基于黔西南雾露洞高分辨率石笋同位素记录,重建了62.0~58.2 ka BP和20.9~15.5 ka BP期间亚洲夏季风水文历史和洞穴岩溶环境变化过程。两支石笋(Wu58和Wu60)实测16个~(230)Th年龄和966组氧、碳同位素数据。结果显示,深海氧同位素3阶段(MIS 3)早期和2阶段(MIS 2)期间,千年尺度delta~(18)O变化非常显著,而delta~(13)C则在稳定的背景值下,呈百年尺度波动。去趋势发现, delta~(18)O指示的百年尺度弱季风事件与delta~(13)C指示的土壤CO_2产率衰减过程变化一致。两组同位素变化呈相似的百年尺度波动,共同周期约300 a,说明本区域土壤CO_2产率变化与百年尺度亚洲夏季风变化密切相关。在变化幅度上, delta~(13)C的振幅远大于delta~(18)O (约1.5~3.5倍),表明碳同位素变化对于气候响应具有放大效应,或者delta~(13)C与delta~(18)O变化具有不同的地球化学行为。通过与大气14C、冰芯10Be记录对比,发现百年尺度季风强弱及岩溶过程变化与太阳活动指标具有相似性,说明太阳活动对百年尺度季风强弱和土壤CO_2产率起到主控作用。可能的途径是,太阳活动通过海-陆热力差,影响夏季风强度和当地土壤湿度水平,并经生态效应进一步放大。然而,在百年尺度上, delta~(18)O振幅仅为0.4,远小于千年尺度变化(1.5)。因此,千年尺度亚洲夏季风突变的诱发因子可能不直接受控于太阳活动,需要其他驱动因素或者气候系统内部放大机制来解释。
英文摘要:
Wulu Cave is in the Yun-Gui Plateau, southern China. Modern climatic conditions in this region are dictated by the tropical Indian and subtropical Asian summer monsoon (ASM). Mean annual temperature at the site is approximately 14 ℃, with a maximum during July (20.8 ℃) and a minimum during January (4.3 ℃). The annual precipitation is approximately 1400 mm, peaking (900 mm) during the summer (June through September) and reaching a minimum (80 mm) during the winter (December through February). The cave is approximately 800 m long and overlain by 40 m of Triassic limestone bedrock. The inner part of the cave chamber (500-800 m from the entrance) is covered by 2-m-high deposits of clay and debris. It is poorly ventilated in the chamber, and the temperature inside approximates the annually averaged surface temperature with a relative humidity of approximately 100%. At this site, current vegetation is mostly composed of deciduous herbs (C4-type vegetation). A thin patchy soil cover (5-10 cm) above the cave is only observed in fractures and karstic depressions. A detailed history of the Asian hydroclimatic and local soil processes during the period 62.0-58.2 ka BP and 20.9-15.5 ka BP was reconstructed from two high-resolution stalagmite isotopic records (Samples Wu60 and Wu58) from Wulu Cave. Constrained by 16 U/Th dates and 966 isotopic subsamples, the two isotopic records show a close coupling of centennial-scale changes in the ASM and soil CO_2 production during early Marine Isotope Stage (MIS) 3 and MIS 2. Over the two time windows, a striking difference can be observed in stalagmitedelta~(18)O and delta~(13)C signals: millennial-scale variability is clear in the delta~(18)O record, while the delta~(13)C signal is characterized by persistent centennial oscillations around the stable mean values. At centennial scale, the detrended delta~(18)O record agrees well with the delta~(13)C, exhibiting a common periodicity of approximately 300 years. During weak ASM episodes, increased delta~(13)C values are observed, implying that decreased soil CO_2 production is closely correlated with centennial-scale ASM variability. Comparatively, the amplitude of the delta~(13)C changes is much larger than (approximately 1.5 to 3.5 times) that of the delta~(18)O signal, lending support to the speleothem carbon isotope being more sensitive to climatic changes, or the delta~(18)O and delta~(13)C changes would have different geochemical behaviors at a centennial scale. When compared to atmospheric 14C and ice-core 10Be records, the two detrended isotopic sequences agree well with the solar activity. Particularly, weak solar activity generally corresponds to weak ASM and a decline in soil CO_2 production. This correlation shows that solar activity plays a dominant role in controlling the ASM intensity and soil CO_2 production. One possible link between them is that external forcing controls the ASM intensity via the thermal contrast between the ocean and land. Subsequently, the balance of soil moisture co-varies with the hydrological responses. Finally, soil CO_2 production is further amplified by an ecological effect. However, changes in detrended delta~(18)O values fall within an amplitude of 0.4, which is significantly smaller than that of millennial-scale events (approximately 1.5), implying that millennial-scale ASM variability is not directly driven by solar activity, and that other forcing factors or amplifying processes of the climatic system are needed.