Asteroid and comet impacts from millions of years ago have encapsulated bits of plants in molten glass. The exquisite fossils offer scientists a snapshot of climate and local environmental conditions of ancient Earth, and the method might uncover ancient life on Mars.
When bolides smash into sediments, the scorching heat melts tons of soil and rock, leaving behind impact melt breccias as they cool. The process is a bit like being deep fried.
A team led by Pete Schultz of Brown University explored large fields of impact glass left during at least seven bombardments that occurred in the Pampas region of eastern Argentina between 9.2 million years and 6,000 years ago. (One of those impacts, around 3 million years ago, coincides with the disappearance of 35 genera of animals.)
“We know these were major impacts because of the shocked minerals trapped inside with plant materials,” Schultz explains in a news release. “These glasses are present in different layers of sediment throughout an area about the size of Texas.” The area is covered with thick layers of windblown sediment called loess, and when an object impacted it, globs of melted material rolled out of the impact area like molten snowballs, he adds. As they rolled, they collected material from the ground. They cooled quickly, and the dust that accumulated on them also helped preserve both the glasses and the stowaways.
The team found fragments of leaves and preserved organic compounds lodged in the glass associated with two impacts: one from 3 million years ago, the other from 9 million years ago. These glasses preserve plant morphology from macro features all the way down to the micron scale,” Schultz says. The encapsulation preserved larger structures -- such as bundles of vein-like structures and papillae, the tiny bumps that line the leaf surfaces -- as well as structures at the cellular level.
The vascular bundles found in several samples are very similar to modern pampas grass, a species common to that region of Argentina. Chemical analyses revealed the presence of organic hydrocarbons (polycyclic aromatic hydrocarbons, to be exact), the chemical signatures of living matter, and the metal oxides they found could be from bacteria acting on the decaying plants.
Fossilization typically occurs over an extended time period as minerals slowly replace organic matter and the stuff around it turns into stone under pressure. The process documented here, however, was instantaneous. So the team tried to replicate the preservation in the lab.
They mixed pulverized impact glass with fragments of pampas grass leaves, then heated the mixture at various temperatures over a range of time. The plant material was preserved when quickly heated to above 1,500 degrees Celsius. They found that water in the exterior layers of the leaves insulated the inside layers, helping them stay intact. “The outside of the leaves takes it for the interior,” Schultz explains. “It’s a little like deep frying. The outside fries up quickly but the inside takes much longer to cook.”
The method could help identify signs of possible life on ancient Mars, the team suggests, since the soil conditions in Argentina that helped with preservation are not unlike soils and loess-like dust found on Mars. According to Schultz, “impact glass may be where the 4 billion-year-old signs of life are hiding.”
The work was published in Geology last month.
Image: Brown
