(Micro)morphological, inorganic–organic isotope geochemisty and microbial populations in endostromatolites (cf. fissure calcretes), Haughton impact structure, Devon Island, Canada: The influence of geochemical pathways on the preservation of isotope biomarkers

Abstract

Endostromatolites (cf. fissure calcretes), which possess microbial evidence for a biogenic origin, are also thought to preserve isotopic biosignatures. In this study, a multi-proxy approach combining (micro)morphological, geochemical and isotopic analyses of middle Holocene age endostromatolites within sub-horizontal fissures in dolomitic limestone outcrops in the Haughton impact crater region (Devon Island, NU) was used to determine their origin (abiotic versus biogenic) and to identify potential isotope biosignatures. The micro-morphologies of the endostromatolites revealed some structures typical of a physico-chemical origin, whereas the presence of rod-shape particles and filamentous structures was more reminiscent of biologically-induced forms. The endostromatolites have δ 13C and δ 18O compositions reaching maximum values of 7.2‰ and − 11.2‰, respectively. Positive relations between the elemental (Mg, Sr) and isotopic ( δ 18O and δ 13C) composition of the endostromatolites are indicative of an evaporative enrichment process of the meteoric water infiltrating the fissures prior to calcite precipitation. However, the positive δ 13C TOC– δ 13C CaCO3 relation in the endostromatolites is strongly indicative that they were microbially-mediated. In support of a microbial origin, mostly aerobic heterotrophic bacteria that have been linked to both carbonate dissolution and mineralization were observed in the microbial diversity of the endostromatolites. However, the results are inconclusive to attribute the formation of the endostromatolites solely to a biologically-induced mineralization, but instead, favor a more complex origin that involved abiotic (evaporation), and to some extent, biological processes prior to and during calcite precipitation. Considering that the endostromatolites result from microbially-influenced mineralization, the effects of physico-chemical processes on the geochemical and isotopic composition of the endostromatolites were much greater than the effect of the biological processes. Thus, it seems that preservation of isotopic biosignatures in secondary carbonate precipitates is dependent on the sequence of processes generating chemical and isotopic modifications of the solution prior to calcite precipitation, the mechanism and condition (equilibrium or kinetic) of formation and post-fossilization alteration processes.