Changes in fire regimes on the Chinese Loess Plateau since the last glacial maximum and implications for linkages to paleoclimate and past human activity

摘要

A high-resolution black carbon (BC) record from 27.5kyr BP to present was reconstructed using a chemical oxidation method on loess and paleosol samples from the Lijiayuan section of the Chinese Loess Plateau. The black carbon mass sedimentation rates (BCMSR) and carbon isotopic record reveal a paleofire history and its relationship with climate and vegetation changes at the study site. The BCMSR record was decomposed into two components: background BCMSR and the BCMSR peaks. The background BCMSR represents regional fires and shows high fire activities occurred contemporaneous with the Younger Dryas, Older Dryas, Heinrich events and Greenland stadials as registered in the loess grain size record. This suggests a rapid response of regional fires on the Loess Plateau to abrupt climate changes. Spectral analysis of background BCMSR showed two meaningful periodicities of 1620 and 1040years, close to the cyclicity of the East Asian monsoon as recorded in the stalagmite δ ~(18)O record in Central China. This indicates a tight control of millennial scale wet-dry changes in the monsoonal climate on regional fires on the Loess Plateau. By contrast, the BCMSR peaks are considered to reflect local fire episodes. The occurrences of local fires were more frequent during the last glacial period, with a maximum frequency of ~6 episodes/1000years during the Last Glacial Maximum (LGM) (22.3 to 14.6kyr BP), when the climate was drier and more continuous grassy fuels existed on the landscape. During the last glacial-interglacial transition (LGIT) period (14.6 to 11.0kyr BP), fire frequency was largely reduced due to an increase in precipitation and more woody vegetation. If the LGIT period is taken as an analog for the projected near future, then future global warming alone may not produce large wildfires in northwestern China. Wildfires remained infrequent during the early-to-middle Holocene. Biomass burning increased after 4.0kyr BP, when the climate became drier and land-use was more intensive. BC carbon isotope ratios may well reflect changes in the vegetation being burnt (i.e., grasses versus trees), yielding results consistent with the associated pollen data in the region.