洞穴碳酸盐微层研究及其发展方向

PROGRESS IN SPELEOTHEM MICROLAYERS RESEARCH

近几年，在高分辨率气候－环境变化研究中，洞穴碳酸盐微层逐渐受到人们重视。洞穴碳酸盐微层的类型较多，主要为年层。微层是在沉积条件发生高频率的周期性改变时形成的，要受到供水条件和碳酸钙浓度等多因素的控制。洞穴碳酸盐微层的稳定同位素组成、生长速率、微层结构与物质组分、微层厚度与灰度和光学特征等可以反映降水、温度、土壤成分及植被变化等信息。应用微层进行古气候重建需要测年技术。

In recent years, the use of stalagmites and other speleothems as high resolution paleoclimatic indicators is advancing with the recognition of annual growth layers in stalagmites. Research has tended to focus on characteristics of microlayers which were formed along the growing axes of stalagmites and can be identified under microscopes. It has been supported that the microlayers almost are annual layers by TIMS U series, AMS 14 C and other dating methods. Speleothem microlayers can be manifested in several manners defined by ①alternating luminescent/non luminescent laminae;②alternating fibrous porosity/density laminae;③alternations of aragonite and calcite laminae. Observed under transmitted light and fluorescent respectively, bi optical microlayers (transmitted light microlayers and luminescent microlayers) can be found in many Holocene stalagmites of China. Microlayers are endured a forming process with high frequency. The growth rate of stalagmites has been demonstrated to be dependent on the calcium concentration of the drip water, the water drip rate, the temperature, the thickness of the water film on the stalagmite cap and the carbon dioxide concentration in the cave air. The presence of annual growth microlayers also has implications for the interpretation of growth rate variations. Annual microlayer is not observed in all stalagmite samples. Previous studies have quoted mixing of groundwater in the aquifer, soil water residence characteristics and depth in the aquifer as a primary cause of the lack of lamina preservation. Great potential exists for the utilization of stalagmites as high resolution proxies in the karsted regions and could potentially allow for acquisition of long annual proxy record of the continent paleoenvironment. Information of precipitation, temperature, paleosoil and vegetation can be got from this kind ideal material. In this paper, several proxies were discussed, such as oxygen and carbon isotopes, growth rate and pattern, chemical components, thickness, gray level, and optical characteristics of microlayers. Paleoenvironmental reconstruction using speleothem samples should ideally use multiple samples. In the near future, critical projects include the calibration of the stalagmite growth lamina record with instruments and historical records and cross calibration with other annually dated proxies such as tree rings. Besides, three aspects of research on speleothem microlayers are needed, ie. improved analytical techniques and instrumentation that allow dating of smaller samples of speleothem calcite with higher precision, an improved understanding of speleothem forming processes, and more accurate information translation.