FOSSIL MICROBIAL SIGNATURES FROM IMPACT INDUCED HYDROTHERMAL SETTINGS; PRELIMINARY SEM RESULTS FROM THE ICDP-USGS CHESAPEAKE BAY IMPACT STRUCTURE DRILLING PROJECT

Impact cratering is a common and fundamental geologic process active throughout the history of all terrestrial planets. Impact-shocked rocks are a common feature of heavily bombarded planets in the earliest stages of prebiotic and biological evolution and it is possible that shock-shattered rocks were among the most common terrestrial habitats in the early stages of Earth and Mars evolution. The Chesapeake Bay impact structure formed about 35.3 million years ago on a multilayered target of seawater, sediments and rocks on the continental shelf (Horton et al. 2005, Poag 1997). The impact excavated a crater ~ 1km deep and nearly 40 km wide. Temperatures within the impacted and deposited materials may have reached over 400°C, peaking at about 10,000 years after impact. Associated elevated temperature conditions would have persisted for about 1 million years (Sanford 2003). Because the structure was covered and preserved by marine sediments, it provides an excellent opportunity to study fossil microbial traces preserved within geologically young, undisturbed post-impact sequences (at depths up to 1.77 km) with a well-developed hydrothermal overprint. A colony of fossil rod-shaped biomorphs associated with Fe and Al precipitates were detected at a depth interval of 1353.9 to 1641 m. The interval is comprised of dark, gray clay and sand in the lower part and suevites traversed by a pyrite/calcite vein the upper part. Association of bacterial activity and Fe and Al elements has previously been reported from anaerobic subsurface environments in relation with reoxidation of Fe2+, under circum neutral pH conditions (i.e. Grantham et al. 1997). Filamentous morphotypes were observed on the surfaces of calcite crystals from a vein sampled at 1353.9 m depth. The pyrite from the same vein was likely utilized as an anchor point by microbial forms. The microbes probably produced biofilms in order to anchor and protect cells from extreme environmental conditions. Precipitation of calcite and pyrite within veins indicates the presence of hydrothermal fluids that may have had high temperatures (~100°C to ~220°C, Horton et al. 2006). Further work is needed in order to verify biogenicity of observed communities by multiple datasets, and to confirm their syngenicity with the hydrothermal overprint.