Horia Mitrofan

Cernei Mountains: Caves Conveying Geothermal Fluids at Băile Herculane

Several small caves located in the southwestern part of the South Carpathians discharge steam or/and chloride-rich hot groundwater derived from a geothermal reservoir situated nearby. The monitoring of natural tracers (heat, certain solutes, stable isotopes) documented uncommon levels for the fluids discharged within the Băile Herculane caves, thus revealing flow processes that ordinary meteoric-derived groundwater would be unable to depict. Furthermore, the exotic cave minerals and their isotopic composition suggest that the caves are the result of combination of hypogene and epigene processes. A rich and peculiar endemic troglobite fauna was identified in the Adam’s Shaft, in which a guano accumulation dated to ~8425 cal yrs BP suggests it is one of Europe’s oldest permanent bat colonies. The unusual warm/wet microclimate of the cave favored both an ideal roosting site for bats and a “tropical” biospeleological oasis.

The Space-Time Distribution of Moderate- and Large-Magnitude Vrancea Earthquakes Fits Numerically-Predicted Stress Patterns

Each of the three major earthquakes (M w ≥ 6.9) recorded within the Vrancea seismogenic body in the years 1977, 1986 and 1990 may have been the result of long-range interactions. The latter seemed to be initiated in coincidence with a moderate (4.7 ≤ m b ≤ 4.9) shock that systematically occurred in the 160–175 km depth-range, 3–4 years in advance of the major earthquake. In addition, each corresponding pair of moderate/major events systematically exhibited, in terms of focal mechanisms, a particular pattern: the latter complied with predictions of a numerical model that had addressed along-strike break-off experienced by a near-vertical slab, which—at the same time—was strongly coupled with its overriding plate along a steeply dipping contact extending on a significant vertical length. It was thus suggested that the Vrancea moderate events of thrust-fault type occurred in the 160–175 km depth-range could correspond to the along-strike propagation of a detachment horizon tip; while the major earthquakes subsequently occurred at shallower depths (85–135 km), were possibly caused by the shearing forces that acted, at the upper-plate/underlying-plate interface, in response to the increased downward pull experienced by the Vrancea slab as the break-off was propagating laterally.

Better constraining silica-enthalpy mixing models in a setting of two separate (karst and non-karst) dilution regimes

It would be possible to elucidate an ambiguity associated with the “silica-enthalpy mixing models” geothermometric technique, provided that an independent estimate was obtained concerning the enthalpy and the dissolved SiO2 content of the hot end-member involved in a mixture between two liquid-phase groundwaters. This would allow one to ascertain whether or not, prior to mixing with the cold parent-water, the hot end-member had undergone a boiling process. The specificities of the Băile Herculane geothermal system (South Carpathians, Romania) provides an opportunity to solve this uncertainty, in contrast to the majority of the so-far investigated natural settings. Advantage has been taken, in this respect, of the fact that a hot brackish Na-Ca-Cl-type parent-water was found to be involved at this site in two separate mixing settings: one which involved only a diluting fluid that was, itself, chloride-rich, and another one which additionally involved, as a third end-member, karst freshwater. By exploiting the two distinct, and converging, linear mixing trends (obtained from the conservatively-behaving natural tracers Cl− and Na+, as well as reasonably tight correlations between the chloride concentration and the contents of both heat and SiO2), the concerned hot brackish end-member contents of heat (enthalpy) and of conservative solutes were quite accurately assessed. This result enabled independent validation of the “no steam loss” hypothesis, out of various alternative scenarios proposed by the “silica-enthalpy mixing models” technique.RésuméIl serait possible d’élucider une ambiguïté associée à la technique de géothermométrie des “modèles de mélange silice-enthalpie”, à condition qu’une estimation indépendante ait été obtenue concernant l’enthalpie et la teneur en SiO2 dissoute du membre de l’extrémité chaude impliqué dans un mélange entre deux eaux souterraines en phase liquide. Cela permettrait de déterminer si, avant le mélange l’eau froide d’origine, le membre de l’extrémité chaude avait subi un processus d’ébullition. Les spécificités du système géothermal de Băile Herculane (Sud des Carpathes, Roumanie) donnent l’occasion de lever cette incertitude, contrairement à la majorité des milieux naturels étudiés à ce jour. L’avantage a été pris à cet égard, du fait qu’une eau chaude originelle saumâtre de type Na-Ca-Cl a été impliquée sur ce site dans deux conditions distinctes de mélange: une impliquant seulement un fluide de dilution qui était, lui-même, riche en chlorures, et une autre impliquant de plus, en tant que troisième membre, une eau douce karstique. En exploitant les deux tendances de mélange linéaire distinctes, et convergentes (obtenues à partir des traceurs naturels conservatifs Cl− et Na+, ainsi que de raisonnablement bonnes corrélations entre les concentrations des chlorures et les teneurs de la chaleur et de SiO2), les teneurs en chaleur (enthalpie) de l’élément de l’extrémité chaud et saumâtre et des solutés conservatifs ont été évaluées de façon très précise. Ce résultat a permis de valider de manière indépendants l’hypothèse de “non perte de vapeur”, à partir de divers scénarios alternatifs proposé par la technique des “modèles de mélange silice-enthalpie”. This result enabled independent validation of the “no steam loss” hypothesis, out of various alternative scenarios proposed by the “silica-enthalpy mixing models” technique.ResumenSería posible dilucidar una ambigüedad asociada con la técnica geotermométrica en los “modelos de mezcla de sílica-entalpía”, siempre que se obtuviera una estimación independiente con respecto a la entalpía y el contenido de SiO2 disuelto del miembro del extremo caliente involucrado en una mezcla entre dos fases líquidas de las aguas subterráneas. Esto permitiría determinar si, antes de mezclarse con el agua original fría, el miembro del extremo caliente se había sometido o no a un proceso de ebullición. Las especificidades del sistema geotérmico de Băile Herculane (South Carpathians, Romania) brindan la oportunidad de resolver esta incertidumbre, en contraste con la mayoría de los escenarios naturales hasta ahora investigados. Se ha aprovechado, a este respecto, el hecho de que se encontró un agua salobre caliente de tipo Na-Ca-Cl en este sitio en dos configuraciones de mezcla separadas: una que involucraba solo un fluido diluyente que era en sí mismo, rico en cloruro, y otro que adicionalmente involucró, como tercer miembro extremo, agua dulce kárstica. Al aprovechar las dos tendencias de mezcla lineales distintas, y convergentes (obtenidas por trazadores naturales, Cl− and Na+, de comportamiento conservador, así como por correlaciones razonablemente ajustadas entre la concentración de cloruro y el contenido tanto de calor como de SiO2), los contenidos de calor (entalpía) del extremo salobre caliente y de los solutos conservadores se evaluaron con bastante precisión. Este resultado permitió la validación independiente de la hipótesis “sin pérdida de vapor”, de varios escenarios alternativos propuestos por la técnica de “modelos de mezcla sílice-entalpía”.摘要如果能获取两个液相地下水混合物中有关热端员的热焓和溶解二氧化硅含量独立估算值, 那么,就有可能阐明与“二氧化硅-热焓的混合模型”地热温度技术相关的含糊情况。这将使人们能够确定与冷的母水混合之前热端员是否经历了沸腾过程。相对于大多数目前调查研究的自然背景来说,(罗马尼亚喀尔巴阡山南部)Băile Herculane地热系统的特异性为解决这个不确定性提供了机会。在这方面占有优势,发现在两个不同的混合背景下都有热的微咸Na-Ca-Cl-类型母水,这为研究提供了优势:一种背景只涉及稀释的液体,这种液体本身富含氯化物,另一种背景还涉及作为第三端员的岩溶水。通过找到两个明显的及会聚的线性趋势(从中规中矩的天然示踪剂Cl− 和 Na+,及氯化物含量与热量和SiO2含量之间的合理严格对比中获取),就可以准确地评价有关热量(热焓)和保守溶质的热微咸端员含量。这个结果能够使人们在“二氧化硅-热焓混合模型”技术提出的众多的可选择的方案中独立确认“无蒸汽损失”假设。ResumoSeria possível elucidar uma ambiguidade associada à técnica geotermométrica de “modelos de mistura de entalpia de sílica”, desde que uma estimativa independente fosse obtida com relação à entalpia e ao conteúdo de SiO2 dissolvido do membro final envolvido em uma mistura entre águas subterrâneas de duas fases líquidas. Isso permite determinar se é ou não, antes de se misturar com a fonte de água fria, o elemento final quente que tivesse sofrido um processo de ebulição. As especificidades do sistema geotérmico Băile Herculane (Cárpatos do Sul, Romênia) oferecem uma oportunidade para resolver essa incerteza, em contraste com a maioria dos cenários naturais investigados até agora. Nesse sentido, foi tirada vantagem do fato de ter sido encontrada uma fonte de água quente salobra do tipo Na-Ca-Cl para ser envolvida neste local em duas configurações de mistura separadas: uma que envolveu apenas um fluido de diluição que era, ela própria, rica em cloreto; e outra que envolveu, adicionalmente, como terceiro membro final, água doce cárstica. Ao explorar as duas tendências distintas, e convergentes, de mistura linear (obtidas dos traçadores naturais de comportamento conservador Cl− e Na+), bem como correlações estreitas entre a concentração de cloreto e o conteúdo de muito calor e SiO2), os conteúdos do membro final quente salobro do calor (entalpia) e dos solutos conservativos foram avaliados com bastante precisão. Esse resultado permitiu a validação independente da hipótese “sem perda de vapor”, a partir de vários cenários alternativos propostos pela técnica de “modelos de mistura de entalpia de sílica”.

Diagnosing Non-Meteorically Induced Variations in the Chemistry of Saline Springs of the Vrancea Seismic Area (Romania)

A monitoring operation conducted over more than seven years has been addressing chemical fluctuations displayed by several cool, low-discharge springs located close to the highly seismic Vrancea area (Romania). Those outflows compositions proved to be strongly controlled by binary, essentially isochemical mixing between a deep-origin brine and meteoric freshwater. By taking advantage of this particular setting, there have been constructed diagrams aimed at investigating how the groundwater-discharges Na–K–Mg geothermometric parameters fluctuated as a function of the chloride content (taken to be an estimator of the freshwater-induced dilution). We made use of the reciprocal algebraic relationships existing between the equations describing the Na–K–Mg geothermometric parameters “Na–K temperature” and “K–Mg temperature” on the one hand, and the so-called “maturity index” (MI) on the other. In an accordingly derived plot of MI against the logarithm of the chloride concentration, a series of data-points being quite uniformly off-set from the MI dilution-curve constructed for an apparently “regular” period, suggested that, episodically, all concerned springwaters were simultaneously subject to some similar changes in the controlling geochemical processes. One such modification intervened 3–4 months before the occurrence of the strongest earthquake (

Lateral detachment in progress within the Vrancea slab (Romania): inferences from intermediate-depth seismicity patterns

M_{\text{w}} = 5.8

Transients of Giggenbach’s ‘Na‐K‐Mg‐Ca Geoindicators’ preceding the 27 October 2004, Mw = 6.0 earthquake in Vrancea area (Romania)

Mw=5.8) of the hydrochemical monitoring period. The consequently derived interpretation was that then, the numerical values of certain geothermometric coefficients were likely altered: such a process could be consistent with changes in the alkali feldspars solubility relationships, possibly in response to episodic Al–Si complexing which might develop within a hypothesised, still active, exhumation-channel.