Fractured rocks of impact craters may be suitable hosts for deep microbial communities on Earth and potentially other terrestrial planets, yet direct evidence has remained elusive. Now, a team of researchers from the Linnaeus University and elsewhere has found evidence for ancient microbial life in rock samples from the Siljan impact crater in Sweden.
The Siljan crater is a circular geological formation located in the province of Dalarna in central Sweden.
The impact that created this structure occurred about 381 million years ago (Devonian period).
Also known as the Siljan Ring, it is the largest known impact crater in Europe and one of the 18 largest known impact craters on Earth.
Mainly eroded today, the original crater is estimated to have been about 32 miles (52 km) in diameter.
“We examined the intensively fractured rock at significant depth in the crater and noted tiny crystals of calcium carbonate and sulfide in the fractures,” said lead author Dr. Henrik Drake, a researcher in the Department of Biology and Environmental Science at the Linnaeus University.
“When we analyzed the chemical composition within these crystals it became clear to us that they formed following microbial activity.”
“Specifically, the relative abundance of different isotopes of carbon and sulfur within these minerals tells us that microorganisms that produce and consume the greenhouse gas methane have been present, and also microbes that reduce sulfate into sulfide. These are isotopic fingerprints for ancient life.”
“We applied newly developed radioisotopic dating techniques to the tiny calcite crystals formed following microbial methane cycling, and could determine that they formed in the interval 80 to 22 million years ago,” added co-author Dr. Nick Roberts, of the British Geological Survey.
“This marks long-term ancient microbial activity in the impact crater, but also that the microbes lived up to 300 million years after the impact.”
“At Siljan we see that the crater is colonized but that it has mainly occurred when conditions, such as temperature, became more favorable than at the impact event,” Dr. Drake said.
“The impact structure itself, with a ring zone of down-faulted Paleozoic sediments, has been optimal for deep colonization, because organics and hydrocarbons from shales have migrated throughout the fractured crater and have acted as energy sources for the deep microbial communities.”
“The preserved organic molecules that we could detect within the minerals give us additional evidence both for microbial activity in the crater, as we find molecules specific to certain microorganisms, but also for microbial biodegradation of shale-derived hydrocarbons, ultimately leading to production of secondary microbial methane at depth,” said co-author Dr. Christine Heim, a scientist at the University of Göttingen.
“Detailed understanding of microbial colonization of impact craters has wide-ranging astrobiological implications,” added co-author Dr. Magnus Ivarsson, from the Swedish Museum of Natural History.
“The methodology that we present should be optimal to provide spatiotemporal constraints for ancient microbial methane formation and utilization in other impact crater systems, such as the methane emitting craters on Mars.”
“Our findings indeed confirm that impact craters are favorable microbial habitats on Earth and perhaps beyond,” Dr. Drake concluded.
A paper reporting the findings was published in the journal Nature Communications.
H. Drake et al. 2019. Timing and origin of natural gas accumulation in the Siljan impact structure, Sweden. Nat Commun 10 (4736); doi: 10.1038/s41467-019-12728-y
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