INFLUENCE OF IMPACTS ON THE DEEP SUBSURFACE BIOSPHERE – PRELIMINARY RESULTS FROM THE ICDP-USGS CHESAPEAKE BAY IMPACT STRUCTURE DRILLING PROJECT

Asteroid and comet impact events are known to cause profound disruption to surface ecosystems. The aseptic collection of samples from a 1.8 km core recovered the deep subsurface of the Chesapeake Bay impact structure has allowed, for the first time, the study of the influence of asteroid and comet impacts on microbial communities in the subsurface biosphere. Microbiological enumeration and culture and culture-independent methods coupled with geochemical data suggest the presence of three major microbiological zones. The upper zone (0-700 m) is characterised by a logarithmic decline in microbial numbers from the surface through the post-impact section of upper Eocene to Pliocene marine sediments and Pleistocene non-marine sediments and across the transition into the upper layers of the impact tsunami resurge sediments and sediment megablocks. The impact-mixing of diverse lithologies within these crater-fill sediments has created a geochemically heterogeneous substrate at many scales. The presence of organisms in this zone is confirmed by microscopy and both culture and culture-independent methods. The middle zone (700-1400 m) corresponds to the region that has low hydraulic conductivity and in its deeper sections may have been sterilized by the thermal pulse delivered during impact. Lack of culturable organisms, no extractable DNA and microbiological enumerations below the limits of detection throughout the middle zone show a biologically impoverished environment, which may be sterile in some locations. The lowest zone (>1,560 m) coincides with a region of heavily impact -fractured, hydrologically conductive target schist/pegmatite in which microbial cell numbers are higher than the middle zone and heterotrophic organisms have been cultured. These results support the hypothesis that impact events cause disruption to the subsurface biosphere that can result in well-defined zones of microbiological colonization linked to the process of impact cratering. This work advances our understanding of the characteristics and evolution of the deep subsurface biosphere through time.