Stable carbon isotope records of black carbon on Chinese Loess Plateau since last glacial maximum: An evaluation on their usefulness for paleorainfall and paleovegetation reconstruction

摘要

Stable carbon isotope composition of soil organic matter (delta C-13(SOM)) has been widely used to infer past environmental or vegetation (C-3/C-4) changes. However, as a carbonaceous product of incomplete burning of biomass, the carbon isotope signature of black carbon (BC) has received little study and a limited usage on this purpose. Moreover, the environmental or vegetation indication of BC carbon isotope composition (delta C-13(BC)) in sedimentary records remains ambiguous although the delta C-13(BC) is supposed to reflect carbon isotope composition of vegetation being burnt. This deserves site-specific studies. Here we analyzed delta C-13(BC) and delta C-13(SOM) on loess-paleosol samples from Lijiayuan and Yangling sections on Chinese Loess Plateau (CLP) spanning the last glacial maximum to decipher the environmental (or vegetation) meaning of delta C-13(SOM) at each study site. Opposite changing patterns were observed on the delta C-13(BC) values for the two sections. The delta C-13(BC) at Yangling (in southern part of the CLP) varied from - 19.18 parts per thousand to - 21.93 parts per thousand (mean: - 20.62 parts per thousand) with more positive values occurred during the middle Holocene than those during the LGM, demonstrating more C-4 plants occupied in the region during the warm-humid middle Holocene. This is consistent with the changing pattern widely-documented in delta C-13(SOM) records over the CLP during the same period. By contrast, the delta C-13(BC) at Lijiayuan (in northwestern part of the CLP) changed within the range of - 21.83 parts per thousand similar to -24.64 parts per thousand (mean: - 23.34 parts per thousand) and displayed more negative values during the early-middle Holocene with respect to the LGM period. The delta C-13(BC) at Lijiayuan were about 2.5 parts per thousand lower than those at Yangling, indicating a northward decrease of C-4 plants on the CLP. The anti-phased changes of delta C-13(BC) at Lijiayuan were considered to ref