石笋矿物类型、成因及其对气候和环境的指示

Phase composition and formation of stalagmite minerals:Indications of climate and environment

洞穴石笋的矿物组成分为方解石、文石和文石-方解石三种类型。本文总结了国内外已有的研究成果,并结合野外观测现象和数据,讨论了影响石笋矿物形成和转变的因素,分析了利用石笋矿物类型特征研究古气候和古环境变化的可行性。洞内滴水饱和度和滴水中Mg/Ca比值是影响石笋矿物类型的主要因素:当滴水Mg/Ca比值较低(Mg/Ca<1或1)时,滴水饱和度较低易形成方解石,而文石沉积则需要更高的滴水饱和度;当滴水Mg/Ca比值较高(Mg/Ca≥1)时,方解石相对文石沉积需要更高的滴水饱和度。洞穴围岩镁含量高且滴水多而稳定的洞内环境是我国南方大量文石笋发育的主要原因。长期处于滴水淋滤环境的文石笋容易向方解石转变,但若滴水中Mg2+浓度较大,此转变过程会受到抑制。在围岩镁含量较低的洞穴中,干旱时期渗流水滞留时间长、滴水速率变慢,会造成滴水中Mg/Ca比值升高并引起文石沉积。因此,石笋矿物类型及矿物相转变可指示气候和环境的变化。

Stalagmite may be composed of calcite,aragonite and aragonite-calcite intergrowth.The factors affecting the mineral formation and transformation of the stalagmite are summed up and the feasibility to apply mineral features of stalagmite in paleo-climate and paleo-environment study is discussed in the paper. The saturation of solution and Mg/Ca ratio in drip water are two principal factors that control the mineral components of stalagmite.When the Mg/Ca ratio in drip water is low（Mg/Ca1 or1）,lower saturation is propitious for calcite to deposit and higher saturation for aragonite.In contrast,when the Mg/Ca ratio in drip water is high（Mg/Ca≥1）,higher saturation favors calcite deposition.High Mg content in bedrock and abundant stable drip water in cave may account for a large number of aragonite stalagmites grow in the caves in southern China.Continuous leaching by drip water may cause aragonite transforming into calcite,but the transition will be restrained by high Mg concentration in drip water.In a cave with lower Mg content in bedrock,the increase of retention time of seepage water and the slowing of drip water rates may increase the Mg/Ca ratios in solution and ultimately cause aragonite deposition during dry period.Consequently,stalagmite mineral features can indicate climate and environment changes.