The book examines empirical and theoretical regularities of hypogene speleogenesis and reveals its hydrogeological significance and the role in karst evolution. It is demonstrated that hypogene karst, along with epigenic karst, is the fundamental and wide spread genetic variety of karst, which nature and peculiar features call for revision and refinement of some basic notions of the general karst paradigm. A new approach is advocated to a definition of the notion of karst, where the latter is viewed as a specific groundwater circulation system with key properties determined by speleogenesis.

It is shown that major distinctions in mechanisms of the development of karstic void-conduit structures (types of speleogenesis) are determined by hydrodynamic peculiarities of confined and unconfined groundwater systems, and by the circulation vector. An evolutionary classification of karst is elaborated, which main categories cumulatively reflect its origin and characterize its most essential properties. Hypogene karst is a natural stage in the evolution of karst groundwater circulation geosystems in the course of regressive lithogenesis and hydrogeological cycles.

The book reveals principal regional regularities and type settings of hypogene speleogenesis, and describes its functional, structural and morphological peculiar features. It demonstrates the significance of hypogene speleogenesis in the formation of hydrogenic deposits of mineral resources and hydrocarbons in soluble strata and adjacent formations, and its role in karst hazards. The genetic and evolutionary approach is outlined and advocated in dealing with karst-related applied issues of hydrogeology, geological engineering, petroleum and ore geology.

Karst aquifers and conduits form by dissolution of carbonate minerals and the slow release of inorganic carbon to the surface environment. As conduits evolve in size, morphology, and position within the aquifer, their function and capacity change relative to the storage and transport of inorganic and organic carbon as sediment. Conduits serve mostly as transport mechanisms in relation to sediments. quantified data are sparse, but for conduits to function effectively there must be at least equilibrium in the amount of sediment entering and exiting the aquifer. If sediment discharge exceeds input, little sediment will remain underground. when natural declines in base level cease removing sediments and only deposit calcite speleothems, these materials are stored until the rock mass is denuded. while sediment storage is mostly transient in hydrologically active conduits, relative differences occur. Aquifers with conduits developed at multiple levels or as floodwater mazes store proportionately greater volumes of sediment. Hypogenic systems should store greater volumes of sediment than epigenic aquifers because they mostly discharge a dissolved load as opposed to both dissolved and suspended clastic loads. However, some hypogenic aquifers are diffusely recharged and receive and store little sediment from the surface. The global volume of sediment and organic carbon stored in karst aquifers is estimated in this study to be on the order of 2x104 km³ and 2x102 km³, respectively. The amount of organic carbon stored in paleokarst is not estimated, but available data indicate it is substantially greater than that stored in modern karst aquifers. Development of such data may suggest that paleokarst petroleum reservoirs might serve as efficient carbon sinks for global carbon sequestration. Hydrocarbon-depleted paleokarst reservoirs should provide substantially more storage per injection well than sequestration in non-paleokarstic rocks.

The classical epigene speleogenetic model in which CO2 is considered the main source of acidity has been challenged over the last three decades by observations that revealed cave passages unrelated to groundwater drainage routes and surface topography. Most of these passages show unusual morphologies, such are cupolas, floor feeders (i.e., inlets for deep-seated fluids), and huge irregular-shaped rooms that terminate abruptly, and often a rich and diverse mineral association. A hypogenetic speleogenetic pathway was proposed for this group of caves.
The presence of abundant gypsum deposits in caves with one or more of the passage morphologies listed above, have prompted scientists to suggest a new theory (i.e., sulfuric acid speleogenesis, SAS) of cave development. In the hypogenic SAS model, the source of acidity is the sulfuric acid produced by oxidation of H2S (originating from sulfate reduction or petroleum reservoirs) near or at the water table, where it dissolves the limestone bedrock and precipitates extensive gypsum deposits. SAS is now thoroughly documented from numerous caves around the world, with the best examples coming from the Guadalupe Mountains (NM), Frasassi caves (Italy), selected caves in France, Cueva de Villa Luz (Mexico), and Cerna Valley (SW Romania).
To date, discrimination between epigene and hypogene speleogenetic pathways is made using cave morphology criteria, exotic mineral assemblages, and the predominantly negative δ34S values for the cave sulfates. This presentation highlights the role sulfur and oxygen stable isotope analyses have in discriminating between epigene and hypogene caves.
Based on a number of case studies in caves of the Cerna Valley (Romania), we found that relatively S-depleted isotopic composition of cave minerals alone does not provide enough information to clearly distinguish SAS from other complex speleogenetic pathways. In fact, δ34S values of SAS by-products depend not only on the source of the S, but also on the completeness of S redox reactions. Therefore, similar studies to this are needed to precisely diagnose SAS and to provide information on the S cycle in a given karst system.
Integrating cave mineralogy, passage morphology, and geochemical studies may shed light on the interpretation of polygenetic caves, offering clues to processes, mechanisms, and parameters involved in their genesis (sulfate-dominated).