TECTONIC AND PALEOCLIMATIC HISTORY RECORDED BY A 8-MYR-LONG LOESS SEQUENCE IN CHINA: PRELIMINARY RESULTS FROM ROCK MAGNETIC AND ZIRCON THERMO- AND GEO-CHRONOLOGY STUDIES

Significant climatic changes have occurred in the past 8 m.y, that might have been caused by tectonic events, such as uplift of the Tibetan Plateau and closure of the Panama Seaway. However, a causal relationship between these tectonic events and climatic changes has not been completely understood. Chinese loess is the result of both global cooling and regional tectonic events. Therefore, a comparison between loess and global paleoclimate records can isolate climatic changes caused by regional tectonic events from global climate. Furthermore constraints on loess provenance may help elucidate the interplay between tectonic and climatic changes. We report new zircon U/Pb and (U-Th)/He results that demonstrate that the provenance of loess deposited around 8-6 Ma on the central Chinese Loess Plateau (CLP) is different from that of younger loess. We attribute this difference to uplift of the Liupan Mt., which is a major sediment source for "loess" of 8-6 Ma and thus "loess" of 8-6 Ma is not eolian. In addition, we report correlation patterns between two magnetic grain size proxies (χ_ARM/χ_LF and χ_ARM/SIRM) and two climate parameters (annual mean temperature and rainfall) for surface soils across the CLP. We find that χ_ARM/SIRM is sensitive to rainfall variations, but not to temperature variations. In contrast, χ_ARM/χ_LF is sensitive to both rainfall and temperature variations. Thus, applying these two proxies to the loess sequences has the potential to obtain both the paleo-rainfall and paleo-temperature history on the CLP. Indeed, such reconstructions indicate that the paleo-temperature of the CLP area is coherent with ice volume changes since 6 Ma. In contrast, although rainfall follows ice volume changes during the intervals 6 to ~4.6 and 2.7 to 0 Ma, rainfall has an intensification trend from 4.6 to 2.7 Ma, likely resulting from tectonic forcing (closure of the Panama Seaway and/or uplift of the Tibetan Plateau) of the Asian monsoon.