Microbial communities and their primary to early diagenetic mineral phases; the record from Neoproterozoic microbialites of Qarn Alam, Oman

Abstract

The 10 m-scale shallowing-up Neoproterozoic carbonate cycles at Qarn Alam (Oman) provide a record of microbial textures and the communities responsible for them. This is documented for four major microbialite facies. Despite their age, these microbialites show extremely fine preservation of microbial fossils and mineral associations (primary calcite and dolomite with minor phosphate, glauconite, palygorskite, hematite and goethite) and they are the record of a suite of microbial communities, from pellicular biofilms (planar laminites) to mats and gels (crinkly laminites, and stromatolitic layered and massive thrombolites) and a more complex microbial community in bushy thrombolites possibly involving a sponge-like form. Mineralized extra-cellular polymeric substances (EPS) resembles that of modern microbial mats. The mineral associations, as well as cathodoluminescence attributes, indicate oxic to suboxic conditions during deposition and early diagenesis for planar laminites and crinkly laminites, but more evaporitic to saline conditions during development of thrombolites of the upper part of a cycle. Early cementation under variable redox conditions sealed the organomineralized phases.

Evidence of a Role for Aerobic Bacteria in High Magnesium Carbonate Formation in the Evaporitic Environment of Dohat Faishakh Sabkha in Qatar

“…The dumbbell and spherical morphologies of the minerals formed have been recognized as microbially induced carbonates in previous reports (Sánchez-Román et al, 2008;Mckenzie and Vasconcelos, 2009;Bahniuk et al, 2015;Mettraux et al, 2015). The co-existence of hydromagnesite and spheroidal dolomite suggests that both may be indicators of microbial dolomite, as previously suggested by Sánchez-Román (2006).…”

Microbial carbonates in space and time: introduction

“…Many of the papers in this volume address this question and windows into past microbial mats and their associated precipitates are occasionally preserved (taphonomic windows) as far back as the Neoproterozoic (Mettraux et al 2015). Such rare studies as this indicate the similarities between ancient and modern microbial mats that have driven actualistic studies, many of which are also presented in this volume (e.g.…”

“…Rare exposures of these Neoproterozoic microbialites are found in surface-piercing diapirs in Oman (Homewood et al 2013;Mettraux et al 2015). These reveal 10 m scale laminate -stromatolite-thrombolite -exposure surface shallowingupward cycles but, more significantly, contain remarkably well-preserved microbial textures, early mineralization, microfossils and what appear to be mineralized microbial mats.…”

What do we really know about early diagenesis of non-marine carbonates?

“…In parallel with the challenge to define and recognize primary fabrics, the search for criteria to distinguish sedimentary precipitates that result solely from physicochemical processes and those that formed in relation to biota is raised (Freytet and Verrecchia, 1998;Sugitani et al, 2007;Oehler et al, 2009;Wacey, 2009;Jones, 2010;Knoll et al, 2013;Mettraux et al, 2015;Nutman et al, 2016). Early diagenesis may strongly complicate the determination of the biogenecity of a fabric (Brasier et al, 2015).…”