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Petrogenesis and tectonic setting of Late Devonian high-Mg andesites in the Beishan orogen, NW China
No full textHigh-Mg andesites (HMAs) are crucial for the reconstruction of plate tectonics, continental margin formation and lithospheric evolution. In this study, we present new fossil age, whole-rock geochemical and Sr-Nd isotope data on the newly discovered Dundunshan Group HMAs in the Dundunshan area of the Beishan orogen (central-southern Central Asian Orogenic Belt). The Dundunshan HMA samples are characterized by high MgO (6.47-7.02 wt%) contents and high Mg-# values (67.27-68.77), with SiO2 (58.57-62.13 wt%), Al2O3 (14.49-16.07 wt%) and CaO (5.05-6.24 wt%) resembling typical HMAs. The Dundunshan HMA samples are calc-alkaline and strongly enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), with slightly negative Eu anomaly and high-field-strength element (HFSE) depletions. Their (Sr-87/Sr-86)(i) ratios (0.7041-0.7057) and epsilon(Nd(t)) (3.73-5.59) indicate that the Dundunshan HMAs were mainly formed by the interactions between subducted oceanic sediment-derived melts and mantle peridotites. Fossil evidence and published radiometric age data constrain the formation of the Dundunshan HMAs to early Late Devonian time. Sedimentological features of the Middle Devonian Sangejing Formation and regional tectonic correlation suggest that the Hongliuhe-Niujuanzi-Xichangjing Ocean in the Dundunshan area was likely closed during late Middle Devonian time, and that the Dundunshan HMAs were formed in a post-collision extensional setting
Eccentricity-paced monsoon variability on the northeastern Tibetan Plateau in the Late Oligocene high CO2 world
No full textConstraining monsoon variability and dynamics in the warm unipolar icehouse world of the Late Oligocene can provide important clues to future climate responses to global warming. Here, we present a similar to 4-thousand year (ka) resolution rubidium-to-strontium ratio and magnetic susceptibility records between 28.1 and 24.1 million years ago from a distal alluvial sedimentary sequence in the Lanzhou Basin (China) on the northeastern Tibetan Plateau margin. These Asian monsoon precipitation records exhibit prominent short (similar to 110-ka) and long (405-ka) eccentricity cycles throughout the Late Oligocene, with a weak expression of obliquity (41-ka) and precession (19-ka and 23-ka) cycles. We conclude that a combination of eccentricity-modulated low-latitude summer insolation and glacial-interglacial Antarctic Ice Sheet fluctuations drove the eccentricity-paced precipitation variability on the northeastern Tibetan Plateau in the Late Oligocene high CO2 world by governing regional temperatures, water vapor loading in the western Pacific and Indian Oceans, and the Asian monsoon intensity and displacement
Evidence of a Large Bias in Rooftop Measurements of Atmospheric Ammonia
No full textIn contrast to Europe and the U.S., human excreta in urban China are mainly stored in septic tanks beneath buildings, and the generated odors (including NH3) are released into the atmosphere through connecting pipes on the rooftops, that is, ceiling ducts. Here, we show that the levels of NH3 in ceiling ducts were 3 to 5 orders of magnitude higher than urban background concentrations. Unfortunately, urban air quality monitoring sites in China are mostly set on the roofs of various buildings. As the first attempt to investigate the influence of this overlooked NH3 source on the measurements of ambient NH3, hourly measurements of NH3 were performed on the rooftops of four buildings in Nanjing and Shanghai megacities. Although the two buildings in Nanjing are located in the same university campus (1 km apart), NH3 concentrations measured on the rooftop of a teaching building with dense ceiling ducts were on average 2.8 times higher than those of another building without ceiling ducts. In Shanghai, the exhaust vents of two ceiling ducts at a supersite were deliberately extended to a higher position and bent toward a direction downwind of our NH3 instrument. The resulting ambient NH3 concentration was 40% lower than that of another academic building without modification of its ceiling ducts. Besides, the measured NH3 peaks were consistently associated with air plumes from ceiling ducts. Realizing this strong NH3 source is important to avoid its significant influences on the measurement of NH3 and to interpret a large spatial heterogeneity of NH3 concentrations in urban China
Evidence of a Large Bias in Rooftop Measurements of Atmospheric Ammonia
No full textIn contrast to Europe and the U.S., human excreta in urban China are mainly stored in septic tanks beneath buildings, and the generated odors (including NH3) are released into the atmosphere through connecting pipes on the rooftops, that is, ceiling ducts. Here, we show that the levels of NH3 in ceiling ducts were 3 to 5 orders of magnitude higher than urban background concentrations. Unfortunately, urban air quality monitoring sites in China are mostly set on the roofs of various buildings. As the first attempt to investigate the influence of this overlooked NH3 source on the measurements of ambient NH3, hourly measurements of NH3 were performed on the rooftops of four buildings in Nanjing and Shanghai megacities. Although the two buildings in Nanjing are located in the same university campus (1 km apart), NH3 concentrations measured on the rooftop of a teaching building with dense ceiling ducts were on average 2.8 times higher than those of another building without ceiling ducts. In Shanghai, the exhaust vents of two ceiling ducts at a supersite were deliberately extended to a higher position and bent toward a direction downwind of our NH3 instrument. The resulting ambient NH3 concentration was 40% lower than that of another academic building without modification of its ceiling ducts. Besides, the measured NH3 peaks were consistently associated with air plumes from ceiling ducts. Realizing this strong NH3 source is important to avoid its significant influences on the measurement of NH3 and to interpret a large spatial heterogeneity of NH3 concentrations in urban China
Mid-Miocene Lake Level Fluctuations in the Lunpola Basin, Central Tibetan Plateau
No full textKnowledge of paleolake evolution is highly important for understanding the past hydroclimate regime on the Tibetan Plateau and associated forcing mechanisms. However, the hydrological history of paleolakes on the central plateau, the core region of the plateau, remains largely inconclusive. Here we present new biomarker records from lacustrine deposits of the Lunpori section in the Lunpola Basin to reconstruct detailed lake-level fluctuations during the mid-Miocene. A set of n-alkane indexes, including the proportion of aquatic macrophytes (P-aq), average chain length and carbon preference index as well as the content of n-alkanes, vary substantially and consistently throughout the studied interval. Our results altogether show relatively low lake level at similar to 16.3-15.5 Ma and high lake level before and after the interval, which is in line with the lithological observations in the section. Further comparison with existing regional and global temperature records suggests that lake level fluctuations can be largely linked to global climatic conditions during the mid-Miocene, with lake expansion during relatively warm periods and vice versa. Therefore, we infer that global climatic changes might have controlled the lake-level fluctuations in this region during the mid-Miocene, whereas the tectonic uplift likely played a subordinate role on this timescale
The roles of aqueous-phase chemistry and photochemical oxidation in oxygenated organic aerosols formation
No full textThe formation mechanism and evolution process of secondary organic aerosol (SOA) remain poorly understood due to measurement uncertainties and chemical complexities. Here, we present the characterization of non-refractory fine particles (NR-PM2.5) analyzed by means of a time-of-flight ACSM (ToF-ACSM) at a rural site in the North China Plain during winter. Our results show that air quality in rural areas was heavily polluted by primary organic aerosols (POA), which accounted for 83% of total organic aerosol (OA). The oxygenated organic aerosols (OOA) were mainly generated from local emissions. As pieces of evidence, firstly, less-oxidized OOA (LO-OOA) had good correlations with primary species like biomass burning OA (BBOA) (R2 = 0.54), EC (R2 = 0.46), and chloride (R2 = 0.46), implying LO-OOA was formed with the POA emission processes; secondly, there was a significant increase in OOA concentration after sunrise, indicating a lot of local generation during the daytime; thirdly, the potential source region of OOA was restricted in that of POA, implying that OOAs were converted from where primary emission exists. The formation processes of LO-OOA and more-oxidized OOA (MO-OOA) are different. Aqueous-phase chemistry had a dominant effect on the formation of LO-OOA due to the better correlation between aerosol liquid water content (ALWC) with LO-OOA (R2 = 0.54) than MO-OOA (R2 = 0.15). In comparison, both aqueous -phase and photochemical processes acted on the MO-OOA formation, when odd oxygen (Ox = O3+NO2) was low (Ox 35 ppb), photochemical oxidation played an essential role in MO-OOA formation
Is Current Research on How Climate Change Impacts Global Food Security Really Objective?
No full textGlobal food insecurity is becoming more severe under the threat of rising global carbon dioxide concentrations, increasing population, and shrinking farmlands and their degeneration. We acquired the ISI Web of Science platform for over 31 years (1988-2018) to review the research on how climate change impacts global food security, and then performed cluster analysis and research hotspot analysis with VosViewer software. We found there were two drawbacks that exist in the current research. Firstly, current field research data were defective because they were collected from various facilities and were hard to integrate. The other drawback is the representativeness of field research site selection as most studies were carried out in developed countries and very few in developing countries. Therefore, more attention should be paid to developing countries, especially some African and Asian countries. At the same time, new modified mathematical models should be utilized to process and integrate the data from various facilities and regions. Finally, we suggested that governments and organizations across the world should be united to wrestle with the impact of climate change on food security.</p
Global-scale altitude effect on leaf wax n-alkane delta D values in terrestrial higher plants
No full textNumerous field studies have shown that leaf wax n-alkane delta D values (delta Dn-alkane) can be used as a proxy for paleoelevation reconstruction. However, the lack of systematic global analysis of delta Dn-alkane values with respect to altitude limits the reliability of delta Dn-alkane values for paleoaltimetry at the global scale. In this study, we analyzed the variation of delta Dn-alkane values with altitude in terrestrial higher plants based upon our compiled global delta Dn-alkane database. The general decrease in delta Dn-alkane values with altitude corresponds to the variation in precipitation delta D (delta D-prec) with altitude, suggesting that the delta Dn-alkane values effectively reflect delta D-prec values. When eliminating the effect of latitude at the global scale, the global delta Dn-alkane-altitude lapse rate for all species was -1.01 +/- 0.10 parts per thousand (100 m)(-1), while these lapse rates were -1.03 +/- 0.13 parts per thousand and -0.47 +/- 0.13 parts per thousand (100 m)(-1) for dicots and monocots, respectively. Our study reveals that it is reliable to use delta Dn-alkane values to gain information about paleoelevation at the global scale, and the global delta Dn-alkane-altitude lapse rates provide an alternative to reconstruct paleoelevation at some delta Dn-alkane-uninvestigated areas
Soil erosion fluxes on the central Chinese Loess Plateau during CE 1811 to 1996 and the roles of monsoon storms and human activities
No full textEvaluating the effects of soil and water conservation on the Chinese Loess Plateau (CLP) during the past 60 years depends mainly on understanding the historical processes of soil erosion fluxes and their mechanism. However, little was known about the historical variation in soil erosion and its driving factors due to the lack of few ideal geological carriers on the CLP. A representative Heshui (HS) landslide-dammed reservoir that formed 200 years ago was drilled with a 31.39 m length sediment sequence. The HS sequence was composed of 165 deposition couplets with 56 annual freeze-thaw layers based on the elemental distributions by X-ray fluorescence core scanners. Mean couplet and annual specific sediment yields (CSSY and ASSY) were 1.44 x 10(4) t/km(2) and 1.28 x 10(4) t/km(2)/yr, respectively. The HS sequence was deposited over CE 1811-1996, which was dated by multi-methods crossing among Cs-137 activities, annual freeze-thaw layers, CSSY, recent rainfall records, and historical flood index grades. Besides, the threshold value of hydrological events that generated sediments in the HS reservoir was greater than 40 mm. As a result, there were eight periods of high ASSY between CE 1811 and 1996, although the total tendency of the ASSY was decreased, especially under the management of soil and water since the 1960s on the CLP. Interannual change of monsoon storms under El Nino-Southern Oscillation events was the primary factor affecting the variations in ASSY and storms in the HS region. Besides, human activities, such as policies and wars, also had significant influences on the historical soil erosion processes in this region. These results not only shed light on the historical changes in soil erosion fluxes and corresponding hydrological events during the past 200 years in the plateau-gully area of the CLP, but the active mechanisms under the different driving factors were also better understood
Stable-isotope tracing of vadose-zone water transport in Achnatherum splendens grassland of the Qinghai Lake Basin, NE Qinghai-Tibet Plateau, China
No full textThe vadose zone serves as a connection and transformation link between the atmosphere, plants, soil, surface water and groundwater. It is considered to be an important component of the geosphere and has sensitive interactions with the hydrosphere, biosphere, and atmosphere, which greatly impacts the environment and human health. This study was conducted in the Achnatherum splendens grassland of the Qinghai Lake Basin, NE Qinghai-Tibet Plateau, China, and can provide references and guidance for research on the mechanisms governing the circulation and evolution of vadose-zone water in cold arid regions. Precipitation, dustfall, and vadose zone samples were collected to investigate transport in the vadose zone water using stable isotopes of hydrogen (delta H-2) and oxygen (delta O-18) as well as chloride as environmental tracers. The results showed that precipitation was the main source of chloride in the research area, accounting for 86.5% of total chloride deposition. The soil water in the vadose zone was mainly recharged by local precipitation. Strong evaporation occurred before the precipitation infiltrated and recharged soil water. The soil water, chloride content, and isotope values in the soil profile varied across different depths. The annual potential recharge from precipitation to the vadose zone soil water was 9.20 mm/yr, accounting for 2.17% of the local precipitation. The actual recharge from precipitation to groundwater was 26.29 mm/yr, accounting for 6.19% of the local precipitation. The infiltration recharge from preferential and piston flow accounted for 66.40% and 33.60% of the total infiltration recharge to groundwater, respectively, indicating that preferential flow was the main source of groundwater recharge in the studied grassland. The results of this study were compared with those of other relevant studies. The recharge mechanisms of the vadose zone soil water and groundwater varied between different regions and were controlled mainly by the spatiotemporal distribution of precipitation, topographic characteristics, and soil structure. These findings provide a reference and guidance for research on the circulation and evolution of vadose-zone water in the Qinghai Lake Basin and Qinghai-Tibet Plateau