Astrobiological potential of glendonite growth after Ikaite: a Carboniferous glaciomarine example

Backgorund: Ikaite (Ca-carbonate hexahydrate) is a metastable mineral that only forms under specific marine and freshwater environmental conditions of near-freezing temperatures, high alkalinity, and elevated concentration of organic matter and orthophosphate. Due to its instability, evidence of its former presence is only preserved in the sedimentary record by glendonite, a calcite stable pseudomorph after ikaite. Thus, the presence of glendonite is commonly considered to be a good paleoclimatic and paleoceanographic phosphate-rich paleoenvironments indicator. Moreover, in the marine environment ikaite crystals have been shown to grow close to the sediment-water interface (Kapland, 1979) probably in association with microbial methanotrophic sulfate reduction at hydrocarbon seeps (Greinert et al., 2004). This conclusion has recently been endorsed by Teichert and Luppold (2013) using stable isotope analysis. Results: Preliminary results of a combined mineralogical, petrographical, and geochemical study (optical microscopy, XRD, SEM-EDX, NanoRAMAN, EMPA, C-O stable isotopes) of glendonites recovered in the Late Carboniferous, glaciomarine deposits of Dwyka Group will be presented. Glendonites from South Africa were firstly reported by McLachlan et al. (2001). The texture and composition, presence of pyrite in the pore space, and the geochemical signatures of these glendonites suggest an early diagenetic phase related to microbially-mediated methane oxidation via sulphate reduction processes possibly associated with paleo-hydrocarbon seepage. We will also outline the mineralogy of ikaite and the glendonite pseudomorph, and then illustrate the use of glendonite as a paleoenvironmental indicator with a special emphasis on its potential role in identifying and elucidating methane paleoseeps. Astrobiological implication: The ability to recognize and characterize peculiar calcite pseudomorphs after ikaite in terrestrial methane-related carbonate deposits may significantly contribute to their interpretation and paleo-seep reconstruction and elevate such deposits as analogues for methane-related carbonate seepage on Mars. An improved understanding of the significance of terrestrial glendonite formation and the potential role of microorganisms in its formation will provide criteria for recognizing input to such deposits on Mars, where an ancient, cold, alkali-rich ocean is postulated to have been conducive to the formation of ikaite. The rock recently imaged by the Spirit rover, nick-named “Sushi” show features strongly resembling glendonite crystals. Moreover, the peculiar condition of formation of ikaite could be relevant to astrobiological searches at other locales in the Solar System, e.g., within icy moons, such as Europa and Enceladus.