In this study, grain-size of 507 bulk samples from the QA-I Mioceneloess-soil sequence at Qinan were analyzed, and the grain-size features are compared with those oftypical Quaternary loess and soil samples, represent...In this study, grain-size of 507 bulk samples from the QA-I Mioceneloess-soil sequence at Qinan were analyzed, and the grain-size features are compared with those oftypical Quaternary loess and soil samples, representative lacustrine and fluvial samples. Theresults indicate that the grain-size distribution pattern of the Miocene loess is essentiallysimilar to that of Quaternary loess, but greatly differs from the lacustrine and fluvial sediments.Loess layers are regularly coarser than soil layers, indicating cyclical climate changes. Mediangrain-size along the 253.1 m sequence varies from 6 to 13 μm and the 】 63 μm fraction representsonly 5.3% in maximum, 0.9% in average. Long-term grain-size variations are consistent with the loessaccumulation rate at Qinan and with the eolian mass accumulation rate in the North Pacific. Thesefeatures firmly indicate an eolian origin of the studied sequence, and also reveal a coeval changesbetween the long-term changes of eolian grain-size and continental aridity in the dust sourceregions.展开更多
This study generated a high-resolution paleoweathering record of the loess-soilsequence from the central Loess Plateau covering the last 220 ka using the ratio between the CBD（citrate-bicarbonate-dithionite） extract...This study generated a high-resolution paleoweathering record of the loess-soilsequence from the central Loess Plateau covering the last 220 ka using the ratio between the CBD（citrate-bicarbonate-dithionite） extractable free Fe<sub>2</sub>O<sub>3</sub> （Fed） and total Fe<sub>2</sub>O<sub>3</sub> （Fet）. The new proxyshows a series of millennial oscillations, which are not necessarily documented by magneticsusceptibility from the same site. Because chemical weathering in the region depends mainly uponsummer precipitation and temperature, we interpret these changes to be a result of millennial-scalevariations in the strength of the East-Asian summer monsoon. Over the last glacial period, there isgeneral agreement between the loess weathering record and the GRIP ice δ<sup>18</sup>O record. The firstsuggests a rather stable summer monsoon pattern during the Last Interglacial period (marine δ<sup>18</sup>Ostage 5e). Large amplitude millennial oscillations of the summer monsoon seem to be particularlycharacteristic of the last glacial period whereas the fluctuations for the older periods are smoother.展开更多
The accurate radiocarbon dating of loess-soil sequences plays an essential role in the reconstruction of the environmental and climatic changes in continental settings during the last glaciation and Holocene. However,...The accurate radiocarbon dating of loess-soil sequences plays an essential role in the reconstruction of the environmental and climatic changes in continental settings during the last glaciation and Holocene. However, our knowledge about the reliability of radiocarbon ages of various fractions of soil and loess samples is still insufficient. Here, we present our study results on radio- carbon ages based on bulk organic matter, humin fraction, and carbonate of samples collected from a loess-paleosol section in the western Chinese Loess Plateau. We compare these observations with the optically stimulated luminescence ages and charcoal radiocarbon ages to evaluate the reliability of these fractions. We observed that the radio- carbon ages of humin fraction are very close to those of charcoal and are consistent with the optically stimulated luminescence ages within the experimental errors. We observed a significant deviation in the radiocarbon ages of carbonate and bulk organic matter from those of charcoal and optically stimulated luminescence ages, likely due to the dilution of these fractions during the pedogenetic process. Our results reveal that, except for charcoal, the humin fraction may yield reliable laC ages for the Chinese loess-soil sequence.展开更多
基金This work was supported by the Chinese Academy of Sciences(Grant No.KZCX3-SW-139)the National Natural Science Foundation of China(Grant Nos.40231001 and 40272088)the'973'Project(Grant No.2004CB720203).
文摘In this study, grain-size of 507 bulk samples from the QA-I Mioceneloess-soil sequence at Qinan were analyzed, and the grain-size features are compared with those oftypical Quaternary loess and soil samples, representative lacustrine and fluvial samples. Theresults indicate that the grain-size distribution pattern of the Miocene loess is essentiallysimilar to that of Quaternary loess, but greatly differs from the lacustrine and fluvial sediments.Loess layers are regularly coarser than soil layers, indicating cyclical climate changes. Mediangrain-size along the 253.1 m sequence varies from 6 to 13 μm and the 】 63 μm fraction representsonly 5.3% in maximum, 0.9% in average. Long-term grain-size variations are consistent with the loessaccumulation rate at Qinan and with the eolian mass accumulation rate in the North Pacific. Thesefeatures firmly indicate an eolian origin of the studied sequence, and also reveal a coeval changesbetween the long-term changes of eolian grain-size and continental aridity in the dust sourceregions.
文摘This study generated a high-resolution paleoweathering record of the loess-soilsequence from the central Loess Plateau covering the last 220 ka using the ratio between the CBD（citrate-bicarbonate-dithionite） extractable free Fe<sub>2</sub>O<sub>3</sub> （Fed） and total Fe<sub>2</sub>O<sub>3</sub> （Fet）. The new proxyshows a series of millennial oscillations, which are not necessarily documented by magneticsusceptibility from the same site. Because chemical weathering in the region depends mainly uponsummer precipitation and temperature, we interpret these changes to be a result of millennial-scalevariations in the strength of the East-Asian summer monsoon. Over the last glacial period, there isgeneral agreement between the loess weathering record and the GRIP ice δ<sup>18</sup>O record. The firstsuggests a rather stable summer monsoon pattern during the Last Interglacial period (marine δ<sup>18</sup>Ostage 5e). Large amplitude millennial oscillations of the summer monsoon seem to be particularlycharacteristic of the last glacial period whereas the fluctuations for the older periods are smoother.
基金Acknowledgements This work was supported by the National Natural Science Foundation of China （Grant No. 40971056） and the Fundamental Research Funds for the Central Universities （Grant No. Lzujbky-2010-120）. We would like to thank Dr. Loukas Bardon of the University of Chicago and Dr. Yanxia Zhong of the Ningxia University for their assistance during fieldwork. We thank Dr. Saeed Doroudiani and Dr. Shiyong Yu for their critical reading and editing of the manuscript.
文摘The accurate radiocarbon dating of loess-soil sequences plays an essential role in the reconstruction of the environmental and climatic changes in continental settings during the last glaciation and Holocene. However, our knowledge about the reliability of radiocarbon ages of various fractions of soil and loess samples is still insufficient. Here, we present our study results on radio- carbon ages based on bulk organic matter, humin fraction, and carbonate of samples collected from a loess-paleosol section in the western Chinese Loess Plateau. We compare these observations with the optically stimulated luminescence ages and charcoal radiocarbon ages to evaluate the reliability of these fractions. We observed that the radio- carbon ages of humin fraction are very close to those of charcoal and are consistent with the optically stimulated luminescence ages within the experimental errors. We observed a significant deviation in the radiocarbon ages of carbonate and bulk organic matter from those of charcoal and optically stimulated luminescence ages, likely due to the dilution of these fractions during the pedogenetic process. Our results reveal that, except for charcoal, the humin fraction may yield reliable laC ages for the Chinese loess-soil sequence.
