Margaret V. 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.

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.

Hypogene Speleogenesis in the Guadalupe Mountains, New Mexico and Texas, USA

The Guadalupe Mountains consist of an uplift of Permian carbonate shelf deposits in a semiarid landscape. A variety of speleogenetic processes, mostly hypogene, have made them one of the world’s best-known cave regions. The most notable caves are Carlsbad Cavern, which contains the largest known cave room in the USA, and Lechuguilla Cave, now the world’s 7th longest. Because the caves are no longer active, there was early confusion about their origin. This was resolved when long-dormant sulfuric acid processes were recognized, with H2S supplied by nearby oil fields. Potassium-argon dating of the by-product mineral alunite in the Guadalupes indicates speleogenetic ages from 12 to 4 million years, decreasing with lower elevation. Caves show abundant evidence for subaerial corrosion, both by sulfuric acid and carbonic acid in water films. Many seemingly phreatic features have resulted from this subaerial process. Microbial alteration of bedrock has contributed to weathering. There is evidence that isolated caves of greater age, lined by large scalenohedral calcite, were formed by supercritical CO2 in deep thermal water.

Sulfuric Acid Caves of the Bighorn Basin, Wyoming

The Bighorn Basin of Wyoming is a region of thermal springs and caves, some with lethal levels of H2S and CO2. It also contains many productive oil wells. In one of these caves, “sulfuric acid speleogenesis” was first recognized and documented in North America by Egemeier in 1973. He proposed that most of the cave dissolution was subaerial and the result of H2S oxidation to H2SO4. Later studies by microbiologists have refined his measurements and show that much of the H2SO4 is generated in the stream by sulfur-oxidizing bacteria, by the process of “microbial sulfuric acid speleogenesis.” Other caves with similar chemistry are known in the region, but they have only been partly explored because of the locally high sulfide concentrations in their atmospheres.

Carbonate Replacement of Sulfate--New Mechanism for Porosity Generation in Carbonate Rocks Marginal to Evaporite Basins: ABSTRACT

Production of macropores by gypsum replacement of carbonate, and of intergranular porosity by pervasive dolomitization constitute two important mechanisms for generating high porosity in carbonate rocks in and adjacent to evaporite basins. Exposures of Upper Permian Capitan Group carbonate beds in Carlsbad Caverns and other caves of the Guadalupe Mountains, New Mexico, indicate that one of the earliest stages of speleogenesis was the massive replacement of carbonate by sulfate. Field relations indicate that replacement took place in a mixing zone between a meteoric freshwater lens and gypsum-saturated brines. Replacement is most pronounced along joints, indicating a possible correlation with the rate of freshwater input. Replacement probably began with the development of the lens following orogenic uplift of the Guadalupe Mountains, or solutional deflation of basinal evaporites. As uplift and deflation continued, hydrologic base level fell with respect to replacement gypsum pods, exposing them first to freshwater phreat c conditions, and later to vadose conditions, causing their partial or complete recrystallization or dissolution and creating enormous voids. These underwent limited enlargement in the freshwater phreatic zone, and collapsed following draining of the caves. Replacement of the gypsum may be recognized by primary carbonate structures preserved as remnant inclusions. Replacement gypsum crystals are generally equant with complex boundaries, but may derive their size and shape from carbonate macrostructures replaced End_Page 512------------------------------ (usually laminae within pisolites). Replacement crystals range in diameter from 0.1 to 10 mm. Inclusions which reproduce primary fabrics are most commonly small (to 50 µm), irregular rounded blebs of unknown composition, but are rarely dolomite rhombs. Generally associated with sulfate replacement zones are massive sucrosic dolomites displaying abrupt lateral transitions with unaltered limestones. These dolomites may form because of increased magnesium:calcium ratios in mixing-zone pore waters, because during replacement magnesium is not accommodated in the gypsum lattice. Work is under way on the recognition of secondary carbonate fabrics related to these mixing-zone environments which would allow identification of gypsum replacement in rocks in which gypsum has totally dissolved or recrystallized. End_of_Article - Last_Page 513------------