Groundwater as a geologic agent: An overview of the causes, processes, and manifestations

 The objective of the present paper is to show that groundwater is a general geologic agent. This perception could not, and did not, evolve until the system nature of basinal groundwater flow and its properties, geometries, and controlling factors became recognized and understood through the 1960s and 1970s. The two fundamental causes for groundwater's active role in nature are its ability to interact with the ambient environment and the systematized spatial distribution of its flow. Interaction and flow occur simultaneously at all scales of space and time, although at correspondingly varying rates and intensities. Thus, effects of groundwater flow are created from the land surface to the greatest depths of the porous parts of the Earth's crust, and from a day's length through geologic times. Three main types of interaction between groundwater and environment are identified in this paper, with several special processes for each one, namely: (1) Chemical interaction, with processes of dissolution, hydration, hydrolysis, oxidation-reduction, attack by acids, chemical precipitation, base exchange, sulfate reduction, concentration, and ultrafiltration or osmosis; (2) Physical interaction, with processes of lubrication and pore-pressure modification; and (3) Kinetic interaction, with the transport processes of water, aqueous and nonaqueous matter, and heat. Owing to the transporting ability and spatial patterns of basinal flow, the effects of interaction are cumulative and distributed according to the geometries of the flow systems. The number and diversity of natural phenomena that are generated by groundwater flow are almost unlimited, due to the fact that the relatively few basic types are modified by some or all of the three components of the hydrogeologic environment: topography, geology, and climate. The six basic groups into which manifestations of groundwater flow have been divided are: (1) Hydrology and hydraulics; (2) Chemistry and mineralogy; (3) Vegetation; (4) Soil and rock mechanics; (5) Geomorphology; and (6) Transport and accumulation. Based on such a diversity of effects and manifestations, it is concluded that groundwater is a general geologic agent. Received, December 1998 · Revised, January 1999 · Accepted, January 1999     

Physical and numerical simulations of the presence of a forsterite transitional zone at high temperature and pressure: insight for scCO2 geological storage

Geological sequestration is one of the most effective ways to reduce greenhouse gases, such as carbon dioxide (CO₂). The deep oceanic crust dominated by ultrabasic rock could store CO₂ permanently. However, the storage mechanism has not been thoroughly understood because of the limited amount of research and experiments conducted. This study explored the reactive mechanisms of water–rock–gas in an ultrabasic system under different conditions. Forsterite, the most dominant mineral found in ultrabasic reservoirs, was used to conduct laboratory physical simulation experiments. Two experimental systems were designed including an scCO₂–forsterite–water system and an N₂–forsterite–water system. All experiments were performed for 1000 h at an experimental temperature of 150°C and a pressure of 150 bar, respectively, to mimic the geological conditions. The liquid products from the experiments were analysed by inductively coupled plasma-optical emission spectrometry, whereas the solid samples were analysed by scanning electron microscopy with energy disperse spectroscopy. Results showed that: (1) in the early stage during scCO₂/N₂–forsterite–water interaction, forsterite was dissolved with a reactive transitional zone forming on the surface, which caused H⁺ to enter into the silicate framework and accelerated the reaction; (2) in the N₂ system, the dissolution of forsterite was inhibited by the Mg²⁺ concentration after reaching its saturation in the late stage; and (3) in the scCO₂ system, magnesite was precipitated as a secondary mineral during the late stage, which promoted the dissolution of forsterite. As a result, the degree of dissolution of forsterite in the scCO₂ system was far higher than in the N₂ system. The experimental results are consistent with the numerical simulation using TOUGHREACT, a geochemical simulation procedure, which showed that CO₂ promotes the dissolution of forsterite greatly at high temperature and pressure.     

Compositional diversity of soluble mineralizing matrices in some recent coral skeletons compared to fine-scale growth structures of fibres: discussion of consequences for biomineralization and diagenesis

 In contrast to the purely crystalline model of the coral fibre, a transversal zonation is made visible within fibres by specific preparations (enzymatic or light acidic etchings). This growth zonation that corresponds to the successive positions of the secretory ectoderm during septal development shows that crystallization of coral fibre is controlled not only by an external organic coating of fibres, but also includes intra-fibrous organic components. This results in a micron-scaled internal structure of fibres, with a noticeable diversity depending on species. In parallel, soluble matrices also exhibit significant differences, although the global high content in acidic amino acids is always observed. Gathering these two sets of data allows the current ideas concerning structures and growth modalities of coral septa to be discussed. Basically, the respective role of centres of calcification and fibres during septal development appears clearly different from the current ideas, resulting in significant change in interpretation of coral structures, with concerns in both skeleton formation and understanding of diagenetic processes. Received: 2 November 1998 / Accepted: 10 May 1999     

Zircon ages and Nd isotopic and chemical compositions of orthogneisses from the Black Forest, Germany: evidence for a Cambrian magmatic arc

 Single zircon U–Pb dating combined with ²⁰⁷Pb/²⁰⁶Pb ages obtained by the evaporation method constrains the emplacement of tonalitic, trondhjemitic, and granodioritic orthogneisses of the Moldanubian zone in the Black Forest between 500 and 510 Ma. Two detrital zircon populations of 1.9 and 1.6 Ga indicate Early-Middle Proterozoic material in the former setting of the basement. The initial e_Nd values range from –0.1 to –3.4 and mean crustal residence ages of 1.0–1.4 Ga are consistent with involvement of Early-Middle Proterozoic crust, and a subordinate juvenile component probably originating from subduction-related melting of the mantle. The orthogneisses have fractionated REE patterns and slightly higher K₂O/Na₂O ratios than typical low-K tonalite–trondhjemite–granite suites. The chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust. The emplacement of the orthogneisses was superceded by a high-temperature metamorphic event at ∼480 Ma which we interpret as a result of lithospheric thinning in a marginal basin behind a Cambrian magmatic arc. Received: 29 March 1999 / Accepted: 25 August 1999