Chesapeake Bay Impact Analysis

From the far reaches of the solar system, a giant asteroid or comet plunged into Earth near what is now the Chesapeake Bay 35 million years ago (shown in this artist's conception). Credit: © The Virginian-Pilot.

From the article...

The true impact of an asteroid or comet crashing near the Chesapeake Bay 35 million years ago has been examined in detail for the first time. The analysis reveals the resilience of life in the aftermath of disaster.

The impact crater, which is buried under 400 to 1,200 feet (120 to 365 meters) of sand, silt and clay, spans twice the length of Manhattan. The sprawling depression helped create what would eventually become Chesapeake Bay. About 10,000 years ago, ice sheets began to melt and once-dry river valleys filled with water. The rivers of the Chesapeake region converged directly over the buried crater, according to the U.S. Geological Survey (USGS).

Gregory Gohn of the USGS and his colleagues analyzed samples from two deep holes drilled into the crater near its center.

"I think what we wanted to do is drill into the central part of the crater and get as long of a section as we could and understand the processes that put them in the order we found them," Gohn told SPACE.com.

Within seconds of the object's touchdown, rocks were flung high into the air. The force of the impact carved a colossal cavity and caused temperatures to skyrocket, turning brittle rocks into taffy. Then, material along the cavity's rim surged downhill into the bowl-shaped depression like an avalanche.

The extreme heat, the researchers say, killed off most life. However, they found abundant microbes living today in the deepest parts of the crater. Some of the ancient bacteria would have survived the impact, the researchers say, because their little hideouts didn't feel the brunt of the heat. The rest of the abundant and newly discovered microbial life is thought to have recolonized the zapped area possibly tens of thousands of years following the impact when temperatures dropped to habitable levels.

"The impact broke up and disaggregated all of these blocks of rock," said researcher Mary Voytek, a microbiologist at the USGS, "and that actually creates space for [the microbes] to colonize and it also creates new routes for water and material to move though, which is always good for bugs."

So a catastrophic event like this could actually be a boon to microbes, at least in the long-run, Voytek said. The impact breaks up compacted rock to create nooks and crannies for bacteria to reside in, and it also brings in a fresh supply of food.

"It's somewhat analogous to whale falls," when a whale carcass eventually settles on the sea floor, Voytek said. "All of a sudden it's a restaurant for these bugs."

Understanding the biological effects of this asteroid impact will shed light on the potential for life deep underground during Earth's Archaean period, 3.8 billion to 2.5 billion years ago, when impacts were more frequent than today. The results also have implications for predicting life in the deep biosphere on Mars.

"If we're going to find life [on Mars], everyone agrees a good place to look is in the subsurface," Voytek said.

The project, which is detailed in the June 27 issue of the journal Science, was funded by the USGS, NASA, the National Science Foundation, the Austrian Science Foundation and DOSECC Inc.

"Life Survived Catastrophic Space Rock Impact"
Jeanna Bryner
Space.com
26 June 2008

Link: Space.com article

-----

ABSTRACT FROM SCIENCE

Science 27 June 2008:
Vol. 320. no. 5884, pp. 1740 - 1745
DOI: 10.1126/science.1158708

Deep Drilling into the Chesapeake Bay Impact Structure
G. S. Gohn,1* C. Koeberl,2 K. G. Miller,3 W. U. Reimold,4 J. V. Browning,3 C. S. Cockell,5 J. W. Horton, Jr.,1 T. Kenkmann,4 A. A. Kulpecz,3 D. S. Powars,1 W. E. Sanford,1 M. A. Voytek1

Samples from a 1.76-kilometer-deep corehole drilled near the center of the late Eocene Chesapeake Bay impact structure (Virginia, USA) reveal its geologic, hydrologic, and biologic history. We conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact-melt creation and distribution, transient-cavity collapse, and ocean-water resurge. Comparison of post-impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long-term sedimentation patterns in the mid-Atlantic region. Salty connate water of the target remains in the crater fill today, where it poses a potential threat to the regional groundwater resource. Observed depth variations in microbial abundance indicate a complex history of impact-related thermal sterilization and habitat modification, and subsequent post-impact repopulation.

1 U.S. Geological Survey, Reston, VA 20192, USA.
2 Department of Lithospheric Research, Center for Earth Sciences, University of Vienna, Althanstrasse 14, Vienna A-1090, Austria.
3 Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ08854, USA.
4 Museum of Natural History (Mineralogy), Humboldt-University Berlin, Invalidenstrasse 43, Berlin 10115, Germany.
5 Centre for Earth, Planetary, Space, and Astronomical Research, Open University, Milton Keynes MK7 6AA, UK.

Link: Science Abstract