A STABLE CARBON AND OXYGEN ISOTOPE RECORD FROM PEDOGENIC CARBONATE FOR THE LATEST OLIGOCENE (EARLIEST MIOCENE ?) OF NORTHWEST CHINA AND ITS PALEOCLIMATIC AND PALEOECOLOGICAL IMPLICATIONS

A continental chemostratigraphic record for the latest Oligocene of northwest China has been developed using the stable isotopes of carbon and oxygen from pedogenic carbonate nodules. The section is located in Xinjiang, China in the foothills of the Tian Shan range. Stratigraphic control for the section is based on the presence of two latest Oligocene fossil mammals. Approximately in the middle of the section, the indricothere, Dzungariotherium orgosensis (Perissodactyla, Mammalia) and Lophiomeryx have been reported.

Carbon isotopic values range from -7.06 to -9.67‰ and have a mean of -8.11‰ (mean = -7.82‰ excluding excursion values). Using Holocene carbon isotopic values from C3 ecosystems as a comparison the results indicate C3 plants existing at their moisture stress limit and the possible presence of a C4 component. Oxygen isotopic ratios range from -8.84 to -11.47‰ with an average of -10.10‰ and the overall trend is towards more positive values suggesting increasing arid conditions and possibly temperatures. In addition, there is a negative excursion in both carbon and oxygen ratios. During the excursion δ¹³C ratios range from -7.06 to -9.67‰ and δ¹⁸O values range from -10.41 to -11.47‰. It is possible that this is a continental record of a negative excursion recorded in the marine record from approximately 23.5 to 23.0Ma but tighter stratigraphic control is needed. In short, the data suggest a shift towards drier conditions in northwest China occurred during the latest Oligocene and earliest Miocene. The Tian Shan range was not at an elevation during this time that could have affected regional climatic conditions and it is suggested that the retreat of the Paratethys was causing the onset of drier conditions in northwest China during this time.