C-14 DATING OF SOIL ORGANIC CARBON (SOC) IN LOESS-PALEOSOL USING SEQUENTIAL PYROLYSIS AND ACCELERATOR MASS SPECTROMETRY (AMS)

The chemical extraction of soil organic carbon (SOC) fractions from soils often does not produce satisfactory results for radiocarbon dating. In this study, a sequential pyrolysis technique was investigated. The soil was pyrolyzed at temperatures of 200, 400, 600, and 800 degrees C to partition organic carbon into pyrolytic volatile (Py-V) and pyrolytic residue (Py-R) fractions. The preliminary results show that the C-14 dates of both fractions become progressively older as the pyrolysis temperature is increased. In addition, the ages of the Py-V fractions are consistently younger than the corresponding Py-R fractions extracted at the same temperature. Experimental results of known-age paleosol samples indicate that the Py-V fractions obtained between 600 and 800 degrees C yield the most reliable ages. This technique provides a new approach to improve the accuracy of C-14 dating of loess-paleosol sequences.</p

The last 130 ka precipitation reconstructionfrom Chinese loess 10Be

Cosmogenic 10Be is a promising precipitation index, because its fallout flux in sediments is mainly controlled by wet precipitation after its production in the atmosphere. Here we report on a new study for reconstructing precipitation during the last 130 ka using 10Be measurements from Chinese loess, with multivariable linear regression to remove the geomagnetic field modulation and dust flux dilution effects from the loess 10Be record. The broad similarity between our result and speleothem &delta;18O indicates that the new precipitation record is robust. It also records an interesting increase in precipitation that occurred during Marine Isotope Stage 3 (MIS 3), exhibiting a similar rainfall amount with that of MIS 5, suggesting that MIS 3 is a special period with strengthened summer Monsoon intensity. By comparison with a stacked marine isotope record and a summer insolation record, our precipitation data clearly show a close correspondence with Northern Hemisphere summer (June, July, and August) solar insolation changes on orbital timescales. During MIS 3, our record follows the insolation differential between 30&deg;N and 30&deg;S, suggesting that rising rainfall changes during MIS 3 are a response to the interhemispheric summer insolation differential forcing.</p

Spatiotemporal relationship between temperature and precipitation in inland China from meteorological data and paleoclimatic implications for lacustrine oxygen records

The factors controlling climate in China&#39;s western interior are numerous and quite complex. These vary on seasonal to multi-millennial time scales as a result of changing dominance of influence from the Westerlies and the Asian and Indian Monsoon systems. These in turn respond to additional factors such as the dynamics of the Walker and Hadley circulation in the Pacific and Indian Ocean basins, the strength of the Siberian and Northwest Pacific Highs, and on long timescales to changes in the distribution of solar insolation resulting from orbital forcing. Here, we use the Empirical Orthogonal Function (EOF) analysis to investigate the spatiotemporal patterns of climate in China&#39;s western interior during the last 15 ka as revealed from a suite of 17 lacustrine carbonate delta O-18 records. Based on similarities to the EOF analysis of the modern data, we interpret the first two leading eigenmodes of the paleodata sets as P and T, respectively. Other climatic proxy data from this region appear to agree well with this interpretation. Our analysis reveals a discrete change in the spatial EOF patterns occurring at circa 7.5-6 ka BP with respect to the patterns observed prior to this. These patterns suggest that the arid NW region of the study area became significantly wetter following this juncture, but that the Asian Monsoon influenced (SE) region of the study area along the eastern flank of the Tibetan Plateau became drier. A more complex pattern of change is revealed by the second rank EOF, interpreted as temperature. This shows a discrete change at similar to 7.5-6 ka BP from cooler to hotter temperatures on the central Tibetan Plateau, but slightly cooler temperatures in the lowland Monsoon affected area (SE) after 6 ka BP. We interpret both these sets of changes to result from more persistent high pressure over the Tibetan Plateau during Boreal summer, with a resultant enhancement of moisture transport by the westerlies and diminished monsoonal flow following similar to 6 ka BP.</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

14C CHRONOSTRATIGRAPHY FOR QINGHAI LAKE IN CHINA

Radiocarbon accelerator mass spectrometry (AMS) techniques were used to date total organic carbon and plant seeds in the 1Fs core sequence (36&deg;48&prime;N, 100&deg;08&prime;E) from Qinghai Lake, China. This core was drilled ~18 m into Qinghai Lake sediments as part of an international cooperative research project, &ldquo;Scientifc Drilling at Qinghai Lake in the Northeastern Tibetan Plateau: High-Resolution Paleoenvironmental Records of Eastern Asia Linked to Global Change,&rdquo; which began in 2004. Based on the differences in lithology and total organic content (TOC) in core 1Fs, the core was divided into 3 sections for age-modeling purposes: the upper ~499 cm lacustrine silty clay to clay; the middle unit of silty clay with silt layers from 499&ndash;901 cm; and the lower 901&ndash;1861 cm silty clay, loess-like silt, and fne sand layers. Three different approaches are applied to the reservoir age problem. First, a simple linear regression gives an offset of 1342 yr. If the core is divided into three sections, linear regressions can be applied separately for the three segments, which results in an age estimate for the average hardwater effect of ~135 yr BP for the surface section up to 499 cm. If extrapolated for deeper sections, these results imply a higher reservoir offset for those two sections, which may be as much as 1143 and 2523 yr, but this assumes that there are no discontinuities in the core. A third approach using a wiggle-matching approach gave an offset of 196 yr. This study concludes that the reservoir age of Qinghai Lake is complex, but these new data add to our understanding of the 14C chronology of Qinghai Lake for the last 32 ka.</p

Tracing fossil fuel CO2 using D14C in Xi'an City, China

Radiocarbon can be used to trace fossil fuel CO2 (CO2ff) in the atmosphere, because radiocarbon has been depleted in fossil fuels. Here we present our study on the spatial distribution and temporal variations of CO2ff in Xi&#39;an City, China using D14C of both green foxtail (Setaria viridis, L. Beauv.) leaf samples and urban air samples collected in the recent years. Our results show that the CO2ff indicated by green foxtail ranged from 14.7 &plusmn; 1.7 to 52.6 &plusmn; 1.7 ppm, reflecting high CO2ff mole fractions in downtown, industrial areas, and at road sites, and low CO2ff mole fractions in public parks. Meanwhile, the monthly CO2ff reflected by air samples showed higher value in winter (57.8 &plusmn; 17.1 ppm) than that in summer (20.2 &plusmn; 9.8 ppm) due to the enhancement usage of coal burning and the poor dispersion condition of atmosphere. This study displays that the increased fossil fuel emission is associated with the fast development of Xi&#39;an City in China. It is worth mentioning that the green foxtail samples can be used to map out the CO2ff spatial distribution on large scale quickly and conveniently, while the air samples can be used to trace the CO2ff temporal variations with high resolution effectively. Therefore the D14C of both green foxtail and air samples is a good indicator of CO2ff emission.</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

Observations of Atmospheric Δ14CO2 at the Global and RegionalBackground Sites in China: Implication for Fossil Fuel CO2 Inputs

Six months to more than one year of atmospheric &Delta;14CO2 were measured in 2014-2015 at one global background site in Waliguan (WLG) and four regional background sites at Shangdianzi (SDZ), Lin&#39;an (LAN), Longfengshan (LFS) and Luhuitou (LHT), China. The objectives of the study are to document the &Delta;14CO2 background levels at each site and to trace the variations in fossil fuel CO2 (CO2ff) inputs at regional background sites. &Delta;14CO2 at WLG varied from 7.1&plusmn;2.9&permil; to 32.0&plusmn;3.2&permil; (average 17.1&plusmn;6.8&permil;) in 2015, with high values generally in autumn/summer and low values in winter/spring. During the same period, &Delta;14CO2 values at the regional background sites were found to be significantly (p &lt; 0.05) lower than those at WLG, indicating different levels of CO2ff inputs at those sites. CO2ff concentrations at LAN (12.7&plusmn;9.6 ppm) and SDZ (11.5&plusmn;8.2 ppm) were significantly (p &lt; 0.05) higher than those at LHT (4.6&plusmn;4.3 ppm) in 2015. There were no significant (p &gt; 0.05) seasonal differences in CO2ff concentrations for the regional sites. Regional sources contributed in part to the CO2ff inputs at LAN and SDZ, while local sources dominated the trend observed at LHT. These data provide a preliminary understanding of atmospheric &Delta;14CO2 and CO2ff inputs for a range of Chinese background sites.</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