Contradicting Barrier Reef relationships for Darwin’s Evolution of reef types

The Darwinian progressive subsidence model for the evolution of fringing reefs, barrier reefs and atolls has been generally accepted following the indisputable proof of subsidence provided by drilling results in the Pacific. Nonetheless, there are data that do not fit the expectations of the model, such as the similar lagoon depths of barrier reefs and atolls as opposed to the subsidence theory’s implicit prediction that atolls should have significantly greater depths. In contrast, a great deal of evidence supports the influence of meteoric solution on barrier reef morphology. For example, the maximum lagoon depth of 56 modern barrier reefs is statistically correlated with the lagoon catchment area for modern annual rainfall. These modern rainfall patterns would seem to be a reasonable proxy for relative geographic differences in glacial lowstand rainfall, even though the absolute amounts of such rainfall are unknown. The correlation therefore suggests the importance of Pleistocene subaerial solution in contributing to barrier reef morphology. Further support for antecedent influence occurs in the form of barrier reef passes in which the depth of the reef pass is correlated with onshore drainage volumes. On a larger scale, the Cook Island of Mangaia provides evidence that solution can produce barrier reef morphology independent of reef development. In contrast, there are no examples of the subsidence-predicted lagoon transition of fringing reefs to barrier reefs to atolls. Moreover, the common occurrence of fringing reefs within barrier reefs negates subsidence as a causal factor in their ‘presumed progressive evolutionary development. Consequently, the evidence to date suggests that a solution morphology template has been accentuated by reef construction to produce the diagnostic barrier reef morphology we see today. The importance of subsidence would seem to be in accounting for the overall thickness of the resulting carbonate caps of oceanic examples and in contributing to lagoon depth variation among the larger continental entities.

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A physical analogue model to analyse interactions between tensile stresses and dissolution in carbonate slopes

A physical analogue model was developed to analyse the relationship between tensional states in rock masses, seepage and karst processes. Use was made of an experimental apparatus consisting of two hydraulic circuits realized by drilling two holes into each of two blocks of sampled limestone from the central Apennines (Italy). A static load (208.85 kg) was applied to one of the blocks in order to elicit tensile stresses within it. Physical and chemical monitoring data showed that the main process involved was temperature-dependent CaCO₃ dissolution. This process was more marked in the loaded block circuit, as Ca⁺⁺ concentration in circulated water reached 54 mg/L, whereas only 28 mg/L was reached in the unloaded one. The interaction between load and dissolution caused the observed opening of microcracks, as confirmed by further increase of water loss and by dilution in the loaded block circuit, resulting in a decrease of Ca⁺⁺ concentration. These findings were confirmed by recording additional water losses after increasing the load up to 445.05 kg. A finite difference numerical model showed that tensile stresses (max 20 kPa) within the loaded block were clustered at the intersection of the main joints with the flowpaths, thus representing points of preferential and accelerated dissolution.This study is part of a research project on analogical and numerical modelling for geological risk mitigation being conducted at CERI, Research Centre for Geological Risks of the University of Rome “La Sapienza” (Valmontone, Roma, Italy; http://www.ceri@uniroma1.it).     

Contribution to the identification of the sea intrusion mechanism of brackish karst springs

A method of identifying the dominant hydrodynamic sea-intrusion mechanism of brackish karst springs is presented. A karst spring becomes brackish when tubes, which bring the freshwater to the spring (freshwater discharge), intersect other tubes that come from the sea and bring saltwater to the freshwater tubes (saltwater discharge) when the saltwater pressure at the intersection is higher than the freshwater pressure. There are two potential seawater intrusion mechanisms. The first one is the difference between the freshwater density and the seawater density, and the second is the venturi effect. Both mechanisms are present but it is a matter of great significance to know which mechanism dominates. In order to find out the dominant mechanism, the seawater discharge versus the freshwater discharge was charted using the MODKARST model, which estimates these discharges. The model determines how the freshwater discharge affects the saltwater discharge estimating thereby the dominant seawater intrusion mechanism. Application was made to the “Almiros” and “Makaria” springs in Greece.

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Resumen Se presenta un método para identificar el mecanismo hidrodinámico dominante de la intrusión marina en manantiales salobres cársticos. Un manantial cárstico se vuelve salobre cuando los conductos, que suministran agua dulce al manantial (caudal de agua dulce), interceptan a otros conductos provenientes del mar que traen agua salada a este (caudal de agua salada), bajo la condición que la presión del agua salada en la intersección es mayor que la del agua dulce. Hay dos mecanismos potenciales de intrusión salina. El primero es la diferencia de densidades entre agua salada y agua dulce, mientras que el segundo es el efecto venturi. Ambos mecanismos están presentes en cualquier caso, sin embargo es de gran importancia establecer cual de ellos predomina. Con el fin de hallar el mecanismo dominante se tabularon los valores de caudal de agua salada versus el de agua dulce. Lo anterior fue posible mediante el uso del modelo MODKARST, el cual estima estos caudales. A partir de esta tabla se puede examinar como el caudal de agua dulce afecta al de agua salada, estimando como resultado de lo anterior el mecanismo dominante en la intrusión marina. Esto se aplicó a los manantiales “Almiros” y “Makaria” en Grecia.

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Résumé On présente une méthode pour déterminer le mécanisme hydraulique dominant de l’intrusion de l’eau salée marine dans le cas des émergences karstiques saumatres. Une émergence karstique devienne saumatre á l’intersection des conduites de l’eau douce avec des autres conduits qui apportent l’eau salée de la mer. Ce processus est possible si la pression de l’eau salée dépasse la pression de l’eau douce. Il y a deux mécanismes potentiels de l’intrusion de l’eau salée. A la base du premier mécanisme se trouve la différence entre les densités de l’eau douce et de l’eau salée pendant que dans le deuxième mécanisme il s’agit de phénomène Venturi. Afin de trouver le mécanisme dominant on a cartographié avec le modèle MODKARST le rapport entre les décharges de l’eau douces et respectivement de l’eau salée. A partir de la carte résultée on peut estimer comment la décharge de l’eau douce influence celle de l’eau salée en estimant en même temps le mécanisme dominat de l’eau salée. On a appliqué la méthode dans le cas des émergences d’Almiros et Makaria de la Grèce.

     

滇东岩溶区表层岩溶水源地特征及其开发利用

表层岩溶水源地是解决分散农村生活用水困难的重要水源，而且分布广泛，水质较好。但水源地含水介质的富水性不均匀，岩溶水动态变幅大，脆弱性突出，易受污染和生态环境变化的影响。由此，表层岩溶水源地生态环境建设及保护尤为重要。表层泉以池、管调蓄系统开发最为有效。表层富水块段适宜打浅井开发，建设庭院式供水工程。采用地质雷达验证地质认识，确定井位，是经济有效的技术手段。     

探地雷达在隐伏岩溶勘察中的应用

在介绍探地雷达(GPR)方法原理基础上,根据岩溶发育特征及其地球物理性质,总结了GPR反射波特征,结合在岩溶发育区成功应用GPR技术方法的工程勘察实例,说明GPR方法是岩溶区岩土工程勘察和评价的有效手段。     

单斜构造岩溶含水层及其岩溶渗漏条件分析——以湖北水江坪河水电站河湾地块为例

通过对湖北水江坪河水电站河湾地块寒武系下统龙王庙组上段(∈21l)单斜岩溶含水系统的水动力、温度、水量均衡以及岩溶发育特征分析,提出在单斜岩层中存在2层岩溶结构和2层岩溶水系统,上层岩溶水系统的水位高于正常蓄水位(475m),而深层岩溶水系统的水位低于正常蓄水位(475m)。在建库条件下,库水可通过深层岩溶水系统向河湾下游径流排泄,即河湾地块存在岩溶渗漏问题。该结论已得到深孔勘探以及钻孔彩色录像初步查证。     

Hydrodynamic characteristics of a typical karst spring system based on time series analysis in northern China

In order to study the hydrodynamic characteristics of the karst aquifers in northern China, time series analyses (correlation and spectral analysis in addition with hydrograph recession analysis) are applied on Baotu Spring and Heihu Spring in Jinan karst spring system, a typical karst spring system in northern China. Results show that the auto-correlation coefficient of spring water level reaches the value of 0.2 after 123 days and 117 days for Baotu Spring and Heihu Spring, respectively. The regulation time obtained from the simple spectral density function in the same period is 187 days and 175 days for Baotu Spring and Heihu Spring. The auto-correlation coefficient of spring water level reaches the value of 0.2 in 34–82 days, and regulation time ranges among 40–59 days for every single hydrological year. The delay time between precipitation and spring water level obtained from cross correlation function is around 56 days for the period of 2012–2019, and varies among 30–79 days for every single hydrological year. In addition, the spectral bands in cross amplitude functions and gain functions are small with 0.02, and the values in the coherence functions are small. All these behaviors illustrate that Jinan karst spring system has a strong memory effect, large storage capacity, noticeable regulation effect, and time series analysis is a useful tool for studying the hydrodynamic characteristics of karst spring system in northern China.© 2021 China Geology Editorial Office.     

册亨县秧庆万重山喀斯特景观旅游资源特征

为把贵州建成"公园省",提高贵州旅游资源的知名度,同时为了给"研学旅行"建立必要的基地,省委省政府高度重视省级地质公园的建设工作。依据2016年旅游普查成果,如何筛选适宜做地质公园的地区,是大家普遍关心的问题。本文对册亨县秧庆万重山喀斯特地貌景观区旅游资源特征进行总结分析,为册亨县旅游资源开发与保护提供理论依据。