution carbon-isotope curve derived from Upper Cretaceous hemipelagic sediments cropping out at Tingri, southern Tibet, shows similarities to patterns established on other continents, notably in the presence of a well-defined positive excursion across the Cenomanian–Turonian boundary where δ13C values exceed 3.5‰. From the upper Turonian to the lower Campanian, δ13C values generally decline, apart from a minor positive excursion in the middle Coniacian: a trend that departs from that recorded from Europe. Relatively low δ13C values (c. 1‰) at the Santonian–Campanian and Campanian–Maastrichtian boundaries in Tibet define a prominent broad positive excursion centred in the middle Campanian and terminated by an abrupt fall towards the close of the stage. When compared with data from Europe and North Africa, the δ13C values of the Tibetan section are generally lower by c. 1.5‰, except for the middle Campanian positive excursion where values (δ13C c. 2‰) are comparable with those documented from Europe and North Africa. These differences are interpreted as reflecting variable mixing of water masses carrying different carbon-isotope signatures, such that areas close to the major sinks of marine organic carbon recorded higher δ13C values than those located in more distal regions. Oxygen-isotope ratios, albeit affected by diagenesis, may record a palaeotemperature signal.

Carbon isotope signatures of pedogenic carbonates from SE China: rapid atmospheric pCO2 changes during middle–late Early Cretaceous time

Lower Cretaceous pedogenic carbonates exposed in SE China have been dated by U–Pb isotopen measurements on single zircons taken from intercalated volcanic rocks, and the agesn integrated with existing stratigraphy. δ 13 C values ofn calcretes range from –7.0‰ to –3.0‰ and can be grouped into five episodes ofn increasing–decreasing values. The carbon isotope proxy derived from these palaeosoln carbonates suggests p CO 2 mostly in the range 1000–2000 partsn per million by volume (ppmV) at S ( z ) (CO 2 contributed by soil respiration) = 2500 ppmV and 25°C during the Hauterivian–Albiann interval ( c . 30 Ma duration). Such atmospheric CO 2 levels aren 4–8 times pre-industrial values, almost double those estimated by geochemical modellingn and much higher than those established from stomatal indices in fossil plants. Rapid risesn in p CO 2 are identified for early Hauterivian, middlen Barremian, late Aptian, early Albian and middle Albian time, and rapid falls forn intervening periods. These episodic cyclic changes in p CO 2 aren not attributed to local tectonism and volcanism but rather to global changes. Then relationship between reconstructed p CO 2 and the development ofn large igneous provinces (LIPs) remains unclear, although large-scale extrusion of basaltn may well be responsible for relatively high atmospheric levels of this greenhouse gas.n Suggested levels of relatively low p CO 2 correspond in timingn to intervals of regional to global enrichment of marine carbon in sediments and negativen carbon isotope ( δ 13 C) excursions characteristic of the oceanicn anoxic events OAE1a (Selli Event), Kilian and Paquier events (constituting part of the OAEn 1b cluster) and OAE1d. Short-term episodes of high p CO 2 coincide with negligible carbon isotope excursions associated with the Faraoni Event andn the Jacob Event. Given that episodes of regional organic carbon burial would draw downn CO 2 and negative δ 13 C excursions indicate then addition of isotopically light carbon to the ocean–atmosphere system, controls on then carbon cycle in controlling p CO 2 during Early Cretaceous timen were clearly complex and made more so by atmospheric composition also being affected byn changes in silicate weathering intensity.

The Cenomanian-Turonian anoxic event in southern Tibet: A study of organic geochemistry

The Cenomanian-Turonian oceanic anoxic event (C/T OAE) is developed in southern Tibet. Organic geochemical study of the Cenomanian-Turonian sediments from the Gamba and Tingri areas shows that the mid-Cretaceous black shales in southern Tibet are enriched in organic carbon. The molecular analyses of organic matter indicate marine organic matter was derived from algae and bacteria. In the Gamba area, the organic matter is characterized by abundant tricyclic terpanes and pregane, which are predominant in 191 and 217 mass chromatograms, respectively. Pristane/phytane (Pr/Ph) ratios in the C/T OAE sediments are less than 1, demonstrating the domination of phytane. The presence of carotane can be regarded as a special biomarker indicating oxygen depletion in the C/T OAE sediments in the Tethyan Himalayas. In anoxic sediments, β-carotane and γ-carotane are very abundant. The β- and γ-carotane ratios relative to nC17 in the Cenomanian-Turonian anoxic sediments vary from 32.28 ∼42.87 and 5.10∼11.01.

Does Submillisecond Pulsar XTE J1739−285 Contain a Weak Magnetic Neutron Star or Quark Star?

The possible detection of the fastest spinning nuclear-powered pulsar XTE J1739–285, of frequency 1122 Hz (0.8913 ms), arouses us to constrain the mass and radius of its central compact object and to infer the stellar matter composition: neutrons or quarks. Spun up by the accreting materials to such a high rotating speed, the compact star should have either a small radius or short innermost stable circular orbit. By the empirical relation between the upper kilohertz quasi-periodic oscillation frequency and star spin frequency, a strong constraint on mass and radius is obtained of 1.51 M⊙ and 10.9 km, respectively, which excludes most equations of state (EOSs) of normal neutrons and strongly hints the star promisingly to be a strange quark star. Furthermore, the star magnetic field is estimated to be about 4 × 107 G < B < 109 G, which reconciles with those of millisecond radio pulsars, revealing the clues of the evolution linkage of two types of astrophysical objects.

Age of Initiation of the India‐Asia Collision in the East‐Central Himalaya: A Discussion

In their recent article, Zhu et al. (2005) provide new data on the stratigraphy and provenance of the Paleogene terrigenous strata in the Zhepure Shan Range of the Tethyan Himalayas, southern Tibet, using petrographic and geochemical whole-rock and single-grain techniques. Observed changes in the sandstone composition, according to the authors, indicate the onset of the India-Asia collision and the first development of the foreland basin immediately south of the India-Asia suture zone, which occurred at Ma. The authors pre50.6 0.2 sent valuable new geochemical data for the Paleogene clastic rocks in southern Tibet, including their source areas and tectonic affinity, which they use to interpret the geotectonic history of the Tethyan Himalaya. The aim of the present discussion is to point out several erroneous claims and conclusions made by the authors that have a considerable effect on the interpretation of regional stratigraphy, lithofacies, the closing of the Tethys seaway, and the initiation of the India-Asia collision. The authors reject most of the conclusions reached by Wang et al. (2002), which were derived from a study of the same stratigraphic sequence at the same location (Qumiba section; for details, see fig. 1). The location of the section, as given in Wang et al. (2002), was based on a Chinese military map, and the location given by Zhu et al. (2005), using a GPS locator, was not correct, but their section is the same one studied by Wang et al. (2002; see fig. 1). The main objective of the Wang et al. (2002)

Composition and sediment dispersal pattern of the Upper Triassic flysch in the eastern Himalayas, China: significance to provenance and basin analysis

The paleogeography and basin type of Upper Triassic flysch (Langjiexue Group) in the eastern Himalayan Orogen are disputed. In order to shed new light on the flysch’s origin, we applied different sedimentological methods. Assemblages of heavy minerals and clastic components of sandstones were utilized to determine the primary depositional composition. Heavy mineral indices, S/M ratios (thickness of sandstonexa0+xa0siltstone “S” versus slate/mudrock “M”), and paleocurrent data were combined to reveal the sediment dispersal pattern and the location of the source areas. In the analyzed sandstones, heavy minerals such as zircon, rutile, tourmaline, apatite, and anatase are most common, and zircon is predominant (most over 60xa0%). ZTR values range from 60 to 98xa0% and systematically increase southward. As a provenance-sensitive parameter, RuZi values vary in large magnitude and are significantly higher in both the east and west (>20xa0%) than in the center. The majority of S/M ratios decrease from north to south, suggesting an overall decrease in grain size to the south. Paleocurrent directions vary between 120° and 270° (main vector 205°, and 185° after 20° counterclockwise correction), displaying a radial-curved pattern. Variable heavy mineral assemblages indicate different sources, and the sandstones fall in the “recycled” and “mixed-arc orogeny” fields of Dickinson triplots, together supporting the view of multiple sources. Results of the ZTR values, S/M ratios, and paleocurrent directions illustrate a dispersal pattern, corresponding to a submarine fan system. The provenance and submarine fan dispersal pattern along with the basin configuration (deep basin with oceanic affinities) suggest that the Langjiexue Group accumulated in a remnant basin between Lhasa, Greater India, and Australia, where the sediments dispersed into the basin toward the developing orogen/suture zone and not away from the orogen, challenging the provenance direction for the traditional remnant basin model.