Speleogenesis

(Submitted Abstract to the 2002 Karst Waters Institute Meeting, Gainesville, Florida)

Subaqueous and subaerial speleogenesis in a sulfidic cave

 

Libby Stern, Annette Summers Engel, Philip C. Bennett

Department of Geological Sciences

University of Texas at Austin

Austin, Texas 78712

 

Megan L. Porter

Department of Zoology

Brigham Young University

Provo, Utah 84602

 

INTRODUCTION

 

Sulfidic, thermal ground waters enter Lower Kane Cave, Bighorn Basin, Wyoming, where sulfide oxidizes to sulfuric acid via abiotic and microbial metabolic pathways. Sulfuric acid production occurs in both subaqueous (cave stream) and subaerial (cave-wall surface) settings by the autooxidation of reduced sulfur compounds and by sulfur-oxidizing bacteria occupying both habitats.  Thick microbial mats have formed in the cave stream, and molecular and enrichment culture techniques identify sulfur-oxidizing communities related to Thiobacillus, Thiothrix, and Thiovulum spp., as well as other aerobic and anaerobic metabolic groups. In contrast, the microbial community on the subaerial cave-wall surfaces is much simpler, and only acid-producing thiobacilli have been identified to date.  These two environments have unique geochemical and biochemical constraints that result in substantially different mechanisms of speleogenesis.

 

METHODS

The cave waters and atmosphere were sampled Summer 2000, and Spring, Summer, and Winter 2001, and the analytical results were compared to microbiological characterization of the microbial mats and biofilms done during the same sampling periods.  Water samples were collected from spring orifices and along the cave stream channels, with immediate analysis of unstable constituents (temperature, pH, Eh, DO, H₂S, NH₄⁺, alkalinity, ferrous iron), rapid analysis of dissolved gasses by gas chromatography, and laboratory analysis of anions by ion chromatography, metals by ICP-MS, and carbon species by carbon analyzer.  In addition to continuous gas sampling by a toxic-gas meter, gas samples were collected in the cave and analyzed immediately for carbon dioxide by non-dispersive IR detection, and within 24 hours for N₂, O₂, CO₂, and reduced sulfur gases by gas chromatography.  During the Winter 2001 sampling we will bring the gas chromatograph into the cave for immediate analyses of atmospheric gasses, and analysis of trace sulfur compounds by trap concentration/gas chromatography.  Samples of biological material and rock specimens were collected and preserved for examination by conventional scanning electron microscopy (C-SEM), and by environmental SEM (E-SEM).

 

OBSERVATIONS

Subaqueous Environment

 

Spring waters emerge at three locations in the cave and feed a small stream that flows most of the length of the cave.  Dry gypsum mounds surround spring orifices and pools, with only an occasional limestone fragment from breakdown.  In contrast, the spring orifices have exposed bedrock limestone and a thin veneer of sediment that grades into a residual chert cobble bed along the spring outflow channel.  Microorganisms colonize both the sediment and chert cobble substrata. The water that issues from the cave springs is anaerobic (DO <0.01 ppm), pH ~ 7.3, H₂S concentrations from 1-3 ppm, and are only slightly undersaturated with respect to calcite (calculated logSI ~^–0.2) and significantly undersaturated with respect to gypsum (logSI ~ ^–1.5) using PHREEQC.  The calculated equilibrium partial pressure of carbon dioxide (log Pco₂) of the spring water is ^–1.9.  The water comes to approximate equilibrium with calcite a few meters down stream of the spring orifice as pH increases to ~7.5 and log Pco₂ increases to ~^–2.4, compared to a cave atmosphere at log Pco₂ of ^–2.7 to ^–3.0.

 

However, while the water is in approximate equilibrium with calcite, limestone breakdown blocks that have fallen into the cave stream are deeply etched at the contact between the limestone and microbial mats, suggesting that the microbial populations, particularly the sulfur-oxidizers, enhance limestone dissolution.  An additional mechanism for limestone dissolution in the cave stream is from acidic drip waters from the subaerial environment.  The drops that fall on exposed (above water) portions of a limestone block produce focused dissolution pitting that contribute to the eventual removal of that fragment of rock.

 

It is unclear, however, if the subaqueous limestone dissolution is a “bulk” phenomenon, whereby acid generated from both biotic and abiotic sulfur oxidation drives the water to slight undersaturation with respect to calcite, i.e.

 

CaCO₃ + H⁺ à Ca²⁺ + HCO₃^– (1)

 

Alternatively, the dissolution may be directly related to contact with the microbial mats where focused oxidation of reduced sulfur results in a more localized, acidic and aggressive geochemical environment.  To test the importance of biotic sulfide oxidation to calcite dissolution and cave formation, we are measuring calcite dissolution rates from the cave stream with sterile and non-sterile microcosms containing chips of Iceland spar calcite.  After one week of exposure, there is no detectable bulk dissolution of the calcite, but there is detectable dissolution where microbial filaments have attached to the surface.  The surfaces will also be examined after 5 months of exposure, and the mass loss of calcite will be directly measured on sterile and non-sterile chips.

 

Subaerial Environment

 

The cave walls are coated with gypsum secondary to the dissolution of the host limestone by sulfuric acid, i.e.:

 

H₂S + 2 O₂ à H₂SO₄ (2)

 

CaCO₃ + H₂SO₄ + H₂O à CaSO₄^.2H₂O + CO₂ (3)

 

Discontinuous patches of organic biofilms occur on the gypsum throughout the cave, with the highest abundance of biofilm found near spring discharge areas where the atmospheric concentrations of reduced sulfur gasses are highest.  The biofilm is composed predominately of carbon and sulfur, with some silicon as authigenic quartz. There is no detectable calcium within the biofilms, indicating that sulfur is present as elemental sulfur and not gypsum.  The high concentrations of carbon reflect the visible microbial colonization of the wall crusts, determined by DAPI-staining for nucleic acids, and cultures for sulfur-oxidizing bacteria.

 

Condensation droplets are found on walls throughout the cave on different materials, including freshly exposed limestone, gypsum, and on the biofilms covering the gypsum substratum.  We hypothesize that the droplets form in the water-saturated cave environment from the slight difference in temperature between the emerging spring water and the cave walls.  Droplets on limestone are slightly acidic, ranging from 5.5 to 4, with lower pH drops found associated with fresh replacement gypsum (gypsum moonmilk) via reaction (2) and (3).  Droplets on the thicker gypsum crusts are very acidic (pH ~1 to 3), with the higher pH drops found directly on gypsum, while the lowest pH droplets occur exclusively on the organic biofilms.  Dissolved geochemical constituents were analyzed for all the droplets, and the gypsum droplets are extremely undersaturated with respect to calcite, and in equilibrium with gypsum, with the pH buffered to approximately 2 by the sulfate-bisulfate weak acid/base pair, i.e.:

 

HSO₄^– à H⁺ + SO₄⁼ pK=1.92                                              (4)

 

As sulfuric acid is produced via sulfide oxidation, the solution in contact with gypsum will approach pH 2, but typically not exceed it due to the buffering by sulfate from gypsum.

 

H₂SO₄ + CaSO₄^.2H₂O à 2 HSO₄^– + Ca²⁺ (5)

 

In contrast, the acid droplets on the organic biofilm have pH values below this critical bisulfate-sulfate pK, and we infer that the biofilm acts as a hydrophobic layer between the solution and the underlying gypsum, allowing the pH to pass the bisulfate buffer point.  This is supported by the equilibrium calculations of the condensation waters, which shows that the droplets at pH > 2 are all in equilibrium with gypsum, while the droplets at pH < 2 are all undersaturated with respect to gypsum.

 

These acidic droplets also contribute to calcite dissolution on the cave floor, as breakdown calcite has dissolution pitting that appears to be caused by acid droplets from the cave wall.  The acid droplets would also contribute to the overall proton production, and in theory the bulk calcite dissolution of the cave, but the mass contribution of proton from this source is likely small compared to the buffer capacity of the water.

 

Mechanisms of Sulfuric Acid Speleogenesis

 

Sulfuric acid on the cave wall is initially produced on the limestone bedrock, quickly converting the calcite to gypsum with the release of CO₂ (reaction 1,2).  This reaction can occur through both biotic and abiotic mechanisms, but probably initiates from abiotic autooxidation of H₂S.  Initially pH is buffered by calcite, but as gypsum develops, the reaction front retreats from the exterior water droplet, creating a pH and sulfuric acid gradient, with low pH at the surface and higher buffered pH at the calcite/gypsum interface.  As the low pH habitat develops, sulfur-oxidizing bacteria colonize the new surface, probably accelerating sulfur oxidation.  As the biofilm continues to develop on the gypsum crust, we hypothesize that the diffusion of sulfuric acid though the gypsum to the underlying limestone becomes limited or is shut off completely, with the condensation droplets now separated and out of equilibrium with respect to the underlying gypsum. While the occurrence of sulfur-oxidizing bacteria on these surfaces may continue to increase the rate of sulfide oxidation and production of sulfuric acid, the accumulation of a thick organic biofilm eventually creates an impermeable layer, precluding additional cave wall dissolution.  Only after the gypsum has sloughed off and exposed fresh limestone will speleogenesis be reinitiated, and the replacement-colonization cycle start again.

 

When the gypsum crusts slough off, gypsum debris drops to the cave floor in three distinct geochemical regions.  Gypsum and associated chert from the original limestone accumulates in mounds exceeding 3 meters in thickness in areas away from the cave stream, and the mounds probably remain until floods of sufficient intensity dissolve and remove the alteration breakdown.  In other areas away from sources of dissolved sulfide and in the absence of sulfur-oxidizing bacteria, the gypsum falls into the stagnant cave water and dissolves, pushing that area of stream toward calcite supersaturation and precipitation via the common ion effect, i.e.

 

CaSO₄^.2H₂O + HCO₃^– à CaCO₃ + SO₄⁼+ 2 H₂O + H⁺ (6)

 

This can be seen at the back of the cave where calcite crusts and lily pads are found near the water surface, but only in stagnant stream reaches in the absence of microbial mats.  Gypsum that falls into the flowing portion of the main stream channel rapidly dissolves in the undersaturated water, and the solutes are quickly flushed out of the cave.  In this environment, the extensive microbial mats of sulfur oxidizers produce acidity, but there is little remaining exposed limestone, and only limestone breakdown in contact with the mats visibly dissolves.  The stream bed is now composed almost exclusively of residual chert fragments, and it is unclear if there is active speleogenesis under that layer.

 

IMPLICATIONS

 

While from a microbiological perspective the subaqueous environment is more interesting, it is unclear what the eventual contribution of these organisms will be to the rate of cave development.  These mats certainly generate acid by sulfide oxidation, but there is little exposed limestone now on the cave floor where the mats are found, and only breakdown limestone is obviously dissolved in this environment.  Additionally, it is not yet clear whether the acidity produced by these mats acts to limit the back precipitation of calcite as gypsum breakdown into the stream pushes the system toward calcite supersaturation, as is seen in the back of the cave.  On the cave walls, in contrast, the microbial community is relatively simple, but the effect is obvious, with intense weathering, gypsum development, and visibly defined geochemical and biochemical environments.  As with the subaqueous environment, however, it is not yet clear what the direct contribution of the subaerial microbial community is to cave development, compared to the abiotic autooxidation of sulfide.