The Role of Biological Processes in Modem Microbialite Morphogenesis: A Field and Experimental Approach to the Stromatolitic/Thrombolitic Subtidal Mounds near Lee Stocking Island, Bahamas

Degree Year

2002

Document Type

Thesis - Oberlin Community Only

Degree Name

Bachelor of Arts

Department

Geology

Abstract

Stromatolites dominate the fossil record for 3.5 billion years and therefore provide a powerful key to understanding the interactions between life and the environment on early Earth and throughout most of geologic time. However, to accurately use stromatolites as indicators of paleoenvironment, as windows into organismal evolution, and as markers of life itself we must first understand stromatolite morphogenesis. Biological processes may play an important role in first understand stromatolite morphogenesis. Biological processes may play an important role in morphogenesis, but an understanding of this role is complicated by the lack of interpretable, preserved information concerning the complex microbial community that once inhabited the stromatolite. The discovery of modem structures resembling stromatolites provides the opportunity to investigate the relationships between microbial communities, the environment, and the potentially preservable morphology and texture of the stromatolite mound. Field and laboratory methods were employed to learn more about the relationships between microbial community and textures on and within microbialite mounds in a tidal channel off of Lee Stocking Island, Bahamas. Nine cyanobacterial genera and sixteen morphotypes were brought into culture and isolated, indicating the presence of a rich cyanobacterial community on these mounds. The relative distance from each mound surface to the sediment/water interface changes as migrating dunes bury and expose the mounds. Evidence from surficial cores and surface observations indicates that this change in sedimentation and exposure drives a community succession in which coccoidal cyanobacteria and metazoan encrusters increase in diversity and abundance with increasing height above the sediment/water interface. A smaller scale community succession may also take place as different types of lithifying communities may be able to establish themselves within centimeters of one another on the same surface due to different microenvironments created by topography and encrusters. Different lithifying mats on the same mound surface and internallithified textural data suggest that thrombolitic and stromatolitic fabrics may form within the same surface and may be heavily influenced by the presence of metazoan encrusters. The Lee Stocking Island mounds may therefore be more accurately defined as microbialites with interspersed stromatolite and thrombolite fabrics. The surface data behind the insight into the stromatolite/thrombolite gradient, as well as observations of Pleurocapsan boring traces suggest that connections can be made between modem surface textures and internal mound structure.

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