重庆丰都雪玉洞群洞穴现代监测与古环境研究回顾和展望

重庆丰都雪玉洞群包括羊子洞、雪玉洞和水鸣洞，西南大学的研究团队从2008年开始对雪玉洞群开展了系统的现代过程监测，以了解洞穴系统气候和环境信息的传输、转化和记录过程。通过对雪玉洞洞穴内外的大气、植被、土壤、基岩、滴水和洞穴沉积物等的动态监测，研究了碳酸盐沉积过程的水化学指标变化，揭示了现代洞穴滴水的影响因素和变化过程，以及碳酸盐沉积物对现代气候环境变化的响应，也为古气候的定量化研究提供了基础支撑。现代过程监测记录表明:雪玉洞CO₂主要来源于上覆土壤，其季节变化受降水的影响较大；在短时间尺度上受到游客旅游活动的影响明显，但幅度远远小于自然过程引起的变化。雪玉洞内次生沉积物的沉积速率具有明显的旱季、雨季特征，不同滴水点下方沉积物的沉积速率变化较大。雪玉洞群三个洞穴的石笋古环境记录研究表明，本区石笋的230Th/232Th比值较适合高精度铀系测年；部分石笋沉积速率较快，平均沉积速率达到0.25 mm·a?1，可以开展高分辨率的气候和环境变化研究。羊子洞YZ1石笋的年龄范围在116~3 ka B.P.之间（平均测年精度2σ，269年），覆盖了整个末次冰期，δ18O和δ13C的变化曲线和东亚季风区的其他记录具有明显的一致性。同位素测试的时间分辨率平均为88年，成功记录了一些百年－千年尺度的气候突变事件，如Heinrich事件、7.2 ka事件、小冰期等。在精确年代学的基础上，雪玉洞群石笋具有重建高分辨率气候环境变化的潜力。 

Review and prospect of modern monitoring and paleoclimate research in Xueyu cave group,Fengdu, Chongqing

Karst caves are well developed due to karstification in carbonate reservoirs. Speleothems in the caves are of great significance to evaluate the evolutionary history of past climate and environment as speleothems record all climatic and environmental information when they were formed. A comprehensive link between paleoclimate change and modern climate based on detailed monitoring, precise chronology and high-resolution records is essential to explore the correlation and interaction between climate and the earth system in the geological history. Here, we review a series of modern monitoring and stalagmite-based Asian Summer Monsoon (ASM) records spanning the past 120 thousand years (ka B.P.) from Xueyu cave group, southwestern China. The Xueyu cave group includes Yangzi cave, Xueyu cave, and Shuiming cave, Xueyu cave is also a famous show cave. The three caves are all located in the Triassic limestone strata on the left bank of the Dragon river, which is the tributary of the Yangzi river, Fengdu, Chongqing. The overlying vadose zone is thick, and the elevations of the three caves are 10-100 m above the level of the Dragon river. Yangzi cave, Xueyu cave and Shuiming cave are respectively located at high-, middle- and low-elevation. These caves from different altitudes are excellent for modern monitoring and paleoclimate reconstruction. Since 2008, regular modern process monitoring has been carried out in the Xueyu cave to observe the transmission, transformation, and recording process of climate and environmental information. External air, overlying vegetation and soil, rock, precipitation, cave atmosphere (temperature, humidity, CO_2, etc.), hydrochemical properties of the subterranean river, drip water, and cave deposits are monitored to understand the detailed mechanisms connecting cave environmental variables and properties of cave deposits. A monitoring program conducted from 2009 to 2016 recorded the characterization of external air temperature, precipitation, and the subterranean water temperature and PCO₂ dynamics in the cave. Monthly measurements of cave air CO₂ in the Xueyu cave show regular seasonal variations. High-resolution monitoring of cave air CO₂ revealed the effects of rainfall events and tourist activities. Precipitation is one of the main factors responsible for transferring the CO₂ signal from the soil to the cave. On a short time scale, PCO₂ is significantly affected by tourist activities, but the magnitude is far less than that caused by seasonal or rainstorm-driven variation. Cave drip-water flow can be classified into two types-'fast flow' and 'slow flow', which reflects climatic events in different ways and may result in distinct speleothem paleoclimate records. Variations in drip water chemistry (Mg/Ca, and Sr/Ca) indicate that the Prior Calcite Precipitation (PCP) may occur when drip rates are slow. Only the 'slow flow' could reflect wet-dry variations as the drip rate variability can provide the primary control on trace element variations that are finally trapped in speleothems. The deposition rates are relatively high in winter, which could reach up to 0.33 mm·a?1. The isotopic and chemical variability observed in Xueyu cave drip waters also support previous interpretations of speleothem records from Yangzi cave and Shuiming cave where a series of stalagmite proxies is published. The stalagmite records of the Shuiming cave and Yangzi cave indicate that the stalagmites in this area have fast depositional rates. From 120 ka B.P. to 3 ka B.P., the variation of δ18O is basically consistent with the change curve of solar radiation at 65°N.The resolution of the dating in some periods was 269a, while the time resolution of isotopic composition is close to 88a, which successfully records some abrupt climate events at the scale of hundreds of or thousands of years , such as the Heinrich event, the 7.2 ka event, and the Little Ice Age. δ13C records of the stalagmite confirmed that the '7.2 ka event' started from 7.29±0.03 ka B.P. and its summit was 7.1±0.04 ka B.P. and the main drought period sustained about 50a. During the Little Ice Age, especially from 1,400-1,650 AD, δ18O records of stalagmites showed severe fluctuation, which indicated Asian summer precipitation was also in the fluctuating period. To sum up,the comprehensive comparison of paleoclimate changes based on accurate chronology and high-resolution records has good research potential in the Xueyu cave group.