Mid-Miocene terrestrial carbon isotope shift driven by atmospheric CO2 in the Xining Basin, NE Tibetan Plateau

The Mid-Miocene Climatic Optimum (MMCO; ~17-15 Ma) possesses boundary conditions including many flora and fauna close to modern but extreme warming occurred. Clarifying the response mechanism of terrestrial ecosystem to the MMCO can provide important clues to our ecological change under the future global warming. However, the MMCO is mostly captured by abundant deep-sea deposits, related terrestrial ecosystem records are scarcely reported because of depositional hiatuses. Continuous sediments from Tashan Borehole (TSB) in the Xining Basin, northeastern Tibetan Plateau (NETP) have been dated as 18.5-13.5 Ma by high-resolution paleomagnetic studies, covering the entire MMCO. Sediment organic carbon isotope (δ13CTOC) could sensitively record its overlying plants and less restricted by preservation. Therefore, TSB δ13CTOC record was recovered in this study to investigate the NETP terrestrial ecosystem response to the MMCO. Firstly, TSB has developed a typical distal floodplain deposit and no change of local provenance appeared during 18.5-13.5 Ma, indicating that surrounding land plants served as the dominant organic source for TSB sediments. TSB δ13CTOC values range from -28.03‰ to -25.16‰, combined with the analysis by isotope mass-balance model, we deduced that C3 plants are the dominant component for the mid-Miocene terrestrial vegetation in the NETP, no obvious C4 plants have grown. Secondly, an obviously negative shift of 0.6‰ occurred in TSB δ13CTOC during ~17.6-14.2 Ma, coincides with the positive excursion of ~0.7‰ in global benthic δ13C. TSB δ13CTOC values vary negatively with the global benthic δ13C values, both in long-term Monterey Excursion event and short-term carbon maxima events. We deduced that increasing atmospheric CO2 during the MMCO lead to the higher temperature, rise of sea level and ultimately increased organic carbon burial, and simultaneouslyenhanced the carbon isotope fractionation during terrestrial C3 plants’ photosynthesis. Finally, terrestrial ecosystem in Xining Basin captured the MMCO exactly at 17.6-14.2 Ma. In a scenario of warm and humid, the carbon isotope cycle of terrestrial C3 plants is dominantly controlled by atmospheric CO2 concentration, may be also superimposed by the effect from enhanced global warming-driven precipitation.