Lower temperature as the main cause of C4 plant declines during the glacial periods on the Chinese Loess Plateau

The distribution of C3 and C4 plants changed in regionally contrasting ways during the last glacial period. C4 plant expansion in low-latitude Africa and America coincided with C4 plant decreases in Mesoamerica and the US Great Plains. This C4 plant expansion has been attributed to lower pCO2 and increased aridity and the decline in C4 plants is believed to have been caused by increased winter precipitation and lower temperatures. However, it is still difficult to generalize whether pCO2, temperature, or aridity was mainly responsible for C3 vs. C4 vegetation changes during glacial periods. Paleoclimate conditions on the Chinese Loess Plateau (CLP) offer opportunities to further evaluate the role of temperature in the C3 vs. C4 competition. Detailed climate records have shown that the CLP region was drier and colder during glacial periods, with aridity favoring C4 expansion and lower temperatures favoring C3 expansion. Here, we present two high-resolution compound-specific carbon isotope records of n-alkanes from the CLP as vegetation biomarkers to estimate the relative abundance of C4 plants during the last two glacial/interglacial transitions. More negative N13C values during the glacial periods from two CLP sites clearly show that, in contrast with low-latitude sites, C4 plants declined. This decrease in C4 abundance was caused by lower temperature, despite the lower pCO2 and increased aridity on the CLP during glacial time. 5 2003 Elsevier B.V. All rights reserved.