石笋微层气候学的几个重要问题

气候动力和土壤过程的年季旋回造成石笋碳酸盐沉积不连续界面(如有机质)或矿物(如文石与方解石)、结构(如舒松层和致密层)交替转换从而形成了石笋微生长层,石笋微层不但为古气候时间序列提供了高分辨率的时间标尺,而且其本身的厚度变化在一定条件下也记录了气候变化,成为高分辨率的气候代用指标。石笋微层气候学就是在研究石笋微层与气候变化关系的基础上,采用石笋微层定年、利用石笋微层厚度变化定量重建气候历史的科学方法。现在国内外已经公布了数个石笋层厚时间序列,但序列的构成差距很大,而且有的序列已经受到质疑,因此有必要建立一个获取和提供数据的科学规范,以增加石笋微层重建气候工作的可信度,这是石笋微层气候学方法得以顺利发展首先要解决的问题。而在何种条件下气候能够成为影响石笋微层层厚变化的主导因素,可能是石笋微层气候学需要解决的基本问题。此外,文章还讨论了微层界面形成及碳酸盐沉积的时间、层厚曲线平底、层内虚假的灰度、伪年层和年层缺失、平行样品交叉定年的困难原因等石笋微层气候学必须回答的问题。

SIGNIFICANT ISSUES OF STALAGMITE LAYER-CLIMATOLOGY

Some stalagmites contain climate-respondent annual growth layers that then provide long and continuous chronologies from which annually climatic information can be extracted. However, the work on stalagmite layers has not been widely accepted. Betancourt et al. (2002) statistically compared variations in layer thickness from a Late Holocene stalagmite in Carlsbad Cavern, Southern New Mexico, USA with three independent tree-ring chronologies from the same region and found no correspondence. They then suggested that laminated stalagmites should be held to the same rigorous standards in chronology building and climatic inference as annually resolved tree rings, corals, and ice cores. This argument resulted in a serious consideration to the methodology of high-resolution stalagmite/climate reconstruction work. Methodology of quantitatively reconstructing climatic variation with stalagmite annual layer based on the understanding of the relation between changes in climate and stalagmite layer thickness is here shortly termed as“stalagmite layer-climatology”. Cyclic deposition shown by luminescent or visible divisions produced by annually or seasonal climate cycles forms stalagmite micro-layers, which supply high-resolution time scale for paleoclimate researches, and the change in the thickness can reflect climate variations. However, it appears that published stalagmite layer thickness datasets are great different for their composition, which has thrown this research into confusion. Therefore, the dataset that is/will be prepared must be standardized, e.g. , errors derived from thickness measurement or layer counting should be supplied. This is the most important issue that must be addressed otherwise growth layer/climate data of stalagmite could not be reliable. A basic issue for stalagmite layer-climatology is distinguishing special cave environmental conditions under which climate plays a dominant factor controlling the changes in layer thickness. Other important issues of layer-climatology are also discussed in this article such as timing and possible mechanism of organic substance formation in stalagmite layers, flat bottom of the time series of layer thickness which is produced by the too small difference between very thin layers, false gray level of inter-layer that is produce by layer division oblique to thin section, false annual layer which may be presented by sub-annual or supra-annual layers, loss of annual layer that may be due to unknown severe dry years. Moreover, cross-dating method couldn't work unless the thickness of the growth layers of different stalagmites from the same cave show the same trend in their changes, and by comparing these samples, geologists may overcome the interference from false annual growth layers and loss of annual layers. However, in most cases this is unusual because different stalagmites in a cave may reflect different environmental factors. The difficulty of obtaining a large number of stalagmite samples is another problem. As a new scientific method, there is a long way to go for stalagmite layer-climatology. How far back we can go depends on the quality of our sampling, and how good the work we can do depends on the quality of our methodology. We therefore need scientific standards, which consist of three parts: statistical standard, chronological standard and climatological standard, and all the three must be conformed.