Semester

Summer

Date of Graduation

2021

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Geology and Geography

Committee Chair

Kathleen C. Benison

Committee Member

James C. Lamsdell

Committee Member

Christopher J. Russoniello

Abstract

Modern shallow brine environments contain an abundance of microorganisms. Microorganisms, mineral crystals, and air bubbles can be trapped with those brines in primary fluid inclusions in bedded halite as it grows in shallow saline surface waters. Primary fluid inclusions and their contents can be preserved, unaltered, for millions of years.

The ~830 million-year-old Browne Formation of central Australia contains intervals of bedded halite with unaltered primary fluid inclusions. Intervals of bedded halite were sampled in the Empress 1A core from ~1480 m – 1520 m depths. Here, I used petrography to describe suspect microorganisms and organic compounds that were trapped in primary fluid inclusions at the time the halite was precipitating in the Neoproterozoic. This is the first study to focus on suspect organic material in primary fluid inclusions in the Browne Formation, and the first study to look for microorganisms in halite of this age.

Suspect microorganisms were generally classified as either suspect algae or suspect prokaryotes based on their optical properties, including size, shape, color, and response to UV-vis light. Suspect prokaryotes were the most common suspect microorganism in Browne Formation halite. They are generally ~0.5 - 1 µm cocci, are bright with high relief, and can appear white or pale orange to pale blue when viewed under plane transmitted light. They exhibited pale yellow or white fluorescence under UV-vis light. In contrast, suspect algae are 3 - 5 µm spheres that are pale orange, pale yellow, or clear. Many have a thick, dark outer boarder or a dark dot in the center. Some appear to have a dimpled surface texture, while others have a ‘halo’ of a viscous liquid, such as glycerin or beta-carotene. Suspect algae fluoresce white to gold or pale red under UV-vis light. Suspect organic compounds occur as viscous liquid halos around suspect algae and air bubbles. They also appear as irregular, blobby shapes that are colorless to pale yellow. They may fluoresce pale white, pale blue, pale yellow, or pale red and are often observed in the same fluid inclusions as suspect microorganisms.

Fluorescent response was not ubiquitous in all suspect organic material and therefore should only be considered as supporting evidence that a given suspect is organic in origin. The discrepancy in may be due to age and/or burial depth. Future work should utilize laser Raman spectroscopy to chemically characterize suspect organic material.

In addition to suspect organics, primary fluid inclusions also contained accidental daughter crystals. Common daughter crystals are clear acicular to tabular/platy minerals, which are likely gypsum/anhydrite. Other, rare daughter crystals have an unknown composition. Minerals were also observed as solid inclusions in halite. Mineral inclusions may represent crystals co-precipitating in shallow saline waters or detrital grains transported by wind.

This study shows that microorganisms and organic compounds can be preserved for hundreds of millions of years in primary fluid inclusions in halite. These prokaryotic and algal cells, found together with organic compounds in remnant Precambrian surface brines, demonstrate exceptional microfossil preservation for a long geological time period. These results from the Neoproterozoic Browne Formation also have implications for the detection of potential biosignatures on Mars. Any chemical sediments returned from Mars should use optical methods for preliminary investigations.

Embargo Reason

Publication Pending

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