BIOSPHERIC CONSEQUENCES OF THE CHESAPEAKE BAY BOLIDE IMPACT

Biostratigraphic and chemostratigraphic studies of sediment cores document biospheric effects from the Chesapeake Bay impact: (1) local effects near the impact site (mid-Atlantic continental shelf of late Eocene North America); and (2) far-field, possibly global, effects, resulting from atmospheric perturbations. Seven continuously cored boreholes in and near the Chesapeake Bay crater provide a variety of paleoenvironmental data, especially microfossil assemblages and stable isotope (del ¹⁸O and del ¹³C) measurements. The initial postimpact deposit is a dead zone, a thin (~20-cm), laminated, sand-silt-clay unit, lacking indigenous microfossils. During accumulation of the dead zone, deleterious bottom conditions precluded development of normal marine biota. Estimates of the duration of these hostile conditions (derived from sediment accumulation rates) range from <1 ky to ~36 ky. The dead zone is succeeded by a species-rich, deep marine (~300 m paleodepth) assemblage of microfossils, which characterizes the late Eocene Chickahominy Formation. The impact site was quickly repopulated after suitable conditions returned, and resultant high-diversity microfossil assemblages contain the same species that occupied the site prior to impact. No significant biotic turnover or mass extinction appears to have been caused by the impact. On the other hand, long-term, far-field effects have been inferred from changes in fossil assemblages and stable isotopes, both at Chesapeake Bay and at many other terrestrial and marine locations. The impacts at Chesapeake Bay, Popigai (Northern Siberia), and, possibly, Toms Canyon (New Jersey Continental Shelf) may have been part of a comet shower that accelerated accumulation of ³He on Earth during the final ~2 my of the late Eocene. The postimpact del ¹⁸O record from calcareous microfossils in two different cores at Chesapeake Bay is consistent with such a scenario. Three warm intervals of global climate can be identified between the top of the dead zone and an early Oligocene mass extinction event, which is associated with abrupt global cooling. The Chesapeake Bay impact and its companion impacts may have produced a delayed, but crucial stress on the early Oligocene biosphere, by warming the late Eocene and, thereby, steepening the gradient of the early Oligocene temperature drop.