Oxygen Production in Archean Stromatolites

When did oxygenic photosynthesis evolve?

Whether oxygenic photosynthesis evolved 2.4 billion years ago (Ga) or earlier remains a contested topic in geobiology. Using an NSF Graduate Research Fellowship, I studied 2.7 Ga lacustrine stromatolites with abundant bubble fenestrae formed from microbial metabolisms. Using geochemistry, petrography, and metabolic modeling, I concluded that oxygen was the most likely gas to produce fenestrae, placing morphological evidence for oxygenic photosynthesis 300 million years before the Great Oxidation Event.

Techniques Used

1) sedimentology and stratigraphy of lacustrine stromatolitic cherts from Hartbeesfontein, South Africa

2) petrography, micro-scale XRF and Raman analysis of detailed bubble fenestrae

3) modeling metabolic rates in Archean mats using parameters from modern microbial communities and ancient stromatolites

Carbonate Precipitation in Hydrothermal Mats

Is carbonate precipitation in mats controlled by biology, abiotic chemistry, or both?

A long-standing question in microbialite research is whether carbonate precipitation is induced by certain metabolic processes, or by surrounding environmental chemistry. Based on incubation experiments from the 2015 International Geobiology Course, our group observed concurrent biogenic and abiogenic precipitation in mat layers, but found no correlation with autotrophy.

Techniques Used

1) field analyses of mat chemistry (temperature, pH, major elements, etc.) in Little Hot Creek, California

2) DNA sequencing and microscopic analyses of microbial communities

3) labeled incubation experiments to determine the relationship between autotrophy and carbonate precipitation

Bubble Preservation in Cambrian Oncoids

How are delicate bubble fabrics preserved in mobile oncoid grains?

Oncoids are detached microbialite grains which form by persistent motion and mineral precipitation in agitated environments. In 2015, I published the first known desription of bubble fenestrae in oncoids (525 Ma, Bayan Gol Formation, Mongolia). The unique preservation of such delicate textures on wave-agitated mobile grains provides rare constraints on carbonate precipitation rates in ancient microbialites.

Techniques Used

1) petrographic analysis of oncoid thin-sections, including bubble fenestrae and microfossils

2) modeling oncoid growth rates using sedimentary and petrographic evidence

Diverse Microbialites in Mesoproterozoic Lakes

How widespread were non-marine microbial communities in the Precambrian?

Precambrian lacustrine deposits are relatively rare and difficult to identify compared to marine environments, leaving a sparse record of early non-marine life. In 2013, I published the first description of clastic microbialites from the 1.1 Ga Midcontinent Rift System of North America. In 2015, I assisted in a re-description of stromatolites from the same formation, which showed alternating biogenic and abiogenic carbonate precipitation. Both studies provide an detailed snapshot of geobiology within a Proterozoic lake system.

Techniques Used

1) sedimentology and stratigraphy of lacustrine stromatolites and clastic microbialites from the Copper Harbor Conglomerate, Michigan

2) petrographic and Raman analyses to determine the biogenicity of observed structures