A new modern analogue reference dataset and its application to the 430-kyr pollen record from Lake Biwa

This study presents a newly compiled dataset of modern pollen and climate data from 798 sites across Japan and the Russian Far East. This comprehensive reference dataset combined with the modern analogue technique (MAT) provides a powerful tool for pollen-based reconstruction of the Quaternary Northwest Pacific climate. Pollen-derived reconstruction of the modern climate at the reference pollen-sampling sites matches well with the estimated modern climate values (R2 values vary between 0.79 and 0.95, and RMSEP values vary between 5.8 and 9.7% of the modern climatic range for all nine tested variables). The successful testing of the method encourages its application to the fossil pollen records. We used a coarse-resolution pollen record from Lake Biwa to reconstruct glacial-interglacial climate dynamics in central Japan since ~438 kyr and compared it to the earlier reconstruction based on a less representative reference dataset. The current and earlier results consistently demonstrate that the coldest glacial intervals experienced pronounced cooling in winter and moderate cooling in summer, supporting the growth of cool mixed forest (COMX) where warm mixed forest (WAMX) predominates today. During the last glacial, maximum (~24 kyr BP) mean temperatures of the coldest (MTCO) and warmest (MTWA) month were about -13 °C (RMSEP = 2.34 °C) and 21 °C (RMSEP = 1.66 °C) respectively, and annual precipitation (PANN) was about 800 mm (RMSEP = 158.06 mm). During the thermal optimums of the interglacial intervals, the temperatures of the coldest and warmest month were above 0 °C and 25 °C respectively, leading to the reconstruction of WAMX and temperate conifer forest (TECO). Although both these vegetation types grow in the southern part of Japan today, WAMX requires warmer space. The presence of WAMX during marine isotope stages (MIS) 11 and 1, and its absence during MIS 9 and MIS 5 contradict the marine isotope and Antarctic ice records, suggesting that the latter two interglacials were the warmest of the last 800 kyr. The apparent contradiction allows at least three different explanations including low temporal resolution of the pollen record; different trends in CO2 concentrations during 'short' and 'long' interglacials; and regional climate variability and non-linear response of different regions to the global forcing. More definitive conclusions will be possible on the basis of forthcoming high-resolution pollen records from central Japan.本研究全新编制了一套涵盖日本及俄罗斯远东地区798个采样点的现代孢粉与气候数据集。该综合性参考数据集结合现代类比技术（Modern Analogue Technique, MAT），为基于孢粉的西北太平洋第四纪气候重建提供了强有力的研究工具。参考孢粉采样点的现代气候经孢粉反演结果，与实测现代气候值拟合度极佳：9个测试变量的决定系数（R²）介于0.79至0.95之间，均方根预测误差（Root Mean Square Error of Prediction, RMSEP）占现代气候区间的5.8%至9.7%。该方法经测试效果良好，为其在化石孢粉记录中的应用奠定了基础。我们利用日本琵琶湖的粗分辨率孢粉记录，重建了约43.8万年来日本中部的冰期-间冰期气候动力学特征，并将其与此前基于代表性不足的参考数据集得到的重建结果进行了对比。本次研究与前期结论均表明，最冷冰期时段冬季出现显著降温，夏季则呈温和降温趋势，这为冷温带混交林（cool mixed forest, COMX）在如今以暖温带混交林（warm mixed forest, WAMX）为主的区域扩张提供了气候学支撑。末次冰盛期（约2.4万年前），最冷月份平均温度（Mean Temperature of Coldest Month, MTCO）与最热月份平均温度（Mean Temperature of Warmest Month, MTWA）分别约为-13℃（RMSEP=2.34℃）与21℃（RMSEP=1.66℃），年降水量（Annual Precipitation, PANN）约为800毫米（RMSEP=158.06毫米）。在间冰期的气候适宜期，最冷月份平均温度与最热月份平均温度分别高于0℃与25℃，据此反演出的植被类型为暖温带混交林（WAMX）与温带针叶林（temperate conifer forest, TECO）。尽管如今这两类植被均分布于日本南部，但暖温带混交林（WAMX）对气候温暖程度要求更高。深海氧同位素阶段（Marine Isotope Stages, MIS）1与11时期存在暖温带混交林（WAMX），而MIS 9与MIS 5时期却无此类植被分布，这一现象与深海同位素及南极冰芯记录相悖，表明后两个间冰期是过去80万年来最为温暖的时段。这一看似矛盾的现象至少可通过三种不同机制解释：一是孢粉记录的时间分辨率较低；二是‘短周期’与‘长周期’间冰期的二氧化碳浓度变化趋势存在差异；三是区域气候变异性与不同区域对全球强迫的非线性响应。未来基于日本中部即将获得的高分辨率孢粉记录，将有望得出更具确定性的研究结论。