Microbial And Metazoan Influences on Microbialite Growth Structures: Insights from Recent Lacustrine Microbialites in Pavilion Lake, BC, and Cambrian Thrombolites from the Great Basin, CA and NV

Microbialites provide a record of microbial ecosystems and sediment-organism interactions through much of Earth history, with abundant microbialites in Precambrian carbonates and microbialites forming today in a wide range of environments. Interpreting the geologic record of microbialites requires understanding how they reflect microbial communities and metazoan interactions as well as post-depositional and diagenetic processes. To better understand how microbialite fabrics and textures reflect ancient ecosystems, I investigate how microbial communities influence mineral fabrics and microbialite growth (Chapter 1) and how metazoans interactions are preserved and expressed in microbialite fabrics (Chapter 2). Finally, I evaluate microbial and metazoan influences on diverse thrombolite textures (Chapter 3). To evaluate how microbial communities influence mineral fabrics and microbialite growth, I characterized microfabrics in modern lacustrine microbialites in Pavilion Lake, BC, that were actively mineralizing under the influence of photosynthetic communities. The suite of microfabrics in Pavilion Lake microbialites demonstrates that microfabric variations can reflect differences in lithification processes as well as microbial community morphology and spatial arrangement. Even when communities are not well preserved in microfabrics, the community arrangement is captured in microbialite growth structures. Some microbialite growth structures can be influenced by metazoans when microbial and metazoan communities co-occur. To investigate how microbial-metazoan interactions are preserved and expressed in microbialite fabrics, I documented Cambrian stromatolites, thrombolites, and dendrolites that grew in shallow marine environments (Great Basin, California and Nevada) closely associated with metazoan burrowers and grazers. Cambrian microbialites preserve evidence of microbial processes and microbial-metazoan interactions in different ways, and include microbial growth structures that formed without an obvious influence of metazoans, growth structures that were obscured by syn-depositional disruption, and examples where neither the growth structure nor features indicative of metazoan interactions could be definitively identified. I propose criteria for distinguishing the degree to which the growth structure is captured in the microbialite fabric versus overprinted by metazoan interactions. Both metazoan interactions and microbial growth have been previously suggested as the dominant influence on thrombolites, microbialites with clotted or patchy fabrics. To evaluate the role of microbial and metazoan influences versus environmental and diagenetic processes on thrombolite origins, I systematically characterized thrombolites in Cambrian carbonates (Great Basin, California and Nevada), and identified eight thrombolite classes based on their internal textures. Several thrombolite classes contain distinct growth components that reflect variable styles of microbial growth, but other thrombolites lack a distinct growth structure and reflect bioturbation of an originally clotted or patchy growth structure. The suite of Cambrian thrombolites demonstrates that thrombolites are much more diverse than has been widely appreciated, both in their internal fabrics and in how they form. Overall, results from modern lacustrine microbialites and Cambrian microbialites indicate that interpreting the geologic record of microbialites requires identifying features that reflect microbial growth, variations in lithification, and metazoan interactions. Teasing apart these various influences requires detailed observation at multiple scales, and is essential for interpreting patterns in microbialite ecosystems throughout Earth history.