BIOGEOCHEMICAL EVIDENCE FOR IN SITU MICROBIAL METABOLISM IN THE CHESAPEAKE BAY IMPACT STRUCTURE

From a microbiological perspective the significant outcomes of the Chesapeake Bay Impact event were a potentially biologically sterilizing pulse caused by the impact which potentially “reset” the subsurface microbial communities, a reorganization and restructuring of lithological units potentially providing new habitat, and an inundation of seawater and terrestrial materials that potentially introduced fresh electron acceptors and donors. Because of the very limited movement of groundwater in and above the crater, detailed chemistry profiles should provide a record of both the biotic and abiotic influences on groundwater since the time of impact. Subcores were collected during the ICDP-USGS drilling project in the Chesapeake Bay Impact Crater near Cape Charles, Virginia in the fall of 2005 for the purpose of analyzing pore waters and sediments for evidence of microbial metabolism, i.e., the distribution and behavior of suitable electron acceptors and donors. The pore waters were analyzed for ammonium, nitrate, sulfate, total dissolved organic carbon (DOC), aromatic components of the DOC and methane. The solids from each sample were also analyzed for Fe2+,, Fe3+, and total carbon and nitrogen. Nitrogen compounds were low or undetectable and sulfate was generally high throughout the crater. DOC concentrations were highest in the post-impact sediments and lowest in the tsunami breccia. In addition, the biodegradability of the DOC was assessed using excitation/emission matrix spectra (EEMS) to quantify and characterize the aromatic components of the DOC. EEMS results varied with depth but were consistent within similar lithologic units and showed that the bioavailability of carbon did not correspond to the concentration of DOC. Methane was detected at depths above and below the post-impact sediment/breccia contact and is likely biogenic. Reduced iron was detected in almost all of the samples with the highest concentrations associated with the schist pegmatite material with generally correspondingly low concentrations of Fe3+. These features suggest a potential microbial involvement in the subsurface geochemistry and thus support the possibility of microbial metabolism in this deep subsurface environment.