峰丛石山的岩溶作用及生态环境元素迁移

中国西南岩溶区的峰丛石山是一种非常脆弱的环境。抑制石漠化趋势，恢复生态平衡，帮助石山区人民脱贫是当前紧迫的任务。本文通过选择广西弄拉、桂林岩溶试验场、忻城石叠等典型地区进行野外勘测和采样实验分析，并结合前人的研究成果，从地球系统的角度，对峰丛石山的岩溶作用特征，岩溶环境的元素迁移规律以及地质生态效应进行了初步的探讨。南方湿热的季风气候迭加于隆起的碳酸盐岩地区，使峰丛石山系统远离平衡态，CO2（气）-水-钙三相不平衡，岩溶作用强烈，低浓度Ca2+含量（＜5mg／l）的雨水，流经峰丛系统后，仅几小时即可达到100mg／1左右Ca2+浓度的岩溶水。桂林地区年雨水溶蚀速度可达91.21m3／km2·a。这种强烈的岩溶作用不但导致深的表层岩溶带、奇特的峰丛景观的形成，也是岩溶环境土壤贫瘠和石漠化的主要自然原因。溶蚀作用在峰丛的不同地貌部位、不同水动力带和不同的生态环境差异很大。以裸露石山不同地貌部位而言，洼地底部的溶蚀量最大，垭口次之，峰顶又次之，峰坡溶蚀量最小。而对于不同水动力带来说，以表层岩溶带溶蚀量最大，向下部包气带溶蚀作用迅速减弱：下部饱水带的岩溶作用则主要发生于流水环境的岩溶管道中，静水环境溶蚀作用则很弱。差异溶蚀过程是造成我国壮观奇特岩溶形态的重要过程，也是峰丛石山发展演化的关键。岩溶动力系统中的CO2-水-钙循环引起了岩溶作用，也驱动了岩溶环境元素的迁移。在强溶蚀动力条件下，元素发生随水溶解的正向迁移，岩溶泉水的暂时硬度一般11-15德国度，弄拉地区郁闭森林下的最高值可达18.9德国度，且伴随岩石的溶解，难溶的Fe、Si、Al、P、Mn元素在岩溶水中也有一定的含量。如果岩溶水中CO2的逸出，水中元素伴随碳酸钙的沉淀而发生反向迁移。元素迁移的结果使峰丛区形成富钙且偏碱的地球化学环境，如产生化学组成与岩溶作用密切相关的地域性石灰土，较高硬度的岩溶水，以及富钙的植物。岩石的地球化学背景对其环境的元素迁移具有重要的影响。白云岩地区，在地形相对平缓的地段有比较完整的土壤剖面，且土壤剖面B层以下，主要上壤成分和微量元素含量具有随深度而增加的一般规律性，反映了原地残积和逐步淋溶风化的特征；而灰岩区的土壤剖面普遍缺少母质层，土壤剖面元素迁移的渐变性特征也不明显。弄拉泥盆系东岗岭组泥硅质白云岩具有相对丰富的微量元素背景值，元素迁移的结果，水、土、植物等环境均具有相对较高的矿质营养元素含量，对于生态的发展具有重要意义。峰丛石山地区特殊的元素迁移规律，产生了特有的岩溶地质生态。如岩溶植被的喜钙性、岩生性、旱生性和分布的区域性。而且，具有特殊地质背景的岩溶区，如广西弄拉，其环境的元素迁移有利于多种植物对元素的吸收而使植物多样性得到发展，并形成许多名、特、优果树和药材。
活动房
Fengcong stone mountain environments in the karst areas of southwest China,about 500,000 km2 , are very fragile. Now Chinese governments face up to theproblems to stop the deforestation tendency, rebuild forest environments and helpthe local people take off poverty. This paper picked up some typical areas, such asNongla in Mashan county, the karst experimental site in Guilin, Shidie in Xinchengcounty, Guangxi autonomous region and Puding, Maolan in Guizhou province, forfield investigations, surveys and collecting samples for experimental analyses in thelaboratory. On the basis of former research results and by means of the systematicanalysis, the karst processes, elemental migration law and the ecological effectsin the fengcong karst areas are approached.The humid and warm monsoon climate and the carbonate rock uplift make thefengcong stone moutain system far away from equilibrium. So there usually arestrong karst processes in the CO2-water-carbonate rock system. Passing through afengcong depression system, the rainwater with low Ca2+ content（less than 5mg/l） becomes rich in Ca2+ up to 70-110 mg/l only in Several hours. The solutionrate in Guilin fengcong area is 91.21 m3/km2·a. Such rapid and strong solutionprocesses not only promote the formation of both deep epikarst zone and themacroscopic tower or cone stone peak landforms, but also result in the rare soil andbare rocks. However, the solution rate is different in different geomorphologicpositions, hydrodynamic zones and ecological environments. For differentgeomorphologic positions, there is the highest solution rate on the bottom ofdepression, the second highest in the saddle between peaks, the third highest on the top of peaks and the lowest on the steep slope of peaks. For differenthydrodynamic zones, the solution rate is high in the epikarst of vadose zone andgoes down quickly with increase of depth. The karst processes in saturation zoneoccur in the rock near flow water in karst conduit. The quiet water only can lead aweak solution process. The evident differential solution is very important forChinese marvellous fengcong karst landforms. It is also a key factor to understandthe development and evolution of the fengcong stone mountains.The cycle of CO2-water-calcium in the karst dynamic system can drive theelement migration of the karst environments. Under the strong karst dynamicconditions, a forward element migration happens with dissolution of the carbonaterocks, and the temporary hardness of the karst spring water usually is 11-15 Germandegree. But under the dense forest at Nongla, the highest temporary hardness canreach up to 18.9 German degree, and the insoluble Fe, Si, Al, P and Mn can also bepartly dissolved into karst water. If some carbon dioxide escapes from the karst water,a backward element migration happens with the deposition of calcium carbonate.With the element migration, the fengcong karst areas become calcium-rich and a littlealkaline geochemical environments. For example, there are calcareous soil which itschemical composition has a close relation with the karst processes, high hardnesskarst water, and enriched calcium trees.The geochemical background of rocks has an important role in the elementmigration of the environments. In dolomite areas, there is a complete soil profilestructure（from D, C, B to A） on the gentle slope of mountain. And below B layer,there is a general law that main consituents and some trace elements increase with thedepth, which show that the soil is the monochronogenous soil and is leached andweathered gradually. However, no C layer and the gradual change law of elements isin the limestone soil profiles. There are relative abundant background value of traceelements in the argillaceous and silicaeous dolomite of the Donggangling Group ofDevonia in Nongla, which, with element migration, results in the relative highmineral nutritious elements in water, soil and vegetation.The special element migration law in fengcong stone mountains results in thekarst geological ecology. The vegetation is characterized by calciphile, xerophyte andlithophyte. And the species is quite different in different geological regions. Somespecial karst areas, such as Nongla in Guangxi, have enough mineral nutritiouselements for promoting development of species diversity. There are a lot of good fruits and precious herbs in Nongla, which help the local people take off their poverty.