The remains of microbial activity discovered in 3.45-giga-year-old volcanic glass have been considered the oldest fossil traces of life. But a recent reassessment of the trace fossils suggests otherwise: They’re not biological in origin.
Specifically, the fossils in question are filamentous titanite microtextures -- tiny tubes of calcium titanium silicate mineral -- found in subseafloor pillow lavas in the Barberton greenstone belt of South Africa. After isotopic aging, temperature mapping, and shape analysis, scientists argued that the fossil traces represent the mineralized remains of tunnels made by ancient bacteria or other microbes in the seafloor during the Archean Eon over three billion years ago. This process of microbes etching volcanic glass is called bioalteration.
But now, according to Eugene Grosch and Nicola McLoughlin from the University of Bergen in Norway, the little tubes were created by the cooling of volcanic rock hundreds and millions of years later, challenging the “bioalteration model” proposed to have occurred. Their work was published in the Proceedings of the National Academy of Sciences last month. Pictured here, titanite microtextures:
The duo drilled 180 meters into the rock where the microtextures were found and measured hundreds of the textures they found throughout the core. The filaments had huge diameters and a very large size distribution compared with those of the miniscule tunnels formed by microbes in oceanic crust today, Grosch explains to Live Science.
Furthermore, looking at the decay of uranium and lead isotopes and also modeling the cooling conditions in nearby pillow lava, the researchers estimated a much younger age for the titanite and conclude that the microtextures could have been formed by conditions in the cooling rock. As Live Science explains, about 2.9 billion years ago, magma intruded into the rock, heating it up, and forming the titanite structures as it cooled.
Images: Nicola McLoughlin via Phys.org (top), E.G. Grosch & N. McLoughlin, PNAS 2014 (middle)