The Cenozoic climatic and tectonic history of Asia [electronic resource]
Keywords: Earth System Science.

There remains substantial controversy surrounding how and if mountain uplift influences climate. The uplift of major mountain ranges is hypothesized to affect both regional climate by altering atmospheric circulation and global climate by removing atmospheric CO2 through increased silicate weathering. Herein, I use the Cenozoic Era as an ideal period with which to explore both of these hypotheses: (1) I address the role of tectonics in the evolution of climate in Asia (Chapters 1--4), and (2) I quantify the behavior of the silicate weathering negative feedback and reconcile tectonic explanations of Cenozoic cooling with carbon mass balance constraints (Chapter 5). Links between tectonics and regional climate have been extensively studied in Asia. However, despite decades of research, the relative roles of tectonics and global climate in pacing the evolution of climate in Asia remain disputed. Today, climate in Asia is characterized by large-scale monsoonal circulation over India and southern China and exceptionally arid deserts across much of Central Asia that, to the north, yield to the wetter conditions of the steppe and taiga. Understanding how, why, and if these boundaries have changed during the Cenozoic has been limited by the lack of paleoclimatic proxy data that can be broadly applied over space and time. I use a large compilation of pedogenic and lacustrine carbonate stable oxygen (δ18O) and carbon (δ13C) isotope data, which record large-scale atmospheric circulation and primary productivity, respectively, to reconstruct climate in Asia. The δ18O data indicate that atmospheric circulation has remained similar to today for at least 55 million years. Southern Tibet has always received monsoonal moisture, and this monsoonal moisture has never extended northward of the central Tibetan Plateau. In contrast, northern Tibet and Central Asia have received moisture dominantly via the mid-latitude westerlies, which have maintained a semi-arid to arid climate in Central Asia since the early Eocene. Thus, large-scale atmospheric circulation appears largely unperturbed by contemporaneous large changes in India-Asia collisional tectonics and in global climate. The δ13C data support this interpretation, indicating that during the Neogene, Central Asia has remained exceptionally arid, particularly compared with surrounding regions, such as northern India and southern China, which have received abundant precipitation due to the presence of monsoonal moisture. However, increases in δ13C across much of Central Asia—particularly Mongolia and Kazakhstan—during the late Neogene imply that primary productivity has declined across much of Asia. This large-scale "de-greening" is attributable to the combined effect of late Cenozoic global cooling and the recent uplift of the Tian Shan and Altai mountains, which block westerly moisture. The uplift of these ranges in the late Miocene is further substantiated by decreasing δ18O data in eastern Kazakhstan and increasing δ13C data in western Mongolia. These regional data demonstrate that the Tian Shan and Altai reached sufficient elevations to interact with the westerly jet by the late Miocene, establishing a substantial rain shadow and shifting precipitation to spring and fall in western Central Asia. Thus, though large-scale atmospheric circulation has remained relatively unchanged since the early Eocene, uplift of ranges in northern Central Asia have contributed to increased aridity during the late Cenozoic. Uplift of mountains—particularly the Himalayas and Tibetan Plateau—during the Cenozoic has also often been linked to hypothesized increases in global weathering fluxes. However, the need for carbon mass balance in the ocean-atmosphere system over geologic timescales requires a concurrent increase in solid Earth degassing for which evidence is sparse. I use Cenozoic carbon cycle data and a box-model of the ocean-atmosphere carbon cycle to calculate the mass imbalance between the input and output fluxes of carbon to the ocean-atmosphere. I find that the imbalance is approximately zero over the past 65 million years, implying the existence of a strong silicate weathering negative feedback on climate. Using additional carbon cycle data, I demonstrate that the strength of the silicate weathering feedback has increased, meaning that silicate weathering in the modern produces the same flux of weathered material as in the Eocene, but at a lower atmospheric CO2 level. This strengthening of the feedback results in invariant weathering fluxes even as climate cools. I attribute increasing feedback strength to an increase in the reactivity of rock on Earth's surface, possibly driven by increased uplift in the late Cenozoic. This mechanism reconciles evidence for a tectonic control on weathering with the need to maintain carbon mass balance and may explain why there are periods in the geologic record when the Earth system appears to have been more sensitive to carbon cycle perturbations. These 5 chapters illustrate some of the ways in which mountains have influenced climate during the past 65 million years. In Asia, proxy data demonstrate the resiliency of atmospheric circulation in spite of large changes in tectonics, but also show the role of recent uplifts in altering local climate. In comparison, on a global scale, increasing Earth surface reactivity—possibly due to increased uplift—has made silicate weathering more efficient. Consequently, the Quaternary Earth has a greater capacity to increase weathering fluxes in response to large carbon cycle perturbations than in the geologic past.