Thursday, August 29, 2019

Earth’s Earliest Signs of Life: If we found it, could we recognize it?

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.

1 comment:

  1. Great story about Tom Hoering identifying newspaper ink as the source of the organic matter!


Rounding Third Base and Heading Home

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