Evidence for archaeal methanogenesis within veins at the onshore serpentinite-hosted Chimaera seeps, Turkey

Serpentinite-hosted ecosystems are potential sites where life may first have evolved on Earth. Serpentinization reactions produce strongly reducing and highly alkaline fluids that are typified by high concentrations of molecular hydrogen (H2) and methane (CH4), which can be used as an energy source by chemosynthetic life. Low-temperature serpentinization at slow-spreading mid-ocean ridges provides an ideal environment for rich microbial communities. Similar environments have also been discovered on land, where present-day low temperature serpentinization occurs during the circulation of groundwater through exposed ophiolites, triggering the production of CH4 and H2, as well as the precipitation of secondary carbonate minerals. The rock samples analyzed here are from the Chimaera seeps in Turkey, representing serpentinized peridotites that are cross-cut by veins composed of brucite and hydromagnesite. Hydromagnesite features a mean δ13C value of −19.8‰ caused by kinetic isotope fractionation during air-groundwater exchange of CO2, followed by CO2 hydroxylation to bicarbonate within the groundwater. Geochemical modeling revealed that mixing of Mg- and Ca-rich groundwaters is required for hydromagnesite formation at the expense of brucite. Within the carbonate-hydroxide veins the lipid biomarkers pentamethylicosane (PMI) and squalane with δ13C values of +10‰ and +14‰, respectively, and unsaturated derivatives thereof were identified. Archaeol, sn2-hydroxyarchaeol, and sn3-hydroxyarchaeol are other prominent archaeal biomarkers in the veins, also revealing high δ13C values from +6 to +13‰. These isotope patterns combined with the absence of crocetane – a biomarker for methanotrophic archaea – reveal that the microbial communities of the Chimaera seeps performed methanogenesis from a CO2-limited pool rather than methanotrophy. Moreover, bacterial dialkyl glycerol diethers (DAGEs) with unusually high δ13C values (−9 to −2‰) and minor monoalkyl glycerol monoethers (MAGEs) were identified, suggesting that bacterial sulfate reduction is also active at the Chimaera site. This study reveals that archaeal methanogenesis and bacterial sulfate reduction may be prominent at onshore peridotite-hosted sites, and that biogenic CH4 may contribute to abiotic CH4 emissions from terrestrial seeps

Engineering geological characterization of the Antalya karstic rocks, southern Turkey

This study encompasses engineering geological characterization of the Antalya karstic foundation rocks, particularly tufa, whose mechanical behavior is highly variable. The Antalya tufa rock has no well-developed discontinuity systems. It is variably porous, and is composed of different rock types with variable structures. To reveal the engineering geological parameters and to develop a thorough engineering geological database, which is not available in the literature for the Antalya tufa rock, geological observations, engineering geological site investigations, mineralogical analyses, field (plate load tests) and laboratory geomechanics tests have been performed. The physico-mechanical properties such as porosity, seismic wave velocity, uniaxial compressive strength, Young's modulus, deformation modulus, tensile strength, cohesion, angle of internal friction and other petrographic characteristics such as organic matter content and rock fabric that are expected to have a significant influence on its behavior were determined. Regression analyses have been performed to obtain relationships between the engineering geological parameters of the Antalya tufa rock. A number of good correlations (R-2 >= 0.75) obtained by regression analyses indicated that the Antalya tufa rock types could be differentiated better by using the strength parameters (namely, uniaxial compressive and tensile strength) and the index parameters (namely, unit weight and porosity)

Gypsum Karst Landscape in the Sivas Basin

The Tertiary Sivas Basin, Central Anatolia, includes one of the most outstanding gypsum karst terrains in the world, covering an area of 2140 km2. This gypsum karst significantly contributes to enrich the diversity of karst landscapes in Turkey and constitutes an excellent natural laboratory for understanding their evolution because it develops and degrades much faster than carbonate karst landscapes. The ENE–WSW trending Sivas gypsum karst terrain is 280 km long and 55 km wide. The karst landscapes are mainly developed on Oligocene gypsum deposits. Sivas gypsum karst terrain has a wide variety of well-developed karst features such as karren, different types of dolines (solution, collapse and suffosion), blind valleys, karst springs, swallow holes (ponors), karstified paleo-valleys, caves, unroofed caves, natural bridges, gorges, uvalas and poljes. Solution dolines, which riddle a large part of the area, are the most common landform. The Kızılırmak River and its tributaries drain the Sivas Basin. Therefore, Quaternary evolution of the Kızılırmak River has played an important role in the long-term evolution of the karst landscape in the basin. Karst development in some parts of the basin has also been affected by halokinetic structures