Alkane variation in peat reveals palaeohydrological changes since the Little Ice Age in eastern China

The East Asian summer monsoon (EASM) system plays a crucial role in regulating climatic and hydrological variability in East Asia. The hydrological effects of the EASM in eastern China during the Little Ice Age (LIA) remains controversial, however. Here, we established a relatively high-resolution palaeohydrological record over the last 700 years in East China via analyzing n-alkanes from a peat core. Mid- and long-chain (C21–C25 and C27–C31) n-alkanes account for more than 95% of the total odd‑carbon-numbered n-alkanes, with short-chain n-alkanes (C17–C19) accounting for only 2.3% on average, suggesting that organic matter in the peat core originated primarily from vascular plants. Organic geochemical indices, including Paq (an indicator of contributions from aquatic plants), the average chain length (ACL) of long-chain n-alkanes, and C31/C27 and C23/C29 n-alkane ratios, suggest a transition from dry conditions during the middle LIA to wet conditions in the late LIA. Comparing Paq records from the study profile with previous records from the EASM region, we found that the northern and southern parts of eastern China showed contrasting changes in humidity during the LIA. Specifically, the southern part shifted from mid-LIA dry conditions to late-LIA wet conditions, whereas the northern part shifted from mid-LIA wet conditions to late-LIA dry conditions. This spatial heterogeneity in hydroclimate could be attributed to La Niña-like conditions and intense summer monsoons during the middle LIA and El Niño-like conditions and weak summer monsoons during the late LIA. Our study demonstrates the importance of understanding spatial heterogeneity of hydroclimatic changes to probe the mechanisms whereby EASM modulates regional climatic changes during the LIA.