You might consider volcanic activity an efficient way of destroying living tissues, but volcanic glasses are often used to study early life. Certain microbial corrosion textures in volcanic glass and in Cenozoic seafloor basalts have been interpreted as evidence of a deep biosphere, dating back to the earliest periods of life on Earth. Similarly, textures found in volcanic glass from the Barberton Greenstone Belt in South Africa and the Pilbara Craton in Australia also suggest the presence of a deep oceanic biosphere on Earth as early as 3.35 billion years ago.

But a group of researchers had challenged the idea that these traces resulted from biological activity. Now, in a strong response, another group has defended the interpretation. We seem to have a genuine scientific controversy on our hands.

Is this life?

Volcanic glass from Cenozoic seafloor volcanics sometimes contain what are called "biotextures." These typically take on two major forms, either microscopic spherical cavities or tubules that extend into the volcanic glass. Biogenicity, substances produced by life processes, is the most widely accepted explanation for these features. These textures can provide us with valuable information about the types of microorganisms that existed long ago, and shed light on biological processes and how those processes may have evolved over time.

To make the case that these feature are produced by organisms, scientists argue that cavities found in volcanic glass have complex textures that could not have formed in the absence of living organisms. Their biogenicity has been established in part through studies of natural and manmade rock surfaces, which makes it a reasonable explanation for the biocorrosion features in volcanic glass. In both cases, scientists think that the cavities can be attributed to the excretion of organic acids by microbes.

But this evidence is indirect compared to typical fossils where the actual organisms is preserved. Because of this distinction, arguments for demonstrating biogenicity need to focus on explaining the textures. The indirect nature of these fossils also allows room for debate on their real origin.

A recent study focused on the types of textures found in a 180-m drilled core drilled through lavas of the Barberton Greenstone Belt, the second-oldest region of the planet we know about. Its authors, Grosch and McLoughlin, rejected the hypothesis that the textures found in this volcanic glass are due to biogenicity.

To address this controversy, another group of scientists recently evaluated what is known about how volcanic glass can be altered by living organisms, as well as how these types of trace fossils may be preserved. The team thinks that previous evidence for biogenicity in titanite-based textures remains the most likely interpretation for the tubular textures found in samples from the Barberton Greenstone Belt and the Pilbara Craton. The alteration of basaltic glass by life leads to the passive accumulation of titanium, which has been found in the walls of the tiny tubules in the volcanic glass.

From life to titanium

The scientists proposed a two-stage model for the origin of these titanite textures. First, the walls of the microbial excavations in volcanic glass become enriched with Ti. The second stage is then marked by the titanite crystallization stage that occurs as the material is transformed to another type off rock in and around the zones of the Ti enrichment. The titanites then draw additional Ti from the surrounding glass as it is transformed to a mineral called chlorite.

In contrast, Grosch and McLoughlin suggested that the titanite crystallization is caused by the breakdown of clinopryoxene, ilmenite, and quartz during the rock's cooling. If this process starts with source material in a complex shape, the results will resemble bioalteration textures. From that starting point, they say the titanite grows outward, increasing the length of the apparent tubular textures.

The new paper attempts to pick apart this argument. It points out that there's no observational evidence for the geochemical steps needed for this process. And there's no detailed description of a volcanic rock that would support the presence of key reactants. The paper's authors point out that Grosch and McLoughlin's model requires a process called "contact metamorphism" to work; a number of the early samples don't seem to have undergone this process.

Grosch and McLoughlin rejected biogenicity of titanite textures in their drill core partly because they did not observe specific “signature” textures—complex coils, ornamented, or annulated shapes. “Signature” textures have shapes that are commonly associated with biological systems, but are typically not formed as a result of abiotic dissolution or weathering of volcanic glass.

The team that cross-examined their evidence believes that Grosch and McLoughlin did not find signature textures because they are scarce and rarely preserved. When looking at bioalteration textures as a whole, granular textures make up 90 percent of those observed; tubular cavities make up the remaining 10 percent (or less). The most distinctive “signature” textures of tunnels, spirals, annulated, decorated, or bifarcting tubules make up a tiny fraction of that 10 percent. Thus, the chances of finding such features are extremely low especially with a limited sampling effort.

The rarity and variety of textures that can be found underscores the difficulty with analysis of these indirect fossils. But, as they may be one of the better indications of life at great ages, it's well worth the hassle.

PNAS, 2015. DOI: 10.1073/pnas.1421052112 (About DOIs).