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

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

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