Panxian Dadong, South China: Establishing a Record of Middle Pleistocene Climatic Changes

Article excerpt

STUDY OF THE CHINESE AND SOUTHEAST ASIAN PLEISTOCENE ENVIRONMENT has been greatly enriched by the combined use of geological, geographical, bio-stratigraphic, and chemical analyses. A major focus has been on understanding the influence of the uplift of the Qinghai-Tibet Plateau on the development of the East Asian monsoon climatic regime and the identification of climatic oscillations throughout the Pleistocene. The Quaternary climates of South and Southwest China are not as well known as those of the east or north, but it appears that the effects of the environment on the formation of the extensive and remarkable southern karst landscape were not as marked as they were in the north. Most notably, the strong scouring actions of the continental ice sheets are absent in the south (Yuan et al. 1995). Instead, as the Qinghai-Tibet Plateau rose during the Quaternary, the karst landscape of South and Southwest China was largely sculpted by major rivers that had their origins in the high plateau mountain ranges (Sweeting 1995).

Most reconstructions of Asian Pleistocene climate change are based on interpretations of loess-paleosol sequences (especially the Luochuan loess of North China [Liu et al. 2000]), where loess deposits are viewed as forming under drier climatic conditions and paleosols are seen as forming during wetter, warmer periods. Therefore, paleoclimatic reconstruction in South China currently depends on making use of information about widespread climatic effects such as the monsoon weather patterns. Paleoclimatic inference is becoming more refined, however. Studies incorporating magnetic susceptibility (Chen et al. 2003) and mineralogic correlations with magnetic susceptibility (Ji et al. 2001) have been very useful for more precise paleoclimatic reconstruction. All of these different techniques are documenting major climatic oscillations and rapid environmental transitions. For example, the work of Ding et al. (1999) on Middle Pleistocene climatic instability utilized a technique based on grain size of particles in soil cores to reconstruct a high-resolution record of climate changes during the last two glacial-interglacial cycles. They found frequent, large magnitude climatic oscillations during the penultimate glaciation (ending between 130-140 kya) that were not characteristic of the last interglacial, illustrating that these glacial periods had much more variable climate. Other evidence for climatic fluctuations during these times is documented in changes in the mollusk species distributed throughout the paleosol-loess sequence. These fossils were also found to be good indicators of monsoon variability. Rousseau and Wu (1999) provide evidence for drier environments based on the presence of xerophilous (dry-loving) taxa at 180 kya, 154 kya, and 138 kya. Species associated with warmer, wetter climate (hygrophilous, or water-loving taxa) were present between approximately 242 and 232 kya and at 210 kya, 164 kya, and 140 kya years ago. There is also substantial ice core evidence for rapid, abrupt climatic transitions during the last few hundred thousand years (Adams et al. 1999; Alley 2000). The time scale of these transitions is on the order of a few hundred years or even decades, suggesting that Middle Pleistocene faunal and human populations had to adapt to environmental change on a regular basis.

As relatively 'protected' environments, cave- and rockshelters are potentially important sources of paleoclimatic data. Microstratigraphic examinations of cave and rockshelter deposits have proven to be useful for identifying abrupt climatic change (cf. Courty and Vallverdu 2001). In addition, geological formations that are unique to cave environments, such as speleothems (also called 'cave calcites,' as calcite is the primary mineral component of speleothems and limestone), are important for dating cave deposits as well as providing information about paleoclimate. Speleothems are features that result from slow-moving water containing calcium carbonate, and the rate of their formation is dependent upon a dynamic system involving the carbon, water, and calcium cycles in the cave and surrounding environment (Yuan et al. …