Climate Change And The Oxygen Isotope Of Carbonate In Loess Deposit Of Northeastern Tibetan Plateau Since LGM

The carbonate in loess deposit is abundant,and the pedogenic carbonate records the pedogenic environment,especially the moisture in the past,can be used to qualitatively and/or quantitatively reconstruct paleoclimate.In this paper,the carbonate content and the carbon and oxygen isotopes of the loess samples from the Ledu section of the northeastern Tibetan Plateau(NETP),the Sanbahuo section of the northeastern China,and TEH section of Xinjiang were measured,and their climatic implication was discussed.The results showed that the loess deposits of the studied regions have abundant carbonates since the Last Glacial Maximum(LGM).The carbonate content of the bulk samples from Xinjiang is the most,followed by Ledu samples,and that from samples from NETP is the least.The change of carbonate content of the bulk samples might reflect difference of precipitation.The change of carbonate content in the loess sequence in NETP could show the increase of the precipitation and the related soil humidity from LGM to Holocene.The components of<2?m(including secondary carbonate)were extracted from the bulk samples.The result of the Fourier Transform infrared spectroscopy(FTIR)shows that the carbonate minerals in the<2?m are mainly calcite,and almost no dolomite.This indicates that most of the carbonate components are pedogenic.By contrast,the ?13C and ?18O values of<2?m carbonate in loess and domolite are quite difference,which may confirm that<2?m carbonate in loess are pedogenic calcite,but not detritus dolomite.The ?18O values of the pedogenic carbonate in loess from Ledu during LGM and early Holocene,are 5.74‰ and 1.54‰,respectively,higher than the theoretical value of ?18O under the condition of modern monsoon climate.However,the temperature difference was not enough to cause this significant magnitude change of the ?18O values in the pedogenic carbonate.Compared with that in East Asian monsoon region including the Chinese Loess Plateau and the northeastern China,the values of the?18O of the pedogenic carbonate in loess from Ledu during LGM and the early Holocene are~2.0‰—6.2‰ higher,but similar to that in the westerly regions such as Xinjiang.The distribution characteristics and spatial differences of the ?18O values of the pedogenic carbonate in loess in different region may reflect that the East Asian summer monsoon was not the controlling factor of the precipitation during the LGM and early Holocene in the northeastern Tibetan Plateau(NETP),while the westerly rainfall and/or local vapor evaporation cycle had an important impact in this period.The climate changing from dry and cold to warm and humid,could be the reason for the gradual decrease of the ?18O values of the pedogenic carbonate in the loess from LGM to early Holocene in NETP.The ?18O values of the pedogenic carbonate in the loess from different climatic zones(NETP,the East Asian monsoon region and the westerly zone)are significant different,with which climatic indication is worth for further investigation.