[meteorite-list] Researchers Push Back Dates of First Life on Earth

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Mar 6 11:20:04 2006
Message-ID: <200603061618.k26GI3817881_at_zagami.jpl.nasa.gov>

http://deseretnews.com/dn/view/0,1249,635189628,00.html

Researchers push back dates of first life on Earth
By Joe Bauman
Deseret Morning News
March 6, 2006

      In the last few years, scientists have discovered that the early
Earth cooled much faster than had been believed earlier, a finding that
could affect our understanding of how quickly life appeared.
      John W. Valley, professor of geology and geophysics at the
University of Wisconsin, Madison, will speak on discoveries by himself
and others, Wednesday at the Frontiers of Science Lecture, University of
Utah. The free public talk will begin at 7:30 p.m. in the Aline Wilmot
Skaggs Biology Building Auditorium.
      Earth formed about 4.5 billion years ago as the solar system was
condensing from interstellar gases, according to a press release from
the University of Utah. At first "it was an inferno of boiling metals,
minerals and gases.
      "Red-hot oceans of molten rock, massive meteorite strikes and even
brimstone in the atmosphere justify calling this time period the
Hadean." The word refers to Hades, a biblical term for the underworld.
      Before life could appear, the atmosphere had to cool enough for
liquid water to form. The question is when that happened.
      The earliest known fossils are dated in deposits from 3.5 billion
years ago, the release adds. Earlier evidence from geology "placed the
boundary between the Hadean and the subsequent Archean Eon at about 3.8
billion years ago."
      Now Valley and colleagues have pushed back the era when the
environment was cool enough for life by nearly half a billion years.
      "Life could have formed 400 million years earlier than we thought
previously," Valley said in a telephone interview from his home in
Wisconsin.
      The discovery was made by examining zircon crystals from western
Australia, dating to 4.1 billion and 4.4 billion years old. This is a
highly stable mineral, retaining elements from the time it formed in
molten rock.
      Although that rock was itself exceptionally hot, it was formed of
matter that had cooled earlier, and the record of the cooling is
retained in such information as the ratio of certain oxygen ions.
      "I've been interested in ions for about 12 years, from all
different ages and different localities from around the Earth," Valley
said. He and a then-graduate student, William Peck (now an assistant
professor at Colgate University, Hamilton, N.Y.), "hypothesized it would
be interesting to analyze the oxygen isotope ratio of the oldest oxygen
we could find on earth."
      These ratios provide information about the temperature at which
minerals formed.
      "I had attempted several times to get old zircons from people,
with no success," until a "very serendipitous meeting" at a conference
in Beijing in 1998, where he met Simon Wilde.
      Wilde, another researcher, had been interested in ancient zircon
crystals for a long time. "I asked Simon if he had any of the 4.1
billion-year-old zircons that had been reported from western Australia,"
he said.
      Wilde replied that he didn't have any that had been dated, but he
had the sample they came from and thought he could locate another.
      He provided several tiny crystals from a conglomerate sample. A
determination of their ages showed most were 4.1 billion years old,
while one was dated to 4.4 billion years.
      Wilde sent zircon samples encased in epoxy. Peck and Valley took
them to Edinburgh, Scotland, where they used an Ion Microprobe to
measure the oxygen isotope ratios in the minute crystals.
      "We discovered that the oxygen isotope ratio was distinctly
different from what we expected, and indicative of a low-temperature
prehistory of the zircons." By prehistory, he meant the material that
was present before the igneous rock melted.
      "That was a total shock. We never expected this."
      The ratios "meant liquid water" was present. To be liquid, it had
to be below the boiling point ??? that is, not from the scorching Hadean
period.
      Of the five samples the researchers had, three had elevated
isotope values, meaning formation when water was present. Since then,
nearly 200 ancient zircon crystals have been analyzed "and many of them
have these elevated oxygen isotope ratios."
      Conditions under which life could have survived, Valley said,
"existed at 4.2 billion years ago."
Received on Mon 06 Mar 2006 11:18:03 AM PST