• Dating Paleosol and Animal Remains in Loess Deposits

      Zhang, H. C.; Li, B.; Yang, M. S.; Lei, G. L.; Ding, H.; Niu, Jie; Fan, H. F.; Zhang, W. X.; Chang, F. Q. (Department of Geosciences, The University of Arizona, 2006-01-01)
      Accurate and reliable dating of paleosols, animal remains, and artifacts is of crucial importance in reconstructing environmental change and understanding the interrelationship between human activities and natural environments. Dating different materials in the same sample can help resolve problems such as soil carbon sources and carbon storage state. Conventional radiocarbon dating of soil (inorganic and organic matter) and accelerator mass spectrometry (AMS) dating of animal remains (fossil bones and teeth) result in different ages for materials from the same sample position in a typical loess section at Xinglong Mountain, Yuzhong County, Gansu Province in NW China. Inorganic matter is ~3400 yr older than organic matter, 4175 +/175 cal BP to 3808 +/90 cal BP. A 1610-yr difference between the 14C ages of fossils (animal bones and teeth) and soil organic matter suggests that a depositional hiatus exists in the studied profile. The varying 14C ages of fossils and soil organic and inorganic matter have important implications for paleoclimate reconstructions from loess sections. It is critical to consider the meaning of the variable 14C ages from different material components from the same sample position in terms of soil organic and inorganic carbon storage, vegetation history reconstruction, archaeology, and the study of ancient civilizations.
    • Dilemma of Dating on Lacustrine Deposits in an Hyperarid Inland Basin of NW China

      Zhang, H. C.; Ming, Q. Z.; Lei, G. L.; Zhang, W. X.; Fan, H. F.; Chang, F. Q.; Wünnemann, B.; Hartmann, K. (Department of Geosciences, The University of Arizona, 2006-01-01)
      Conventional and accelerator mass spectrometry (AMS) radiocarbon, TL, OSL, and IRSL dating results on samples from the cores D100 and I70 from Ejina Basin, one of the most important inland basins in arid-hyperarid NW China, show that it is difficult to determine the ages of sediments at different depths. AMS ages of core D100 samples demonstrate that the sediments at depths from 10 to 90 m were formed between 14 to 30 kyr BP. The inverted ages from both the D100 and I70 cores imply that there was a strong reworking of the sediments during and after deposition processes. The inverted ages also indicate drastic fluctuations of groundwater bearing soluble organic matters, which might be related to neotectonic activities and climate changes during the period. Consequently, it is impossible to establish an accurate and reliable chronology for the cores based only on these dates. All AMS ages, if they are reliable and acceptable, indicate a high deposition rate (5~8 mm/yr), and since all TL, OSL, and IRSL ages are much older than those given by AMS, it makes these methods questionable for determining the ages of lacustrine-fluvial-alluvial deposits.