Spatial patterns of vegetation and climate on the Chinese Loess Plateau since the Last Glacial Maximum

摘要

In order to identify the spatial patterns of vegetation and climate on the Chinese Loess Plateau since the Last Glacial Maximum (LGM), pollen analysis was conducted for loess deposits from seven sites, which constitute two north-south and west-east transects across the Plateau. Results show that steppe prevailed both in the LGM and the Holocene Optimum. During the LGM, vegetation in the northwest mainly consisted of Artemisia, Echinops-type, Taraxacum-type, and Chenopodiaceae, and vegetation in the southeast was characterized by the same types but with a slight increase in Poaceae. During the Holocene Optimum, vegetation was more diverse, with Poaceae, Artemisia, Echinops-type, and Chenopodiaceae dominant in the northwest, and Pinus, Corylus, Poaceae, Artemisia, and Selaginella sinensis dominant in the southeast. Our records also show that from the LGM to the Holocene Optimum, the presence of Echinops-type and Taraxacum-type pollen decreased, while Poaceae pollen became more prevalent. As it has been found that most species of Poaceae prefer relatively humid conditions and abundances of Echmops-type and Taraxacum-type increase in desert vegetation, a new proxy, Poaceae/(Echinops-type + Taraxacum-type) ratio (P/(E + T)), was thus developed to compare the spatial differences in climate between the LGM and the Holocene Optimum, with high P/(E + T) values indicating relatively warm and wet conditions. The P/(E + T) ratio shows an overall southward and eastward increase for both the LGM and the Holocene Optimum. Furthermore, it exhibits a prominent north-south and west-east gradient during the Holocene Optimum but displays a flat one during the LGM, indicating more pronounced spatial contrasts in climate during the Holocene than in the LGM. This phenomenon may be attributed to the unique geographic location of the Loess Plateau, which was located in the marginal zone of the East Asian summer monsoon during interglacials but was dominated by the winter monsoon during glacials due to a southward displacement of climate zones.