环境地球化学国家重点实验室知识库

The intensity variation of the East Asian Summer Monsoon during the Younger Dryas (YD) cold period remains an important scientific issue requiring clarification. In our article published in Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 214–222, based on the Hani peat cellulose δ¹³C sequence from Northeast China, we have documented the abrupt strengthening of the East Asian Summer Monsoon during that period. Our findings support the occurrence of wet conditions in the north and of dry conditions in the south of the Chinese Mainland, and they indicate behavior in anti-phase with the Indian Ocean Summer Monsoon that weakened during the same period. This result agrees with the previously revealed anti-phase variations of the two monsoons during the ice-rafted debris (IRD) cold events occurring in the North Atlantic Ocean. We have also proposed a theory for the anti-phase variations of the monsoons and for the anomalous distribution of precipitation. We have attributed these events to the occurrence of an El Niño-like phenomenon in the Equatorial Pacific Ocean during the YD. G. Schettler has argued that “this palaeo-climatic interpretation cannot be substantiated by chemical data from the nearby Lake Sihailongwan or by biomarker studies of the Hani peat. The inferred inverse correlation between the summer monsoon strength in northeast and southern China during the Younger Dryas appears questionable.” However, after carefully examining Schettler's [G. Schettler, Comment on “Anti-phase oscillation of Asian monsoons during the Younger Dryas period: Evidence from peat cellulose δ₁₃C of Hani, Northeast China” by B. Hong, Y.T. Hong, Q.H. Lin, Y. Shibata, M. Uchida, Y.X. Zhu, X.T. Leng, Y. Wang and C.C. Cai [Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 214–222], Palaeogeography, Palaeoclimatology, Palaeoecology, this issue] water chemistry data from Lake Sihailongwan, we find considerable accumulation ratios of biosilica during the YD period that clearly indicates abrupt strengthening of the East Asian Summer Monsoon. We point out that Schettler referred incorrectly to Zhou's research results. Zhou et al. have clearly indicated that elevated n-alkane P_aq values and C₂₃/C₂₉ ratios serve to record an especially moist local climate in YD. We also confirm that the influence of an increased relative contribution of Sphagnum species to Hani peat composition on peat cellulose δ¹³C is negligible. Therefore, our conclusions that, during the YD cold period, the EASM abruptly strengthened and that, moreover, an anti-phase occurred between the two Asian monsoons during the YD period, are both, in fact, substantiated not only by biomarker studies of the Hani peat but also by chemical data from the nearby Lake Sihailongwan. We therefore consider Schettler's statement to be untenable.