Advisors: Melissa E. Lenczewski.Committee members: Justin Dodd; Mike Konen.The Antarctic Geological Drilling (ANDRILL) researchers obtained a rock core from below the sea-floor in South McMurdo Sound (SMS), Antarctica. During the core retrieval, approximately 5.6 x105 liters of drilling fluid was lost in the subsurface. The loss of this fluid, which contained biodegradable components, may significantly alter the in situ microbial ecology as well as the geochemistry of the subsurface. The introduction of drilling fluid into the subsurface can be viewed as a source of contamination. However, it also provides the opportunity to indirectly study the microbial communities of the subsurface through analyses of the return drilling fluids. This study focused on the differences between new (uncirculated) fluids and the return drilling fluids circulated at depth, along with the chemical and microbial changes that took place during the biodegradation of drilling fluid. Drilling fluid samples were incubated for forty days at 4, 20 and 50°C representing the in situ temperatures, under aerobic and anaerobic conditions. Geochemical analyses were completed and microbial community fingerprints were constructed and compared. The return fluids show significant differences in the geochemistry compared to the new fluids specifically in regards to the ions Barium, Thallium, Lead, Manganese, Iron, and Boron. The return drilling fluids have higher DNA concentrations than the new fluids counterparts within the first four days of the biodegradation experiment. The use of Biolog Ecoplates(TM) revealed that microbes in the return fluids utilized significantly less carbon substrates than those in the new fluids. RAPD-PCR results showed the return fluids exhibited significantly less diversity and species evenness than the microbes in the new fluids. DNA concentrations fluctuated over time, with a direct relationship among fluid type. There are shifts in the community structure and function as biodegradation occurs that varies between the different incubation environments. In situ temperatures ≤4°C were recorded for the top ~21 meters of the borehole. In anaerobic environments and temperatures ≤4°C, slower microbial metabolism may increase the residence time of the biodegradable drilling fluid components, allowing these components the opportunity to penetrate further into the surrounding formation through conduits. The 20°C temperatures recorded at ~360-~384 mbsf may be the most favorable conditions for the biodegradation of the drilling fluids allowing the least amount of migration of the biodegradable components into the surrounding formation. At a depth of ~1026 mbsf, 50°C in situ borehole temperatures were recorded. From this depth through to the final depth of ~1139 mbsf and 57°C, the higher temperatures may result in smaller microbial populations that are able to metabolize the drilling fluid, leading to further drilling fluid migration into the surrounding formation.M.S. (Master of Science
