For fifty years, the Tibetan Plateau has been recognized as the largest topographic feature that perturbs atmospheric circulation. It serves as an ideal field laboratory for understanding the geodynamic processes that build high terrain. Accordingly, the growth of the plateau should have altered atmospheric circulation and therefore written an evolving paleoclimatic signature not only on eastern Asian regional climates, but on global climate as well. Despite many recent studies, we still do not know precisely when the Tibetan Plateau reached its current dimensions and how it perturbs atmospheric circulation. This project brings together geodynamicists, atmospheric scientists, and paleoclimatologists in a multidisciplinary study of the when and the how.
One of the major goals of the project is to quantify the extent to which Tibet has grown by crustal thickening, by thrust faulting and folding, by flow within the crust that redistributes material there, or by replacement of cold mantle lithosphere with hotter material (all in a state of isostatic equilibrium). Such quantification will take big steps toward the understanding of how high plateaus are built and how continental lithosphere deforms, topics at the forefront of geodynamics.
Determining how Tibet has grown will require determining when crustal shortening and thickening occurred, using basic field methods and modern laboratory techniques, and quantifying paleoaltitudes with new isotopic tools. Applying such paleoaltimetric techniques, however, requires an understanding not only of how the atmosphere transports isotopes, but how the evolving high terrain affected surface temperatures at times in the past. Even if the project?s focus were solely on how Tibet has grown, a meteorological component of the study, focused particularly on eastern Asia?s hydrological cycle, would be necessary.
Most continental paleoclimatic indicators are thought to be more sensitive to precipitation than to temperature, and among the unknowns of future climate, the hydrological cycle stands out. Accordingly, a major focus will be on understanding how high terrain like Tibet affects the hydrological cycle of eastern Asia, and China in particular. These studies will focus on: (1) how the plateau, as both a topographic obstacle and a sink for solar radiation, affects atmospheric circulation; (2) how the atmosphere transports stable isotopes (ä18O and äD); (3) how it affects mid-latitude climate variability, including how, via lee cyclogenesis, it lofts and transports dust, and (4) how vegetation feeds back on atmospheric circulation and the hydrological cycle. As links from geologic processes occurring at multi-Myr time scales to those on human time scales, the Principal Investigators plan studies that specifically examine paleoprecipitation over the past few hundred thousand years, using both loess deposition and speleothems that quantify paleoclimate.
Like radiocarbon (14C), 10Be is a rare and long lived radioactive isotope produced in the atmosphere by cosmic rays. Unlike radiocarbon however, 10Be does not form a gasseous species, and instead becomes adsorbed onto atmospheric dust, most of which is carried to the ground by rain or snow. Modern studies have shown that the annual flux of 10Be is proportional to rainfall amount. We have used this knowledge to reconstruct past variations in rainfall by measuring 10Be concentrations in thick dust (loess) accumulations found in a region of central China know as the Chinese Loess Plateau (CLP). The dust there is derived from the arid deserts of central Asia, and has been accumulating on the CLP for the past several million years. Rainfall on the CLP arrives mainly during the summer months, and is principally associated with the East Asian Summer Monsoon system (EASM). In this study we have used this 10Be proxy to reconstruct variations in EASM rainfall intensity in central China over the last ~500,000 years, and we find that rainfall variations depend strongly on changes in the shape of the Earth's orbit around the sun. These orbital changes include changes in eccentricity, obliquity (tilt of the earth's rotation axis relative to the plane of the ecliptic), and precession of the seasons relative to perihelion. These three orbital phenomena oscillate with regular cyclic frequencies, and interact with each other to control the amount of solar energy arriving at each latitude as a function of time of year. The seasonal and latitudinal distribution of incoming solar energy in turn strongly influence the position and strength of atmospheric circulation patterns such as the monsoons. We evaluated the dependence of paleo-EASM intensity using this rainfall proxy on orbital forcing at high and low latitudes and on interhemispheric insolation gradients. These results are illustrated in the accompanying figure, which shows the variations in eccentricity, precession and obliquity over time ( top 3 curves), their impact on insolation ( incomming solar radiation) at 65°N, 15°N, and two insolation gradients (30°N + 15°S and 30°N - 30°S). The bottom curve shows our paleo-rainfall reconstruction, and its phasing relative to the insolation forcing. We found that while high latitude (65°N) forcing of the EASM is important in terms of controlling global mean temperature and ocean-atmospheric temperature gradient, the inter-hemispheric subtropical insolation gradient is also important for generating cross equatorial moisture transport that enhance the Asian Monsoon. We propose a mechanism involving regulation of cross equatorial moisture transport by the southern hemisphere low to mid-latitude Mascarene and Australian highs during Austral Precession lows, that explains precession modulation of EASM intensity and the phasing of delta18O (i.e. 18O/16O ratio) variations observed in U/Th dated Chinese speleothems, which is also a proxy for where the EASM rainfall comes from.
