Cenozoic organic carbon isotope and pollen records from the Xining Basin, NE Tibetan Plateau, and their palaeoenvironmental significance

Marine oxygen isotope records show that the Cenozoic global climate has undergone remarkable fundamental changes. However, how terrestrial ecosystems respond to these changes remains unclear. The Xining Basin on the northeastern Tibetan Plateau holds continuous fine fluvial-lacustrine sediments from the early Eocene to the early Miocene (between ~ 52 Ma and 17 Ma). This paper first presents total organic carbon isotope (δ13CTOC) and pollen records from the Xiejia section in the Xining Basin, as indicators of the long-term terrestrial paleoecosystem and paleoclimate changes in the Asian interior of NW China. The δ13CTOC record shows a long term persistently decreasing trend (from ~− 20‰ to − 23.5‰) superimposed with obvious cycles of five distinctive phases of heavier and lighter δ13CTOC values, along with short pulses of especially low values occurring at ~ 32.5 Ma, 23 Ma, 21.5 Ma, and 18 Ma. These variations generally correlate with changes of gymnosperm content or the ratio of gymnosperm to angiosperm (G/A); i.e., heavier δ13CTOC values correspond with higher gymnosperm content or higher G/A, as well as with global temperatures (for both long term trends and shorter term fluctuations). We propose that global temperatures might provide the major factor controlling the evolution of the gymnosperm content (and the G/A ratio) that determines the δ13CTOC in the sediments. The East Asian monsoon may not have reached this region during the deposition of the sediments, or alternatively, its signals could not clearly be identified in these low resolution paleoecologic records.