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Physical Chemistry

Geochemical precipitates may fool fossil hunters

Inorganic crystals can mimic biofossils, but they’re formed without biological help

by Elizabeth K. Wilson
March 21, 2017 | A version of this story appeared in Volume 95, Issue 13

Credit: Juan Manuel Garcia-Ruiz
Inorganic precipitates from high-pH spring water resemble biominerals.
A micrograph showing abiotically produced mineral structures that resemble those produced biologically.
Credit: Juan Manuel Garcia-Ruiz
Inorganic precipitates from high-pH spring water resemble biominerals.

Geochemists searching for evidence of ancient life often look to collections of round, twisted, and loopy mineral precipitates they find in soil or water samples as possible microbial fossils.

A team led by Juan Manuel García-Ruiz at the Spanish Higher Council of Scientific Research and the University of Granada and Oliver Steinbock at Florida State University, however, have now made these types of inorganic structures without the help of biology (Sci. Adv. 2017, DOI: 10.1126/sciadv.1602285).

The discovery, which blurs the line between processes that are purely biological and ones that are geological, may complicate efforts to study ancient terrestrial or extraterrestrial biofossils.

The group collected water from the Ney spring in California’s Siskiyou County that has an unusually high pH of 11.9 and is rich in silica. The researchers added solutions of barium chloride dihydrate and calcium chloride dihydrate to the organism-free water and then watched as a gallery of fantastically shaped precipitates formed.

In addition to barium carbonate structures that resemble microbial fossils, the researchers also found “silica gardens” of metal-silicate-hydrate tubes and calcium carbonate “shells.”

Until recently, structures of this sort were thought to be produced via biological mechanisms. Biogeochemists have considered the chemistry and morphology of these crystals as biomarkers for possible extraterrestrial microbial life.

Then a few years ago, García-Ruiz and his colleagues reported that similar biomineral-like structures could be precipitated by adding carbonates to lab-prepared alkaline, silica-rich solutions. They dubbed the precipitates “biomorphs.”

That these conditions can be found in a natural setting such as the Ney spring makes an even stronger case for a possible reframing of biological and geological chemistry, García-Ruiz and his team say.

Resarchers led by Dominic Papineau at University College London recently reported in Nature that they had found microbial fossils in hydrothermal vent precipitates up to 4.22 billion years old (2017, DOI: 10.1038/nature21377). García-Ruiz’s work “is clearly very important to help understand morphological, biological, and nonbiological signatures,” Papineau says.

“It is exactly this kind of science that significantly contributes to the progress of the fields of Precambrian biogeochemistry and exobiology,” he notes.

Biominerals expert Fabien Salport of Paris Diderot University says the work “shows the major difficulty to detect unambiguously records of the most ancient terrestrial biological activity. The ability to synthesize crystal structures that mimic biominerals makes it even more difficult to search for the oldest traces of life on Earth.”



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