How “wet islands” form – A case study of the Qilian Mountains on the arid northern Tibetan Plateau during the Middle Miocene

Abstract

In arid regions, mountains usually exhibit diverse climates and complex ecological niches, fostering the formation of “wet islands”. However, the timing and mechanisms behind the formation of such “wet island” remain poorly understood, particularly in the central East Asia arid region (CEAA). This study focuses on the Qilian Mountains in the northern Tibetan Plateau (NTP), adjacent to the CEAA, which constitute an alpine “wet island” with a mean annual precipitation (MAP) exceeding 500 mm. Miao et al. (2012) synthesized climate records spanning Eurasia and the oceans since the Middle Miocene and proposed a conceptual hypothesis: global temperature decreases drive aridification, while mountain uplift shapes regional climate humidity. In this study, we explore the Middle Miocene (16–12 Ma), a period characterized by significant global cooling and intense NTP uplift, to quantify how these processes contributed to the formation of the Qilian Mountains “wet island”. First, we integrated typical climate records from the Westerlies, Asian monsoon, Plateau basin, and Qilian Mountains. The results show continuous aridification in the first three regions driven by a cooling-induced reduction in moisture transport. In contrast, the Qilian Mountains experienced a wetting trend due to orographic uplift. Second, this differential climate evolution led to divergent vegetation patterns between the Qilian Mountains and Qaidam Basin: conifers became dominant in the mountains, while the basin interior exhibited a complex vegetation response to both cooling and uplift. The moisture disparity between the mountains and basin also widened, with MAP differences widening from ∼100 mm at 16–15 Ma to ∼470 mm at 13–12 Ma. This growing disparity indicates that the formation of the Qilian Mountains “wet island” occurred during the Middle Miocene Climatic Cooling period (14–12 Ma). Third, we conducted a regional climate model (RegCM 4.6) simulation at a 30-km resolution, testing temperature sensitivity (a decrease of ∼2 °C) and comparing the results with a topography sensitivity test (uplift from one-third of the present elevation to current level) from Miao et al. (2022a). The model results show that the cooling-driven precipitation reduction in the CEAA (−100 %) was much greater than the precipitation increase (+30 % to +80 %) caused by uplift. Conversely, the Qilian Mountains experienced a substantial precipitation increase (+100 %) due to uplift, which mitigated the slight cooling-driven decrease (−10 %). These results suggest that global cooling and mountain uplift were pivotal factors in the formation of the Qilian Mountains “wet island”, within a context of overall drying in the CEAA. After the NTP reached its present elevation in the late Middle Miocene, global climate primarily governed the evolution of climate and environment in the interior of Asia. In summary, this study provides a model for understanding the geological formation of “wet islands” and contributes to evaluating biotic and environmental changes in arid regions in response to future climate change.