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Climatic controls on travertine deposition in southern Tibet during the late Quaternary
No full textLarge volumes of travertine deposits are preserved at hydrothermal spring sites on the Tibetan Plateau (TP). Yet, most of these deposits are under-researched with respect to their diagenetic and depositional history and there is still very limited understanding of the tectonic and climatic influences on travertine precipitation in the arid high-altitude setting of Tibet. In this study, a detailed uranium-series dating campaign was carried out for the Qiusang travertine (similar to 4270 m above sea level), southern Tibet that has been previously dated back to 486 thousand years ago (ka). Based on 42 new Th-230/U ages, combined with geomorphological and sedimentological investigations, several travertine zones were identified and distinct travertine depositional phases constrained: 11.7-6.8 ka and similar to 13.4 ka (zone 1), 128-122 ka (zone 2),-193 ka (zone 3),similar to 292 ka and 324 ka (zone 4), > 317 ka (zone 5), ca. 415 to 470 ka (zone 6) and ca. 419 to 445 ka (zone 7). Comparison of these depositional phases with local and regional proxy records suggests that travertine accumulation at Qiusang occurred during main interglacials when monsoon precipitation peaked on the TP. This coincidence, together with a sensitive response of Tibetan hydrothermal spring activity to meteoric recharge, implies that climate controls the precipitation of large travertine volumes on orbital timescales on the plateau. We propose that (i) tectonic activity is of subor-dinate importance and influences travertine precipitation on the TP only episodically and on significantly shorter (i.e. centennial to millennial) timescales related to the recurrence rates of large earthquakes and that (ii) intensive monsoonal-driven groundwater recharge is required on top of tectonic activity for generating volu-metrically significant travertine accumulations. Because of the high precipitation rates typical for hydrothermal spring carbonates, we conclude that travertine deposits on the TP could be utilized as valuable high-resolution proxy records of peak monsoon conditions in the currently arid to semi-arid landscape.& nbsp;Furthermore, the Qiusang travertine zone 7 is terraced and the travertine layers adjusted to a paleo-riverbed elevation similar to 30 m above the current river, allowing us to constrain fluvial incision to-0.07 m/ka for the south -central sector of the TP since the Mid-Pleistocene. The abundant travertine occurrences in Tibet in combination with uranium-series dating can thus also provide detailed insights into earth surface dynamics and landscape evolution on the world highest plateau
Efficient charge separation of a Z-scheme Bi5O7-delta I/CeO2-delta heterojunction with enhanced visible light photocatalytic activity for NO removal
No full textIn this work, a novel Z-scheme Bi5O7-delta I/CeO2-delta heterojunction photocatalyst was prepared by combining hydrothermal synthesis and thermal treatment methods. Oxygen vacancies were in situ generated on the surface of Bi5O7-delta I and CeO2-delta nanostructures. The existence of oxygen vacancies significantly improved the light absorption of CeO2-delta and inhibited the recombination of photogenerated carriers. Based on the photocatalytic activity, the as-prepared Bi5O7-delta I/CeO2-delta photocatalyst exhibited excellent photocatalytic NO removal performance under visible light irradiation. The results of the capture experiment and electron spin resonance (ESR) spectroscopy show that super oxygen radicals (O-2(-)) and hydroxyl radicals (OH) are the main active species in the overall photocatalytic reaction process. Density functional theory (DFT) calculations and ESR results demonstrate that a Z-scheme heterojunction is formed between Bi5O7-delta I and CeO2-delta. Compared with the traditional type-II heterojunction, the Z-scheme heterojunction exhibited more efficient charge separation and robust redox capacity in the photocatalytic reaction
Hydrochemistry and source apportionment of boron, sulfate, and nitrate in the Fen River, a typical loess covered area in the eastern Chinese Loess Plateau
No full textFen River Basin (FRB) is water-deficient and strongly influenced by human activities in the eastern Chinese Loess Plateau. The spatio-temporal variation and controlling factors of hyrochemistry and quality, sources of high boron, sulfate, and nitrate of surface waters in FRB were unclear. Major ions, delta 11B, delta 15N, and delta 18O in surface waters in dry season and wet season of FRB were analyzed and correlation analysis (CA), principal component analysis (PCA), self-organizing map (SOM), forward model, and Bayesian isotope mixing model (MixSIAR) were used to solve above problems. Results showed that average riverine delta 11B, delta 15N, and delta 18O of FRB was 7.8%o, 11.2%o, and 1.3%o (1SD), respectively. Dissolved solutes ranked midstream > downstream > upstream with water type of Na +-Cl-, Ca2+-Mg2+-Cl-, and Ca2+-HCO3-, respectively. Low dissolved solutes were in forest areas while high values were in cropland and city areas. SOM analysis indicated that hydrochemistry was both influenced by natural (upstream) and pollutional input (midstream and downstream) and variation between dry season and wet season was minor. The abnormally high boron concentrations were mainly from silicate weathering (43%) and evaporites dissolution of loess (32%), urban and industrial input contributed 15% of riverine boron. High SO42- (207 +/- 267 mg/L, 1SD) was mainly from sulfates. delta 15N and delta 18O analysis indicated that nitrification was the primary N cycling process. Further, MixSIAR showed that NO3- was mainly from municipal sewage (-67%) and the total contribution of chemical fertilizer and soil nitrogen was -30% with slightly higher values in upstream and wet season. Influenced by land-use types, evaporite dissolution, and anthropogenic input, water quality below midstream was worse and strict sewage reduction policies must be developed. This study highlights the significant influence of evaporite dissolution of loess and anthropogenic input (urban and industrial input for B and sewage for NO3- ) on hydrochemistry and water quality
The Seasonally Altered Atmosphere Moisture Circulations With Rainfall and Rainfall Isotopes in Southwest China
No full textTo interpret the climatic signals of precipitation/speleothem delta O-18, it is critical to identify the importance of the factors affecting the precipitation delta O-18. This study presents new stable isotope data for precipitation delta O-18 and delta D in the site of Shenqi cave, southwest China, from November-2015 to October-2016 (the "Super-El Nino " event), to investigate the regional-scale climate forcing on precipitation delta O-18. The precipitation delta O-18, delta D and d-excess have an obvious seasonality, relatively low values in the wet season and high in the dry season. The further analysis of seasonally altered LMWL and moisture circulations suggested that changes in atmosphere moisture circulations would be the key factor underlying the precipitation/speleothem delta O-18 fluctuations in our study area at least on seasonal timescales. Combined with the seasonal-monthly variations of the IsoGSM delta O-18, GPCP/CRU rainfall and NCEP/NCAR moisture fluxes, we detected that the super-El Nino of 2016 have changed the distributions of monthly rainfall in wet season through the Western Pacific Subtropical High, but not mainly the precipitation isotopic compositions and moisture circulations in our study area
Novel methods of resolving daily growth patterns in giant clam (Tridacna spp.) shells
No full textSclerochronology is a powerful tool for high resolution paleoclimate and paleoenvironment reconstruction. However, we can only observe annual growth bands using traditional technology for most marine bivalves, hampering the ability to establish daily resolution chronology in order to reconstruct past weather changes. Giant clams (Tridacna spp.) are the largest bivalves in the world. Their hard and dense aragonite shells have clear annual and even daily growth bands, creating the potential for daily resolution chronology establishment and paleoweather reconstruction. In this study, we present two new and reliable methods that resolve daily growth patterns in giant clam shells: (1) Fluorescence image method. Measuring the width between two bright fluo-rescent bands of the Tridacna shell with the help of CooRecorder 9.0 software based on the clear daily growth bands obtained by laser scanning confocal microscope (LSCM). (2) Ultrahigh resolution Sr/Ca method. Counting the amount of Sr/Ca ratio data in each daily cycle derived from Tridacna shell, and then calculating the daily growth increment based on the spatial resolution of Sr/Ca data. The results show the variation of daily growth increments profiles obtained by the two methods synchronize, and the chronology uncertainty is statistically acceptable over a long-term record, indicating that both methods can estimate the daily growth increment of Tridacna shell reliably. The methods developed here lay the foundation for paleoweather reconstruction using daily growth increments of Tridacna spp
Nonlinear Responses of Droughts Over China to Volcanic Eruptions at Different Drought Phases
No full textPrevious studies show that volcanic eruptions can intensify and extend drought events triggered by internal variability over Eastern China. However, it has remained unclear whether volcanic eruptions occurring in different drought phases have different impacts. Here, based on multiple reconstructions, simulations, as well as volcanic sensitivity experiments with volcanic forcing imposed in the early and late phases of droughts, we propose a nonlinear effect of volcanic eruptions on drought events. Late-phase volcanic eruptions exert greater impact on drought persistence and intensity while early-phase volcanic eruptions induce modest and weaker impacts. The evolutions of drought differ substantially from the typical volcanic-only influence or the linear combination of the drought triggered by internal variability and volcanic-only influences, which are hypothesized to be associated with positive feedbacks of soil moisture to precipitation, as well as its interaction with the evolution of the East Asia Summer monsoon
How is the El Nino-Southern Oscillation signal recorded by tree-ring oxygen isotopes in southeastern China?
No full textPrevious studies universally found that tree-ring oxygen isotope (delta O-18(TR)) in southeastern China (SEC) significantly recorded the El Nino-Southern Oscillation (ENSO) signal. However, the correlation between the climate in SEC and ENSO is insignificant, so the local climatic response of SEC delta O-18(TR) fails to explain the significant positive correlations between these delta O-18(TR) records and ENSO. In this study, based on four delta O-18(TR) series from SEC, meteorological data, simulated precipitation oxygen isotope (delta O-18(P)) data, and a moisture uptake atlas deduced from Lagrangian backward trajectory experiments, a conceptual model was established to explain how the ENSO signal is recorded by SEC delta O-18(TR). During the El Nino decaying years, the Northwest Pacific (NWP) convective activity weakens; the convective weakening areas coincide with the main moisture sources for SEC, which enriches O-18 in moisture that is transported to SEC, resulting in positive anomalies of delta O-18(P) in SEC; finally, the ENSO-modulated delta O-18(P) signal is inherited by SEC delta O-18(TR). The opposite situations occur in the La Nina events. It was also indicated that the change in the moisture contribution percentage from different sources may not be a primary factor that connects SEC delta O-18(TR) with ENSO. These findings contribute to understanding climatic signals represented by delta O-18(TR) and delta O-18(P) in the East Asian monsoon region
OH-initiated atmospheric degradation of hydroxyalkyl hydroperoxides: mechanism, kinetics, and structure-activity relationship
No full textHydroxyalkyl hydroperoxides (HHPs), formed in the reactions of Criegee intermediates (CIs) with water vapor, play essential roles in the formation of secondary organic aerosol (SOA) under atmospheric conditions. However, the transformation mechanisms for the OH-initiated oxidation of HHPs remain incompletely understood. Herein, the quantum chemical and kinetics modeling methods are applied to explore the mechanisms of the OH-initiated oxidation of the distinct HHPs (HOCH2OOH, HOCH(CH3)OOH, and HOC(CH3)(2)OOH) formed from the reactions of CH2OO, anti-CH3CHOO, and (CH3)(2)COO with water vapor. The calculations show that the dominant pathway is H-abstraction from the -OOH group in the initiation reactions of the OH radical with HOCH2OOH and HOC(CH3)(2)OOH. H-abstraction from the -CH group is competitive with that from the -OOH group in the reaction of the OH radical with HOCH(CH3)OOH. The barrier of H-abstraction from the -OOH group slightly increases when the number of methyl groups increase. In pristine environments, the self-reaction of the RO2 radical initially produces a tetroxide intermediate via oxygen-to-oxygen coupling, and then it decomposes into propagation and termination products through asymmetric two-step O-O bond scission, in which the rate-limiting step is the first O-O bond cleavage. The barrier height of the reactions of distinct RO2 radicals with the HO2 radical is not affected by the number of methyl substitutions. In urban environments, the reaction with O-2 to form formic acid and the HO2 radical is the dominant removal pathway for the HOCH2O radical formed from the reaction of the HOCH2OO radical with NO. The beta-site C-C bond scission is the dominant pathway in the dissociation of the HOCH(CH3)O and HOC(CH3)(2)O radicals formed from the reactions of NO with HOCH(CH3)OO and HOC(CH3)(2)OO radicals. These new findings deepen our understanding of the photochemical oxidation of hydroperoxides under realistic atmospheric conditions
Late Holocene Orbital Forcing and Solar Activity on the Kuroshio Current of Subtropical North Pacific at Different Timescales
No full textThe North Pacific subtropical gyre (NPSTG) redistributes heat and moisture between low and high latitudes and plays a key role in modulating the global climate change and ecosystem. Recent evidence suggests intensification and poleward shift of the subtropical gyres over the last decades due to global warming, but insufficient observations have hampered insight into the integrated effects of ocean-atmosphere interactions at longer timescales. Here we present the first high-resolution (similar to 12 years) grain-size record from Core CF1 in the Okinawa Trough, western subtropical North Pacific, to reconstruct the evolution of the western boundary Kuroshio Current (KC) of NPSTG during the Late Holocene. Our results indicate the KC slow-down during 4.6-2.0 ka, followed by quick enhancement after 2.0 ka, with centennial-scale variabilities (500-700 years) superimposed on the long-term trend. Over millennial timescales, gradually increased pole-to-equator thermal gradient, due to orbital forcing mechanisms, resulted in long-term enhanced KC, whereas solar activity triggered phase changes in the tropical Pacific mean state and controlled KC anomalies on centennial timescales. We suggest that both forcing mechanisms resulted in ocean-atmosphere feedback provoking concurrent changes in mid-latitude westerly and subtropical easterly winds over the North Pacific, alternating their dominance as source regions causing the dynamic changes of KC at different timescales. Our findings offer insight into the role of external forcing mechanisms in the NPSTG changes before the Anthropocene, which have profound implications for the deeper understanding of changes in ocean gyres under global warming scenarios
Late Holocene Orbital Forcing and Solar Activity on the Kuroshio Current of Subtropical North Pacific at Different Timescales
No full textThe North Pacific subtropical gyre (NPSTG) redistributes heat and moisture between low and high latitudes and plays a key role in modulating the global climate change and ecosystem. Recent evidence suggests intensification and poleward shift of the subtropical gyres over the last decades due to global warming, but insufficient observations have hampered insight into the integrated effects of ocean-atmosphere interactions at longer timescales. Here we present the first high-resolution (similar to 12 years) grain-size record from Core CF1 in the Okinawa Trough, western subtropical North Pacific, to reconstruct the evolution of the western boundary Kuroshio Current (KC) of NPSTG during the Late Holocene. Our results indicate the KC slow-down during 4.6-2.0 ka, followed by quick enhancement after 2.0 ka, with centennial-scale variabilities (500-700 years) superimposed on the long-term trend. Over millennial timescales, gradually increased pole-to-equator thermal gradient, due to orbital forcing mechanisms, resulted in long-term enhanced KC, whereas solar activity triggered phase changes in the tropical Pacific mean state and controlled KC anomalies on centennial timescales. We suggest that both forcing mechanisms resulted in ocean-atmosphere feedback provoking concurrent changes in mid-latitude westerly and subtropical easterly winds over the North Pacific, alternating their dominance as source regions causing the dynamic changes of KC at different timescales. Our findings offer insight into the role of external forcing mechanisms in the NPSTG changes before the Anthropocene, which have profound implications for the deeper understanding of changes in ocean gyres under global warming scenarios