Abstract Relict soils provide insights into Quaternary soil formation and erosion on the Edwards Plateau of central Texas and into soil-forming processes in karst terranes. Late Quaternary climate-driven soil erosion produced a mosaic of thick and thin soils on the Edwards Plateau landscape. Thick soils on uplands of the Edwards Plateau are interpreted to be relicts of a formerly more extensive soil cover that was eroded during the late Pleistocene to middle Holocene. The relict, thick soils are silicate-rich and most commonly overlie the relatively silicate-poor Cretaceous Edwards Limestone, which supports the idea that the thick soils did not form from weathering of the underlying limestone. Other potential sources of silicates for the relict soils include dust, alluvial sediments, and the Del Rio Clay, a stratigraphically higher but locally eroded clay-rich unit. Here we investigate the geographic distribution, texture, clay-sized mineralogy, rare earth element geochemistry, and neodymium isotope composition of the relict soils. We have found that the relict, thick soils are deeply weathered soils that occur dominantly over the Edwards Limestone and have a high clay content and a neodymium isotope composition that is similar to that of the Del Rio Clay. Thus, we propose that in situ weathering of the Del Rio Clay, along with partial weathering of the upper portion of the underlying Edwards Limestone produced thick chert- and clay-rich soils over resistant limestone. In areas like the Edwards Plateau, where pure limestones are interbedded with clay-rich strata, the overlying clay-rich strata must be considered as a possible silicate source to soils on pure limestone bedrock.
Filamentous microbial mats from three aphotic sulfidic springs in Lower Kane Cave, Wyoming, were assessed with regard to bacterial diversity, community structure, and ecosystem function using a 16S rDNA-based phylogenetic approach combined with elemental content and stable carbon isotope ratio analyses. The most prevalent mat morphotype consisted of white filament bundles, with low C:N ratios (3.5-5.4) and high sulfur content (16.1-51.2%). White filament bundles and two other mat morphotypes had organic carbon isotope values (mean delta13C=-34.7 per thousand, 1sigma=3.6) consistent with chemolithoautotrophic carbon fixation from a dissolved inorganic carbon reservoir (cave water, mean delta13C=-7.4 per thousand for two springs, n=8). Bacterial diversity was low overall in the clone libraries, and the most abundant taxonomic group was affiliated with the "Epsilonproteobacteria" (68%), with other bacterial sequences affiliated with Gammaproteobacteria (12.2%), Betaproteobacteria (11.7%), Deltaproteobacteria (0.8%), and the Acidobacterium (5.6%) and Bacteriodetes/Chlorobi (1.7%) divisions. Six distinct epsilonproteobacterial taxonomic groups were identified from the microbial mats. Epsilonproteobacterial and bacterial group abundances and community structure shifted from the spring orifices downstream, corresponding to changes in dissolved sulfide and oxygen concentrations and metabolic requirements of certain bacterial groups. Most of the clone sequences for epsilonproteobacterial groups were retrieved from areas with high sulfide and low oxygen concentrations, whereas Thiothrix spp. and Thiobacillus spp. had higher retrieved clone abundances where conditions of low sulfide and high oxygen concentrations were measured. Genetic and metabolic diversity among the "Epsilonproteobacteria" maximizes overall cave ecosystem function, and these organisms play a significant role in providing chemolithoautotrophic energy to the otherwise nutrient-poor cave habitat. Our results demonstrate that sulfur cycling supports subsurface ecosystems through chemolithoautotrophy and expand the evolutionary and ecological views of "Epsilonproteobacteria" in terrestrial habitats.
Research Article| May 01, 2004 Microbial contributions to cave formation: New insights into sulfuric acid speleogenesis Annette Summers Engel; Annette Summers Engel 1Department of Geological Sciences, Research Group for Microbial Geochemistry, University of Texas, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar Libby A. Stern; Libby A. Stern 1Department of Geological Sciences, Research Group for Microbial Geochemistry, University of Texas, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar Philip C. Bennett Philip C. Bennett 1Department of Geological Sciences, Research Group for Microbial Geochemistry, University of Texas, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Annette Summers Engel 1Department of Geological Sciences, Research Group for Microbial Geochemistry, University of Texas, Austin, Texas 78712, USA Libby A. Stern 1Department of Geological Sciences, Research Group for Microbial Geochemistry, University of Texas, Austin, Texas 78712, USA Philip C. Bennett 1Department of Geological Sciences, Research Group for Microbial Geochemistry, University of Texas, Austin, Texas 78712, USA Publisher: Geological Society of America Received: 23 Oct 2003 Revision Received: 02 Jan 2004 Accepted: 10 Jan 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2004) 32 (5): 369–372. https://doi.org/10.1130/G20288.1 Article history Received: 23 Oct 2003 Revision Received: 02 Jan 2004 Accepted: 10 Jan 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Annette Summers Engel, Libby A. Stern, Philip C. Bennett; Microbial contributions to cave formation: New insights into sulfuric acid speleogenesis. Geology 2004;; 32 (5): 369–372. doi: https://doi.org/10.1130/G20288.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The sulfuric acid speleogenesis (SAS) model was introduced in the early 1970s from observations of Lower Kane Cave, Wyoming, and was proposed as a cave-enlargement process due to primarily H2S autoxidation to sulfuric acid and subaerial replacement of carbonate by gypsum. Here we present a reexamination of the SAS type locality in which we make use of uniquely applied geochemical and microbiological methods. Little H2S escapes to the cave atmosphere, or is lost by abiotic autoxidation, and instead the primary H2S loss mechanism is by subaqueous sulfur-oxidizing bacterial communities that consume H2S. Filamentous "Epsilonproteobacteria" and Gammaproteobacteria, characterized by fluorescence in situ hybridization, colonize carbonate surfaces and generate sulfuric acid as a metabolic byproduct. The bacteria focus carbonate dissolution by locally depressing pH, compared to bulk cave waters near equilibrium or slightly supersaturated with calcite. These findings show that SAS occurs in subaqueous environments and potentially at much greater phreatic depths in carbonate aquifers, thereby offering new insights into the microbial roles in subsurface karstification. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
In contrast to the thin rocky soils that typify the modern Edwards Plateau landscape, thick red soils occur on isolated Edwards Limestone-capped uplands in central Texas. These thick soils are interpreted to be relicts of a formerly more extensive soil cover that was eroded away during the late Pleistocene to middle Holocene. Red clay deposits in central Texas caves and fossils of burrowing mammals contained in these cave-fill sediments provide evidence that thick red soils were once more extensive on the Edwards Plateau (Toomey an others, 1993). Sr isotope variations among fossil plants and animals contained in sediments in Hall’s Cave, Kerr County, Texas provide a record of the gradual erosion of the former thick soil cover from 21,000 to 5,000 cal. years before present (Cooke and others, 2003).
A surface water and precipitation transect across the southern Patagonian Andes at 47°–48°S was conducted to assess how mountains affect the isotopic composition of precipitation. This westerly wind region derives its moisture that falls as precipitation from the Pacific. Orographic uplift of air over the Andes causes pseudoadiabatic cooling and orographic precipitation on the western side and a strong rain shadow on the eastern side of the mountains. These processes also produce a profound isotopic rain shadow, with δ 18 O values ∼4‰ lower in the east compared to the west. On the windward western side of the Andes, the isotope values of precipitation and surface waters show weak, although systematic, trends with elevation, but a pronounced correlation with distance from the main source of moisture (Pacific Ocean). A Rayleigh simulation of precipitation δ 18 O values as a function of condensation altitude matches our data well, suggesting that other processes, such as mixing of water sources and postcondensation evaporation, are negligible, and that topography is the dominant control on the isotope ratio of precipitation. In contrast, isotope ratios of waters on the leeward eastern side of the southern Patagonian Andes vary neither with elevation nor with distance from the mountains, consistent with much of the precipitation on the leeward side being derived from precipitation carried over the Andes by winds. Thus, paleoprecipitation isotopic composition on the leeward side of orogens in similar climatic settings (single moisture source and cool climate) yield useful estimates of the elevation of these orogens, but not local elevation.
