我国北方过去15万年来主要温暖期增温幅度与变率的耦合同位素研究

At present, a large uncertainty still exists regarding the extent of temperature rising in the future global warming and the ecological and environmental impacts under different warming scenarios remain unclear. The key to resolve these questions lies in studying the analogs of global warming in the geological past. In this project, we would reconstruct a highly precise paleo-temperature history during the past 150 ka in North China by analyzing carbonate clumped isotopes of land snail fossils collected from Luoyang and Ningxian loess sections above L2 loess horizon. We emphatically study the extents and rates of temperature increases in typical warm periods therein. Meanwhile, stable carbon and oxygen isotopes will be measured on those snail shells to research into eco-physiological habits of different land snail species. Furthermore, the oxygen isotopic compositions in the fluids of the land snails (δ18Ofluid) could be worked out based on the oxygen isotopes of land snail shells and the temperatures at which shell carbonates precipitated as provided by the clumped isotopes. The δ18Ofluid values can be a real reflection of oxygen isotope compositions of monsoonal precipitation. On the basis of the above data, we would finally elucidate the changes in the eco-physiological habits of the land snails and the varied strength of summer monsoon under the different extents of temperature increases during the selected warm periods. As a result, we could have an opportunity to further discuss the linkages of the land snail eco-physiological habits and the monsoonal hydrological cycles with the temperature. The research findings produced in this project would present a vivid example to better understanding the impacts of future global warming on the monsoonal climates in our country and the ecological environment in semi-arid region. It would also provide very important scientific data to our government attending future climate negotiation.

未来全球变暖的增温幅度仍存在很大的不确定性，不同增温场景下生态环境效应也并不明确，解决问题的关键在于研究地质历史时期的‘相似型’。本项目拟选择洛阳和宁县黄土剖面L2以上地层中的蜗牛化石为研究对象，通过碳酸盐耦合同位素分析重建过去15万年来我国北方地表温度的变化历史，并着重研究该时段主要增温期的升温速率和温暖期的增温幅度。同时，我们还将分析蜗牛化石壳体碳、氧同位素来研究不同种类蜗牛生态习性，并根据耦合同位素所恢复的壳体碳酸盐形成温度和壳体氧同位素计算出蜗牛体液的氧同位素组成，进而更精确地给出夏季风降雨的氧同位素特征。基于上述数据，我们最终可以恢复不同增温幅度下蜗牛生态习性演变和夏季风强度变化，深入探讨温度与季风降水循环特征和蜗牛生态习性的联系。项目产出的研究成果将会为我们理解未来全球增温对我国季风气候和干旱－半干旱区生态环境的影响提供一个生动范例，也会为我国应对未来气候谈判提供重要科学数据。

地质增温期古温度精确重建是古气候研究的重中之重，可以帮助准确预测未来气候增温幅度和评估未来全球变暖的生态环境效应提供关键科学数据。然而，以往的温度指标由于受多种因素影响存在很大的不确定性。碳酸盐团簇同位素是国际上新近研发的古温度计，由于只与温度有关，可精确重建古温度。本项目设计以黄土地层中蜗牛壳体化石为研究对象，分析其碳酸盐团簇同位素组成，进行古温度重建研究。首先，我们从我国南至三亚北到哈尔滨共68个研究点，系统地采集了现生华蜗牛和巴蜗牛壳体，测定了这些蜗牛壳体团簇同位素，确定它们与生长季温度具有较好的相关性，从而建立蜗牛壳体团簇同位素—温度转换方程。同时，发现华蜗牛的团簇同位素温度要比巴蜗牛高出3˚C左右，表明不同种属之间可能存在生态习性的差别。然后，通过测定洛阳黄土-古土壤剖面中华蜗牛属Cathaica pulveratrix壳体团簇同位素，并应用以上温度转换方程重建了过去两个冰期旋回以来的蜗牛生长季（4-10月）温度变化。结果表明：MIS3阶段温度平均为20˚C左右，要比MIS2阶段总体高出6˚C。相比而言，MIS4阶段温度平均为18˚C，略低于MIS3阶段。MIS3阶段可以分为3个亚段，其温度均值分别为18.6˚C、17.6˚C和22.1˚C。在MIS7到MIS6阶段过渡期，蜗牛生长季温度18.5˚C降低到10.8˚C。据蜗牛壳体δ18O和团簇同位素温度计算出的蜗牛体液δ18O（δ18OBW），总体上呈现出与温度较强的正相关（R2=0.65），反映了受温度影响的蒸发效应，因而不能完全记录降雨δ18O（δ18OP）信息。但是，在温度变化不大的时段，还是能据此追踪δ18OP变化。例如，MIS3阶段晚期，δ18OBW平均在-5.6‰左右，而在温度相当的MIS4阶段δ18OBW变为-4.6‰，反映出δ18OP偏正而降雨量明显减少的过程。本项目建立了世界首条季风区生物壳体团簇同位素温度转换方程，为应用蜗牛壳体化石进行古温度重建奠定了基础。同时，证实蜗牛壳体化石团簇同位素重建古温度的有效性，精确重建了MIS3阶段的增温幅度，为评估该时期变暖的生态环境效应提供重要温度数据。