Droughts and broad-scale climate variability reflected by temperature-sensitive tree growth in the Qinling Mountains, central China

The relationship between temperature and drought was investigated using the temperature-sensitive growth of Larix chinensis Beissn in the Qinling Mountains, central China. Extremely high tree-ring width index values (TRWI) agreed well with dry conditions defined by the dryness-wetness index (DWI) obtained from data in Chinese historical documents and climate-related papers between 1814 and 1956 (before the short of instrumental measurements); the reverse applied to extremely low TRWI values. The main severe drought epochs occurred from the late 1850s to the 1870s, the 1920s to 1930s and in the 2000s, whereas wet spells occurred from 1817-1827 and 1881-1886. The droughts in the 2000s exhibited a similar pattern as the ones from the 1920s to 1930s, with obviously an increasing temperature. The variation of tree growth agreed well with other reconstructed temperature series from nearby and remote regions, suggesting that Larix chinensis could respond to broad-scale climate variability. The longest cold interval, 1817-1827, could be associated with the influence of the Tambora eruption in 1815.</p

Analysis of low-level I-129 in brine using accelerator mass spectrometry

An improved solvent extraction procedure for iodine separation from brine samples has been applied at Xi&#39;an Accelerator Mass Spectrometry (AMS) center. Oil in the brine sample has to be removed to avoid appearance of the third phase during solvent extraction and to improve the chemical yield of iodine. The small amount of oil remained in the water phase was first removed by phase separation through settling down sufficiently based on their immiscibility, and then by filtration through a cellulose filter, on which oil was absorbed and removed. After oil removed, extraction recovery of iodine could achieve more than 90 %. The sodium bisulfite as an effective reductant should be added before acidification to avoid loss of iodine by formation of I-2 in sample via reaction of iodate and iodide at pH 1-2, and then pH was adjusted to 1-2 to reduce the iodate to iodide followed by oxidation of iodide to I-2 and solvent extraction to separate all inorganic iodine. As a pre-nuclear era sample, I-129/I-127 ratio in brine is normally more than two orders of magnitude lower than that in present surface environmental samples, so prevention of cross-contamination and memory effect in apparatus during processing procedure are very critical for obtaining reliable results, and monitoring the procedure blank is very important for analytical quality of I-129. The I-129/I-127 isotopic ratio in the brine samples and procedure blank of iodine reagents were measured to be (1.9-2.7) x 10(-13) and 2.08 x 10(-13), respectively, 3-4 orders of magnitudes lower than that in environmental samples in Xi&#39;an, and the result of procedure blank is in the same level as the previous experiments in past 3 years, indicating contamination is not observed in our method.</p

A preliminary study of small-mass radiocarbon samplemeasurement at Xi’an-AMS

To meet the measurement demands on small-mass radiocarbon (carbon content at 10&minus;6 g level) which are becoming increasingly significant, Xi&rsquo;an-AMS has made improvements to the existing method of sample loading and has upgraded the Cs sputter ion source from the original SO-110 model. In order to study the feasibility of small-mass samples in Xi&rsquo;an-AMS and evaluate the radiocarbon sample preparation ability using existing routine systems of H2/Fe and Zn/Fe, the small-mass samples prepared by four different methods are tested. They are the mass division method, mass dilution method, H2/Fe reduction method and Zn/Fe reduction method. The results show that carbon mass above 25 &micro;g can be prepared using the existing Zn/Fe system, but no less than 100 &micro;g is required using the existing H2/Fe system, which can be improved. This indicates Xi&rsquo;an-AMS are now able to analyze small-mass radiocarbon samples.</p

Performance of Accelerator Mass Spectrometry for I-129 using AgI-AgCl carrier-free coprecipitation

I-129 has been successfully applied as tracer in environmental, geological, and oceanographic research. For samples with low stable iodine concentration and ultra low level I-129, the sample preparation technique to separate iodine prior to AMS measurement has been a bottleneck, limiting the applicability of I-129 We have reported a carrier-free method, using coprecipitation, to avoid the potential introduction of I-129 through the use of stable iodine carrier iodine. In this work, the detection limit and the analytical uncertainty of this method are investigated and minimum sample amount required to obtain reliable analytical results are estimated. The method is validated with a series of samples in ranges of known iodine concentrations and I-129/I-127 ratios. The results confirm our previous conclusion that an AMS target containing 5.0 mu g iodine can be used for analyzing samples with I-129/I-127 &gt; 10(-12), and that for samples with I-129/I-127 &lt; 10(-13) more than 25 mu g iodine is necessary.</p

Speciation and migration of 129I in soil profiles

A method has been developed for speciation analysis of ultra low level I-129 in soil using sequential extraction combined with coprecipitation for separation of carrier free iodine and AMS measurement of I-129. Two loess profiles collected from northwest China were analyzed for species of I-129 and I-127. Similar partitioning of I-129 and I-122 was observed in the loess profiles, the distribution of iodine isotopes followed an order of organic &gt; leachable &gt; reducible &gt; residue. The I-129 concentrations and I-129/I-127 ratios decreased exponentially with the depth, and 2 orders of magnitude lower in the deepest layer (60 and 90 cm) compared with the top layer, indicating a significant contribution of anthropogenic input in the upper layer, and high retention of I-129 in soil. The mobility of I-129 in different fractions decreased in an order of leachable &gt; organic &gt; oxides &gt; residue. The results suggest that migration of iodine downwards in the soil profile is a slow process; the oxides and residue are the less mobile fractions of iodine.</p

Analysis and environmental application of I-129 at the Xi'an Accelerator Mass Spectrometry Center

The newly established 3 MV Accelerator Mass Spectrometry (AMS) facility in Xi&#39;an, with an instrument background of 2 x 10(-14) for I-129/I-127 ratio, provides efficient analytical capability to carry out I-129 environmental tracing studies. Chemical separation methods of iodine from different types of samples have been established at the Xi&#39;an AMS Center, including solvent extraction and combustion followed by extraction or coprecipitation depending on sample types. A carrier free method for iodine separation and AMS measurement of ultra low level I-129 in samples with low total iodine concentration has been established, which can be used for analysis of geological samples for I-129 dating. Some environmental samples collected in China have been analyzed using the developed methods. The analytical results show I-129/I-127 ratios of (0.9-1.1) x 10(-10) for seawater collected adjacent to a nuclear power plant, and (3.02 similar to 5.43) x 10(-19) for soil samples collected in a less than 10 km area surrounding the NPP. These values are not significantly different from those measured in remote areas, reflecting a safe nuclear environment in terms of I-129 level.</p

HIGH-LEVEL (14)C CONTAMINATION AND RECOVERY AT XI'AN AMS CENTER

A sample with a radiocarbon concentration estimated to be greater than 10(5) times Modern was inadvertently graphitized and measured in the Xi&#39;an AMS system last year. Both the sample preparation lines and the ion source system were seriously contaminated and a series of cleaning procedures were carried out to remove the contamination from them. After repeated and careful cleaning as well as continuous flushing with dead CO(2) gas, both systems have recovered from the contamination event. The machine background is back to 2.0 x 10(-16) and the chemical blank is beyond 50 kyr.</p

Level and source of 129I of environmental samples in Xi'an region, China

Iodine-129 is widely used as a tracer in various environmental practices such as monitoring of nuclear environmental safety, seawater exchange and transport, geochemical cycle of stable iodine and dating of geological events. The spatial distribution of (129)I concentration varies significantly on global scale because of anthropogenic input from nuclear activities coupled with scarcity of data on environmental (129)I variability in many parts of the world including Asia. Here we report new data on (129)I and (127)I concentrations in soil, vegetation, river water and precipitation collected from Xi&#39;an area, China. The results indicate values for environmental (129)I/(127)I ratios in the investigated area range from 1.1 x 10(-10) to 43.5 x 10(-10) with a mean of 20.6 x 10(-10), which is 1-3 orders of magnitude lower than the ratios observed in Europe, but comparable with those observed in the locations far from direct effect of point release sources and at similar latitude. The main source of (129)I in the investigated area is attributed to the global fallout of both atmospheric nuclear weapons testing and long distance dispersion of fuel reprocessing releases. (C) 2011 Elsevier B.V. All rights reserved.</p

Determination of Low Level 129I in Soil Samples Using Coprecipitation Separation of Carrier Free Iodine and Accelerator Mass Spectrometry Measurement

The accurate determination of ultra low level (129)I in sample is critical and essential for the application of natural (129)I in geological dating and environmental tracer studies. In this work, iodine was first separated from soil by combustion at high temperature; the released iodine was collected in an alkali trap solution. AgI-AgCl coprecipitaiton was used to separate carrier free iodine from the trap solution and to prepare target. (129)I in the target was then measured using a 3. 0 MV accelerator mass spectrometer. The recovery of iodine during the combustion is higher than 95%. An iodine recovery of about 75%-85% was obtained in the coprecipitation and the total recovery of iodine is above 70%. The developed method has been successfully used to determine (129)I in soil sample with low iodine content. A (129)I/(127)I atomic ratio as low as 10(-11) in the deep soil has been determined. The determination of (129)I/(127)I was ratio down to 10(-12) in solid samples.</p