TECTONICALLY FORCED CARBONATE OXYGEN AND CARBON ISOTOPE SHIFTS SINCE 15 MA IN THE TIAN SHAN FORELAND, TARIM BASIN CHINA

Located between the Tibetan Plateau, the Tian Shan, and the Pamir ranges, the Taklimakan Desert formed during the Cenozoic and evolved to become one of the largest deserts on Earth. New stratigraphic evidence from a 3800m thick section (WK) in the northwestern Tarim Basin suggests the desert formed at ~12 Ma, when an erg system developed abruptly above wetlands. We infer this desertification to be linked to rainshadow development east of the growing Pamir and Tian Shan. To test this hypothesis, we present new stable isotope data from sedimentary carbonates from fluvio-lacustrine (~15-12 Ma), eolian dune and interdune palustrine (12-7 Ma), and floodplain and palustrine (7~2 Ma) strata within the WK section. Average δ¹⁸O_c values shift from -8.43‰ before 13 Ma to -7.40‰ before erg-system development. From 13-7 Ma, δ¹⁸O_c values decrease to as low as -10‰, before abruptly increasing at 7 Ma and moving to more positive values as high as -6.32‰. Overall, δ¹³C_c values are relatively constant from 15-5 Ma (-0.90 ± 0.77‰), when they shift to more positive values as high as 1.47‰. In paleosol carbonates, relatively positive and constant δ¹³C_c values throughout the section indicate low-respiration rates in soils developed in arid conditions. From ~12-7 Ma, deposition of >1200 m-thick eolian sand is linked to a ~2‰ negative shift in interdune palustrine δ¹⁸O_c, when detrital zircon populations indicate a change in fluvial source region from the Pamir to the emerging Tian Shan. This negative isotopic shift resulted from rainout of isotopically heavier water on the windward side of the growing Tian Shan. After ~7 Ma, a trend to more positive values of both δ¹³C_c and δ¹⁸O_c is concurrent with a shift to palustrine and fluvial environments, and indicates enhanced evaporation in an increasingly arid environment, perhaps associated with increased basin isolation. Increasingly positive δ¹⁸O_c and δ¹³C_c values continued to at least 3.5 Ma in shallow ground water carbonates. Our data suggest tectonic forcing of δ¹⁸O_c to more negative values in the emerging rainshadow of the Tian Shan, and illustrates that careful consideration of depositional environments and provenance is crucial to understanding the tectonic influence on variable δ¹³C_c and δ¹⁸O_c values of sedimentary carbonates.