MtDNA of common vole from Grotta della Ferrovia site. Integration of direct radiocarbon dating, genetic studies and taxonomy of small mammals to investigate the chronology of past climatic oscillations: The Last Glacial Maximum sequence of Grotta della Ferrovia (Fabriano, Italy)

The recent developments in palaeoecological reconstruction methods, collagen extraction of small bone samples and ancient DNA analyses led us to test new approaches to enhance the chronological resolution of past climate reconstructions inferred from small mammal assemblages. Grotta della Ferrovia (Fabriano, Ancona, Italy, 40°25’36’’N, 13°0’11’’E, 215 m a.s.l.) is a small cave that opens on the southern bank of the Esino river, around 45 km west of Ancona. Discovered in 1966, it is famous among scholars because it is the first locality where the fossil birch mouse was recorded in Italy and because the lithic assemblage found in the cave was related to the Late Epigravettian. Across the small mammal sequence, divided into seven layers (from GDF7 to GDF2), two main phases can be recognised: in the first phase, from GDF7 to GDF5, Microtus arvalis is dominant over Microtus agrestis, in a poorly diversified assemblage. The sequence gradually changes in GDF4 and a second phase can be observed in GDF3 and GDF2, where Apodemus gr. sylvaticus-flavicollis dominates, but combined with a higher number of taxa. From GDF7 to GDF3, few individuals of Sicista cf. subtilis and Alexandromys oeconomus are recorded. Environmental and climatic reconstructions calculated with the Habitat Weighting and the Bioclimatic methods show a dominance of open meadows (GDF7 to GDF4) that were gradually replaced by closed or semi-closed forests (GDF3 and GDF2) in a context of gradually increasing temperature, going from a cold phase to conditions similar to the current ones in the Grotta della Ferrovia area. Direct radiocarbon dates on 18 rodent bones and aDNA analyses on three Microtus arvalis teeth helped us to better understand the significance of the oscillation inferred from the small mammal sequences. A robust series of 9 radiocarbon measurements from GDF7 to GDF5 shows that this part of the sequence accumulated between ~24,600 - 19,600 cal BP (95.4% probability), which includes the final phases of the Last Glacial Maximum. The ancient DNA from Microtus arvalis confirmed the chronological framework and the presence of italic populations in this area during the LGM, similar to the slightly more ancient Grotta del Sambuco on the western side of the central Peninsula. The small mammal radiocarbon dates from GDF4 and above are less consistent with their stratigraphic position, probably related to reworking of the sediment, but suggest the last part of the sequence accumulated during the Late Glacial. Our results show that it is now possible to validate and enhance paleoclimatic and paleoenvironment inferences from small mammal sequence studies with direct radiocarbon dates and ancient DNA analyses, using these integrated methods as a high resolution tool for studying the past.

近年来，古生态重建方法、小型骨样本胶原蛋白提取技术以及古DNA分析技术的发展，促使我们测试新的研究方法，以提升基于小型哺乳动物组合体重建古气候的年代分辨率。格罗塔德拉费罗维亚洞穴（Grotta della Ferrovia，位于意大利安科纳省法布里亚诺，地理坐标为北纬40°25′36″、东经13°0′11″，海拔215米）是一处小型洞穴，洞口开在埃西诺河南岸，距安科纳以西约45公里。该洞穴于1966年被发现，在学术界颇具知名度：它不仅是意大利境内首个发现化石桦鼠的遗址，洞内出土的石制品组合还与后格拉维特文化晚期（Late Epigravettian）相关联。这套小型哺乳动物地层序列共分为7个层位（GDF7至GDF2），可识别出两个主要演化阶段：第一阶段为GDF7至GDF5层，此时普通田鼠（Microtus arvalis）在物种多样性较低的组合中占据优势，种群数量超过根田鼠（Microtus agrestis）；GDF4层时序列发生显著渐变，第二阶段见于GDF3与GDF2层，此时林姬鼠-黄颈姬鼠类群（Apodemus gr. sylvaticus-flavicollis）占据主导，伴生类群的数量更为丰富。在GDF7至GDF3层中，还记录到少量疑似纤细蹶鼠（Sicista cf. subtilis）与沼泽东方田鼠（Alexandromys oeconomus）的个体。通过栖息地加权法（Habitat Weighting）与生物气候法（Bioclimatic）开展的环境与气候重建结果显示，GDF7至GDF4层以开阔草甸占主导；随后在温度逐步升高的背景下，区域环境从冷期逐步向接近现今格罗塔德拉费罗维亚洞穴地区的气候条件转变，GDF3与GDF2层逐渐被郁闭或半郁闭森林取代。对18件啮齿动物骨骼的直接放射性碳测年，以及对3颗普通田鼠牙齿的古DNA分析，帮助我们更深入地理解小型哺乳动物序列所反映的气候波动意义。针对GDF7至GDF5层的9组高精度放射性碳测年结果显示，该段地层的沉积年代介于约24600至19600校准年前（95.4%概率区间），这一时期涵盖末次冰盛期（Last Glacial Maximum, LGM）的最后阶段。普通田鼠的古DNA分析结果验证了该年代框架，同时证实末次冰盛期期间该区域存在意大利本土种群，这与亚平宁半岛中部西侧年代稍早的桑布科洞穴（Grotta del Sambuco）的研究结果一致。GDF4及以上层位的小型哺乳动物放射性碳测年结果与地层埋藏位置的对应性较差，这可能与沉积物再沉积作用有关，但仍表明该序列的上部沉积于晚冰期。本研究结果表明，借助直接放射性碳测年与古DNA分析技术，如今已能够验证并优化基于小型哺乳动物序列研究得出的古气候与古环境推论，可将这些整合方法作为研究古环境的高分辨率工具。