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See Inside July 2008

Chemical Fossils Preserved in Lava Reveal Remains of Ancient Sea Life

Searching for microfossils inside igneous rocks

Adorf, Germany—After a five-hour drive south from the University of Bremen that got them in at half-past midnight, the two researchers visiting this small village were happy to sit and talk with tavern patrons about the volcano just up the street. Gathered around a map, they listened intently as geobiologists Joern Peckmann and Benjamin Eickmann pointed to the extinct volcano, Arnstein Hill, and explained that the forested region had been underwater 400 million years ago, during the Devonian. Flooded lands were something these German townsfolk could relate to. The completion of the Edersee Dam in 1914 put the neighboring village of Asel at the bottom of a lake.

Peckmann and his students are investigating chemical fossils from the interior of seafloor basalts—solidified lava—from the Devonian. Peckmann, who first reported the findings in March, believes that they have uncovered a previously unknown niche for microbial life, “one that existed in the past, occurs in the present and has the potential to have existed since the beginning of Earth’s history,” he says. The work could also contribute to investigations of possible fossils in Martian basalt.

Scientists have looked for evidence of life in rocks before but only on the surface of basalt rocks or in sedimentary layers. Igneous rocks, which form under high temperature, are not considered ideal homes. Peckmann is trying to prove otherwise.
On Arnstein Hill, what was once a horizontal seafloor was uplifted to a vertical position and folded multiple times. Rocky outcrops exposed along the face of the cliffs reveal explosions from the Devonian where the lava erupted into the ocean and formed pillow basalts, much like those that form underwater around the shores off Kilauea, Hawaii, today. The seawater quickly cooled the exterior of the lava into a black glass crust of obsidian.

It is this dark rind that Peckmann and Eickmann are looking for as they scramble over boulders and shards along the base of the 60-foot tall cliff. Unlike porous, lightweight pumice, which forms when gas-filled lava cools quickly in the atmosphere, basalt is dense, heavy stuff. Instead of the standard geologic pickax, the researchers carry a sledgehammer. (Only later does Peckmann acknowledge that our hard hats are pretty much useless against any falling basalt bigger than a softball.) After several heavy hits, Eickmann succeeds in breaking open a large sample. Inside, the rock is rife with tiny crystals of carbonate. It is like looking at a loaf of dark German bread baked full of sesame seeds. In the case of the obsidian-crusted pillow basalt, the interior cooled more slowly than the exterior and trapped gas bubbles that later filled with seawater, forming the carbonate crystals, called amyg­dules, throughout its matrix. “You can’t be better protected” than in an amyg­dule, Peckmann exclaims.

Examining thin sections through a microscope reveals tubular or filamentlike growths that branch away from the amygdule walls. These growths are the chemical fossils, molecules that the putative microbes, or cryptoendoliths, produced that are stable over geologic time. Katharina Behrens, one of Peckmann’s students, is finding similar signs with fresh basalts from Kolbeinsey Ridge in the North Atlantic. In the Devonian rocks chemical analyses revealed that the minerals inside the amygdules precipitated from seawater, rather than from hydrothermal fluid, which is hostile to life. This is key for convincing others that the microbes were using the basalt as a home.

Although other scientists have found microbes boring into the glassy rinds before, the work by Peckmann and his colleagues  marks “the first time that apparently mineralized microbial filaments have been found in gas vesicles” and voids in basalts, says astrobiologist Roger Buick of the University of Washington. “Thus, the study suggests a new style of preservation for microbial fossils” that could potentially be applied to older rocks, thereby “providing scientists with new tools to search for signs of life very early in Earth’s history.” Moreover, because basalts are common on Mars, the techniques “could even be applied to the search for extraterrestrial fossilized life,” Buick remarks.

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