Dried roll-ups from Bardenas Reales, Spain,
located on a muddy slope close to a vernal
pool. Blade width = 1.5 cm. Source.
Who hasn’t walked along a previously flooded area and seen flakes of dried mud cakes magically curled up in geometrical shapes? At times, the curls are so pronounced that they make complete scrolls. The area looks like a field of shards. No big deal, just dried mud you might say (as you reach down, tempted to pick up some pieces to play with). Au contraire, such curlicue structures, known as roll-ups, are of considerable interest to geologists. Roll-ups are sedimentary structures capped with a surface layer of clay and organic material that form when the surface dries out. Once formed, they are quite resistant to wetting and weathering, the open curls much less so.
Microbial architecture of a roll-up. Cross-section of a roll-up
from Bardenas Reales, Spain. Layers of cyanobacteria
(increasing in abundance toward the surface) can be seen
intercalated within layers of fine sediment (arrows).
Bar = 0.5 mm. Source.
To explain why we're playing with mud cakes on this blog, it turns out that the roll-ups are largely biogenic. Their surface includes distinct populations of cyanobacteria, which are much less common in the adjacent, non-rolled soil. The cyanobacteria are largely filamentous and of the non-heterocyst forming variety. How do they contribute to the curling? According to H. Beraldi-Campesi and F. Garcia-Pichel of Arizona State, the authors of a recent study, they provide the organic material in the form of bacterial extracellular polysaccharide (EPS). Enough there and it alone can be the reason for the curvature. With less biomass present (<1% of the total), the microbial filaments in combination with their EPS and some fine sediment suffice to do the curling. The authors could reproduce credible roll-up formations by using a clay that swells on watering (smectite) or agar (we hope they enjoyed doing it).