Deep-seated geochemical environments in karst: An introduction

Abstract

As the rapid growth of scientific information forces us into specialized niches, it is increasingly important that we retain a broad view of geological systems and the diverse interactions within them. Perhaps no other field in geology shows this dichotomy more vividly than the interpretation of deep karst systems, in which dissolution and precipitation are driven by sulfur-related redox processes. Our conventional view of karst is limited to its geomorphic aspects. We take comfort in the CO2-H20CaCO 3 system, because we can see it in action right before our eyes. Few geologists are even aware of the more cryptic sulfur-related karst. The typical sulfur cycle begins in anoxic subsurface environments with the reduction of sulfates, in the presence of hydrocarbons, to hydrogen sulfide. In aqueous or gaseous form, hydrogen sulfide can migrate upward along faults or permeable beds toward oxygen-rich zones where it is oxidized to sulfuric acid, occasionally with sulfur as an intermediate step. The resulting solution is highly aggressive toward carbonate rocks and is responsible for considerable cavernous porosity having little relation to the overlying surface. This deep-seated, hypogenetic form of karst is of great importance to the fields of economic geology and petroleum geology. Many of these redox processes operate only with the aid of microbes. For example, at low temperatures, the reduction of sulfate is limited by a kinetic threshold that can be overcome only by bacterial mediation. Fossil microorganisms provide clues to former redox processes that are no longer active. An important aspect of this kind of karst is that it is a focal point where geology, geochemistry, hydrology, and biology all meet on common ground. It is impossible to understand the system without integrating all four fields. Toward this goal, a multidisciplinary symposium was sponsored in February 1994 by the Karst Waters Institute of Charles Town, West Virginia, USA, in cooperation with the University of Colorado at Colorado Springs, entitled Breakthroughs in Karst Geomicrobiology and Redox Geochemistry. Invited speakers from all four fields were given the opportunity to compare observations and ideas on the application of microbiology to the understanding of geologic processes. Topics of discussion included redox environments and microbial processes in karst, chemoautotrophy, thermodynamics, cave origin in sulfatesulfide-carbonate terranes, microbial mediation of carbonate precipitation, and new frontiers in geomicrobiology. Extended abstracts are available in Karst Waters Institute Special Publication No. 1 (P.O. Box 490, Charles Town, WV 25414). Selected papers from this conference are presented in this issue of Environmental Geology. Several others will appear individually in future issues. In this issue, papers by Carol Hill (ed.), Kimberley Cunningham and others, Yuri Dublyansky, and Stephen Worthington and D.C. Ford describe some of the most bizarre subsurface terranes imaginable, in which karst processes take place in environments entirely foreign to most geologists. In the past decade, karst research has emerged from the confines of a narrow geomorphic specially to what is now recognized as the crux of many diverse studies in hydrology, engineering, stratigraphy, and economic geology. Discoveries in sulfur-related karst have now drawn microbiology into the fold. By expanding into this formerly unrecognized field, environmental geology has gained a new partner.