Chronology of the shell bar section and a discussion on the ages of the Late Pleistocene lacustrine deposits in the paleolake Qarhan, Qaidam basin |
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| Authors: | Hucai Zhang Guoliang Lei Fengqin Chang Yang Pu Hongfang Fan Yanbin Lei Mingsheng Yang Wenxiang Zhang and Lunqing Yang |
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| Institution: | (1) Key Laboratory of Lake Sedimentation of Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, 210008, China;(2) Key Laboratory of Western China Environments Systems (Ministry of Education), College of Earth and Environments Sciences, Lanzhou University, Lanzhou, 730000, China |
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| Abstract: | Shell bar, composed of abundant fossil shells of Corbicula fluminea müller and Corbicula largillierti philippi and located at the southeastern end (36°30′N, 96°12′E) of the paleolake Quarhan, is one of the most prominent features
in the Qaidam basin. It is the highest site where such species of fossil shells have been found in the Late Pleistocene age.
A 2.6-m-thick fresh profile was manually excavated to determine the formation ages and the scope of the high paleolake levels.
Accelerator mass spectrometry (AMS), conventional radiocarbon dating, and sector inductively coupled plasma-mass spectroscopy
230Th methods were used to investigate the reliability and accuracy of dating results in different laboratories. Ages of various
components (e.g. acid residual and acid soluble fraction of the organic matter) from the same sample were determined. Age
differences of a variety of materials (e.g. organic matter, fossil shells, and salt crystals), and age differences at the
same sampling position were evaluated. Dating shows that the AMS ages given by alkali residue and acid soluble fractions vary
greatly, e.g., from 124.5 cm upwards. The dates given by the acid soluble fraction were normal. Down the column, the dates
show a reverse pattern and those of alkali residues, especially the lower part of the section, show an unstable pattern. These
imply that organic matter had been influenced by two separate processes. One possible explanation is that the alkali residual
fraction most likely had been contaminated by dead carbon-bearing reworked material because the study section is located near
the edge of the paleolake and could have been easily influenced by old eroded deposits. Another is that the acid soluble fraction
could have been contaminated by upward-flowing groundwater containing soluble organic matter. The amount of this soluble organic
material should be very small because the strongest age reversal is in the lower part, where the TOC content remains low,
meaning that the change of soluble organic fraction alters neither TOC content nor the δ
13C dramatically. It is concluded that a uniform mega-paleolake developed in the Qaidam basin in the northeastern Tibetan plateau
between 39.7 and 17.5 14C kaB.P. During the period when the high paleolake level of Qarhan was formed, the huge paleolake covered a vast area with
dramatic lake level fluctuations. It is found that the ages determined may be influenced by either radioactive 14C variations of repeated deposition or content variations in the atmosphere. The ages given by fossil shells are 15–18 ka
older than those given by organic matter. The large differences between the ages of fossil shells and organic matter might
have resulted from the large water areas and huge water volume and the special location of the high-elevation Qaidam basin.
This study also shows the reliability and accuracy of the 230Th dating method on the salt crystals but further study is needed to determine whether this method could be applied to the
study area.
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Translated from Quaternary Sciences, 2007, 27(4): 511–521 译自: 第四纪研究] |
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| Keywords: | Qaidam basin shell bar section traditional and AMS dating |
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