Today, Asia is a remarkably diverse place. Tectonically, Asia hosts some of the highest and largest mountains on the planet, including the Himalayas and Tibetan Plateau, the Tien Shan, the Pamir, and the Altai mountains. Climatically, Asia experiences every year an array of impressive weather systems, from the monsoonal systems over Southeast Asia to the intense Siberian High that sits atop Mongolia in the winter, suppressing precipitation and producing the highest recorded sea-level surface pressures on Earth. Ecologically, Asia is even more diverse, with tropical and sub-tropical forests spanning much of Southeast Asia, expansive deserts across much of the length of China and Mongolia, and with the taiga spanning Siberia. How this tectonic, climatic, and ecological diversity has changed over the past 65 million years remains a hotly-debated and active area of research.
I study two aspects of this system: (1) the uplift histories and attendant climatic impacts of the Asian northern bounding ranges, including the Altai and the Hangay mountains and; (2) how changes in the supply of moisture by the mid-latitude westerlies has altered climate in Central Asia over the course of the Cenozoic Era.
a. Uplift of the Altai and Hangay mountains
The Altai and Hangay mountains form part of the modern-day "Mongolian Plateau", an area of high topography deep in the continental interior though substantially smaller than the Tibetan Plateau. The Altai mountains straddle the border between Mongolia, China, Russia, and Kazakhstan, while the Hangay lie entirely within Mongolia. Today, these mountains–particularly the Altai–form a prominent rain shadow and block moisture delivered by the mid-latitude westerlies, causing substantially more precipitation on their northern and western flanks than on their southern flanks.
We are interested in both when these mountain ranges formed and how their uplift subsequently impacted climate and ecosystems in Mongolia. We traveled to Mongolia in both 2011 and 2012 to collect samples from terrestrial basins lying in the rain shadow of these mountains. We primarily collected Eocene-to-modern carbonate samples from palesols in the Valley of Lakes--a deep, inward-draining depression that separates the Hangay Mountains from the Altai and Gobi Altai mountains. Back at the Stanford Biogeochemistry Laboratory, we measured the oxygen and carbon isotopes of these paleosol carbonates. Interestingly, the oxygen isotopes show no change over time, while the carbon isotopes–at all sites–increased dramatically over the same period. In research published in the American Journal of Science (Caves et al., 2014), we demonstrate that Mongolia has become substantially more arid over the past 30 million years and, in particular, in the last 10 million years. We attribute this increasing aridity to progressive uplift of, first, the Hangay mountains and, second, the Altai mountains, which blocked westerly moisture from reaching Mongolia. We tentatively estimate that precipitation has decreased by approximately 50% in much of Mongolia, causing a large decrease in primary productivity.
b. Westerlies-driven evolution of Cenozoic Central Asian climate
Our work in Mongolia led us to investigate how the primary sources of moisture to Central Asia have changed over the Cenozoic. Oxygen isotopes are sensitive recorders of both the type of atmospheric moisture transport and the sources of moisture (Winnick et al., 2014). Remarkably, the oxygen isotope values of pedogenic and lacustrine carbonate in Mongolia are statistically indistinguishable from oxygen isotope values derived from across all of Central Asia. Even more remarkably, the spatial distribution of reconstructed precipitation d18O in Central Asia since the late Paleocene is statistically indistinguishable from the modern spatial distribution. In Earth and Planetary Science Letters (Caves et al., 2015), we demonstrate that this is due to long-term stability of the moisture transport pathways to Central Asia. For nearly the entire Cenozoic, Central Asia has received moisture from the mid-latitude westerlies, while southern Tibet has received moisture from the South Asian monsoon. Even more remarkably, the moisture from these two dominant climatic systems have always mixed in the central Plateau.
These results imply that (1) southern Tibet has always been receiving moisture from a southerly, "monsoonal" source, and this moisture has always traversed a high, "Himalayan"-style barrier (often termed the Gangdese Arc), and; (2) paleoclimatic proxies in Central Asia reflect changes in the flux of moisture delivered by the mid-latitude westerlies. In short, uplift of the Tibetan Plateau appears to have little effect on climate in Central Asia, and only a high "Himalayan" ridge is necessary to both create monsoonal circulation and block that moisture from reaching Central Asia.