Abstract
The complex nature of growth and decomposition in microbial mats results in a broad range of microbial preservation. Such taphonomic variability complicates both the description of microbial elements preserved within geologic materials and the potential interpretation of microbial biomarkers. This study uses a taphonomic assessment to explore the preservation of different microbial components within silicified microbial mats of the late Mesoproterozoic (~1.0 Ga) Angmaat Formation, Bylot Supergroup, Baffin Island. The Angmaat Formation consists of unmetamorphosed and essentially undeformed strata that represent intertidal to supratidal deposition within an evaporative microbial flat. Early diagenetic silicification preserved microbial communities across a range of environments, from those episodically exposed to persistently submerged. Here, we present the development of a new methodology involving the use of high-resolution image mosaics to investigate the taphonomy of microfossils preserved in these mats. A taphonomic grade is assigned using a modified classification that accounts for both the taphonomic preservation state (good, fair, poor) of individual microfossils, as well as the degree of compaction of the overall mat. We show that although various taphonomic states occur within each of the silicified mats, the overall taphonomic assessment differentiates between well-preserved mats that are interpreted to have been silicified during active growth, to highly degraded and compacted mats that are interpreted to represent preservation during later stages of biological decomposition. These data indicate that even small changes in the timing of silicification may have substantial implications on our identification of microbial biomarkers and, therefore, our interpretation of early Earth ecosystems.
Highlights
Microbial mats are highly resilient organosedimentary systems [1,2,3,4] in which mat communities are represented by a dynamic and complex ecosystem comprised of photoautotrophic, photoheterotrophic, chemoautotrophic, and heterotrophic populations [4,5]
600 microfossils were classified for each of the seven samples that were classified as representing a range of mesoscale mat fabrics (Table 1)
Island demonstrated the preserved complexity of microbial mats in the fossil record
Summary
Microbial mats are highly resilient organosedimentary systems [1,2,3,4] in which mat communities are represented by a dynamic and complex ecosystem comprised of photoautotrophic, photoheterotrophic, chemoautotrophic, and heterotrophic populations [4,5] These microbial communities commonly show a laminated structure that reflects the occurrence of functionally distinct groups of organisms responding to a combination of physical and chemical gradients, such as temperature and light availability, oxygen concentration, pH, and salinity [6]. Organic matter produced in the photosynthetic upper layers of the microbial mat provide a ready carbon source for heterotrophic organisms at deeper levels of the mat [10] These deeper mat regions can be dominated by populations of fermenters, sulfate reducers, metal reducing microbes, or methanogens, depending on the composition of pore fluids. With this view in mind, it is easy to envision that the biological and geochemical gradients within a microbial mat should be reflected in both the taphonomy of mat components and, potentially, in chemical signatures associated with preserved materials
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