Strengthened East Asian summer monsoons during a period of high-latitude warmth? Isotopic evidence from Mio-Pliocene fossil mammals and soil carbonates from northern China

Abstract

The East Asian monsoons have fluctuated in concert with high-latitude warmth during the past several hundred thousand years, with humid summer monsoon-dominant climates characterizing warm intervals, including interglacials and interstadials, and arid winter monsoon-dominant climates characterizing cool intervals, including glacials and stadials. Of the states comprising the mid-Pleistocene to recent climatic regime, interglacials are most similar in terms of high latitude ice volumes and temperatures to those extant during the late Miocene and early Pliocene. Thus, an important question is whether Mio-Pliocene climates in northern China were analogous to a hypothetical ‘prolonged interglacial state,’ with increased summer monsoon precipitation and expansion of forest and steppe environments at the expense of desert environments. We utilize new and previously published carbon isotopic data from fossil teeth and soil carbonates to place constraints on paleovegetation distributions and to help infer the behavior of the monsoon system between ~7 and 4 Ma. We find that plants using the C_4 photosynthetic pathway -— which today are largely grasses found in regions with warm season precipitation -— were present in northern China by late Miocene time, demonstrating that the C_4 expansion in China was not significantly delayed compared to the global C4 event. During the late Miocene–early Pliocene interval, soil carbonate and tooth enamel δ^(13)C data indicate: 1) that nearly pure C_3-plant ecosystems existed in the southern Chinese Loess Plateau (CLP), and therefore ecosystems there were dominated by woody dicot, herbaceous dicot, or cool-season grass vegetation (or a combination of these), and 2) that the CLP was characterized by a pattern of northward-increasing C_4 vegetation and aridity. Utilizing a broadened conceptual model for interpreting δ^(13)C data, and citing independent faunal, floral, and lithostratgraphic data, we suggest that these patterns reflect northward expansion of forest and steppe ecosystems and relatively humid monsoon climates during the late Miocene and early Pliocene. An important implication of this interpretation is that the forcing mechanism illuminated by the temporal correlation during the Pleistocene between warm high latitudes and strong East Asian summer monsoons is a robust feature of the Eurasian tectonic–climatic system that predates the Plio-Pleistocene climatic reorganization.