Dina Bower (left), unidentified young astrobiologist, Jim Cleaves, Verena Starke--the next generation learning to look at ancient rocks |
Since the
early 1970s, scientists have been measuring the amount and nature of organic
carbon from almost 4 billion year old Precambrian rocks. Geologists were
searched the world over for older and older rocks resting exposed on the
surface that might contain evidence of the first signs of life. Greenland’s
coasts have a couple of small deposits of some of the Earth’s oldest rocks. For
example, the Isua formation was thought to have formed in a sedimentary
environment 3.85 billion years ago. In the 1980s, Cyril Ponneperuma and his
student Cliff Walters of the University of Maryland examined the organic
geochemistry of these rocks to find the evidence of the first living organisms.
Professor Ponneperuma wanted Cliff to
discover something revolutionary. Cliff struggled at the University of Maryland
to find any molecules that did not look like modern contamination, but his
professor pressured him to “discover” something big. Fortunately for Cliff, he
sought out the wisdom of Tom Hoering. Hoering’s reputation for careful,
exacting work was well known in the geochemistry community, particularly after
he debunked an earlier study on “Precambrian” hydrocarbons, which turned out to
be ink from the newspapers wrapping the rock specimens. Walters finally
concluded that any molecular signals in these samples were contamination. He
went on to become a very successful petroleum organic geochemist at Exxon
Mobil, having learned from Tom Hoering about stringent lab procedures.
A decade later, UCLA scientists,
including Stephen Mojzsis, used more sophisticated instrumentation to measure carbon
isotope signals directly in the rocks (i.e. in situ) with an instrument called an ion probe, a
multi-million dollar combination mass spectrometer and microscope. A beam of strong positive ions, charged
particles, bombard the polished surface of a rock sample. Elements from the
rock were sputtered off the surface then accelerated through a high vacuum
flight tube where they were separated and measured. The ion probe was promoted
as the solution to answering the question of whether the carbon in ancient
rocks was indigenous to the sample or was caused by contamination. The UCLA
group measured carbon isotope signals (Mojzsis et al., 1996) in Isua samples,
concluding that they were in the range of similar measurements from much
younger, firmly established Precambrian stromatolite samples.
The problem with the ion probe
measurements was that there were no comparable working standards. As time went
on, ion probe users realized they needed to be much more careful about how
their instruments were standardized. Dominic Papineau, a postdoctoral fellow at
the Geophysical Laboratory, compared “conventional” elemental analyzer methods
with ion probe methods to learn more about the standards needed for accurate
and precise carbon isotope analyses (Papineau et al., 2010b) using Akilia
rocks, from southwestern Greenland.
Clark Johnson studying Canadian banded iron formations, 2007 |
Dominic Papineau, a French Canadian,
was a graduate student at the University of Colorado training with Stephen
Mojzsis, now a professor there. Dominic wrote to me midway in his doctoral work
and asked if would be one his dissertation committee. I readily agreed. Steve
Mojzsis has quite a reputation for speaking his mind at scientific conferences.
A bright, well-spoken man, he can argue a point with great skill, which he
does. Dominic, in learning from his professor, tried to emulate Mojzsis, but as
a student, he wasn’t ready to take on senior scientists in public. Papineau
came to the Geophysical Lab as a postdoc working with me and opened many new
doors for research and collaboration. With a bit of a swagger, he worked hard
trying to deal with opinions and speculations about the Earth’s oldest rocks.
We remain colleagues to this day.
Several papers were published on the
Akilia “rocks” (e.g., McKeegan et al., 2007), however, there are only a handful
of specimens from this location and no real outcrop that can be studied by the
community. Therefore, few samples can be shared among labs. Speculation and debate about what type of
rocks these are and how they were formed abounds. Dominic obtained a couple of
the Akilia specimens from his Ph.D. advisor Stephen Mojzsis. Our first paper together
was based primarily on microscopic analyses using Raman spectroscopy,
transmission electron microscopy, and Synchrotron X-ray based microscopy
(Papineau et al., 2010a). Papineau studied graphite inclusions in association
with apatite crystals, phosphate minerals common in many types of rocks.
Andrew Steele and I encouraged him to
quantify the occurrences of graphite-apatite pairs rather than loosely
describing them. Were these common features? Were there only one or two within
a thin section? Did they all present the same appearance? About one-sixth of
the apatite crystals were associated with a graphite coating. The graphitic
carbon was primarily ordered graphite, with a much smaller amount of disordered
carbon. Raman spectroscopy was also used to determine that the graphite in
these rocks was crystallized at very high temperatures during metamorphism
(>650°C). The carbon was severely reordered making it impossible to
determine if it was originally biogenic or abiogenic carbon.
It is important for scientists to
debate and ultimately come to an agreement on the first conclusive evidence of
life on Earth. Many researchers use the carbon isotope compositions of graphite
from Earth’s oldest rocks as firm evidence that photosynthesis was an active
process 3.85 billion years ago. Others argue that owing to metamorphic
processes, graphitic carbon in ancient rocks could result from numerous types
of abiogenic reactions that show carbon isotope compositions similar to
photosynthetic ones. This distinction is important because we want to know how
to identify very old signs of life after we have sufficient samples from Mars
and other planetary bodies. We still struggle to find an unambiguous signal of
first life on Earth.
Great story about Tom Hoering identifying newspaper ink as the source of the organic matter!
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