中全新世持续近600年的湖北神农架石笋年纹层*

根据湖北神农架青天洞QT9石笋ICP-MS测年和纹层计数结果,建立了约6.7~6.1kaB.P.期间连续发育近600年石笋年纹层时间序列。年纹层厚度平均300μm,在数十年尺度上,变幅达350μm。在紫外、透射和反射3种光照条件下,年纹层呈现基本对应的季节性反差特征。年纹层厚度与原始灰度序列在数十年尺度旋回上呈现较为一致变化规律,表明暗色物质与年层生长速率共同响应于某一洞穴气候环境因子。基于中国东部不同气候期洞穴石笋年层具有的显微结构共性研究,推测这一共性是洞穴外部气候要素与洞穴内部环境耦合作用的结果。年层厚度与原始灰度两个序列的功率谱分析揭示50a,24a,9.1a以及2~7a等周期成分,其中,5.9a,4.2a和3.3a共同周期的存在,暗示中全新世存在不同频率的ENSO事件。

CONTINUOUS ANNUAL LAMINATION OF ABOUT 600 YEARS RECORDED IN A MIDDLE-HOLOCENE STALAGMITE FROM SHENNONGJIA AREA, HUBEI PROVINCE

One stalagmite named QT9, about 18cm in length with a diameter of 8cm, was collected from the Qingtian Cave at an elevation of 1630m in Shennongjia area, Central China. It was formed during mid-Holocene from 6.7kaB.P. to 6.1kaB.P. The cave ventilates poorly, with relative humidity almost close to 100 ％ on each of the several measurements of our visits. In the cave there is a limestone bedrock of approximately 30~100m thickness with a thin soil cover, offering little flowpath for penetrative water from the ground surface to the cave. Mean annual rainfall in the vicinity of Shennongjia area varies from 1500mm to 2000mm. The number of lamina corresponds well to the U-Th dating, implying that the laminations of QT9 stalagmite were formed annually. The average lamina-thickness is 300μm, with the thickest up to 350μm. Changes in initial gray level of each year on the polished surface correspond to visible lithological phases on the polished profile, that is, higher values of gray level are corresponding to milk-white color calcite interval, whereas lower values corresponding to transparent calcite section. Lamina microstructure has different optical characteristics under three kinds of light. It emits stronger fluorescence when the relatively darker banding of lamina is placed under ultraviolet light and high contents of dark materials under transparent as well as light milk-white color under reflex light. The lamina-thickness, initial gray level and annual total gray level time series of an about 600 years continuous Middle Holocene stalagmite have been established by ICP-MS dating and lamination counting. A comparison between lamina-thickness and initial gray level time series demonstrates that both proxies have similar changes at multi-decadal time scales, implying that dark materials and crystal rate of calcite in annual layer correspond to common forcing mechanisms of cave conditions. Based on the common microstructure characteristics of stalagmite lamina during different climatic phase in different caves over East China, we suggest that this common microstructure feature may be resulted from the coupling of external climatic factors and the inner cave conditions. Although the annual total gray level can be used as an indicator of cave environmental change under particular conditions, it should be further verified by other multi-proxies incorporated in the stalagmite lamina when applying this climatic index, because thickness information have been usually superimposed on it. Spectral analyses of stalagmite lamina-thickness and gray level time series have been performed in order to study the mechanism of paleoclimatic changes. The spectra are compared with 95 ％ and 90 ％ false-alarm levels, respectively, resulting in 50, 24, 9.1 and 7~2 year cycles. Among them, the common cycle of 3.3 year is in accord with 3.3 year-ENSO precipitation cycle of East China. In addition, the existence of common cycles of 5.9 and 4.2 year, imply the existence of different frequent ENSO event during Middle Holocene. On the base of the teleconnection between ENSO circulation and precipitation of East China, stalagmite lamina-thickness and gray level, which reflect varieties of meteoric precipitation intensity, could be regarded as indirect proxies of ENSO cycle.