石笋灰度和同位素对末次冰期气候事件的响应

对石笋多指标的研究有利于全面理解季风气候事件的变化特征和机制。以湖北永兴洞YX55石笋为研究对象,通过高精度U/Th定年和多指标分析重建了65~35 ka B.P.石笋灰度和稳定同位素的变化序列。在65~40 ka B.P.时段,石笋灰度、δ¹³C与δ¹⁸O都呈现出数个千年至亚千年尺度的变化,但δ¹³C变化幅度比δ¹⁸O小得多。这说明灰度和δ¹³C这类代表局域气候的指标,都响应于δ¹⁸O指示的Heinrich（H）事件和Dansgaard-Oeschger（DO）旋回,但响应的程度各不相同。H4期间,石笋δ¹³C和灰度呈现不一致变化：δ¹³C不像在其他H事件一样小幅振荡,而是大幅正偏（相当于其他H事件2~3倍）;灰度则显示出和DO旋回一致的特征。这种δ¹³C/灰度与δ¹⁸O异常响应关系可能的机制是：H4期间季风强度急剧减弱,使得洞穴上覆土壤覆盖率大幅降低,从而削弱或阻止了土壤—岩溶系统的过滤能力,使石笋中杂质含量异常增加。

The response of stalagmite gray-level and isotopes to the climatic events during the last glacial period

The study of various proxies of stalagmites is conducive to understand the characteristics and mechanisms of monsoon climate events comprehensively. Here, we present a high-resolution and multi-proxy stalagmite record from Yongxing Cave, Central China, spanning the interval between 65 and 35 ka B.P. based on high precision U/Th dating method and multi-proxy analyses. A total of 371 subsamples were drilled for oxygen and carbon isotopic measurements along the growth axis at a spacing of 0.5-mm-diameter carbide dental burrs. The analyses were conducted using a Finnigan MAT 253 mass spectrometer linked to a Kiel Carbonate Device at the School of Geography, Nanjing Normal University. The precision is 0.06‰ for δ¹⁸O and 0.05‰ for δ¹³C at the 1σ level. For the gray-level measurements, the polished surface of the studied section was scanned at a resolution of 1200 dpi using an EPSON Perfection 4990 Photo Scanner. The gray level was then obtained from the image using the Environment for Visualizing Images (ENVI 4.8) software while traversing down the growth axis. The values range between 0 and 442, with a spatial resolution of 20 μm. The experimental results show that during the period of 65-40 ka B.P., the stalagmite records of gray-level, δ¹³C and δ ¹⁸O all presented millennial to sub-millennial changes, but the amplitude of δ ¹³C is much smaller than that of δ ¹⁸O. This indicates that the local climate indicators of δ ¹³C, gray-level and lithology also respond to the Heinrich (H) events and Dansgaard-Oeschger (DO) events, but their sensitivities to events are different. During the H4 episode (about 40-38 ka B.P.), δ ¹³C and the gray-level of the stalagmite showed anomalous changes. The significantly positive shift of δ ¹³C reached an amplitude of 4.5‰ during H4, which was 2-3 times than other H events, while the gray-level presented the same characteristics to that of the DO cycles. We imply that the state of soil erosion may play an important role in such correspondence during H4. When the monsoon weakened to a certain threshold, the soil cover above the permeable cave was greatly deteriorated, thus weakening and/or preventing the infiltration ability of the soil-karst system and increasing the impurity contents in the stalagmite.