【作者】 李俊云；

【导师】 王建力；

【作者基本信息】 西南大学 ， 自然地理学， 2013， 博士

【摘要】 洞穴沉积物作为重建古气候研究的重要载体之一,具有高分辨率连续记录和高精度定年的特征,从洞穴沉积物中可以提取大量记录古气候和古环境演变的信息,且代用指标多样化。尽管微量元素具有来源多样性和影响因素复杂性的特征,在重建古气候研究中的应用不如δ180广泛,但微量元素可以揭示地下水在洞穴上覆表层岩溶带的运移过程与地表环境的关系,研究微量元素的影响因素是解释洞穴沉积物记录的环境信息的重要依据。目前研究微量元素的来源和控制因素的主要方法是洞穴监测。本文选取重庆市武隆县芙蓉洞作为研究区域,监测洞穴所在区域的大气降水量、气温、土壤环境和洞穴环境,并收集降水、土壤、基岩、土壤渗透水、洞穴水和洞穴现代沉积物,重点分析地表环境对各类样品中的Ca、Mg、Sr的浓度和Mg/Ca、Sr/Ca、Mg/Sr比值的影响,并监测土壤渗透水、洞穴水和洞穴现代沉积物地球化学性质对2009/2010年的西南地区冬春干旱气候事件的反映。经过研究得出以下结论：(1)芙蓉洞上覆土壤的粒度分析结果显示<16μm粒径组的平均比重达到63.2%,是芙蓉洞上覆土壤颗粒的主要粒径组。0-10cm的表层土壤中>63μm的颗粒平均比重9.07%,深度在10cm以下的土壤中其平均比重为5.14%；随着深度的增加土壤中粘土所占比重呈增加趋势。土壤的粒度组成特征有助于降水的渗透,并减缓地下水在土壤中的下渗速度。土壤中Ca、Mg、Sr的浓度随着深度的增加而增加,Sr的浓度与粘土含量呈正相关关系。但各土壤剖面中的元素浓度有较大差异,可能与土壤中含有碳酸盐碎屑物质有关。土壤的地球化学组成与基岩相似,均表现出Ca>Mg>Sr,基岩中含有白云岩使土壤中的Mg浓度较高。但由于Ca在土壤中的迁移率较大,使土壤的Mg/Ca、Sr/Ca比值大于基岩,Mg/Sr比值小于基岩。(2)通过2010-2012年土壤渗透水量的变化,发现夏秋季节渗透水量大,冬春季竹渗透水少或断流,与研究区域的降水量季节性分布特征相似,说明渗透水量能响应大气降水量的变化。在土壤渗透水的监测过程中还发现,场次降水强度和持续时间,以及土壤渗透水采样点的坡度、土壤的颗粒组成等因素也可能影响土壤渗透水量对降水量的响应。(3)对土壤渗透水的地球化学性质进行了监测,结果显示2009/2010年的冬春干旱气候以后,2010年4月渗透水的Ca、Mg、Sr的浓度普遍较高,在土壤不同深度收集的渗透水Ca、 Mg、Sr的浓度随着深度的增加而上升,说明渗透水在土壤中的滞留时间和渗透距离都对Ca、 Mg. Sr的浓度有重要影响,相同的影响因素导致渗透水中Ca、Mg、Sr之间均呈正相关关系。2011年土壤渗透水的Mg/Ca、Sr/Ca比值同时上升,且Mg/Ca>1,说明碳酸钙提前沉淀(PCP)不但发生在基岩中,可能由于高温使地表蒸发旺盛导致渗透水的C02脱气,在土壤中也出现PCP。土壤渗透水的Mg/Sr比值在一年中出现两次峰值,与降水量的两个峰值有较好的对应关系。在高温多雨的夏季,Mg/Sr比值的峰值能快速响应气温和降水量的变化。但在气温相对较低的初夏或秋季,Mg/Sr比值峰值滞后于降雨量,说明Mg/Sr比值受到气温和降水量的双重影响,但气温高有助于提高Mg/Sr比值响应降水量的灵敏度。(4)芙蓉洞洞穴环境监测数据显示,洞内C02浓度有季节性变化,夏季高于冬季,与土壤C02浓度的季节性变化相似。洞穴滴水的Mg/Ca、Sr/Ca比值在2011年5-8月同时上升,9-11月同时下降,推测原因为2009/2010年冬春干旱气候导致表层岩溶带含水量减少,使岩溶带裂隙和孔隙的通风性增强,导致地下水在表层岩溶带产生PCP,使地下水的Mg/Ca、Sr/Ca比值上升。随着地下水的运移,此气候信息被传递到滴水中,说明芙蓉洞滴水的地球化学性质能够响应极端气候事件。各滴水点的元素浓度和元素比值的变化趋势均一致,说明洞穴滴水的水源和地表环境对地下水的影响有相似性。各滴水的元素浓度和比值在数量上存在差异,可能与地下水的水文地质过程有关。MP7滴水的Ca、Sr的浓度低于其他滴水,且Mg的浓度低于滴水的平均值,但Mg/Ca比值普遍大于1,根据MP7滴水量大于500mL/分钟的实际情况,推测该地下水快速通过了白云岩层。滴水MP5的滴率小于MP4,MP5的Mg/Ca、Sr/Ca比值均大于MP4,因为MP5滴水的地下水在洞穴顶板滞留时间更长,从而产生了PCP,滴水MP5的Ca浓度减小,导致Mg/Ca、Sr/Ca比值增加。(5)在芙蓉洞收集的洞穴现代沉积物的Ca、Mg浓度差异较小,但Sr的浓度差异较大。样品20100226-MP2、20110802-MP3中Sr的浓度都超过80μg/g,可能沉积物中混入了细小的粘土颗粒。样品20111201-MP4、20111201-MP5的Sr的浓度分别为5.37μg/g和7.03μg/g,原因有待进一步分析。滴水携带的水文地质信息可以传输到洞穴沉积物中,如滴水MP5的Mg/Ca, Sr/Ca比值均大于MP4,在MP4、MP5两个滴水点沉积的洞穴现代沉积物的Mg/Ca、Sr/Ca比值也呈现相应特征。根据采集的芙蓉洞现代沉积物的Ca、Mg、Sr的浓度以及Mg/Ca、Sr/Ca比值的变化,没有发现记录2009/2010年冬春干旱气候事件的相应信息,说明在洞穴沉积物形成过程中还有其他未知因素的影响,导致芙蓉洞洞穴现代沉积物没有记录地表环境极端气候信息。利用野外监测数据得出的洞穴现代沉积物的KMg1平均值0.0288±0.0038,通过经验公式计算的KMg2平均值为0.01856±0.01389,根据KMg1和KMg2计算的芙蓉洞平均气温,分别为16.9±2.2℃和10.9±8.2℃,在芙蓉洞监测记录的洞穴平均气温为16-16.8℃,说明野外监测数据与实际温度更接近,且变化幅度较小,因此可以利用芙蓉洞穴监测数据进一步研究芙蓉洞石笋微量元素记录的重庆地区古气候和古环境的变化。更多还原

【Abstract】 Speleothems are increasingly used to provide high resolution, accurately dated paleoclimate information to reconstruct paleoclimate and paleoenvironment. Speleothems contain a number of suitable proxies to record the environmental information. Although unlike oxygen isotopes, trace elements have been widely used for the variety of sources and complex factors, trace elements are used to reveal the groundwater runoff in the epikarst overlying the cave with the process and the surface environment. Study of the factors controlling the trace elements in water-soil-cave system is an important basis work to explain the environmental information recorded in speleothems. Currently, cave monitoring is a common method to study the source of trace elements and their controlling factors.We have installed equipment to monitor the rainfall and air temperature outside Furong Cave which is located at Wulong County in Chongqing. The content of CO2in the soil, temperature, humidity and concentration of CO2in Furong Cave were also monitored. The rainfall, soil, bedrocks, soil infiltrating water, cave water and modern speleothems were collected. In this paper, studying on the response of the concentration of trace elements and the ratios of Mg/Ca, Sr/Ca and Mg/Sr of the samples to the surface environment is the main objective. The other goal is to monitor geochemical properties of soil infiltrating water, cave water and modern speleothems reflecting the arid climate at southwestern region in winter and spring from2009to2010. The conclusions are as follows:1) The data of soil particle size showed that the diameter of main particles is<16μm which counts for63.2%in all the soil particles. The proportion of particles that have diameter of>63μm is9.07%in the depth of0-10cm while the proportion is5.14%in the depth below10cm indicating the proportion of clay (the particle diameter<4μm) increased with soil depth. The concertrations of Ca, Mg and Sr in the soil are also increased with soil depth. The Sr concentration is positively correlated to clay content. The difference of the element concentration in every soil profile was inferred that there was much unweathered carbonate debris in the soil. The chemical composition of soil was similar to the bedrocks that was the concentrations of Ca>Mg>Sr. However,the mobility of Ca was higher than Mg and Sr in the soil which resulted in the ratios of Mg/Ca and Sr/Ca in the soil were larger than the bedrocks. The higher concentration of Mg in the soil was attributed to the dolomite., More Mg was dissolved in water during the weathering of bedrock so that the ratios of Mg/Sr in the bedrocks was higher than the soil.2) The volume of soil infiltrating water varied seasonally and was corresponding to the precipitation and other factors, such as the intensity and the duration time of precipitation, the slope gradient and the soil particle composition.3) The concentrations of Ca, Mg, Sr of soil infiltrating water collected in April,2010, were higher than the samples collected in other months. This was explained that the soil infiltrating water was resident in the soil for longer time ascribing to the arid climate at southwestern region in winter and spring from2009to2010. The concentrations of Ca, Mg, Sr of soil infiltrating water were increased along with increasing soil depth. The results indicate that the concentrations of trace elements of soil infiltrating were controlled by the residence time and the distance flowing in the soil which resulted to the positive relationship of Ca, Mg, Sr with each other. The ratios of Mg/Ca and Sr/Ca were increased and the ratios of Mg/Ca were more than1in2011. It was argued that the prior calcite precipitation (PCP) was occurred in the soil because the higher temperature and more intense evaporation resulted in CO2degassing from the soil infiltrating water. The effect of PCP was to remove Ca from the water in the proportion. The maximum value of Mg/Sr was appeared in summer when temperature was highest and the precipitation was the most. In early spring and autumn, the peak value of Mg/Sr was behind the rainfall for the lower temperature. It was illustrated that the ratio of Mg/Sr of soil infiltrating water was controlled by rainfall and temperature. Higher temperature enhanced the sensitivity of Mg/Sr to rainfall.4) The content of CO2in cave was more in summer than in winter as same as in soil. The ratios of Mg/Ca and Sr/Ca of drip water were increased from May to August while decreased from September to November in2011. It was estimated that less water in the epikarst zone for the arid climate in winter and spring from2009to2010enhanced the ventilation in rock fracture and PCP was occurred as groundwater flowing in the rocks. The information of the arid climate was transferred by groundwater and responded to the geochemical character of drip water.For the drip water of all monitoring sites, the concentration of Ca,Mg, Sr and the Mg/Ca and Sr/Ca ratios presented similar variation in this study. This was attributed to the similar sources and the factors of influence of surface environment to groundwater. It was presumed that the difference of hydrogeology condition was the main factor to result in the different values of concentration and ratios of trace elements. The ratios of Mg/Ca of MP7drip water was more than I which was explained the groundwater flew across the dolomite. The dripping rate of MP5drip water was slower than MP4and the residence time of MP5was longer resulting in CO2degassing from drip water. PCP was occurred and the ratios of Mg/Ca and Sr/Ca of MP5were higher than MP4.5) The concentrations of Ca and Mg of the modern speleothems were small difference, while the maximum concentration of Sr was87.87μg/g and the minimum was5.37μg/g. The ratios of Mg/Ca and Sr/Ca of MP5modern speleothems were higher than MP4which was the same as drip water. It showed that the hydrogeology information could be transferred by drip water to the speleothems. Nevertheless, it was not found that the information about the arid climate at southwestern region in winter and spring from2009to2010was recorded in the modern speleothems.The partition coefficient of Mg (KMgl) of speleothems was0.0288±0.0038which was calculated by using the data of cave monitoring. The other partition coefficient of Mg (KMg2) was0.01856±0.01389by using the formula. The mean cave temperature was16.9±2.2℃(calculated by KMgl) and10.±8.2℃(calculated by KMg2), respectively. The average temperature of Furong Cave monitored by instrument was16-16.8℃. It showed that the monitoring data approached the actual data.更多还原