Zicheng Peng

Pb-, Sr- and Nd-isotopic systematics and chemical characteristics of Cenozoic basalts, eastern China

Abstract Forty-eight Paleogene, Neogene and Quaternary basaltic rocks from northeastern and east-central China have been analyzed for major-element composition, selected trace-element contents, and Pb, Sr and Nd isotopic systematics. The study area lies entirely within the marginal Pacific tectonic domain. Proceeding east to west from the continental margin to the interior, the basalts reveal an isotopic transition in mantle source material and/or degree of crustal interaction. In the east, many of the rocks are found to merge both chemically and isotopically with those previously reported from the Japanese and Taiwan island-arc terrains. In the west, clear evidence exists for component(s) of Late Archean continental lithosphere to be present in some samples. A major crustal structure, the Tan-Lu fault, marks the approximate boundary between continental margin and interior isotopic behaviors. Although the isotopic signature of the western basalts has characteristics of lower-crustal contamination, a subcrustal lithosphere, i.e. an attached mantle keel, is probably more likely to be the major contributor of their continental “flavor”. The transition from continental margin to interior is very pronounced for Pb isotopes, although Sr and Nd isotopes also combine to yield correlated patterns that deviate strikingly from the mid-ocean ridge basalt (MORB) and oceanic-island trends. The most distinctive chemical attribute of this continental lithosphere component is its diminished U/Pb as reflected in the Pb isotopic composition when compared to sources of MORB, oceanic-island and island-arc volcanic rocks. Somewhat diminished Sm/Nd and elevated Rb/Sr, especially in comparison to the depleted asthenospheric mantle, are also apparent from the Nd- and Sr-isotopic ratios.

The δ18O variation of a stalagmite from Qixing Cave, Guizhou Province and indicated climate change during the Holocene

A stalagmite from Qixing Cave, Guizhou Province was dated with the TIMS U-series method and its oxygen isotope composition was analyzed. On the basis of the ages and the variations of the δ18O of the stalagmite, the climate change of the last 7.7 ka has been reconstructed in this area: 7.7–5.8 ka, summer monsoon maximum period; 5.8–3.8 ka summer monsoon weakening period; 3.8–0.15 ka, weakened summer monsoon and high amplitude climate fluctuations period. We put forward that the increasing trend of δ18O of the stalagmite reflected not only the weakening of the summer monsoon, which was caused by the decreasing of solar radiation in the Northern Hemisphere, but also the possibly decreasing contribution of the southwest monsoon to the rainfall of this area. These results are consistent with the output of the numerical climate-model modeling. The high amplitude fluctuation of the δ18O may imply the quick shift of the contributions of different moisture sources to the precipitation in this area.

Mysterious abrupt carbon-14 increase in coral contributed by a comet

A large and sudden increase in radiocarbon (14C) around AD 773 are documented in coral skeletons from the South China Sea. The 14C increased by ~ 15‰ during winter, and remain elevated for more than 4 months, then increased and dropped down within two months, forming a spike of 45‰ high in late spring, followed by two smaller spikes. The 14C anomalies coincide with an historic comet collision with the Earths atmosphere on 17 January AD 773. Comas are known to have percent-levels of nitrogen by weight, and are exposed to cosmic radiation in space. Hence they may be expected to contain highly elevated 14C/12C ratios, as compared to the Earths atmosphere. The significant input of 14C by comets may have contributed to the fluctuation of 14C in the atmosphere throughout the Earths history, which should be considered carefully to better constrain the cosmic ray fluctuation.

Re-Os dating of the Raobazhai ultra mafic massif in North Dabie

The ultramafic massif at Raobazhai in North Dabie is located in the suture zone between the Yangtze craton and North China craton. The Re-Os isotope compositions of the massif are used to decipher the origin and tectonics of the ultramafic rocks involved in continental subduction and exhumation. Fifteen samples were collected from five drill holes along the main SE-NW axis of the Raobazhai massif. Major and trace element compositions of the samples show linear correlations between MgO, Yb and Al2O3. This suggests that the massif experienced partial melting with variable degrees and is from fertile to deplete in basaltic compositions. Nine selected samples were analyzed for Re-Os isotope compositions. Re contents range from 0.004 to 0.376 ng/g, Os contents from 0.695 to 3.761 ng/g,187Re/188Os ratios from 0.022 to 2.564 and187Os/188Os ratios from 0.1165 to 0.1306. These indicate that the massif is a piece of continental lithospheric mantle with variable depletion. Using the positive correlations between187Os/188Os and Yb and Al2O3 respectively, a proxy isochron age of 1.8±0.1 Ga is obtained for the Raobazhai ultramafic massif, which is interpreted to represent a fragment of the ancient subcontinental lithospheric mantle. During Triassic subduction of the Yangtze craton beneath the North China craton, part of the wedge of subcontinental lithospheric mantle was intruded into the subduction belt, and then exhumed to crustal level together with the subducted crustal plate after ultrahigh pressure metamorphism at mantle depths. This ancient lithospheric mantle is now exposed as an orogenic peridotite massif.

Boron concentration and isotopic composition of halite from experiments and salt lakes in the Qaidam Basin

Abstract The concentration and isotopic abundance of boron in salt can be used to trace paleosalinities and depositional environments for marine and non marine evaporites. However, the mechanism of incorporating boron into halite during evaporation of salt lake brines is subject to dispute, and there have been few studies of boron concentrations and isotopic compositions during this process due to the low boron concentration in halite. A group of evaporation experiments from artificial solutions and salt lake brines have been analyzed in this study. The results of boron concentration and isotopic analyses demonstrate that the boron in halite comes mainly from fluid inclusions, with a lesser amount from coprecipitation. The isotopic fractionation factors between the brine and halite are from 0.9857 to 1.0000 for the evaporation experiments, and 0.9945 to 1.0009 for natural samples from the salt lake. The δ11B values of halite from the Qaidam Basin salt lakes vary from −4.7 to 25.8‰, compared to −4.7 to 31.4‰ in the salt lake brines. These values are controlled by the boron isotopic composition of the boron sources, pH values and Na/Ca ratios in the salt lake brines. The variation of boron isotopes in halite may be used to trace the hydrochemical evolution and paleoevaporation environment in salt lakes.

Acceleration of modern acidification in the South China Sea driven by anthropogenic CO2

Modern acidification by the uptake of anthropogenic CO2 can profoundly affect the physiology of marine organisms and the structure of ocean ecosystems. Centennial-scale global and regional influences of anthropogenic CO2 remain largely unknown due to limited instrumental pH records. Here we present coral boron isotope-inferred pH records for two periods from the South China Sea: AD 1048–1079 and AD 1838–2001. There are no significant pH differences between the first period at the Medieval Warm Period and AD 1830–1870. However, we find anomalous and unprecedented acidification during the 20th century, pacing the observed increase in atmospheric CO2. Moreover, pH value also varies in phase with inter-decadal changes in Asian Winter Monsoon intensity. As the level of atmospheric CO2 keeps rising, the coupling global warming via weakening the winter monsoon intensity could exacerbate acidification of the South China Sea and threaten this expansive shallow water marine ecosystem.

Obliquity pacing of the western Pacific Intertropical Convergence Zone over the past 282,000 years.

The Intertropical Convergence Zone (ITCZ) encompasses the heaviest rain belt on the Earth. Few direct long-term records, especially in the Pacific, limit our understanding of long-term natural variability for predicting future ITCZ migration. Here we present a tropical precipitation record from the Southern Hemisphere covering the past 282,000 years, inferred from a marine sedimentary sequence collected off the eastern coast of Papua New Guinea. Unlike the precession paradigm expressed in its East Asian counterpart, our record shows that the western Pacific ITCZ migration was influenced by combined precession and obliquity changes. The obliquity forcing could be primarily delivered by a cross-hemispherical thermal/pressure contrast, resulting from the asymmetric continental configuration between Asia and Australia in a coupled East Asian–Australian circulation system. Our finding suggests that the obliquity forcing may play a more important role in global hydroclimate cycles than previously thought.

Interannual variation of rare earth element abundances in corals from northern coast of the South China Sea and its relation with sea-level change and human activities

Here we present interannual rare earth element (REE) records spanning the last two decades of the 20th century in two living Porites corals, collected from Longwan Bay, close to the estuarine zones off Wanquan River of Hainan Island and Hong Kong off the Pearl River Delta of Guangdong Province in the northern South China Sea. The results show that both coral REE contents (0.5-40 ng g⁻¹ in Longwan Bay and 2-250 ng g⁻¹ in Hong Kong for La-Lu) are characterized with a declining trend, which are significantly negative correlated with regional sea-level rise (9.4 mm a⁻¹) from 1981 to 1996 in Longwan Bay, 13.7 mm a⁻¹ from 1991 to 2001 in Hong Kong). The REE features are proposed to be resulted from seawater intrusion into the estuaries in response to contemporary sea-level rise. However, the tendency for the coral Er/Nd time series at Hong Kong site is absent and there is no significant relation between Er/Nd and total REEs as found for the coral at Longwan Bay site. The observations are likely attributed to changes of the water discharge and sediment load of Pearl River, which have been significantly affected by intense human activities, such as the construction of dams/reservoirs and riverbed sediment mining, in past decades. The riverine sediment load/discharge ratio of the Pearl River decreased sharply with a rate of 0.02 kg m⁻³ a⁻¹, which could make significant contribution to the declining trend of coral REE. We propose that coastal corals in Longwan Bay and similar unexplored sites with little influences of river discharge and anthropogenic disruption are ideal candidates to investigate the influence of sea-level change on seawater/coral REE.

Osmium isotope determination on mantle-derived peridotite xenoliths from Panshishan with N-TIMS

The187Os/188Os ratios of spinel lherzolite xenoliths from Panshishan determined with N-TIMS are lower than that of the primitive mantle, which shows depleted mantle characteristics. Their positive correlation with Al2O3/MgO suggests that the geochemical behavior of trace elements Re and Os is similar to that of Al2O3 and MgO respectively during the magmatic evolution and Re-Os isotopic system is largely immune to mantle metasomatism. A model age of 2.8–3.4 Ga obtained by187Os/188Os-Al2O3/ MgO correlation might represent the homogeneous age of the mantle lithosphere beneath the area.

High-precision uranium-series dating of lacustrine carbonates from Daihai Lake

Ages of small aragonite samples retrieved from sediments from the Daihai Lake are determined by using the Thermal Ionization Mass Spectrometry (TIMS) technique. The ages of the samples range from (8740±601) to (10446 ±489) a, with a precision of 2%–3% for non-corrected dating ages and 4.7%-9.8% for corrected ages respectively, and are consistent within the deviation range. At about 10.5 ka BP, the grain-size of the sediments coarsens sharply, the frequency curves of grain-size distribution show coarse grain dominated single-double kurtosis, indicating cooling, decreased hydrodynamic conditions and increased wind influence procedure. This is in good agreement with previous results obtained from the Daihai Lake, which suggests that the climate was cold and the lake surface was shrinking. The TIMS ages of the sediments are therefore believed to be credible.