Autochthonous Micrite to Aphanodolomite: The Microbialites in the Dolomitization Processes

Abstract

In the present paper, we examine the influence of micrite types, autochthonous or allochthonous, on the dolomitization processes. The recrystallized and dolomitized Carnian samples from Rifugio Vallandro and Alpe di Specie erratic boulders (South Tyrol, Italy) offer a unique example for studying the relationship between microbialites and dolomitization processes. The comparison between the carbonates of the well-preserved erratic boulders of Alpe di Specie and the isochronous, recrystallized, and dolomitized, samples of Rifugio Vallandro, allows for hypothesizing the role of microbialites on dolomitization processes. The Rifugio Vallandro samples represent variously dolomitized boundstone (made of corals, sponges, and peloidal crusts) with a fine texture (aphanodolomite) which contain organic matter relics, suggesting microbial-mediated mineralization. Geomicrobiological characterization of the microbialites from Alpe di Specie indicates that they formed through microbial metabolic activity of sulfate-reducing bacteria, which thrive on organic matter accumulated in the suboxic to anoxic interspaces of the skeletal framework. Similar processes can be hypothesized for the microbialite precursor of Rifugio Vallandro. Extracellular polymeric substance (EPS) and other organic compounds trapped inside the fine crystal matrix can have a role in the dolomitization processes of the microbialites. High pH and high alkalinity, derived from the degradation of organic matter, may be critical in promoting the dolomitization of microbialites because the high pH increases the concentration and activity of the dissolved CO32−, thereby increasing the dolomite supersaturation and reaction rates. This process produces very fine dolomite (aphanodolomite) that replaces the original organic-rich micrite, while the fine crystalline dolomite forming larger euhedral crystals seems to derive from the allochthonous micrite due to the presence of a large amount of siliciclastics and the absence of organic remains.

Preservation of organic carbon in dolomitized Cambrian stromatolites and implications for microbial biosignatures in diagenetically replaced carbonate rock

“…The composition of dolomitizing fluids can be constrained by the relative abundance of major and trace elements in dolomite, including the Mg/Ca ratio, Fe, Mn, Zn, Sr, and Si concentrations (Morrow, 1982;Tucker and Wright, 1990;Gasparrini et al, 2006;Zhang et al, 2009;Guido et al, 2018). The Si concentrations detected within the dolomite are not well understood and are rarely reported in literature.…”

Carbon isotopic composition of Frutexites in subseafloor ultramafic rocks

“…The Raman analyses of the MS showed multiple peaks coinciding with carbonaceous matter (CM), which may have been derived from decomposed biofilms, but that is not possible to discern using 514 nm Raman spectroscopy. The D and G bands are usually used to detect CM of geological samples of various origins, such as microbialites, Frutexites and older fossils (Guido et al 2018(Guido et al , 2019Allwood et al 2006;Greco et al 2018). Close to the vein walls, black mineral structures roughly resembling the MS, were all composed of pure magnetite, likely a product of serpentinization (Fig.…”

Carbon isotopic composition of Frutexites in subseafloor ultramafic rocks

“…The Raman analyses of the MS showed multiple peaks coinciding with carbonaceous matter (CM), which may have been derived from decomposed bio lms, but that is not possible to discern using 514 nm Raman spectroscopy. The D and G bands are usually used to detect CM of geological samples of various origins, such as microbialites, Frutexites and older fossils (Guido et al 2018, Allwood et al 2006, Greco et al 2018. Close to the vein walls, black mineral structures roughly resembling the MS, were all composed of pure magnetite, likely a product of serpentinization (Fig.…”