A 2000‐year lipid biomarker record preserved in a stalagmite from north‐west Scotland

Previous studies on lipid biomarkers preserved in Chinese stalagmites have indicated that ratios of low‐molecular‐weight (LMW) to high‐molecular‐weight (HMW) n‐alkanes, n‐alkan‐2‐ones, n‐alkanols and n‐alkanoic acids can be used as an index of vegetation versus microbial organic matter input to the system and, by extension, a marker of climatic changes, with increases in the proportion of LMW compounds coinciding with colder periods. Here we test whether this hypothesis is equally applicable to a different geographical region (north‐west Scotland), by examining a stalagmite record of the past 200 years, and a wider range of lipid markers. We also test the applicability of other lipid proxies in this context, including the use of n‐alkane ratios, to interpret vegetation changes, and unsaturated alkanoic acid ratios as climatic indicators. The results show that lipid proxies preserved in stalagmites, and especially those related to vegetation, are potentially extremely useful in palaeoenvironmental research. Of particular value is the use of C₂₇/C₃₁ n‐alkane ratios as a proxy for vegetation change, clearly indicating variations between herbaceous and arboreal cover. This proxy has now been successfully applied to samples from diverse environments, and can be considered sufficiently robust to be of use in analysing future stalagmite records. It will be of particular value in areas where reliable pollen records are not available, as is often the case with deeper cave deposits. However, the division between LMW and HMW aliphatic compounds is not a clear‐cut case of microbial versus plant activity, with the changes in LMW compounds relating more closely to those in their HMW analogues than in specific bacterial biomarkers. The use of unsaturated alkanoic acid ratios here gives conflicting results, with the observed variation through time depending on the isomer measured. The discrepancies between the findings of this study and previous work are likely to be due to the varying controls on the lipids (original organic matter input, and compound degradation), which in turn will be affected by whether the main climatic limiting factor on the soil is temperature or precipitation. This suggests that lipid proxies preserved in stalagmites must be interpreted with care, particularly in the case of bacterial compounds which may be derived from within the cave or from the soil. However, many of these issues can be resolved by the use of multi‐proxy studies. Copyright © 2011 John Wiley & Sons, Ltd.     

A new approach to detecting vegetation and land-use change using high-resolution lipid biomarker records in stalagmites

A hundred-year stalagmite lipid biomarker record from Mechara, southeastern Ethiopia, is presented. The record has been recovered at a 10-yr temporal resolution, marking the first time this has been achieved in stalagmite biomarker work and providing the first opportunity to investigate the relationship between stalagmite lipid records and hydrological transport lags, a vital issue in interpreting palaeoenvironmental signals. Preserved plant-derived n-alkanes and n-alkanols show clear changes in composition over time, relating to known land-use changes in the area, particularly the expansion of agriculture in the early twentieth century. The level of environmental detail provided by this technique, combined with the long-term chronological framework offered by stalagmites, holds significant promise for the investigation of early human environments and their associated climatic and anthropogenic controls.     

A paleoclimate record of the last 17,600 years in stalagmites from the B7 cave,Sauerland, Germany

Petrographical and geochemical parameters of stalagmites from the B7 cave in Iserlohn–Letmathe (Northern Rhenish Massif, NW Germany) record Late- and postglacial climate changes (temperature and/or precipitation). Fabrics and microfacies of the stalagmite profiles lead to a differentiation of four hierarchies of rhythms. Clastic layers in the stalagmites are caused by flooding events and are time markers. Twenty-four TIMS Th/U-age-dates provide a time calibration of stalagmite growth phases. One stalagmite reveals an early growth period between 17.6 and 16.7 ka BP. Between 9.6 and 5.5 ka BP (Atlantic episode of the Holocene) the growth rate of the stalagmites was higher than before and after this time, with dominant light-porous microfacial laminae and high δ¹⁸O and δ¹³C values representing partly kinetic fractionation effects. This part of the Holocene is interpreted as a mainly warm episode with frequent interruptions of dripping. Within the past 4 ka the profiles with predominant dark compact facies reveal low isotopic values which may be interpreted as a temperature proxy record. The stalagmite records resemble records from an Irish stalagmite. Correlation with the Δ¹⁴C record from European tree rings suggests that colder periods in the North Atlantic were accompanied by drier winters in central Europe.     

桂林水南洞石笋的沉积学特征

利用组成石笋的方解石成分、岩石化学、微量元素含量以及结构、层面构造、沉积间断或风化壳等特征标志,阐述桂林水南洞石笋的沉积特征以及古滴水动态变化.经铀系法测年,确定石笋形成于中更新世晚期至晚更新世早期,其年龄为8.07万年至22.91万年,经历了14.84万年,沉积速率为2.08~2.82mm/100a(未扣除间断时间).     

Hyperspectral imaging of speleothems

Proxy records from speleothems are used in palaeoclimatic reconstructions, as the factors controlling their growth rate, nature of their internal structure and chemical composition respond to changes in surface climate. Optical analyses of stalagmites include visual observation of stalagmite images obtained from flatbed scanners or digital cameras. Hyperspectral imaging has not been used in stalagmite research, but potentially has many advantages over standard optical imaging techniques. Hyperspectral images of a set of stalagmites have been obtained in order to demonstrate the application of hyperspectral imaging in speleothem research. Our results highlight the following: (1) Spectra obtained for these calcite stalagmites are similar between a group of stalagmites of different ages, hydrological setting within a cave and from different caves. (2) The largest differences in relative reflectance between areas of dark compact calcite (DCC) and white porous calcite (WPC) are in the range 470–590 nm (this may extend to 680 nm for some of the stalagmites). (3) Imaging in the near infrared (NIR) demonstrates a decrease in reflection at water absorbance wavelengths, suggesting there is potential for mapping H₂O.     

Is there a possibility to correct fossil n-alkane data for postsedimentary alteration effects?

The long chain n-alkane composition of plant material can significantly differ between plant groups e.g. trees and grasses. Due to their relative recalcitrance, they have been employed in paleoecological research as molecular proxies for different types of vegetation. Most of those paleoenvironmental studies rely on the assumption that characteristic molecular fingerprints of plant material are preserved in the fossil organic material without significant alteration. However, there exists evidence that n-alkane distributions may change in the course of plant litter degradation. Here, the authors propose and discuss a conceptual approach to the correction of n-alkane patterns in paleosols and terrestrial sediments for postsedimentary alteration effects. This might have potential to improve paleoenvironmental reconstructions derived from these molecular fossils. In soil depth profiles typically a correlation between the OEP (odd over even predominance) and paleoecological valuable long-chain n-alkane ratios (LARs) can be found. Similar relationships have been also obtained from n-alkane records in paleosols. With the OEP serving as a proxy of microbial reworking, the correction procedure applies OEP vs. LAR regression functions to correct fossil LARs for degradation effects. The regression functions have been derived from modern soils. The application of the procedure and its significance for paleoecological interpretations is demonstrated on a case study of a loess-paleosol sequence (∼400–700 ka) in Romania. It is shown that changes in the C27/C31 n-alkane ratio at this site are closely related to degradation effects rather than to changes in the paleovegetation (e.g. tree vs. grass abundance). However, it was found that the C29/C31 ratio is a more suitable paleoenvironmental proxy at the Mircea Voda site. The results indicate that there is a future potential to correct fossil n-alkane ratios via the OEP/LAR relationship, however at the moment a general straight forward application of this approach might be critical due to lack of extended and diverse n-alkane records from modern soils. The need of more systematic n-alkane studies on soil profiles is highlighted to improve knowledge concerning dynamics and actual mechanisms of postsedimentary LAR and OEP changes.     

石笋记录年际、年代际极端天气/气候事件研究进展

近年来随着极端天气/气候事件的频发,应对极端天气/气候事件的要求极其紧迫。目前气象资料对极端天气/气候事件的研究相对较短,由此作者提出利用石笋记录重建历史年际、年代际极端天气/气候事件的构想。通过对目前已有的石笋极端天气/气候事件研究实例分析总结,认为洞穴石笋沉积速率相对较快,石笋中标志性结构构造特征的存在,有利于石笋记录到极端天气/气候事件。同时就目前的研究现状,作者提出建立准确年代标尺、选择生长速率相对较快和存在标志性结构构造特征、能记录到极端气候的石笋,以及提高采样分辨率和与其他记录相互验证等作为石笋极端气候研究的工作要求,同时就文石笋研究极端天气/气候事件提出文石笋可能更加容易记录到极端天气/气候事件的个人新认识。      

石笋记录年际、年代际极端天气/气候事件研究进展

近年来随着极端天气/气候事件的频发,应对极端天气/气候事件的要求极其紧迫。目前气象资料对极端天气/气候事件的研究相对较短,由此作者提出利用石笋记录重建历史年际、年代际极端天气/气候事件的构想。通过对目前已有的石笋极端天气/气候事件研究实例分析总结,认为洞穴石笋沉积速率相对较快,石笋中标志性结构构造特征的存在,有利于石笋记录到极端天气/气候事件。同时就目前的研究现状,作者提出建立准确年代标尺、选择生长速率相对较快和存在标志性结构构造特征、能记录到极端气候的石笋,以及提高采样分辨率和与其他记录相互验证等作为石笋极端气候研究的工作要求,同时就文石笋研究极端天气/气候事件提出文石笋可能更加容易记录到极端天气/气候事件的个人新认识。     

Oxygen and carbon isotopic systematics of aragonite speleothems and water in Furong Cave, Chongqing, China

To understand oxygen and carbon stable isotopic characteristics of aragonite stalagmites and evaluate their applicability to paleoclimate, the isotopic compositions of active and fossil aragonite speleothems and water samples from an in situ multi-year (October 2005-July 2010) monitoring program in Furong Cave located in Chongqing of China have been examined. The observations during October 2005-June 2007 show that the meteoric water is well mixed in the overlying 300-500-m bedrock aquifer, reflected by relatively constant δ¹⁸O, ±0.11-0.14‰ (1σ), of drip waters in the cave, which represents the annual status of rainfall water. Active cave aragonite speleothems are at oxygen isotopic equilibrium with drip water and their δ¹⁸O values capture the surface-water oxygen isotopic signal. Aragonite-to-calcite transformation since the last glaciation is not noticeable in Furong stalagmites. Our multi-year field experiment approves that aragonite stalagmite δ¹⁸O records in this cave are suitable for paleoclimate reconstruction. With high U, 0.5-7.2 ppm, and low Th, 20-1270 ppt, the Furong aragonite stalagmites provide very precise chronology (as good as ±20s yrs (2σ)) of the climatic variations since the last deglaciation. The synchroneity of Chinese stalagmite δ¹⁸O records at the transition into the Bølling-Allerød (t-BA) and the Younger Dryas from Furong, Hulu and Dongge Caves supports the fidelity of the reconstructed East Asian monsoon evolution. However, the Furong record shows that the cold Older Dryas (OD) occurred at 14.0 thousand years ago, agreeing with Greenland ice core δ¹⁸O records but ∼200 yrs younger than that in the Hulu record. The OD age discrepancy between Chinese caves can be attributable to different regionally climatic/environmental conditions or chronological uncertainty of stalagmite proxy records, which is limited by changes in growth rate and subsampling intervals in absolute dating. Seasonal dissolved inorganic carbon δ¹³C variations of 2-3‰ in the drip water and 5-7‰ in the pool and spring waters are likely attributed to variable degrees of CO₂ degassing in winter and summer. The variable δ¹³C values of active deposits from −11‰ to 0‰ could be caused by kinetically mediated CO₂ degassing processes. The complicated nature of pre-deposition kinetic isotopic fractionation processes for carbon isotopes in speleothems at Furong Cave require further study before they can be interpreted in a paleoclimatic or paleoenvironmental context.     

The significance of chemical,isotopic, and detrital components in three coeval stalagmites from the superhumid southernmost Andes (53°S) as high-resolution palaeo-climate proxies

Stalagmites are important palaeo-climatic archives since their chemical and isotopic signatures have the potential to record high-resolution changes in temperature and precipitation over thousands of years. We present three U/Th-dated records of stalagmites (MA1–MA3) in the superhumid southern Andes, Chile (53°S). They grew simultaneously during the last five thousand years (ka BP) in a cave that developed in schist and granodiorite. Major and trace elements as well as the C and O isotope compositions of the stalagmites were analysed at high spatial and temporal resolution as proxies for palaeo-temperature and palaeo-precipitation. Calibrations are based on data from five years of monitoring the climate and hydrology inside and outside the cave and on data from 100 years of regional weather station records.Water-insoluble elements such as Y and HREE in the stalagmites indicate the amount of incorporated siliciclastic detritus. Monitoring shows that the quantity of detritus is controlled by the drip water rate once a threshold level has been exceeded. In general, drip rate variations of the stalagmites depend on the amount of rainfall. However, different drip-water pathways above each drip location gave rise to individual drip rate levels. Only one of the three stalagmites (MA1) had sufficiently high drip rates to record detrital proxies over its complete length. Carbonate-compatible element contents (e.g. U, Sr, Mg), which were measured up to sub-annual resolution, document changes in meteoric precipitation and related drip-water dilution. In addition, these soluble elements are controlled by leaching during weathering of the host rock and soils depending on the pH of acidic pore waters in the peaty soils of the cave’s catchment area. In general, higher rainfall resulted in a lower concentration of these elements and vice versa. The Mg/Ca record of stalagmite MA1 was calibrated against meteoric precipitation records for the last 100 years from two regional weather stations. Carbonate-compatible soluble elements show similar patterns in the three stalagmites with generally high values when drip rates and detrital tracers were low and vice versa. δ¹³C and δ¹⁸O values are highly correlated in each stalagmite suggesting a predominantly drip rate dependent kinetic control by evaporation and/or outgassing. Only C and O isotopes from stalagmite MA1 that received the highest drip rates show a good correlation between detrital proxy elements and carbonate-compatible elements. A temperature-related change in rainwater isotope values modified the MA1 record during the Little Ice Age (～0.7–0.1 ka BP) that was ～1.5 °C colder than today. The isotopic composition of the stalagmites MA2 and MA3 that formed at lower drip rates shows a poor correlation with stalagmite MA1 and all other chemical proxies of MA1. ‘Hendy tests’ indicate that the degassing-controlled isotope fractionation of MA2 and MA3 had already started at the cave roof, especially when drip rates were low. Changing pathways and residence times of the seepage water caused a non-climatically controlled isotope fractionation, which may be generally important in ventilated caves during phases of low drip rates. Our proxies indicate that the Neoglacial cold phases from ～3.5 to 2.5 and from ～0.7 to 0.1 ka BP were characterised by 30% lower precipitation compared with the Medieval Warm Period from 1.2 to 0.8 ka BP, which was extremely humid in this region.     

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。      

Thermochemolysis of organic matter preserved in stalagmites: A preliminary study

The analysis of organic matter (OM) preserved in stalagmites is a growing field, but there have been few studies of biomarker compounds such as lignin phenols that are widely used in other palaeoenvironmental contexts. Here we present a preliminary qualitative study of the OM in six stalagmite samples from contrasting environments, using thermochemolysis in the presence of tetramethylammonium hydroxide (TMAH). The results indicate that a wide variety of products is preserved, including several potential lignin-derived compounds, but also that further research is needed to maximise compound recovery and allow the analysis of dissolved OM preserved in stalagmites to reach its full potential.     

Analysis of the climate signal contained within δO and growth rate parameters in two Ethiopian stalagmites

We combine surface and cave climate monitoring with multiple stalagmite parameters to help understand and calibrate the climate records contained within stalagmites from a region with strong rainfall seasonality. Two actively growing stalagmites from Ethiopia were analysed in order to investigate the climate signal contained within δ¹⁸O and growth rate parameters. The δ¹⁸O and growth rate of the two stalagmites give different responses to surface climate due to variations in the climate signal transfer. Both stalagmites (Merc-1 and Asfa-3) have a climate response that is seasonal; however this signal is subsequently smoothed by the mixing of event and storage water within the aquifer. Merc-1 responds more to high frequency (‘event’) climate, due to a greater ratio of event to storage water in this sample, whereas Asfa-3 responds more to low frequency (‘storage’) climate. In addition, different parameters respond to different seasons. For example, stalagmite Asfa-3, from greater depth from the surface and with a slow drip rate, has a growth rate that responds to the amount of summer rain. In contrast, Merc-1, closer to the surface and with a faster drip rate, exhibits no clear response to surface climate, probably due to a more complex climate signal transfer. δ¹⁸O response varies with stalagmite due to the interplay between rainfall forcing factors (amount, seasonality) and disequilibrium kinetics, with opposing correlations between seasonal rainfall and δ¹⁸O between the samples. Our results demonstrate that analysis of seasonal climate forcing, and transfer functions reflecting the mixing of event and storage water, may be the most appropriate approach to develop of transfer functions appropriate for high-resolution, stalagmite climate reconstruction.     

利用石笋δ13C重建岩溶石漠化研究进展

岩溶石漠化的形成演化机制是被关注的科学问题,对脆弱的岩溶区生态环境恢复具有重要的现实意义。洞穴石笋δ13C受到多种因素影响,能敏感响应地表生态环境以及岩溶水文条件的变化。因此,利用石笋δ13C研究岩溶地区生态环境演变历史成为一个重要方向。本文从地表环境和洞穴沉积两个方面梳理了影响石笋δ13C的主要因素。结合现代洞穴监测及模型模拟研究,分析整理了影响洞穴滴水和沉积物中δ13C的主要因素和机理。在多重因素的影响下,石笋δ13C的环境意义具有多解性,文章从时间尺度、空间分布、沉积环境三方面归纳了石笋δ13C的指示意义。为了准确解释石笋δ13C环境意义,提出了综合分析、现代监测以及模型模拟的解决方案。通过对岩溶石漠化概念、成因、发展过程、以及环境效应的讨论,分析了地表石漠化与石笋δ13C记录的密切联系。总结了已经发表的利用石笋δ13C重建区域石漠化的研究成果,讨论了目前研究中面临的主要问题:(1) 如何正确解译石笋δ13C的指示意义？这是石笋δ13C能够用于重建区域石漠化历史的前提;(2) 在空间上,石笋δ13C记录反映上覆地表的面积是有限的,需考虑石笋能否代表目标研究区域的环境变迁;(3) 石漠化可在年—十年际时间尺度上快速发展,而石笋测年存在一定的年龄误差,石笋δ13C是否能够敏感记录地表的石漠化过程？为了准确重建区域岩溶环境以及石漠化演变历史,提出以下主要建议:(1) 为了避免石笋δ13C重建古环境的不确定性,可加强石笋δ13C与δ18O、微量元素、矿物结构等指标的综合对比分析,与现代监测以及模型模拟的解决方案综合集成,能更加准确重建研究区岩溶水文变化过程,判定石漠化的演化历史;(2) 通过区域和同一洞穴的多根石笋记录对比,减少单一石笋记录的区域代表性问题;(3) 高精度年代控制的高分辨率多指标石笋记录,有助于捕捉快速发生的石漠化过程。      

贵州荔波董歌洞较老石笋ICP-MS~(230)Th测年研究及古气候应用

石笋记录研究的进展与测年技术的发展密不可分,测年新技术的突破不断推动利用石笋重建古气候研究的飞速前进。在利用石笋进行古气候重建过程中,建立精确的时标是首要条件。通过对较老(>250ka)石笋的ICP-MS230Th测年研究,发现贵州荔波董歌洞D6、D7、D9石笋具有比较连续的高分辨记录,最老年龄超过400ka,通过230Th测年数据的分析为建立我国西南地区500ka以来标准剖面打下基础;对董歌洞3根大型石笋135个230Th年龄数据及生长速率的分析研究,发现石笋生长速率在大尺度的变化上与海洋沉积SPECMAP曲线δ18O记录有很好的负相关关系,并与北纬25°夏季太阳辐射能量变化曲线呈一定的正相关关系,石笋生长速率的变化在间冰期处于高值,而在冰期时为低值,在间冰期和冰期转化阶段生长速率呈跳跃式变化。通过生长速率变化分析表明,石笋生长速率记录了过去突变冷暖事件的变化,因此可以作为一个有意义的环境记录替代指标来研究古降水及古气候变化。     

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。     

石笋生长直径的测定及其在古气候重建中的应用

石笋是第四纪陆地气候重建的良好信息载体,对了解过去气候和环境的演变十分重要。中国石笋具有生长相对连续,年代准确,信息丰富等特点,为全球季风及其长期演化提供了不可多得的视窗。基于石笋而开发了一系列气候替代指标,如δ18O、δ13C、微量元素及其同位素组成、有机化合物及其同位素组成、生长速率等,在石笋古气候重建中得到广泛的应用。但是,作为石笋形貌学的基本特征,对石笋的生长直径及其对气候变化的指示意义却鲜有报道。本研究中,作者首先提出一种测定石笋生长直径的方法,然后利用该方法测定了湖北清江和尚洞HS4石笋顶部35 cm的石笋生长直径,并与研究区的气温和洪涝频率记录进行比较,探讨石笋生长直径对气候变化的响应。研究表明,通过纹层的识别和不同深度上纹层宽度的测定而建立的指数回归法较好地表征了石笋的生长直径,适合于一些具有明显纹层的石笋直径测定。HS4石笋的实际生长直径比理论计算值偏小,可能与洞穴结构有关;高的落差增加了岩溶滴水的冲力,水滴飞溅而导致有效水量的损失,即滴水的有效体积减少,石笋直径偏小。与温度相比,降水对石笋直径的影响更加显著,因而石笋直径是一个有效降水的替代指标,有望在石笋古气候的研究中得到应用。      

Oxygen self-diffusion “fast-paths” in titanite single crystals and a general method for deconvolving self-diffusion profiles with “tails”

Like most other minerals, titanite rarely if ever forms perfect crystals. In addition to the point defects that might affect lattice diffusion, there may be extended line- or planar defects along which fast diffusion could occur. During the course of an experimental study of oxygen lattice diffusion in titanite, we found that almost all of the ¹⁸O uptake profiles produced in natural titanite crystals departed from the complementary error function solution expected for simple lattice diffusion with a constant surface concentration. Instead, they exhibited “tails” extending deeper into the samples than expected for simple lattice diffusion. The purpose of this contribution is to report on these features—described as “fast-paths” for oxygen diffusion—and outline a method for coping with them in extracting information from diffusion profiles.For both dry and hydrothermal experiments in which the “fast paths” are observed, ¹⁸O was used as the diffusant. In dry experiments, the source material was ¹⁸O-enriched SiO₂ powder, while ¹⁸O-enriched water was used for the hydrothermal experiments. Diffusive uptake profiles of ¹⁸O were measured in all cases by nuclear reaction analysis (NRA) using the ¹⁸O (p,α)¹⁵N reaction [see Zhang X. Y., Cherniak D. J., and Watson E. B. (2006) Oxygen diffusion in titanite: lattice and fast-path diffusion in single crystals. Chem. Geol.235 105-123].In our experiments, different sizes of “tails” (with varying ¹⁸O concentrations) were observed. Theoretically, under the same temperature and pressure conditions, the sizes of tails should be affected by two factors: the diffusion duration and the defect density. For the same experiment duration, the higher the defect density, the larger the “tail”; for the same defect densities, the longer the diffusion duration, the larger the “tail.”The diffusion “tails” could be a result of either planar defects or one-dimensional “pipe” diffusion. AFM imaging of HF etched titanite surfaces confirmed that the etched features might be caused by either parallel planar defects or parallel pipe defects, but could not differentiate between these possibilities. Through theoretical calculations simulating the tailed diffusion profiles using reasonable assumptions of lattice diffusivities and fast-path diffusivities, and comparing these with tail features measured in our samples, it can be concluded that the “tails” observed in our experiments are caused by planar defects rather than pipe defects.A new method was developed for separating the “fast-path” contribution from the overall composite diffusion profile consisting of both “fast-path” and lattice diffusion. Through this process, the lattice diffusion coefficient could be determined, which is required to analyze the tail. The oxygen diffusion rates in the fast-paths were obtained by traditional graphical analysis methods, using the Whipple-Le Claire equation (for 2-D defects) assuming that the width of the fast-path is 1 nm. Two Arrhenius relations were obtained for the fast-path diffusion phenomenon, one for experiments under dry conditions, and the other for hydrothermal conditions:

     

Modeling stalagmite growth by first principles of chemistry and physics of calcite precipitation

Growth rates and morphology of stalagmites are determined by the precipitation kinetics of calcite and the supply rates of water to their apex. Current modeling attempts are based on the assumption that precipitation rates decrease exponentially with distance along the surface. This, however, is an arbitrary assumption, because other functions for decrease could be used as well. Here we give a process-oriented model based on the hydrodynamics of a water sheet in laminar radial flow spreading outwards from the apex, and the well known precipitation rates F = α(c − c_eq); c is the actual calcium concentration at distance R from the growth axis, c_eq the equilibrium concentration of calcium with respect to calcite, and α is a kinetic constant. This enables us to calculate the concentration profile c(R) for any point of an actual surface of a stalagmite and consequently the deposition rates of calcite there. The numerical results show that under conditions constant in time the stalagmite grows into an equilibrium shape, which is established, when all points of its surface are shifting vertically by the same distance during a time interval. We also show this by strict mathematical proof. This new model is based entirely on first principles of physics and chemistry. The results show that the modeled precipitation rates can be approximated by a Gaussian decrease along the equilibrium surface. In general from the mathematical proof one finds a relation between the equilibrium radius of the stalagmite, Q the supply rate of water, and α the kinetic constant. This is also verified by numerical calculations. An interesting scaling law is found. Scaling all stalagmites by 1/R_eq and presenting them with the origin at their apex yields identical shapes of all. The shapes of the modeled stalagmites are compared to natural ones and show satisfactory agreement. Finally we explore the effect of varying water supply Q and kinetic constant α on the shape of a growing stalagmite, and estimate the minimum period of change that can be imprinted into the morphology of the stalagmite.     

Stalagmites from western Thailand: preliminary investigations and challenges for palaeoenvironmental research

Locating suitable caves and stalagmites for palaeoenvironmental and palaeoclimatic studies can be challenging. Isotopic geochemical analyses, albeit commonly performed for palaeoclimatic reconstruction, are also time consuming and costly. Therefore, petrographic and non‐destructive morphological studies on speleothems are desirable to facilitate sample selection for further analysis. In this study, 20 caves were surveyed in Ban Rai district, Uthai Thani province in western Thailand. After external physical observations in the field, three stalagmite samples were collected from Tham Nam Cave to test their potential for palaeoclimatic research. Firstly, the stalagmites were scanned by X‐ray computed tomography (CT scanning) and subsequently the CT images were compared with petrographic inspections. Columnar fabrics show the highest density, whereas closed and open dendritic fabrics have medium and the lowest densities, respectively. Layers near the top and bottom of the three stalagmites were dated by U‐Th mass spectrometric techniques. All three samples were deposited between c. 87 and c. 105 ka ago; therefore, they are probably the oldest stalagmites that have been reported so far from mainland Southeast Asia. However, their physical features indicate that all the samples have suffered from postdepositional dissolution, and are unlikely to be suitable for palaeoclimatic research. The internal dissolution feature of stalagmites, however, cannot be identified by visual inspection of uncut samples. We hereby argue that CT images are useful to characterize stalagmite petrography, in particular fabric, porosity and density. Such features can be used to select the ideal plane of a stalagmite for sectioning, to maximize the chances of robust climatic reconstruction.