Doctoral Dissertations

Date of Award

8-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Linda C. Kah

Committee Members

Christopher M. Fedo, Andrew D. Steen, Steven W. Wilhelm

Abstract

Meso- to Neoproterozoic peritidal carbonate strata commonly contain early diagenetic chert deposits that can record evidence of mineral and organic precursors. Exceptional morphologic preservation can be used to place time constraints on silicification, often indicating silicification is penecontemporaneous with mat growth. Silica-rich sedimentary deposits have also been suggested as potential targets for the search for extraterrestrial life on Mars. The chapters in this dissertation investigate fluid compositions capable of precipitating primary and secondary silica phases and the extent to which microorganisms are preserved, specifically their morphology and chemistry, by early silicification. Twenty potential fluid compositions were modeled using PHREEQC, and the mineral phases capable of precipitating were compared to the mineralogies observed in thin sections from silicified microbial mats of the Angmaat Formation, Bylot Supergroup. This model indicates that silica phases were capable of precipitating as primary or secondary mineral phases over a wide range of fluid chemistries. However, only brine solutions were capable of precipitating each mineral phase observed in the Angmaat Formation samples, including amorphous silica and silica gel.Hydrous silica phases permeate the sheaths of the organisms, which preserves the individual microorganisms and the complexity of microbial mats. Mat morphologies in the Angmaat Formation preserve a range of taphonomic states, including well-preserved mats that are interpreted to reflect the top layers of mats, to poorly-preserved mats that may reflect buried layers in the mat that were exposed to greater heterotrophic degradation. A taphonomic assessment of the individual microfossils reveals preserved complexity within the microbial mats and a new taphonomic assessment technique is introduced to account for the complexity. Regardless of taphonomic grade, coccoidal microfossils exhibit better preservation than filamentous mat-building organisms. Differential preservation may be related to the motility of the organisms.Organic geochemical characterization of the silicified mats indicates that despite exceptional morphologic preservation, lipids are not preserved in early diagenetic chert. Lipids, specifically hopanes and steranes, could be used to elucidate microbial processes active at the time of silicification and paleoenvironmental conditions. The lack of lipid preservation suggests that silica-rich low-temperature deposits may not be ideal targets for the search of ancient extraterrestrial life.

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