2D and 3D imaging of ‘tunnel-shaped’ weathering features in soil grains: a biological origin?
Authors: Mitchell Ria , Kenrick Paul , Bodey Andrew , Mansfield James , Johnston Richard 
Published: January 2024

The evolution of the first plant-based terrestrial ecosystems some ~450 million years ago had a profound effect on the development of soils and shifts in global biogeochemical cycles, notably drawdown of CO2 from the atmosphere. In some part these shifts were due to biologically mediated weathering of mineral grains, which until plants evolved, had not been a significant contributor to fluctuations in the Earth system. Here, we investigate modern analogues of the earliest plant-based communities to understand what micro-scale biologically mediated weathering processes might have been occurring in the geologic past. We study analogous organisms such as those in cryptogamic ground covers (CGCs), including bryophyte plants, lichens, fungi, algae, and bacteria. These organisms leave specific markings both externally and internally (e.g., tunnel-like features) in substrates and soil grains, however until now study of these has mostly been limited to two dimensions (2D). We use a combination of non-destructive 3D X-ray microscopy (XRM) and synchrotron X-ray microtomography (sr-μCT) imaging to characterize potential biologically mediated weathering by a variety of organisms on a range of substrates, including basalt agglomerate proto-soil grains (liverworts, lichen), limestone (lichen), a rhyolite regolith grain (moss, fungi, lichens), and basaltic scoria (mosses, lichen). We conclude that 2D imaging alone can be misleading and a 3D imaging approach must be performed for accurate characterization of tunnel-like features. From initial exploratory scans and observing the data in 2D as ‘digital thin sections’ (slices), we found tunnel-shaped weathering features in three grain examples (agglomerate, limestone, rhyolite regolith), which appear to mostly radiate from grain surface organics. However, once digitally reconstructed and segmented in 3D, those in the agglomerate and rhyolite grains are in fact flattened, lens shaped voids and not singular, tubular tunnels. In contrast, the features in the limestone are more conclusively networks of interconnected tubular shaped tunnels in 3D, while the scoria lacks tunnels but has evidence of larger ‘caverns’ which have developedNon-peer reviewed EarthArXiv preprintbeneath grain surface organic material. While it is therefore difficult to interpret these diverse features, particularly the flattened and lens-shaped voids, solely as the result of penetrating biological action (e.g., tubes formed from ‘mining’ fungal hyphae), it is plausible that they could be the result of inorganic dissolution along atomic-scale or chemical boundaries, from organic exudates, or from a combination of the three. The identification, careful morphological characterization, and cautious interpretation of these features has implications not only for understanding how the earliest terrestrial soil biotas weathered their substrates here on Earth, but also for understanding similar features, potentially inferred as biological in origin, in highly topical extra-terrestrial rocks (e.g., on Mars), and in engineering applications where the presence of such features may be detrimental (e.g., to nuclear waste glass durability).