Road, we spent a revealing day examining thin sections of Warrawoona chert.
Figure 4.5. Warrawoona chert at Marble Bar, Western Australia. The red (pigmented by iron oxides, gray in picture) and white bands accumulated on the seafloor. In contrast, the black bands cut across other beds and so are younger—and formed in a different way. Warrawoona microstructures interpreted as fossils come from these crosscutting cherts, interpreted as hydrothermal plumbing systems filled by silica.
In Spitsbergen cherts, microfossils are abundant. They have shapes similar to those of living microorganisms, but different from shapes made by purely physical and chemical processes. Most retain at least some of their original organic matter. And some even occur in environmental settings much like those of close living counterparts. No comparable claims can be made about the Warrawoona microstructures.Observed through the microscope in Martin’s lab, the tiny filaments in Warrawoona rocks looked like minerals.
As a boy, I often spent idle summer afternoons gazing at clouds. Most were billowy masses, beautiful but shapeless. Every now and again, however, an unmistakable face appeared in the sky. Or a castle. Or a lion. For a moment, the shapes took striking form, but even as a youngster I was pretty sure that they were, in the end, just clouds. Are the Warrawoona microstructures “just clouds,” as well?
That question is difficult to answer based on a few pictures cut and cropped for publication. It requires context —the framework provided by the overall rock fabric seen in thin sections of Warrawoona chert. It was the rest of the clouds that revealed my air castles as watery illusions, and it is the overall fabric of Warrawoona cherts that casts doubt on those rare features that look biological. Martin and his colleagues painstakingly documented how volcanic and hydrothermal processes shaped the cherts from Chinaman’s Creek. They believe that physical processes can account for all microscopic features of the cherts, including those singled out as fossils. If this interpretation is correct, the Warrawoona microstructures cannot be cellular filaments, only stacked crystals that mimic but do not preserve a record of biology.
Are paleontology’s crown jewels, so old and rare, made of paste? In fairness, Bill Schopf disputes this reading. In a rebuttal to the claim by Brasier and colleagues, Bill and University of Alabama chemist Tom Wdowiak show that the disputed Warrawoona structures contain organic matter at their margins. This, of course, is consistent with the view that they are microfossils, but it doesn’t end the debate. Archean cherts commonly contain the ghosts of early formed minerals whose distinctive shapes are preserved by a veneer of organic matter. My own guess is that most Warrawoona structures are mineral chains draped by an organic film (that may, itself, have a biological origin). Continuing study may yet confirm the presence of fossils in these rocks—the debate is far from over—but I doubt that any such remains will teach us much about early ecosystems. Warrawoona microstructures, like Warrawoona stromatolites, can only suggest that something interesting and important lies just beyond our grasp.
Biomarker molecules are not retained in rocks from North Pole, butisotopic signatures are; the carbon and sulfur isotopes of Warrawoona rocks provide our best indication of life’s deep history. As in Spitsbergen (and nearly everywhere else that sedimentary rocks were deposited in Precambrian times), 13 C/ 12 C ratios in Warrawoona carbonates and organic matter differ by about 30 parts per thousand. This difference is most easily explained by photosynthesis, but given our experience with stromatolites and microfossils, we should ask once more whether physical processes can mimic the effects of biology. Some chemical reactions do form organic molecules depleted in 13 C. Only under carefully controlled experimental
Figure 4.5. Warrawoona chert at Marble Bar, Western Australia. The red (pigmented by iron oxides, gray in picture) and white bands accumulated on the seafloor. In contrast, the black bands cut across other beds and so are younger—and formed in a different way. Warrawoona microstructures interpreted as fossils come from these crosscutting cherts, interpreted as hydrothermal plumbing systems filled by silica.
In Spitsbergen cherts, microfossils are abundant. They have shapes similar to those of living microorganisms, but different from shapes made by purely physical and chemical processes. Most retain at least some of their original organic matter. And some even occur in environmental settings much like those of close living counterparts. No comparable claims can be made about the Warrawoona microstructures.Observed through the microscope in Martin’s lab, the tiny filaments in Warrawoona rocks looked like minerals.
As a boy, I often spent idle summer afternoons gazing at clouds. Most were billowy masses, beautiful but shapeless. Every now and again, however, an unmistakable face appeared in the sky. Or a castle. Or a lion. For a moment, the shapes took striking form, but even as a youngster I was pretty sure that they were, in the end, just clouds. Are the Warrawoona microstructures “just clouds,” as well?
That question is difficult to answer based on a few pictures cut and cropped for publication. It requires context —the framework provided by the overall rock fabric seen in thin sections of Warrawoona chert. It was the rest of the clouds that revealed my air castles as watery illusions, and it is the overall fabric of Warrawoona cherts that casts doubt on those rare features that look biological. Martin and his colleagues painstakingly documented how volcanic and hydrothermal processes shaped the cherts from Chinaman’s Creek. They believe that physical processes can account for all microscopic features of the cherts, including those singled out as fossils. If this interpretation is correct, the Warrawoona microstructures cannot be cellular filaments, only stacked crystals that mimic but do not preserve a record of biology.
Are paleontology’s crown jewels, so old and rare, made of paste? In fairness, Bill Schopf disputes this reading. In a rebuttal to the claim by Brasier and colleagues, Bill and University of Alabama chemist Tom Wdowiak show that the disputed Warrawoona structures contain organic matter at their margins. This, of course, is consistent with the view that they are microfossils, but it doesn’t end the debate. Archean cherts commonly contain the ghosts of early formed minerals whose distinctive shapes are preserved by a veneer of organic matter. My own guess is that most Warrawoona structures are mineral chains draped by an organic film (that may, itself, have a biological origin). Continuing study may yet confirm the presence of fossils in these rocks—the debate is far from over—but I doubt that any such remains will teach us much about early ecosystems. Warrawoona microstructures, like Warrawoona stromatolites, can only suggest that something interesting and important lies just beyond our grasp.
Biomarker molecules are not retained in rocks from North Pole, butisotopic signatures are; the carbon and sulfur isotopes of Warrawoona rocks provide our best indication of life’s deep history. As in Spitsbergen (and nearly everywhere else that sedimentary rocks were deposited in Precambrian times), 13 C/ 12 C ratios in Warrawoona carbonates and organic matter differ by about 30 parts per thousand. This difference is most easily explained by photosynthesis, but given our experience with stromatolites and microfossils, we should ask once more whether physical processes can mimic the effects of biology. Some chemical reactions do form organic molecules depleted in 13 C. Only under carefully controlled experimental
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