Environments Elsewhere
Conspicuous evidence of microbially mediated reduction of sulfate anions (SO42-) by CH4, or, for that matter, reduction of molecular oxygen (O2) by CH4, is uncommonly recognized within the Earth’s terrestrial environments. In rare instances, aerobic bacterial cells (living and dead) are preserved ephemerally at and near the earth’s surface as “paraffin dirt,” which provides direct evidence that CH4 has reacted with O2 (Davis, 1952). Lithologic evidence of anaerobic microbial oxidation of CH4 in terrestrial diagenetic environments is apt to be preserved prominently only where CH4 has leaked into shallow strata having an absence of dissolved oxygen, a favorable concentration of SO42- (e.g., >1000 ppm, Oehler and Sternberg, 1984, their fig. 14), and a favorable range of temperatures (e.g., ~40° to ~85°C). Evidence of such anaerobic oxidation consists of bleaching of redbeds by H2S (such as occurs in some Mesozoic strata of the western USA (E. F. McBride, written communication, 1996) and above the Cement field of south-central Oklahoma (Donovan, 1974)), but especially from a combination of carbonate, sulfur, sulfate, and sulfide, replacements and/or pore- filling cements, and/or crystal growths, commonly with revealing isotopic signatures.
The redox reaction between CH4 and SO42- at several oil and gas fields in south-central Oklahoma had a magnitude that rivals that which occurred at the Culberson sulfur deposit (Kirkland et al., 1995). At the Cement, Velma, Carter-Knox, and several other fields, CH4 migrated upward from complex Pennsylvanian structures across an angular unconformity into gently folded, gypsum-bearing Permian red beds. Sulfate- reducing microbes at the Cement field, for example, consumed an estimated 37 billion m3 (1.3 trillion ft3) of CH4, and in the process generated millions of metric tons of the metabolic by-products, CO2 and H2S. The by- products reacted with associated cations—chiefly Ca2+, Fe2+, and Mg2+—to form “chimneys” of bleached red beds, chiefly sandstone, cemented by calcite, dolomite, and lesser amounts of pyrite and marcasite (FeS2), and, in addition, trace amounts of sphalerite, (Zn,Fe)S and galena, PbS. The carbonate minerals have δ13C values as low as -39.2‰, and the sulfide minerals, δ34S values as low as -37.9‰ (Kirkland et al., 1995). Other terrestrial examples in which anaerobic reduction of SO42- by clearly derived directly or indirectly from CH4 (either
thermogenic, biogenic, or both). The H2S generated at modern cold seeps supports an impressive mat of H2S- oxidizing bacteria as well as an associated seep fauna, including tubeworms. The ecology of the seep fauna is built around bacteria, especially Beggiatoa sp., that obtain their energy from reaction between aqueous O2 and aqueous H2S (e.g., Orcutt et al., 2005). The comprehensive processes that occur at modern cold-seep deposits of the world’s oceans were duplicated, in many respects, by the comprehensive processes that occurred in southeastern New Mexico; these include microbial generation of H2S at the calcite masses and its oxidation at the caves and at the sulfur deposits.
Furthermore, within shallow anaerobic sediments of the world’s oceans, SO42-oxidizes CH4. The oxidation is apparently nearly pervasive in shallow oceanic sediments, but is most vigorous in sediments of continental shelves. Dissolved within seawater, O2 diffuses downward through marine pore water for a few millimeters to more than one meter below the seafloor until it becomes exhausted (e.g., Jørgensen, 1982). Similarly, SO42- within seawater—less reactive and in much greater concentration (>300 times)—diffuses downward even farther. Below its limit of diffusion (typically <1 m), a varied assemblage of anaerobic microbes (including methanogenic archaea) decomposes particulate organic matter by hydrolysis and by fermentation to generate CH4 (e.g., Barker, 1956; McCarty, 1964). The upward diffusing CH4 eventually contacts the downward diffusing SO42 (see Valentine, 2002). At the interface, anaerobic microbes (probably a consortium of archaea and bacteria) then bio-catalyze oxidation of CH4 and the coupled reduction of SO42- to form the metabolic “waste products,” H2S and CO2 (e.g., Boetius et al, 2000; Hinrichs and Boetius, 2002). Most of the sulfur and carbon atoms are incorporated into pyrite and calcite, respectively. Little CH4 escapes anaerobic oxidation to enter either the overlying O2-bearing sediments or the O2-bearing water column, the “sulfate-dependent methane oxidation” acting as a barrier (Valentine and Reeburgh, 2000). The microbially mediated reaction between CH4 and SO42- occurs collectively on a vast scale within the world’s oceans, an estimated 100 trillion grams of CH4 per year (Reeburgh, 1989), the amount of CH4 consumed being approximately equivalent to 5-20% of the total annual flux of CH4 to the atmosphere (Hinrichs and Boetius, 2002).
CH4 has played an important role are calcite-cemented sandstone along the flank of Butler Salt Dome, east Texas (Enos and Kyle, 2002), possibly the Beeri sulfur deposit, southern Israel (e.g., Druckman et al., 1994), and many low-temperature mineral deposits of copper, iron, lead, uranium, vanadium, and zinc within sedimentary strata.
The transient, high-heat flow increased stratal temperatures in Permian and older petroleum-source strata, which, in turn, cracked dispersed crude oil and further decomposed dispersed kerogen to generate copious volumes of CH4. In addition, the episodic and uniform easterly tilting during the late Miocene and early Pliocene (by a cumulative 1-2°) along with a nearly contemporaneous late-phase of Basin and Range crustal extension, created and rejuvenated fractures within the Paleozoic sedimentary section.
In the Delaware Basin near the beginning of the late Miocene, pressurized, nearly fresh artesian groundwater, which originated in the ancestral Guadalupe Mountains, moved upward from sandstone beds of the upper Bell Canyon Formation (Middle Permian) through the new and rejuvenated fractures into the directly overlying Anhydrite I Member (thickness ~50 m) of the Castile Formation. The artesian groundwater dissolved CaSO4, the density of the solvent increased, it became gravitationally unstable, and Ca2+- and SO
42--bearing groundwater sank back into the Bell Canyon. Taking its place, under artesian pressure, the freshest, least-dense water available rose inherently to the highest accessible elevation. The continuous process of free convection created dissolution voids, and, subsequently, fractures and collapse breccias within the Anhydrite I Member through which CH4, aggressive groundwater (rising), and nonaggressive groundwater (sinking) moved freely. Hypogenic groundwater ascended through the solutionally enhanced, transverse pathways through the Anhydrite I Member and eventually contacted the base of the directly overlying Halite I Member (thickness ~125 m) of the Castile Formation. The rising groundwater readily dissolved the bedded NaCl, and the resulting brine sank. Simultaneously, groundwater with the greatest solutional aggressiveness for NaCl (that with the lowest density and the lowest concentration of solutes) rose continuously to the solution front where it, in turn, dissolved additional Castile halite.
The free convective process resulted in chambers being dissolved vertically upward within the Halite I Member until they contacted the intact base of the next overlying bed of anhydrite (namely, the base of the Anhydrite II Member), which dipped uniformly eastward over An immense weight (millions of metric tons) of
microbial hydrogen sulfide (H2S) moved into caves of the Guadalupe Mountains during the late Miocene and early Pliocene (~12-4 million years ago). The H2S reacted with O2 chiefly within subaerial water of condensation to form sulfuric acid (H2SO4)—the primary cave-forming agent in the mountains. The caves formed within Middle Permian reefal limestone (the Capitan Formation) and within adjacent, time-equivalent, shelfal carbonates (particularly, the Seven Rivers Formation).
Pathways previously proposed for transporting the precursor, H2S, to the caves are likely deficient. Neither large quantities of gaseous H2S nor aqueous H2S apparently migrated from the northwestern Delaware Basin updip into the evolving caves through siliciclastics of the Bell Canyon Formation (Middle Permian; Guadalupian series). Furthermore, large quantities of H2S dissolved within artesian groundwater apparently did not migrate from elevated (mountainous) shelfal strata northwest of where the modern Capitan reef escarpment now exists downdip through permeable Middle Permian strata into the evolving caves. Instead, the H2S involved in speleogenesis was probably transported into the evolving caves from the adjoining Delaware Basin through upward- inclined pathways within Castile halite (Upper Permian; lower Ochoan Group). The Castile Formation, a thick (~0.5 km) evaporite unit (originally ~30% halite, ~60% anhydrite, ~10% calcite), is confined to the basin.
Each Castile member, bed, and lamina—whether halite, anhydrite (probably initially gypsum), or calcite (possibly initially aragonite)—that formed by deposition and diagenesis in the Late Permian had, with few exceptions, an extraordinarily uniform thickness, lithology, and contact relationships over many thousands of square kilometers. Two approximately coeval Late Tertiary events superimposed on the consistent Castile stratigraphic framework resulted in intense H2S-H2SO4 speleogenesis in the Guadalupe Mountains. These events were
• high-heat flow, particularly in the western Delaware Basin, and
• eastward tilting of the paleo-Guadalupe tectonic block, a huge homocline that included the Guadalupe Mountains and much of the Delaware Basin.