Dust storms and Chinese loess sources over the last 22 million years

The dust cycle is a fundamental component of climate, but remains one of the least understood aspects of the Earth-system. Dust in the atmosphere affects oceanic productivity and atmospheric carbon dioxide levels, cloud formation and the amount of radiation absorbed or reflected by the atmosphere, all driving climatic change. However, how dust production and emission is controlled by environmental change in the past is poorly understood, preventing quantification of its effects on climate, fact acknowledged in the recent IPCC 2007 report. This project addresses this gap by attempting to pinpoint the precise sources of the world's largest and longest active dust sink, the Chinese Loess Plateau. The Chinese Loess Plateau contains a 22 million year archive of loess (deposited dust) deposition, covering one of the most significant time periods for understanding the Earth's history. Chinese loess has made crucial contributions to understanding past changes in the Asian monsoon and dust storms and is one of the most valuable climate archives available. However, its interpretation relies on detailed constraint of the precise sources of this dust, and the pathways by which it reaches the Loess Plateau. Unfortunately, despite investigation using the geochemical properties of bulk samples there are multiple competing hypotheses over 1) the precise loess source regions, 2) the environmental controls on dust production, 3) whether sources shift through time, 4) the atmospheric mechanisms are for dust transport and indeed 5) whether the oldest part of the record (8-22 Ma) is indeed wind-blown at all. These disagreements severely limit our understanding of the very origin of these deposits, preventing us from constraining the past atmospheric, tectonic and oceanographic conditions responsible for the emission and transport of dust and undermining the use of certain climate proxies in loess. In turn this restricts our understanding of the dominant environmental processes operating in China in the past, and the origin of the current atmospheric circulation systems. Key to overcoming this gap is to properly constrain the sources of loess. Our pilot work has demonstrated that widely applied bulk sediment geochemical analysis of sediment from loess and source regions will mask the detail of the multiple sources of loess dust. Only individual grains of certain heavy minerals (> 2.8 specific gravity) can be source diagnostic as each grain will have one source and certain of its geochemical and geochronological characteristics may be diagnostic of this. Thus analysis of multiple single grains of zircons and other heavy minerals isolated from loess and adjacent desert deposits will be undertaken using a multi-proxy single-grain geochemical approach to maximise the likelihood of success. Until our pilot work this has not been conducted on Chinese loess previously. Samples for these analyses will be taken from multiple, typical loess sequences in China, allowing determination of source variance through time and space. Sampling will concentrate on key intervals, such as the uplift of Tibet, the onset of Ice Age glaciation and the enhanced intensity of glaciation in more recent times. The results will allow us to test between conflicting hypotheses of loess dust source and transport, enabling constraint of the fundamental controls on dust emission and the atmospheric mechanisms involved in their transport. In turn and through comparison with independent records, this will allow us to assess the effect of global and regional climatic, tectonic and oceanographic changes on dust. This is currently a poorly understood yet critical component of the Earth-system. Finally, the results will test the validity of widely used sedimentary proxies such as mass accumulation rates and grain size in reconstructing past environmental changes from loess.