Lower temperature as the main cause of C 4 plant declines during the glacial periods on the Chinese Loess Plateau

Abstract

The distribution of C 3 and C 4 plants changed in regionally contrasting ways during the last glacial period. C 4 plant expansion in low-latitude Africa and America coincided with C 4 plant decreases in Mesoamerica and the US Great Plains. This C 4 plant expansion has been attributed to lower pCO 2 and increased aridity and the decline in C 4 plants is believed to have been caused by increased winter precipitation and lower temperatures. However, it is still difficult to generalize whether pCO 2, temperature, or aridity was mainly responsible for C 3 vs. C 4 vegetation changes during glacial periods. Paleoclimate conditions on the Chinese Loess Plateau (CLP) offer opportunities to further evaluate the role of temperature in the C 3 vs. C 4 competition. Detailed climate records have shown that the CLP region was drier and colder during glacial periods, with aridity favoring C 4 expansion and lower temperatures favoring C 3 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 C 4 plants during the last two glacial/interglacial transitions. More negative δ 13C values during the glacial periods from two CLP sites clearly show that, in contrast with low-latitude sites, C 4 plants declined. This decrease in C 4 abundance was caused by lower temperature, despite the lower pCO 2 and increased aridity on the CLP during glacial time.