PARENT SESSION
Tuesday, August 8, 5:00-6:30 pm
Poster Session 10 - Genetics, evolution, and paleoecology
Exhibit Hall, Ballroom Level, Cook Convention Center

The increasing importance of C₄ vegetation during the late Cenozoic in the Sikouzi basin, NE Tibet.

Hough, Brian^*,1, Garzione, Carmala¹, Peizhen, Zhang², Guangliang, Zhang², ¹ University of Rochester, Rochester, NY² Institute of Geology, China Earthquake Administration, Beijing, China

ABSTRACT- Carbon and oxygen stable isotope ratios from soil carbonates in part reflect the C isotopic compositions of soil organic matter and the O isotopic composition of rainfall at the time of carbonate growth, providing reliable indicators of paleoenvironment. Soil water, derived from rainfall, may experience evaporative enrichment of ¹⁸O. Therefore, changes in soil carbonate ¹⁸O values over time may reflect changes in the source, transport pathway, and/or amount of rainfall, as well as changes in local aridity. ¹³C values of soil carbonates vary depending on soil respiration rates and the proportion of plants utilizing C₃ versus C₄ photosynthetic pathways and, provide reliable indicators of vegetal cover. The Sikouzi section locality in Guyuan basin, NE Tibet lies east of contractional deformation of the Earth’s crust associated with the rise of the Tibetan Plateau and contains a well exposed paleosol succession within an ~3 km thick section of calcareous sandstones and loess. O and C isotope records were constructed based on 136 soil carbonate and cement samples from the Sikouzi paleosol section. Lower in the section, typical ¹³C values of -5 ‰ indicate water stressed conditions in a mixed C₃-C₄ ecosystem, but do not suggest the significant presence of C₄ plants. However, anomalously high ¹³C values (0.5‰ - 2‰ – indicative of C₄ plants) are recorded in soil carbonates near the top of the Sikouzi section. These values are interpreted to reflect the increasing importance of C₄ plants in NE Tibet at ~2 Ma potentially in response to either 1) global changes in climate or 2) local changes in the relative strength of the East Asian/Siberian monsoon.

Key words: stable isotope, C₄, NE Tibet