Search

The objective of this work is to propose a more effective way to prepare an in-house CO2 with known triple oxygen isotope compositions. The major experimental steps include: (1) the O2 is combusted to CO2 on a graphite rod at 750 °C with Pt-catalyst for 3-4 min; and (2) converted CO2 is subsequently purified by two cryogenic traps. The results show high reproducibility of δ13C and δ18O values of the converted CO2 within 0.010-0.020 ‰ and 0.006-0.010 ‰ (1σ, SD), and the identical δ18O value within error with that of the original O2. Additionally, we have measured the triple oxygen isotope compositions of converted CO2 using an O2-CO2 Pt-catalyzed oxygen-isotope equilibration method. The measured δ17O values of CO2 show high reproducibility within 0.006 ‰ (1σ, SD), and are identical within error with the original O2 as well. Notably, our experiments also found that the O2 with heavier oxygen isotope ratios (δ18O > 40 ‰, VSMOW) might have a lesser conversion efficiency, and this effect, combined with the lighter isotope preferential fractionations during the reaction processes of O2 to CO and CO to CO2, may explain the observed lower 17O/16O and 18O/16O ratios of the converted CO2 relative to the original O2.

A speleothem fluid inclusion extraction and isotope analysis system was established based on a vacuum extraction device and a Picarro L2140-i wavelength scanned cavity ring-down spectroscopy (WS-CRDS) water isotope analyzer. WS-CRDS allows to measure the stable oxygen and hydrogen isotope composition ( δ 18O and δ D) of small water samples simultaneously, which is faster, simpler, and requires less maintenance compared to currently used continuous-flow isotope ratio mass spectrometry (CF-IRMS). The precision of water isotopic analysis utilizing the WS-CRDS is often superior to that of CF-IRMS. Therefore, the use of Picarro for isotope analysis has rapidly gained popularity. The oxygen isotopic composition of speleothem calcite ( δ 18Oc) is one of the main proxies of speleothem-based paleoclimate reconstructions, but the interpretation of δ 18Oc remains complex as it can be influenced by many factors. Speleothem fluid inclusions contain the characteristics of cave drip water. The δ 18O and δ D values of fluid inclusion water provide direct information about paleoprecipitation, and can be further used to reconstruct paleotemperature when combined with δ 18Oc. The present paper contains a detailed description of the system as we have designed it, and provides an introduction to the system’s performance. The vacuum extraction device, connected to the Picarro analyzer, is designed to release the water trapped in speleothem fluid inclusions by pneumatically-operated crushing of discrete speleothem calcite chips. The whole device is kept at a temperature of 120°C to ensure the total and instantaneous vaporization of the water without isotopic fractionation. The vaporized water is carried by a flow of high-purity nitrogen to a 40 mL volume for homogenization, and is then introduced into the Picarro analyzer for δ 18O and δ D analysis. A syringe injection unit was added to the vacuum extraction device to be able to measure standard water, with δ 18O and δ D values similar to the speleothem samples, before and during measurements. To verify the precision and accuracy of the system, the same standard water samples were measured with an automated Picarro L2140-i set-up that includes an autosampler and a Picarro evaporation unit. The δ 18O and δ D values of standard waters as measured by both systems are comparable. Using the crushing device, speleothem samples from the East Asian Monsoon region yield paired δ 18O/ δ D values within ±0.5‰ of the Global Meteoric Water Line (GMWL), indicating no isotopic fractionation during the measurements. The precision is a function of the water amount released and of the isotopic composition of the sample. The reproducibility of crushed speleothem samples, whose water vapor concentration is between 2500 and 50000 ppm, is 0.5‰ for δ 18O and 2‰ for δ D (1 SD), similar to results obtained from other published measuring systems.

Triple oxygen isotope compositions have become one of critical proxies in characterizing a wide range of geochemical and hydroclimate processes. However, Δ17O (carbonate 17O anomaly) has only been barely used in the last decade because it is difficult to measure δ17O of natural samples to a sufficient precision in order to resolve small natural variability. In this study, we present triple oxygen isotope data from speleothems obtained by an O2-CO2 Pt-catalyzed oxygen-isotope equilibration method. The high precision (9 per meg or better, 1σ SD) of our new speleothem Δ17O data is sufficient to resolve subtle hydroclimatic signals. Based on this method, we established triple-oxygen-isotope records of TON cave in westerly region since the last 135ka, providing the evolution history of water vapor source and water vapor cycle in the orbit-millennium scale atmospheric precipitation in the Central Asia. In addition, the triple-oxygen-isotope records of speleothem from Asian and South American monsoonal regions were established in the key periods, such as glacial and interglacial periods. Our speleothem Δ17O data indicate a 20 per meg difference between Marine Isotope Stage 5d and 5e in samples from Central Asia, suggesting a shift in moisture source and/or fractionation history. Unexpectedly, there were no measurable Δ17O differences between glacial and interglacial samples from both the South American (western Amazon) and Asian (southern China) monsoon domains, implying consistent moisture-source conditions across glacial and interglacial cycles, at least in terms of relative humidity. Speleothem Δ17O data may thus provide new and important constraints for understanding regional and global hydroclimate dynamics.

In this paper, the bottom fire behaviour of 25 cm × 25 cm × 5 cm natural rubber (NR) latex foam with uniformly distributed 6 mm diameter holes was investigated experimentally in a small-scale experimental platform under bottom ventilation. The bottom fire behaviour was analysed. The results show that the burning process of the thermally thick NR latex foam under bottom ventilation conditions can be divided into three stages: initial growing, full development, and decay. A deflagration covered the entire rear surface was observed at 308 s. The burning balls moving at a speed of 0.15 m/s were observed after the bottom ignition and they moved from the center to the sides along with the expansion. The mass loss rate of the sample was accelerated dramatically from 0.2 g/s to 0.5 g/s when the bottom surface was ignited at 308 s.

Human activities within the context of a drying trend triggered the megafaunal extinctions in Madagascar and Mascarene Islands., Madagascar and the Mascarene Islands of Mauritius and Rodrigues underwent catastrophic ecological and landscape transformations, which virtually eliminated their entire endemic vertebrate megafauna during the past millennium. These ecosystem changes have been alternately attributed to either human activities, climate change, or both, but parsing their relative importance, particularly in the case of Madagascar, has proven difficult. Here, we present a multimillennial (approximately the past 8000 years) reconstruction of the southwest Indian Ocean hydroclimate variability using speleothems from the island of Rodrigues, located ∼1600 km east of Madagascar. The record shows a recurring pattern of hydroclimate variability characterized by submillennial-scale drying trends, which were punctuated by decadal-to-multidecadal megadroughts, including during the late Holocene. Our data imply that the megafauna of the Mascarenes and Madagascar were resilient, enduring repeated past episodes of severe climate stress, but collapsed when a major increase in human activity occurred in the context of a prominent drying trend.

The carbonate 17O anomaly (Δ17O) has recently been developed as a geochemical proxy for estimating the relative humidity of moisture at the source point of evaporation, which can be a vital tool in paleoclimate research. Speleothem Δ17O variability in particular may provide a quantitative constraint on moisture regimes at millennial and orbital timescales—far longer than can be addressed by analyzing 17O in other materials, such as tree rings. Modern observations and calibration studies have established a robust negative correlation between the Δ17O (17O excess) of rainfall and relative humidity, so that Δ17O is enhanced during arid conditions at the moisture source. Herein, we report novel triple oxygen isotope data across the Last Glacial in speleothems collected from Cherrapunji Cave, northeastern India. Triple oxygen isotope measurements were obtained by an O2-CO2 Pt-catalyzed oxygen-isotope equilibration method. Preliminary results suggest lower Δ17O in speleothems during cold periods (e.g. Heinrich Events), which would imply higher relative humidity over the oceanic moisture source. Importantly, higher source relative humidity does not necessarily imply changes in precipitation amount at the cave site. While it is possible that a substantial geographic shift in the moisture source region (or additional contributions from secondary sources) could obfuscate the Δ17O signal, we argue that this explanation is unlikely for our study site. Alternatively, we cannot exclude the effects of excessive moisture recycling in the tropical ocean, which can enhance the 17O anomaly in cloud vapor (particularly if combined with large temperature swings) and thereby alter speleothem Δ17O. To refine our interpretation of the Δ17O signal in Cerrapunji Cave samples, we investigate multiple cold periods, as well as coeval samples from the Asian Summer Monsoon region. Finally, we compare our results with novel data from westernmost Asia, where temperature variations during cold events are more likely to be accompanied by large shifts in predominant moisture source, due to the migration of wintertime westerlies.

High precision triple oxygen isotope measurement of meteoric water is a newly added tracer in hydrological and paleoclimate research. However, it is prerequisite to study the controls on precipitation 17O-excess for proper application of it. Here we report two years highly precise precipitation data from Nanjing, a southeast China station dominated by Asian monsoon. All the water isotopes (δ17O, δ18O and δD) reported here are based on mass spectrometer measurements and optical measurements (cavity ring-down spectroscopy). Nanjing receives moisture from different vapor sources and experiences different rainout mechanisms during various monsoonal sessions. Combined use of above parameters can help us to delineate processes occurring during evaporation, transport, condensation and re-evaporation. Year to year 17O-excess variability is observed in the obtained dataset and no notable seasonal variation is observed. However, the 17O-excess seasonal amplitude is little larger in the first year than the subsequent year. So far, it is known that the precipitation 17O-excess depends on three values: 17O-excess of the source water bodies, amount of 17O-excess gain during evaporation and 17O-excess loss during raindrops evaporation. During dry months 17O-excess gain is balanced by 17O-excess loss, which might lead to the near absence of seasonal cycle at Nanjing. From the comparison of observed data and model simulation, the amount of re-evaporation on falling raindrop is estimated to be about 10% at Nanjing. In addition, correlation with available meteorological parameters has been discussed. Except temperature no significant correlation has been found with other metrological variables (relative humidity and rainfall amount). This study will serve as a baseline to understand some of issues in paleoclimate that have puzzled the scientific community for years.

The “4.2 ka event” is frequently described as a major global climate anomaly between 4.2 and 3.9 ka, which defines the beginning of the current Meghalayan age in the Holocene epoch. The “event” has been disproportionately reported from proxy records from the Northern Hemisphere, but its climatic manifestation remains much less clear in the Southern Hemisphere. Here, we present highly resolved and chronologically well-constrained speleothem oxygen and carbon isotopes records between ∼6 and 3 ka from Rodrigues Island in the southwestern subtropical Indian Ocean, located ∼600 km east of Mauritius. Our records show that the 4.2 ka event did not manifest itself as a period of major climate change at Rodrigues Island in the context of our record's length. Instead, we find evidence for a multi-centennial drought that occurred near-continuously between 3.9 and 3.5 ka and temporally coincided with climate change throughout the Southern Hemisphere.

A large array of proxy records suggests that the “4.2 ka event” marks an approximately 300-year long period (∼3.9 to 4.2 ka) of major climate change across the globe. However, the climatic manifestation of this event, including its onset, duration, and termination, remains less clear in the Indian summer monsoon (ISM) domain. Here, we present new oxygen isotope (δ18O) data from a pair of speleothems (ML.1 and ML.2) from Mawmluh Cave, Meghalaya, India, that provide a high-resolution record of ISM variability during a period (∼3.78 and 4.44 ka) that fully encompasses the 4.2 ka event. The sub-annually to annually resolved ML.1 δ18O record is constrained by 18 230Th dates with an average dating error of ±13 years (2σ) and a resolution of ∼40 years, which allows us to characterize the ISM variability with unprecedented detail. The inferred pattern of ISM variability during the period contemporaneous with the 4.2 ka event shares broad similarities and key differences with the previous reconstructions of ISM from the Mawmluh Cave and other proxy records from the region. Our data suggest that the ISM intensity, in the context of the length of our record, abruptly decreased at ∼4.0 ka (∼±13 years), marking the onset of a multi-centennial period of relatively reduced ISM, which was punctuated by at least two multi-decadal droughts between ∼3.9 and 4.0 ka. The latter stands out in contrast with some previous proxy reconstructions of the ISM, in which the 4.2 ka event has been depicted as a singular multi-centennial drought.

Although the 8.2 ka event is a well-known globally documented abrupt climate event, its detailed expression in tropical southeastern Africa is poorly constrained. Here we present a high resolution and precisely dated record of the regional hydroclimatic variability between 8.5–7.5 ka BP from a polymorphic speleothem (ABC-1) from northwestern Madagascar. In combination with the exclusive recurrences of calcite layers (indicative of wetter conditions), the distinctly negative excursions in the ABC-1 δ18O record manifest a wet 8.2 ka event that spanned from 8.230 to 8.053 ka BP. The event is characterized by the replacement of shorter-term (interannual to interdecadal) δ18O-δ13C coherence with the longer-term (multidecadal to centennial) coherence, suggesting a stable and well-developed forest system during the 8.2 ka event. Our data reveal a two-stage structure of the 8.2 ka event, which is superimposed with two brief pluvial episodes in each stage. The widely documented two peaks/troughs during Stage-I suggest rapid propagation of climate signal through atmospheric processes triggered initially by the freshwater forcing, whereas the divergent behaviors of the peaks/troughs in Stage-II in different climate systems may suggest a different forcing.

The 4.2 ka BP event is widely described as a 200–300 years long interval of major climate anomaly (typically, arid and cooler conditions potentially across the globe), which defines the beginning of the current Meghalayan age in the Holocene epoch. The 4.2 ka event however, has been disproportionately reported from proxy records situated at low-mid latitudes in the Northern Hemisphere. Consequently, the climatic manifestation of the 4.2 ka event in both spatial and temporal domains is still much less clear in Southern Hemisphere. This is particularly the case for the southwest sector of the southern Indian Ocean. Here we present high-resolution and chronologically well-constrained speleothem oxygen and carbon isotopes records of hydroclimate variability between ~ 6 and 3 ka ago from Rodrigues Island, located in the southwest subtropical Indian Ocean, ~ 600 km east of Mauritius. Our records reveal a major shift to drier condition at circa 4 ka BP, which culminated into a multicentennial period of drought (i.e., megadrought) that lasted continuously from ~ 3.9 to 3.5 ka BP. The inferred hydroclimatic conditions between 4.0 and 4.2 ka BP, are however not distinctly distinguishable from the region’s mean hydroclimatic state over the length of our record. Because the precipitation variability at Rodrigues is distinctly modulated by meridional movement of the Inter-Tropical Convergence Zone and the El Nino Southern Oscillation dynamics, our proxy data may ultimately provide critical constraints in our understanding the timing and dynamical forcing of the 4.2 ka event.

Development of magnetohydrodynamic acceleration technology is expected to improve wind tunnel simulation capability and testing capability. The underlying premise is to produce uniform and stable plasma in supersonic air flow, and gas discharge is an effective way to achieve this. A nanosecond pulsed discharge experimental system under supersonic conditions was established, and a pin-to-plate nanosecond pulsed discharge experiment in Mach 2 air flow was performed to verify that the proposed method produced uniform and stable plasma under supersonic conditions. The results show that the discharge under supersonic conditions was stable overall, but uniformity was not as good as that under static conditions. Increasing the number of pins improved discharge uniformity, but reduced discharge intensity and hence plasma density. Under multi-pin conditions at 1000 Hz, the discharge was almost completely corona discharge, with the main current component being the displacement current, which was smaller than that under static conditions.