石笋矿物类型、成因及其对气候和环境的指示

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。      

文石石笋发生矿物重结晶的影响因素及其古气候意义

洞穴沉积物—石笋已成为研究岩溶区环境气候变化历史的重要载体。在我国湘西地区,某些洞穴石笋原始沉积多为不稳定的文石矿物,极易发生重结晶,可能使石笋中相关化学元素含量最终偏离原生矿物的特征,限制了文石石笋某些代用指标在古气候研究中的应用。文章以前人研究成果为基础,总结梳理了文石石笋发生重结晶的影响因素及其对石笋记录古气候的影响:（1）石笋剖面特征、XRD结果、显微镜观察和地球化学元素特征等可作为石笋发生重结晶的判别依据;（2）洞穴滴水和石笋孔隙水饱和度、文石晶体缺陷和晶体之间的方解石胶结物以及岩溶水体中Mg2+浓度等均会影响文石石笋的矿物转变;（3）在文石向方解石转变过程中,石笋铀含量会有一定程度的流失,可导致放射性铀系定年的异常或年代倒序;（4）矿物重结晶可导致δ18O、δ13C及石笋微量元素浓度（或比值）等指标发生改变,其变化特征因洞穴而异,从而影响其作为环境指示器的可靠性;（5）湖南龙山惹迷洞石笋（RM2）发生了不均一的矿物重结晶,自顶部至20.3 cm以放射状为主,20.3 cm至底部主要为糖粒状,并结合年代结果发现文石重结晶对石笋铀系定年产生了影响,而重结晶作用对该石笋其他指标的影响还有待进一步研究。      

重庆芙蓉洞洞穴沉积物δ13C、δ18O特征及意义

利用重庆芙蓉洞内各种新老沉积物的δ13C、δ18O以及对洞穴内的滴水、池水和洞外泉水的长期观测结果,发现芙蓉洞内的次生沉积物中氧同位素变化整体一致,处于稳定温度下(16℃)的平衡分馏状态。而且洞内滴水和池水的氧同位素也相当一致,反映了外界大气降水中氧同位素的年平均状态。芙蓉洞内各种沉积物中碳同位素变化范围很大,从0‰~-11‰均有分布。由于芙蓉洞内各种滴水以及池水中溶解无机碳(DIC)的δ13C变化约在-8‰~-11‰,显著偏轻于部分洞穴沉积物中的δ13C。通过研究从洞穴滴水到形成次生化学沉积物这个过程中的可能影响洞穴沉积物中碳同位素变化的因素,例如：洞穴温度、滴水高度和速率、CO2脱气、生物作用、矿物同质异相转换等,同时参考芙蓉洞内连续生长达37 ka的FR5石笋的碳同位素记录,发现以上可能的影响因素都不能完全解释芙蓉洞内次生沉积物中碳同位素的异常偏重现象。虽然芙蓉洞内广泛存在文石与方解石共存的次生沉积物,但是综合分析表明这些沉积物的氧同位素处于平衡分馏状态,可以用来进行古气候研究。不过在利用石笋碳同位素解释古环境变化时需要慎重,特别是在讨论由文石或文石—方解石混合构成的次生沉积物时。     

洞穴文石石笋古气候环境变化研究进展、存在问题及研究方向

在全球变暖、极端事件频发的背景下,利用地质载体重建过去2 000年来气候变化规律、研究极端事件发生机制是未来气候变化,特别是极端天气/气候事件预测的重要手段。文章综述了高U含量、能够高精度测定年龄的洞穴文石石笋在古气候环境重建的应用:用于研究气候变化与人类文明的关系、反演区域气候差异性、准确重建区域气候变化及极端事件的发生;并针对洞穴文石石笋古气候环境重建中存在的问题提出以下研究方向:综合87Sr/86Sr、δ26Mg、δ18O和δ13C等同位素技术追寻洞穴文石的物源、利用Mg/Ca、Sr/Ca和δ13C、△47综合分析洞穴文石形成的环境及水文过程,加强洞穴文石的结晶学、物理化学研究,查明其形成及转化为方解石的条件。      

洞穴滴水、石笋中元素及元素比值对气候环境变化响应的研究进展

滴水/石笋元素是除δ18O和δ13C,研究气候环境变化的又一重要代用指标。外界气候环境变化通过改变表层岩溶带和岩溶含水层中的水文环境,甚至洞穴环境,进而影响元素的溶解、运移和沉淀过程,从而使得滴水/石笋中元素表现一定的变化规律。本文通过分析影响洞穴滴水元素及元素比值变化的因素：元素来源、水—岩相互作用和滞留时间、差异性淋滤、先期碳酸盐沉淀及分配系数的基础上,从元素对岩溶区“二元结构”和极端天气/气候事件响应的角度,探讨了滴水/石笋中元素的气候环境指示意义。取得了以下认识：① 强降水带来的冲刷作用和溶解作用,促进土壤和基岩中元素、胶体和天然有机质（NOM：Natural Organic Matter）等物质在短时间内快速溶解和运移,使滴水中元素含量增加;但随着降水增多带来的稀释作用,使得滴水/石笋中Mg,Sr和Ba等元素含量降低,因此,单一元素的解译较为复杂;② 但基岩/溶液中元素溶解和沉积的差异,导致元素相对含量的变化,使得元素X/Ca比值对外界环境的响应具有同一性,尤其是Mg/Ca和Sr/Ca比值：在干旱条件下,降水减少导致方解石先期沉积（PCP：Prior Calcite precipitation）作用增强,使Mg/Ca和Sr/Ca比值增大。但目前存在着一些问题：① Mg/Ca和Sr/Ca比值变化对强降水事件的响应并不明显,可能与新、老水混合及元素溶解过程中的溶解比例差别不大有关;② 多源、多期混合水源会导致洞穴滴水元素对极端事件响应减弱;③ Mg/Ca和Sr/Ca的变化为解释δ18O的“雨量效应”及“源效应”提供了见解,但元素变化能否反映季风强度的变化,仍有待进一步的研究。基于以上认识,笔者提出开展更加系统的大气—土壤—包气带—洞穴的监测;开展更高分辨率、更长时间尺度的洞穴监测;开展多区域、多洞穴系统对比研究来更加深入地开展洞穴石笋元素研究。     

石笋矿物由文石转变为方解石后碳、氧同位素组成的变化

石笋矿物由文石转变为方解石,其碳、氧同位素组成如何变化的认识对于古环境研究具有重要意义。选取江西省神农宫洞石笋SN15进行研究,X衍射分析测试证明其主体部分的矿物组成为文石,部分位置文石转变为方解石。相同层位文石和转变方解石的碳、氧同位素组成分析结果表明,转变方解石与原生文石相比,δ13 C值几乎没有变化;而δ18 O值明显偏负,且不是系统性偏负,偏负范围为0.2‰~1.4‰。因此,原生文石记录的降水氧同位素组成在矿物转变时受到扰动,转变方解石的氧同位素组成信息不能用于高分辨率古气候与古环境研究;碳同位素组成虽然变化很小,使用时也需慎重。     

中国南方发现大型文石笋

中国南方大量石笋剖面研究表明,石笋矿物组成有方解石、文石、方解石-文石三种类型。石笋沉积纹(微)层结构构造特征大同小异,但是文石石笋的放射状纹(微)层构造更显著。文石石笋沉积生长的气候环境是滴水多而稳定,水温低、气温略高的冷、湿的洞穴气候条件。文石转变为方解石常保留文石的针状、柱状晶形,并存在于石笋沉积生长的始终。文石结构构造转化在常温或低温、常压或低压下自调整作用的成晶成岩环境进行,其强度取决于所在洞穴气候环境、滴水在石笋的渗流和石笋的含水度。转化作用不影响同位素分馏,石笋同时全面记录古气候环境信息。研究文石石笋对重建古气候环境、成岩成矿作用都有重要理论和实践意义。     

洞穴滴水、石笋中元素及元素比值对气候环境变化响应的研究进展

滴水/石笋元素是除δ18O和δ13C,研究气候环境变化的又一重要代用指标。外界气候环境变化通过改变表层岩溶带和岩溶含水层中的水文环境,甚至洞穴环境,进而影响元素的溶解、运移和沉淀过程,从而使得滴水/石笋中元素表现一定的变化规律。本文通过分析影响洞穴滴水元素及元素比值变化的因素：元素来源、水—岩相互作用和滞留时间、差异性淋滤、先期碳酸盐沉淀及分配系数的基础上,从元素对岩溶区“二元结构”和极端天气/气候事件响应的角度,探讨了滴水/石笋中元素的气候环境指示意义。取得了以下认识：① 强降水带来的冲刷作用和溶解作用,促进土壤和基岩中元素、胶体和天然有机质（NOM：Natural Organic Matter）等物质在短时间内快速溶解和运移,使滴水中元素含量增加;但随着降水增多带来的稀释作用,使得滴水/石笋中Mg,Sr和Ba等元素含量降低,因此,单一元素的解译较为复杂;② 基岩/溶液中元素溶解和沉积的差异,导致元素相对含量的变化,使得元素X/Ca值对外界环境的响应具有一致性,尤其是Mg/Ca和Sr/Ca值：在干旱条件下,降水减少导致方解石先期沉积（PCP：Prior Calcite precipitation）作用增强,使Mg/Ca和Sr/Ca值增大。但目前存在着一些问题：① Mg/Ca和Sr/Ca值变化对强降水事件的响应并不明显,可能与新、老水混合及元素溶解过程中的溶解比例差别不大有关;② 多源、多期混合水源会导致洞穴滴水元素对极端事件响应减弱;③ Mg/Ca和Sr/Ca的变化为解释δ18O的“雨量效应”及“源效应”提供了见解,但元素变化能否反映季风强度的变化,仍有待进一步的研究。基于以上认识,笔者提出开展更加系统的大气—土壤—包气带—洞穴的监测;开展更高分辨率、更长时间尺度的洞穴监测;开展多区域、多洞穴系统对比研究来更加深入地开展洞穴石笋元素研究。     

安徽蓬莱仙洞不同滴水点差异PCP作用及其古气候记录研究意义

正石笋由于分布广泛、定年准确,并且具有丰富的气候和环境代用指标(如氧碳稳定同位素~([1,2])、微层厚度~([3])、微量元素~([4~6])等),因而受到古气候学家广泛关注。其中,微量元素和稳定碳同位素常被用来间接地反映局地干湿变化。当降水减少时,洞穴滴水在洞顶储存时间较长,使岩溶水在补给石笋之前已经发生了方解石沉积作用。由于Mg,Sr和Ba在方解石和溶液中的分配系数远小于1~([7,8]),从而使Ca优先于Mg,Sr和Ba沉积出来,使得溶液(渗流水及其所形成的滴水)和现代次生碳酸盐沉积物中的Mg,Sr和Ba富集,进而导致Mg/Ca,Sr/Ca和Ba/Ca比值增大~([4,5,9]),这种现象被称为方解石前期沉积作用(Prior Calcite Precipitation,简称PCP)。此外,当降水减少,洞穴滴水滴率变慢(即PCP作用增强)时,CO_2脱气时间增长也会导致石笋碳同位素偏     

贵州中西部洞穴水系与碳酸钙沉积物的Mg/Sr比值和地球化学特征

对贵州中西部郑家洞、石将军洞和织金洞三个洞穴中水样和碳酸钙沉积物样品地球化学特征的分析,得出洞穴水Mg /Sr 比平均值分别为1 278± 411、1 155± 207、14. 0± 2. 7。安顺洞穴的水样富Mg 贫Sr,与织金洞的相反,一方面可能与围岩组成的差别有关,另一方面可能与洞穴地表的植被和土壤覆盖有关。碳酸钙样品中的微量元素比值反映了洞穴水的比值,而后者受围岩和地表条件的影响。因此,以Mg /Sr或Mg /Ca 比值来反映温度的变化,必须详细了解各种影响因素的影响作用。通过织金洞洞穴水和现代碳酸钙沉积物中Sr /Ca比值,计算出K Sr值为0. 04～ 0. 134,平均值为0. 073± 0. 035; KMg平均值为0. 021± 0. 010。这些结果可为今后利用石笋Mg /Sr 比值作为古气候代用指标的研究提供基础数据。      

Origin and palaeoenvironmental significance of lamination in stalagmites from Katerloch Cave,Austria

The origin and environmental dependencies of lamination in stalagmites from Katerloch, common in speleothems from other cave sites, are examined in detail. Petrographic observations and chemical analyses (including isotopes) of stalagmites and modern calcite were combined with multi‐annual cave monitoring. All investigated stalagmites are composed of low‐Mg calcite and show white, porous laminae and typically thinner, translucent dense laminae. The binary lamination pattern results from changes in the calcite fabric: white, porous laminae are characterized by a high porosity and abundant fluid inclusions and also by enhanced vertical growth and thinning towards the flanks. Translucent, dense laminae exhibit a compact fabric and constant thickness of individual growth layers. U‐Th dating supports an annual origin of the lamination and the seasonally changing intensity of cave ventilation provides a robust explanation for the observed relationships between lamination, stable C isotopic compositions and trace elements (Mg, Sr and Ba). The seasonally variable air exchange, driven by temperature contrasts between the cave interior and outside atmosphere, modulates the rate and amount of CO₂ degassing from the drip water and affects the hydrochemistry and consequently the fabric of the precipitating calcite. Although cave air composition and drip rate are both major variables in controlling CO₂ degassing from the drip water, the seasonally changing ventilation in Katerloch exerts the primary control and the results suggest a secondary (amplifying/attenuating) influence of the drip rate. Drip rate, however, might be the controlling parameter for lamina development at cave sites experiencing only small seasonal cave air exchange. Importantly, the seasonally variable composition of drip water does not reflect the seasonal cycle of processes in the soil zone, but results from exchange with the cave atmosphere. The alternating porous and dense calcite fabric is the expression of a variable degree of lateral coalescence of smaller crystallites forming large columnar crystals. The white, porous laminae represent partial coalescence and form during the warm season: low calcite δ¹³C values are linked to low δ¹³C values of cave air and drip water during that time. This observation corresponds to times of reduced cave ventilation, high pCO₂ of cave air, low drip water pH, lower calcite supersaturation and typically high drip rates. In contrast, the translucent, dense laminae represent more or less complete lateral coalescence (inclusion‐free) during the cold season (high calcite, drip water and cave air δ¹³C values), i.e. times of enhanced cave ventilation, low cave air pCO₂, increased drip water pH, relatively high calcite supersaturation and typically low drip rates. In essence, the relative development of the two lamina types reflects changes in the seasonality of external air temperature and precipitation, with a strong control of the winter air temperature on the intensity of cave‐air exchange. Thick translucent, dense laminae are favoured by long, cold and wet winters and such conditions may be related closely to the North Atlantic Oscillation mode (weak westerlies) and enhanced Mediterranean cyclone activity during the cold season. Studies of speleothem lamination can thus help to better understand (and quantify) the role of seasonality changes, for example, during rapid climate events.     

Magnesium-isotope fractionation during low-Mg calcite precipitation in a limestone cave - Field study and experiments

The chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings. Despite intensive research, including modelling and novel approaches, speleothem data remain difficult to interpret. A possible way foreword is to apply a multi-proxy approach including non-conventional isotope systems. For the first time, we here present a complete analytical dataset of magnesium isotopes (δ²⁶Mg) from a monitored cave in NW Germany (Bunker Cave). The data set includes δ²⁶Mg values of loess-derived soil above the cave (−1.0 ± 0.5‰), soil water (−1.2 ± 0.5‰), the carbonate hostrock (−3.8 ± 0.5‰), dripwater in the cave (−1.8 ± 0.2‰), speleothem low-Mg calcite (stalactites, stalagmites; −4.3 ± 0.6‰), cave loam (−0.6 ± 0.1‰) and runoff water (−1.8 ± 0.1‰) in the cave, respectively. Magnesium-isotope fractionation processes during weathering and interaction between soil cover, hostrock and solute-bearing soil water are non-trivial and depend on a number of variables including solution residence times, dissolution rates, adsorption effects and potential neo-formation of solids in the regolith and the carbonate aquifer. Apparent Mg-isotope fractionation between dripwater and speleothem low-Mg calcite is about 1000lnα_Mg-cc-Mg(aq) = −2.4‰. A similar Mg-isotope fractionation (1000lnα_Mg-cc-Mg(aq) ≈ −2.1‰) is obtained by abiogenic precipitation experiments carried out at aqueous Mg/Ca ratios and temperatures close to cave conditions. Accordingly, ²⁶Mg discrimination during low-Mg calcite formation in caves is highly related to inorganic fractionation effects, which may comprise dehydration of Mg²⁺ prior to incorporation into calcite, surface entrapment of light isotopes and reaction kinetics. Relevance of kinetics is supported by a significant negative correlation of Mg-isotope fractionation with the precipitation rate for inorganic precipitation experiments.     

洞穴滴水地球化学的空间和时间变化及其控制因素--以北京石花洞为例

通过对北京石花洞滴水地球化学一个水文年的观测,揭示了洞穴滴水水文地球化学季节变化与外界气候变化的关系,3个滴水点的滴率随降雨量的增加都有明显的变化,但不同滴水点滞后时间不同。滴水滴率、Mg²⁺和SO^{2-₄含量的季节变化数据显示,雨季洞穴滴水主要来源于当季降水,但也存在岩层滞留水的混入。滴水中Mg/Ca比值存在明显季节变化,旱季较低而雨季较高,但在雨季初期出现较大的波动。分析洞穴上覆土壤和洞内裂隙土壤数据,认为雨季初期滴水中Mg/Ca比值的波动是由土壤中Mg2+的快速淋溶造成的,上覆土壤结构性质和组分变化均影响滴水地球化学特征。}     

Seasonal dripwater Mg/Ca and Sr/Ca variations driven by cave ventilation: Implications for and modeling of speleothem paleoclimate records

A 4-year study in a central Texas cave quantifies multiple mechanisms that control dripwater composition and how these mechanisms vary at different drip sites. We monitored cave-air compositions, in situ calcite growth, dripwater composition and drip rate every 4-6 weeks. Three groups of drip sites are delineated (Groups 1-3) based on geochemical variations in dripwater composition. Quantitative modeling of mineral-solution reactions within the host carbonate rock and cave environments is used to identify mechanisms that can account for variations in dripwater compositions. The covariation of Mg/Ca (and Sr/Ca) and Sr isotopes is key in delineating whether Mg/Ca and Sr/Ca variations are dictated by water-rock interaction (i.e., calcite or dolomite recrystallization) or prior calcite precipitation (PCP). Group 1 dripwater compositions reflects a narrow range of the extent of water-rock interaction followed by varying amounts of prior calcite precipitation (PCP). Group 2 dripwater compositions are controlled by varying amounts of water-rock interaction with little to no PCP influence. Group 3 dripwater compositions are dictated by variable extents of both water-rock interaction and PCP. Group 1 drip sites show seasonal variations in dripwater Mg/Ca and Sr/Ca, whereas the other drip sites do not. In contrast to the findings of most previous dripwater Mg/Ca-Sr/Ca studies, these seasonal variations (at Group 1 drip sites) are independent of changes in water flux (i.e., rainfall and/or drip rate), and instead significantly correlate with changes in cave-air CO₂ concentrations. These results are consistent with lower cave-air CO₂, related to cool season ventilation of the cave atmosphere, enhancing calcite precipitation and leading to dripwater geochemical evolution via PCP. Group 1 dripwater Mg/Ca and Sr/Ca seasonality and evidence for PCP as a mechanism that can account for that seasonality, have two implications for many other regions where seasonal ventilation of caves is likely: (1) speleothem trace-element records may provide seasonal signals, and (2) such records may be biased toward recording climate conditions during the season when calcite is depositing. Additionally, we use our results to construct a forward model that illustrates the types of speleothem Mg/Ca and Sr/Ca variations that would result from varying controls on dripwater compositions. The model provides a basis for interpreting paleo-dripwater controls from high frequency Mg/Ca and Sr/Ca variations for speleothems from caves at which long term monitoring studies are not feasible.     

Temporal and spatial variations in hydro-geochemistry of cave percolation water and their implications for four caves in Guizhou, China

Soil water and cave drip water from four cave systems in Guizhou, Southwest China, were monitored and sampled monthly from April 2003 to May 2004 to understand spatio-temporal variations in hydro-geochemistry of cave percolation water. The results indicated that among the 5 drip water sites from the Liangfeng Cave (LFC), there were no significant differences among the hydro-geochemical parameters. In the Xiniu Cave (XNC), the drip rates were variable and responded quickly to precipitation events in the 3# (No. 3) drip site with variable water head; both bulk and variation in drip rates were smaller in the 2# with constant water head. However, in the Qixing Cave (QXC) and Jiangjun Cave (JJC), drip rates, concentrations of major ions and saturation index for calcite (SI C ) were smaller, and Mg/Ca ratios in Group I (1#, 2#, 6#, 7# and 8# in the QXC; 2# and 3# in the JJC) were larger than those in Group II (3#, 4#, 5# and 9# in the QXC; 1# and 4# in the JJC). These differences might be the result of different hydrogeological processes above the caves, which are divided into five categories based on hydro-geochemistry data. The formation of some proxies in speleothem, such as Mg/Ca, is likely to be affected by those processes.     

Carbon mass-balance modelling and carbon isotope exchange processes in dynamic caves

Diverse interpretations have been made of carbon isotope time series in speleothems, reflecting multiple potential controls. Here we study the dynamics of ¹³C and ¹²C cycling in a particularly well-constrained site to improve our understanding of processes affecting speleothem δ¹³C values. The small, tubular Grotta di Ernesto cave (NE Italy) hosts annually-laminated speleothem archives of climatic and environmental changes. Temperature, air pressure, pCO₂, dissolved inorganic carbon (DIC) and their C isotopic compositions were monitored for up to five years in soil water and gas, cave dripwater and cave air. Mass-balance models were constructed for CO₂ concentrations and tested against the carbon isotope data. Air advection forces winter pCO₂ to drop in the cave air to ca. 500 ppm from a summer peak of ca. 1500 ppm, with a rate of air exchange between cave and free atmosphere of approximately 0.4 days. The process of cave ventilation forces degassing of CO₂ from the dripwater, prior to any calcite precipitation onto the stalagmites. This phase of degassing causes kinetic isotope fractionation, i.e. ¹³C-enrichment of dripwater whose δ¹³C_DIC values are already higher (by about 1‰) than those of soil water due to dissolution of the carbonate rock. A subsequent systematic shift to even higher δ¹³C values, from −11.5‰ in the cave drips to about −8‰ calculated for the solution film on top of stalagmites, is related to degassing on the stalagmite top and equilibration with the cave air. Mass-balance modelling of C fluxes reveals that a very small percentage of isotopically depleted cave air CO₂ evolves from the first phase of dripwater degassing, and shifts the winter cave air composition toward slightly more depleted values than those calculated for equilibrium. The systematic ¹³C-enrichment from the soil to the stalagmites at Grotta di Ernesto is independent of drip rate, and forced by the difference in pCO₂ between cave water and cave air. This implies that speleothem δ¹³C values may not be simply interpreted either in terms of hydrology or soil processes.     

贵州七星洞系统中水文地球化学特征对滴水δ~(13)C_(DIC)的影响及其意义

2003年4月至2004年5月,笔者对贵州七星洞(QXD)进行了较为详细的监测,逐月采集了土壤水和洞穴滴水等样品,分别测定了样品的稳定碳同位素组成和水文地球化学参数。结果显示,9个滴水点同期的溶解无机碳同位素值(δ13CDIC)之间存在着大的差异,最大达6.9‰;δ13CDIC值偏重的Ⅰ组滴水,其Ca、Sr、HCO3、电导率(EC)和方解石饱和指数(SIC)等水文地球化学指标偏小,而Mg/Ca比值偏大,Ⅱ组则相反;不同滴水点的δ13CDIC值分别与相应滴水的Ca、Sr、HCO3、EC、Mg/Ca和SIC等水文地球化学指标之间存在较好的相关关系。进一步分析表明,与土壤水δ13CDIC平均值-9.9‰相比,Ⅰ组1#、2#、6#、7#和8#滴水点δ13CDIC值偏重4.5‰～5.7‰,主要是由大量的基岩溶解以及前期方解石沉积(prior calcite precipitation,PCP)共同作用的结果;Ⅱ组3#、4#、5#和9#滴水点δ13CDIC值偏重0.6‰～1.6‰,受基岩溶解和PCP过程影响较小,尤其是9#滴水点受影响最小。因此,若不考虑基岩溶解和PCP过程影响作用,将会极大地影响洞穴化学沉积物碳同位素记录的准确解释。     

Large variations of δ13C values in stalagmites from southeastern China during historical times: implications for anthropogenic deforestation

Variations in speleothem δ¹³C values can reflect changes in overlying surface vegetation, which, over historical time scales, may represent the influence of human activities. Here, we examined δ¹³C variations in two stalagmites growing for the last 2200 years in Shennong Cave, Jiangxi Province, SE China. The two δ¹³C records corroborate well one another and show a prominent 6‰ enrichment of the δ¹³C values from AD 700 to 1100. The isotopic equilibrium for modern calcite and negative correlation between δ¹⁸O and δ¹³C values along the growth axis suggest that the influences of kinetic fractionation are negligible. Varied correlations between Mg/Ca and Sr/Ca ratios and divergent changes between δ¹³C values and Mg/Ca and Sr/Ca ratios from AD 700 to 1100 reveal that the prior calcite precipitation (PCP) and water–rock interaction did not dominate the increase of δ¹³C values. It is plausible that the obvious δ¹³C variation was largely influenced by the changes in vegetation cover overlying the cave. Our δ¹³C results, together with the records of climate and human activity from historical documentary records, suggest that: (i) prior to AD 700, small fluctuations in relatively light δ¹³C values reflect the presence of lush forest coverage above the cave, which was minimally disturbed by human activities; (ii) during AD 700–1100, the drastic increase in δ¹³C values indicates persistent and massive deforestation associated with large‐scale immigration into northern Jiangxi after the Rebellion of An & Shi (AD 755–763) in the Tang Dynasty and the subsequent development of agriculture and economic activity; and (iii) since AD 1100, fluctuations in relatively high δ¹³C values suggest that local vegetation during the last millennium has been sparse. Since the Rebellion of An & Shi, southeastern China was progressively developed, coincident with deforestation and vegetation deterioration caused by human disturbance in the form of deforestation and cultivation.