The late Eocene rise of eastern Tibet and its impact on climate and biodiversity

The rise of the Tibet Plateau played a significant role in Asian climate evolution, especially the eastern Tibet which forms the transitional area where the South Asian Monsoon and East Asian Monsoon systems interact, and the formation of modern high-relief topography of eastern Tibet potentially makes its the cradle of Hengduan Mountain biodiversity hotspot.

We reconstruct the uplifting history of the eastern Tibet in three Cenozoic basins, including Gonjo, Relu and Markam basins based on multiple proxies. These basins are NW-SE directed basins with an elevation of ~4000 m at present. Today, the climate in these basins is semi-humid monsoonal with a mean annual air temperature of 0-5 ℃ and annual precipitation of 400-600 mm/yr. Aeolian deposits are pervasively developed at the bottom of the eastern Tibet Cenozoic basins before early Eocene (>50 Ma), especially in the Gonjo and Relu basins. Fluvial and lacustrine strata were deposited in the middle part of Gonjo Basin and the lower part of Relu Basin (50-45 Ma). Large number of lacustrine sediments (45-34 Ma) exists in the middle of the Relu Basin and the top of the Markam Basin. Oxygen and clumped isotopes from the Gonjo Basin suggested an earlier uplift from 0.7 km to 3.8 km during the middle Eocene (50-40 Ma; Xiong et al., 2020). The CLAMP and clumped isotope results for the Relu Basin indicated a rise in elevation from 0.6 km to 3.7 km between 45 to 34 Ma (He et al., 2022). The Markam Basin remained at a moderate elevation of 2.6 km between 42 to 39 Ma, then rose rapidly to 3.8 km by 36 Ma as indicated by CLAMP and oxygen isotope paleoaltimetry (Zhao et al., 2023). Combined with published paleoelevation results, the elevational history of eastern Tibet revealed as: During the early Eocene, it remained as lowland, and then underwent moderate to quick rise in the middle Eocene, approached to near present elevations by the latest Eocene of ~35 Ma.

The rise of the eastern Tibet during warm-house period significantly changed the climate as well as the biodiversity within and around Tibet. Before the rise of eastern Tibet, the climate was dry with typical intermountain desert system. Accompany with the rise of eastern Tibet, a Mediterranean climate developed in eastern Tibet characterized by bi-modal precipitation with two peaks during the spring (MAM) and autumn (SON) seasons, and a lower precipitation in the summer (JJA) seasons (He et al., 2022; Chen et al., 2023). Another line of evidence that supports the Mediterranean-like climate comes from the plant fossils. A typical semi-arid or arid flora that includes Palm, Eucalyptus, Palibinia and Quercus shows some similarity to Mediterranean vegetation. This flora co-evolved with the rise of eastern Tibet in the Relu and Markam basins, and even dispersed to the southeastern China. The high-relief topography, coupled with this distinctive Mediterranean climate system, significantly contributes to the development of the highly diversified species.