Scientists from the United States and Japan have discovered unusual spindle-shaped organic microfossils in 3 billion-year-old rocks from the Pilbara Craton of northwestern Australia.
Spindle-like plankton microfossils found in the Farrel Quartzite, Pilbara Craton, northwestern Australia (Christopher House)
The microfossils are from 20 to 60 microns in length, about the size of fine sand and within the size range of today’s microplankton. They were likely planktonic autotrophs – free-floating, tiny ocean organisms that produce energy from their environment, according to the study published in the journal Geology.
“It is surprising to have large, potentially complex fossils that far back,” said study lead author Prof Christopher House from Penn State University.
The researchers looked at marine sediment rocks from the Farrel Quartzite in Western Australia. “Prof Kenichiro Sugitani from Nagoya University, Japan, and a co-author discovered these unusually shaped microfossils embedded in really old rock,” Prof House said.
To determine if these inclusions were actually biological in origin, they looked at 15 different samples of Farrel Quartzite and determined their stable carbon isotope ratios.
The percentage of carbon 13 in the microfossils was indicative of material produced by biological processes. They found that the carbon 13 percentage in the background organic matter in the surrounding rock was different from that of the microstructures.
“When considered along with published morphological and chemical studies, these results indicate that the Farrel Quartzite microstructures are bona fide microfossils, and support the interpretation that the spindles were planktonic.”
Stable carbon isotope analysis can determine the biological origin of these microfossils because they used carbon dioxide to create energy and incorporated the carbon into themselves. During this process, the organisms selectively incorporate more carbon 12 than carbon 13 from the available carbon, producing a signature of biological origin.
“The spindles appear to be the same as those found in rocks from the Strelly Pool Formation in Western Australia and the Onverwacht Group in South Africa and Swaziland that are both 3.4 billion years old,” said co-author Dr Dorothy Oehler from Astromaterials Research and Exploration Science Directorate, NASA – Johnson Space Center.
“The existence of these microfossils in diverse locations as far back as 3.4 billion years ago suggests that the oceans probably had life in them for a very extended period of time. Moreover, this has implications beyond what we have done here, suggesting the evolution of diverse life proceeded quickly.”
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Bibliographic information: C.H. House et al. Carbon isotopic analyses of ca. 3.0 Ga microstructures imply planktonic autotrophs inhabited Earth’s early oceans. Geology, vol. 41, no. 6, p. 651-654; doi: 10.1130/G34055.1
