New Type of Fossilization Discovered

A previously unrecognized mode of fossilization of ancient microbes may explain how some of Earth’s oldest microfossils formed, according to new research.

Rasmussen Muhling show that the infiltration of liquid hydrocarbons was responsible for the formation of these filamentous microfossils from the Red Dog Zn-Pb-Ag massive sulfide deposit in the western Brooks Range, northern Alaska, the United States. Image credit: Rasmussen Muhling, doi: 10.1130/G46346.1.

Microfossils provide information about the antiquity of life on Earth and the nature of the early biosphere, however some mystery still surrounds how they were preserved.

It generally thought that many of the oldest microfossils formed when silica grew on their cell walls encasing the microorganism.

When the microorganism died, their cells rapidly decayed, leaving behind only traces of carbon in a hollow cavity that molded the shape of the organism.

Professor Birger Rasmussen from the University of Western Australia and the China University of Geosciences and University of Western Australia’s Dr. Janet Muhling worked on the 340-million-year-old Red Dog zinc-lead deposit in northern Alaska, the United States.

They found the carbon filling of semi-hollow microfossils was not derived from the original organism but from migrated oil.

“We found that oil infiltration of silica-entombed bacteria was responsible for producing the structures, which resembled many of the oldest microfossils preserved in the rock record,” Professor Rasmussen said.

“Using high-resolution electron microscopes, we found microfossils located around oil-carrying fractures were black and filled by a thin film of carbon.”

“In contrast, microfossils between the fractures contained little or no internal carbon and were virtually invisible.”

The formation of carbon-rich microfossils around fractures suggested oil moving through the cracks in the rock had seeped into the semi-hollow molds left after the bacteria died.

“What we have found is a new process of fossilization,” Professor Rasmussen said.

“We are now looking at rock samples from other localities to examine what role oil might have played in the preservation of microfossils on the early Earth.”

“The results may have implications for how we assess whether ancient microstructures are signs of life on early Earth and how we might detect biosignatures on Mars and other rocky planets.”

The study was published in the journal Geology.

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Birger Rasmussen Janet R. Muhling. Organic-rich microfossils produced by oil infiltration of hollow silicified bacteria: Evidence from the ca. 340 Ma Red Dog Zn-Pb deposit, Alaska. Geology, published online September 25, 2019; doi: 10.1130/G46346.1