Geology Reference
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surfaces. So Knoll encouraged her to bolster her case for mats by adding analytical data on
the minerals, biomolecules, and isotopes preserved in her distinctively crenulated layers.
Maybe traces of ancient carbon or concentrations of characteristic minerals could provide
a smoking gun for some of the oldest, yet ambiguous, matlike features. I had worked with
other Knoll students, so I got the call.
The very first specimens that Noffke sent proved to be an object lesson in why such
analyses are important. She had found thin wiggly black layers in a sandy sediment three
billion years old—what would have been the record oldest microbial mat at the time. She
needed confirmation that the black features were carbon-rich with life's requisite isotopic
signature, with about 3 percent less of the heavier carbon-13 isotope compared to the aver-
age crust. She had already written a paper for Science , ready to submit, awaiting only that
single confirmatory value. The rock samples were rushed from Cambridge, Massachusetts,
to the Geophysical Laboratory by FedEx under the highest priority. I was under the gun.
Fortunately my colleague Marilyn Fogel, who is the carbon isotope expert at the Carne-
gie Institution's Geophysical Lab, was willing to help. Marilyn looked at the sample and
told me what to do: crush the rock and grind it to a fine powder, put a few micrograms of
powder into each of several tiny cups of pure tinfoil, weigh the samples, and fold each foil
cupintoatinyballthesizeofaBB.Thesesamplesandcarbonisotopestandardswerethen
fed, one by one, into a furnace that vaporizes any carbon-containing compounds to carbon
dioxide gas. The gas flows into a sensitive mass spectrometer that separates and measures
carbon-12 and carbon-13. It only took a few hours to obtain the telltale ratio.
Nora was hoping for something in the −25 to −35 range, typical of other microbial mats.
Butthemachinespatoutadifferentstory.Theisotopicratiowascloseto0,avaluethathad
nothing to do with biology. Rather, it was characteristic of inorganic carbon, the kind that
rises from the mantle in fluids and is deposited as thin veinlets of black graphite. Bottom
line:theblackfeaturesinNoffke'ssampleswerecarbon-rich,buttheywereunambiguously
not biological.
With that object lesson in mind, we proceeded quickly to analyze thin, blackened fea-
tures in many other promising ancient sediments that Nora had accumulated from her var-
ied field areas—from South Africa, from Australia, from Greenland. Time and again we
measured carbon isotopes in the −30 range befitting microbial mats and found other con-
vincing evidence that microbes had flourished along the sandy shorelines of ancient Earth
morethanthreebillionyearsago.Andunliketinyblacksmudgesortracesofbiomolecules,
you could see Noffke's evidence in the field, at the scale of an outcrop. Her evidence, you
could hold in your hand.
But a core question remains: did the mat microbes produce oxygen, or did they use sun-
light for simpler photochemistry? Microbes evolved a variety of sun-harvesting strategies,
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