Arthur N. Palmer

Microbiology and geochemistry in a hydrogen-sulphide-rich karst environment

Abstract Cueva de Villa Luz, a hypogenic cave in Tabasco, Mexico, offers a remarkable opportunity to observe chemotrophic microbial interactions within a karst environment. The cave water and atmosphere are both rich in hydrogen sulphide. Measured H 2 S levels in the cave atmosphere reach 210 ppm, and SO 2 commonly exceeds 35 ppm. These gases, plus oxygen from the cave air, are absorbed by freshwater that accumulates on cave walls from infiltration and condensation. Oxidation of sulphur and hydrogen sulphide forms concentrated sulphuric acid. Drip waters contain mean pH values of 1.4, with minimum values as low as 0.1. The cave is fed by at least 26 groundwater inlets with a combined flow of 200–300 l/s. Inlet waters fall into two categories: those with high H 2 S content (300–500 mg/l), mean P CO 2 =0.03–0.1 atm, and no measurable O 2 ; and those with less than 0.1 mg/l H 2 S, mean P CO 2 =0.02 atm, and modest O 2 content (up to 4.3 mg/l). Both water types have a similar source, as shown by their dissolved solid content. However, the oxygenated water has been exposed to aerated conditions upstream from the inlets so that original H 2 S has been largely lost due to outgassing and oxidation to sulphate, increasing the sulphate concentration by about 4%. Chemical modelling of the water shows that it can be produced by the dissolution of common sulphate, carbonate, and chloride minerals. Redox reactions in the cave appear to be microbially mediated. Sequence analysis of small subunit (16 S ) ribosomal RNA genes of 19 bacterial clones from microbial colonies associated with water drips revealed that 18 were most similar to three Thiobacilli spp., a genus that often obtains its energy from the oxidation of sulphur compounds. The other clone was most similar to Acidimicrobium ferrooxidans , a moderately thermophilic, mineral-sulphide-oxidizing bacterium. Oxidation of hydrogen sulphide to sulphuric acid, and hence the cave enlargement, is probably enhanced by these bacteria. Two cave-enlarging processes were identified. (1) Sulphuric acid derived from oxidation of the hydrogen sulphide converts subaerial limestone surfaces to gypsum. The gypsum falls into the cave stream and is dissolved. (2) Strongly acidic droplets form on the gypsum and on microbial filaments, dissolving limestone where they drip onto the cave floors. The source of the H 2 S in the spring waters has not been positively identified. The Villahermosa petroleum basin within 50 km to the northwest, or the El Chichon volcano ~50 km to the west, may serve as source areas for the rising water. Depletion of 34 S values (−11.7‰ for sulphur stabilized from H 2 S in the cave atmosphere), along with the hydrochemistry of the spring waters, favour a basinal source.

Incision history of Glenwood Canyon, Colorado, USA, from the uranium-series analyses of water-table speleothems

Uranium-series analyses of water-table-type speleothems from Glenwood Cavern and “cavelets” near the town of Glenwood Springs, Colorado, USA, yield incision rates of the Colorado River in Glenwood Canyon for the last ~1.4 My. The incision rates, calculated from dating cave mammillary and cave folia calcite situated 65 and 90 m above the Colorado River, are 174 ± 30 m/My for the last 0.46 My and 144 ± 30 m/My for the last 0.62 My, respectively. These are consistent with incision rates determined from nearby volcanic deposits. In contrast, δ234U model ages (1.39 ± 0.25 My; 1.36 ± 0.25 My; and 1.72 ± 0.25 My) from three different samples of mammillary-like subaqueous crust collected from Glenwood Cavern, 375 m above the Colorado River, yield incision rates of 271 +58/-41 m/My, 277 +61/-42 m/ My, and 218 +36/-27 m/My. These data suggest a relatively fast incision rate between roughly 3 and 1 Ma. The onset of Pleistocene glaciation may have influenced this rate by increasing precipitation on the Colorado Plateau starting at 2.5 Ma. Slowing of incision just before 0.6 Ma could be related to the change in frequency of glacial cycles from 40 to 100 kyr in the middle Pleistocene. This interpretation would suggest that the cutting power of the Colorado River prior to 3 Ma was smaller. An alternative interpretation involving tectonic activity would invoke an episode of fast uplift in the Glenwood Canyon region from 3 to 1 Ma.

The Kaskaskia paleokarst of the northern Rocky Mountains and Black Hills, northwestern U.S.A.

The Kaskaskia paleokarst, part of the Mississippian-Pennsylvanian unconformity in North America, is typified by sink-holes, fissures, and dissolution caves at and near the top of the Kaskaskia Sequence (Madison Limestone and equivalents) and is covered by basal Absaroka siliciclastics (Chesterian to Morrowan). In the Rocky Mountains and Black Hills of the northwestern U.S.A. it post dates earlier features produced by sulfate-carbonate interactions, including breccias, dissolution voids, bedrock alteration, and mineralization. Both the paleokarst and earlier features have been intersected by post-Laramide caves. Ore deposits, aquifers, and petroleum reservoirs in the region are also concentrated along both the paleokarst horizons and earlier sulfate-related features. Each phase of karst modified and preferentially followed the zones of porosity and structural weakness left by earlier phases, producing an interrelated complex of now-relict features. All should be considered together to explain the present aspect of the paleokarst.

Hypogene Karst Regions and Caves of the World

Signs of hypogenic speleogenesis have been detected in a number of caves of the Murcia Region (SE Spain), in some cases revealing active speleogenetic mechanisms rarely observed in hypogene cavities elsewhere in the world. Here, we investigate the hypogenic morphologies and speleothems of four caves in this region, namely Sima de la Higuera, Sima Destapada, Cueva del Agua and Cueva del Puerto. Also, other ten caves showing evidence for hypogenic speleogenesis has been preliminary described. Processes related to ancient and current hydrothermal activity, the discordance of permeability structures in the adjacent beds and the spatial arrangement of the regional hydrogeology have given rise to maze patterns and typical subaqueous hypogenic morphologies. These include spongework mazes, rising wall channels and shafts, feeders, bubble trails, solution pockets, megascallops and rising of chains cupolas, among others. Carbonic acid speleogenesis is responsible for the formation of most of these cave features; however, evidence of sulfuric acid speleogenesis (SAS) has been observed in Cueva del Puerto and Sima del Pulpo, which host massive secondary gypsum deposits. Speleothems typically linked to hydrothermal water upwelling and CO2 degassing close to the water table are present in most of these cavities, including folia, calcite spar crystals, cave clouds, calcite rafts deposits and several types of cave raft cones. The wide variety of hypogenic speleogenesis indicators and speleothems whose genesis is unconnected to meteoric water seepage reveals that the hydrothermal field of the Murcia Region hosts one of the densest active hypogenic subterranean networks in the world.

Petrographic and isotopic evidence for late-stage processes in sulfuric acid caves of the Guadalupe Mountains, New Mexico, USA

INTRODUCTION Caves of sulfuric acid origin retain diagnostic minerals and features that allow reconstruction of their geochemical history (Polyak & Provencio, 2001). This paper centers on caves in the Guadalupe Mountains, New Mexico, specifically their latest transition from H2SO4 speleogenesis to processes dominated by CO2 equilibria. Much cave research today yields paleoclimate data from meteoric speleothems (Fairchild & Baker, 2012). Instead, this paper emphasizes processes and features specific to the caves themselves. Such information can help identify former conditions in similar caves, and is useful for organizing future studies of geochronology and paleoclimate.

Sulfuric Acid Caves

Most caves owe their origin to carbonic acid generated in the soil. In contrast, sulfuric acid caves are produced by the oxidation of sulfides beneath the surface. Although sulfuric acid caves are relatively few, they include some large and well-known examples, such as Carlsbad Cavern, New Mexico. They also provide evidence for a variety of deep-seated processes that are important to petroleum geology, ore geology, tectonic history, and the nascent field of karst geomicrobiology.

Passage Growth and Development

Of all cave types, solution caves have the most complex developmental histories. They are formed by the dissolving action of underground water as it flows through fractures, partings, and pores in bedrock. Such caves must grow rapidly enough to reach traversable size before the rock material that contains them is destroyed by surface erosion. Because of their sensitivity to local landscapes and patterns of water flow, solution caves contain clues to the entire geomorphic, hydrologic, and climatic history of the region in which they are located. At the land surface most of this evidence is rapidly lost to weathering and erosion; but in caves these clues can remain intact for millions of years.

Solution Caves in Regions of Moderate Relief

Caves in regions of moderate relief are widely regarded as the standard to which all others are compared. Tectonic stability is their most significant characteristic. The presence of moderate relief implies that uplift of the land is slow and erosional processes are able to keep pace. As a result, rivers easily erode to their local base levels, and base-level control is reflected in cave passages. The relation between caves and regional geomorphic history is stronger than in any other setting. Examples are given from the Illinois Basin (Kentucky and Indiana), the Ozark Plateau (Missouri), and the Appalachians.

Hypogene Karst Springs Along the Northeastern Border of the Appalachian Plateau, New York State

The northern border of the Appalachian Plateaus in New York State is a limestone–dolomite escarpment with sulfate rocks at depth and hypogene karst springs at its base. All springs contain dissolved carbonates, but many are also exceptionally rich in sulfate, sulfide, or CO2. None connect to traversable caves, but their chemistry provides clues to their internal character, flow depth, and underground processes. Many show evidence for dedolomitization driven by sulfate dissolution, which forces calcite travertine to precipitate at the surface. Isotopic variation and radium content reveal groundwater flow patterns. Chemical contrasts with nearby epigenic caves highlight the nature of the hypogene springs. A nearby but separate spring area, fed by deep flow along faults, illustrates the effects of high-pressure CO2 on carbonate groundwater. Despite the lack of underground access, these karst areas give much insight into hypogene processes.

Hypogenic Versus Epigenic Aspects of the Black Hills Caves, South Dakota

The Black Hills contain several extensive maze caves in the early Carboniferous Madison Limestone. They include Wind Cave and Jewel Cave, which are among the world’s longest and most complex. Their origin is debated, with diverse hypotheses ranging from artesian conditions to rising thermal water. Recent evidence indicates a polygenetic origin including early Carboniferous diagenesis and paleokarst; deep burial by Carboniferous–Cretaceous strata; re-exposure of the limestone by the Laramide Orogeny (early Paleogene); and major cave enlargement in the late Paleogene along old paleokarst zones. Cave enlargement depended mainly on diffuse recharge through overlying sandstone, mixing with lateral inflow through carbonate outcrops. Only a few of these processes were hypogenic, but recognizing them all helps to clarify the limits of that process.