Libby A. Stern

Large kinetic isotope effects in modern speleothems

The application of stable isotopes in speleothem records requires an understanding of the extent to which speleothem calcite isotopic compositions refl ect the compositions of the cave waters from which they precipitate. To test for equilibrium precipitation, modern speleothem calcite was grown on glass plates, so that the carbon and oxygen isotope composition of the calcite and the water from which it precipitated could be directly compared. The plates were placed on the tops of three actively growing stalagmites that occupy a 1 m 2 area in Harrison’s Cave, Barbados, West Indies. Only some of the plate δ 13 C values and none of the plate δ 18 O values correspond to equilibrium values, indicating signifi cant kinetic isotope effects during speleothem calcite growth. We investigate herein mechanisms that may account for the kinetic isotope effects. On each plate, speleothem calcite was deposited with distinct δ 18 O and δ 13 C compositions that increase progressively away from the growth axis, with up to 6.6‰ 13 C and 1.7‰ 18 O enrichments. The positive δ 13 C

Microbial contributions to cave formation: New insights into sulfuric acid speleogenesis

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 H 2 S 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 H 2 S escapes to the cave atmosphere, or is lost by abiotic autoxidation, and instead the primary H 2 S loss mechanism is by subaqueous sulfur-oxidizing bacterial communities that consume H 2 S. 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.

Filamentous “Epsilonproteobacteria” Dominate Microbial Mats from Sulfidic Cave Springs

ABSTRACT Hydrogen sulfide-rich groundwater discharges from springs into Lower Kane Cave, Wyoming, where microbial mats dominated by filamentous morphotypes are found. The full-cycle rRNA approach, including 16S rRNA gene retrieval and fluorescence in situ hybridization (FISH), was used to identify these filaments. The majority of the obtained 16S rRNA gene clones from the mats were affiliated with the “Epsilonproteobacteria” and formed two distinct clusters, designated LKC group I and LKC group II, within this class. Group I was closely related to uncultured environmental clones from petroleum-contaminated groundwater, sulfidic springs, and sulfidic caves (97 to 99% sequence similarity), while group II formed a novel clade moderately related to deep-sea hydrothermal vent symbionts (90 to 94% sequence similarity). FISH with newly designed probes for both groups specifically stained filamentous bacteria within the mats. FISH-based quantification of the two filament groups in six different microbial mat samples from Lower Kane Cave showed that LKC group II dominated five of the six mat communities. This study further expands our perceptions of the diversity and geographic distribution of “Epsilonproteobacteria” in extreme environments and demonstrates their biogeochemical importance in subterranean ecosystems.

Linking phylogenetic and functional diversity to nutrient spiraling in microbial mats from Lower Kane Cave (USA).

Microbial mats in sulfidic cave streams offer unique opportunities to study redox-based biogeochemical nutrient cycles. Previous work from Lower Kane Cave, Wyoming, USA, focused on the aerobic portion of microbial mats, dominated by putative chemolithoautotrophic, sulfur-oxidizing groups within the Epsilonproteobacteria and Gammaproteobacteria. To evaluate nutrient cycling and turnover within the whole mat system, a multidisciplinary strategy was used to characterize the anaerobic portion of the mats, including application of the full-cycle rRNA approach, the most probable number method, and geochemical and isotopic analyses. Seventeen major taxonomic bacterial groups and one archaeal group were retrieved from the anaerobic portions of the mats, dominated by Deltaproteobacteria and uncultured members of the Chloroflexi phylum. A nutrient spiraling model was applied to evaluate upstream to downstream changes in microbial diversity based on carbon and sulfur nutrient concentrations. Variability in dissolved sulfide concentrations was attributed to changes in the abundance of sulfide-oxidizing microbial groups and shifts in the occurrence and abundance of sulfate-reducing microbes. Gradients in carbon and sulfur isotopic composition indicated that released and recycled byproduct compounds from upstream microbial activities were incorporated by downstream communities. On the basis of the type of available chemical energy, the variability of nutrient species in a spiraling model may explain observed differences in microbial taxonomic affiliations and metabolic functions, thereby spatially linking microbial diversity to nutrient spiraling in the cave stream ecosystem.

Precise timing and rate of massive late Quaternary soil denudation

Strontium isotopes are a unique tool to study soil-erosion dynamics. Changes in Sr isotope ratios ( 87 Sr/ 86 Sr) provide a record of late Quaternary landscape denudation of the Edwards Plateau of central Texas, United States. The use of Sr isotopes as a tracer for soil erosion is based on the observation that, in central Texas, the 87 Sr/ 86 Sr ratio of soil correlates with soil thickness. Plants and animals express the 87 Sr/ 86 Sr ratio of exchange- able Sr in the soil. Therefore, we use changes in Sr isotope ratios through a well-dated stratigraphic sequence of fossil plants and animals in Halls Cave, Kerr County, Texas, as a proxy for temporal changes in soil thickness. By using this record we are able to characterize late Quaternary climate-driven soil-erosion dynamics on the Edwards Pla- teau. We find that continuous erosion removed at least 180 cm of soil at a constant min- imum rate of 11 cm/k.y.; this continuous phase of erosion ended ca. 5 ka. The Sr isotope record of soil erosion is consistent with late Quaternary environmental change in central Texas that has been independently modeled by using local and regional climate records. However, the rate of this climate-driven soil-erosion event was an order of magnitude slower than recent soil erosion caused by human land use. These results link erosion to century- to millennial-scale climate change and are cautionary evidence that even greater landscape degradation may result from coincident climatic variability and anthropogenic influences.

Links between Mountain Uplift, Climate, and Surface Processes in the Southern Patagonian Andes

Miocene surface uplift of the southern Patagonian Andes, related to an episode of rapid plate convergence prior to the ∼14–10 Ma collision of the Chile Ridge with the South American subduction zone, has produced one of the most pronounced orographic rain shadows on Earth. Apatite fission track ages from the western flank of this Andean segment imply that 3–4 km of denudation has occurred in this region since ∼17 Ma. The track-length distribution of the studied samples suggests a complex thermal history with initial cooling followed by reheating, presumably owing to the progressive opening and eastward migration of a slab window after the ridge-trench collision, and ultimately more rapid cooling since ∼4 Ma. These thermochronological data are in good agreement with constraints on the elevation history of the southern Patagonian Andes. Based on sedimentological and geochronological data from ∼23 to ∼14 Ma sedimentary rocks in the eastern foreland, and oxygen isotope data from pedogenic carbonate contained in these deposits, we infer that > 1 km of surface uplift of these mountains occurred between ca. 17 and 14 Ma. Carbon isotope data from the pedogenic carbonate samples demonstrate that this led to strong aridification in the eastern foreland and, presumably, strongly increased precipitation rates on the windward western side of the mountains. Because a thicker trench fill promotes weaker coupling along the plate interface, this implies that progressive surface uplift of the southern Patagonian Andes and the increasing sediment flux to the adjacent segment of the South American trench may have contributed significantly to a decrease in compressive deformation and surface uplift.