3-billion-year-old microfossils of plankton were recently identified as such by an international team of researchers. The spindle-shaped inclusions — found in the 3-billion-year-old marine sediment rocks of Farrel Quartzite in Western Australia — were previously thought to be of possible biological origin, but there was no strong evidence for the theory.
But now, researchers have been able to show that the inclusions are actually microfossils of plankton — plankton which very likely lived throughout all of the world’s oceans at the time.
“It is surprising to have large, potentially complex fossils that far back,” said Christopher H. House, professor of geosciences, Penn State, and lead author.
The new research was done by utilizing secondary ion mass spectrometry to perform an isotopic analysis of marine sediment rocks from the Farrel Quartzite in Western Australia. “Ken (Kenichiro Sugitani, professor, Graduate School of Environmental Studies, Nagoya University, Japan, and a co-author) discovered these unusually shaped microfossils embedded in really old rock,” said House.
In order to determine if the inclusions were of biological origin, “the researchers 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,” the researchers state.
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The spindle-shaped microfossils are from 20 to 60 microns in length, about the size of fine sand and within the size range of today’s microplankton.
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.
“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,” said co-author Dorothy Oehler. “Moreover, this has implications beyond what we have done here, suggesting the evolution of diverse life proceeded quickly.”
The findings were detailed in a new paper published in the journal Geology.