Cecilia Howard Dissertation Defense

Microbial ecosystems have shaped and been shaped by Earth’s environments since the origin of life, and microbial sediments and rocks (“microbialites”) can both inform our understanding of the past and complicate our records of it. The sediment record of microbial ecosystems has the potential to preserve information about past climate, biology, mechanics, and more. However, separating individual processes from the complex amalgamation of information that typifies microbial sediments is a continuing challenge for microbial systems from the Archean to the modern. This dissertation investigates the impacts of environmental variations on microbial records at a range of spatial and temporal scales.
In Chapters 2 and 3, I focus on early records of life in the Archean and Paleoproterozoic, prior to the evolution of multicellularity. In Chapter 2, I use a literature review to determine how microbialite depositional environments change across nearly two billion years of the Archean and Paleoproterozoic. This chapter presents the first broad dataset to consider marine and tidal microbialites separately and also reveals the consistent presence of terrestrial microbialites from the earliest records of life. I find that the majority of microbialites formed in tidal environments and the proportion of terrestrially influenced microbialites increased during periods of craton development, suggesting that terrestrially derived nutrients were essential to early life. In Chapter 3, I use microCT scanning to measure and reconstruct 3.48 Ga microbialites, among the earliest accepted evidence of life. These measurements, along with a compilation of carbon and sulfur isotope data, suggest that the microbialites were formed by metabolically diverse communities in a high flow tidal environment, consistent with modeling of Archean tides.
In Chapter 4, I investigate microbialites from a lake in the hothouse climate of the Early Eocene (~50 Ma) using a mixture of morphological and geochemical analyses. This chapter considers how spatial and temporal differences influence microbialites, looking at single beds over 10–20 km distances and samples spanning 3 Ma. I find that microbialite morphology and chemistry records a mixture of large-scale information such as temperatures consistent with a hothouse environment and local conditions such as spring or stream influence and sediment sources, which manifests as lateral variability within beds. Additionally, comparison of atmospheric carbon dioxide reconstructions based on preserved carbon in the microbialites to past estimates supports low to moderate microbial growth rates throughout this time period.
In Chapter 5, I consider how sediments in a modern microbial ecosystem are influenced by climate change using a ten-year timeseries of sediment carbon and nitrogen data from an anoxic sinkhole in Lake Huron, which hosts a diverse microbial mat ecosystem. I integrate this sediment data with climate and lake chemistry parameters, finding that changes in ice cover lead to differences in sediment carbon in the following year. These results suggest that decreasing ice cover in the Great Lakes could lead to rapid but potentially ephemeral effects on sediment carbon.
The results in this dissertation exhibit the array of spatial and temporal scales at which microbial-environmental interactions occur, from local effects such as groundwater altering microbialite chemistry or carbon preservation to global climate and tectonics driving microbialite distribution. These findings can provide a framework for understanding the interactions between microbial ecosystems and their environment and emphasize the importance of environmental context for understanding microbialite records.