Scientists have spent a lot of time developing models to explain this time lag but now Rasmussen and colleagues say the earlier research got it wrong. "You may not need an explanation [for the time lag]. It may be simpler than that," he said. Rasmussen and colleagues have carried out another analysis of Pilbara rocks, including samples used in the biomarker study. They said that carbon-13 to carbon-12 isotope ratios associated with the biomarkers indicate they are younger than organic residues found in the rocks themselves, and they must have become associated with the rock at a later date. "The results we've got suggest that the biomarkers were probably not indigenous to the rock they were extracted from," said Rasmussen. About 2.2 billion years ago the rock was heated as high as 200°C and the biomarkers must be no older than this because they are unlikely to have survived the heat, he said. "They could have entered the rock after that peak heating event, possibly infiltrated from younger sedimentary rock, or even during the drilling process [to obtain rock samples]," said Rasmussen. The original team's method involved crushing the rock and dissolving it in solvents, allowing for greater potential for contamination of the sample, whereas the new study analyzed organic carbon residues in situ by examining ultra-thin slices of the rock. Geologist Malcolm Walter, who heads up the Australian Center for Astrobiology at the University of New South Wales in Sydney, said the latest study has been much anticipated as experts knew there were limitations to the earlier study. But he said "it's definitely not the last word." "I'm keeping an open mind on it at this stage," he said. Walter said there is independent evidence to suggest cyanobacteria was around earlier, including 2.7 to 2. 8 billion-year-old stromatolites found in the same rocks that yielded the biomarkers. "Those particular stromatalites have characteristics that suggest they were built by cyanobacteria," he said. Related Links: |
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