Life’s Wrinkles in the Sand
A new study shows how wrinkle structures can form on a bed of sand when waves and microorganisms are present. Wrinkle structures on sandy bed surfaces are rare on Earth today, but were more common in ancient sedimentary environments. These ancient sediments often have trace fossils and imprints of early animals, and appear in the geological record after some of the largest mass extinctions on Earth.
Some scientists have theorized that wrinkle structures are the remnants of dense colonies of microbes known as microbial mats, but the new study proposes a different origin.
In their experiment, researchers placed microbial aggregates on bare sand in a wave tank. Microbial mats are dense colonies of microorganisms that are often many layers thick and attached to a surface. Microbial aggregates are basically broken-up pieces of microbial mats, which are produced when the mats are damaged in events like storms or strong currents. Animals that feed on microbial mats also help break them apart, making them more susceptible to storms and helping to produce free-floating microbial aggregates.
The team showed that the wrinkle structures are formed in the interaction between microbial aggregates and sandy sediments when waves are present. The waves do not move sand grains directly. Instead, they act on the microbial aggregates to produce these features. Aggregates of microorganisms (about a millimeter in size) are pushed around by waves, which results in the formation of features like ridges and pits.
The team concluded that wrinkle structures are indeed biosignatures (signs of life’s presence), but that they form when microorganisms are present at the interface between sediment and waves, and not beneath microbial mats.
For more details on the study, Astrobiology Magazine spoke with lead author Giulio Mariotti of the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology (MIT) and co-author Tanja Bosak, Professor in Earth, Atmospheric and Planetary Sciences at MIT.
Astrobiology Magazine (AM): What is the difference between a microbial mat, a biofilm, and a microbial aggregate?
Mariotti: A biofilm is composed by many different microorganisms embedded in a gel-like substance and attached to a surface. A microbial mat is like a biofilm, but thicker (> 1mm) and vertically stratified. A microbial aggregate is a round-shaped, mm-size piece of a microbial mat.
AM: Wrinkle structures on sandy bed surfaces are rare on Earth today, but where are they found? Do wrinkle structures form on other surfaces?
Bosak: They are found in some coastal environments, but they do not last long, so there are very few reports of modern wrinkle structures. Our mechanism takes only a couple of hours to form these structures, so it would be hard to see them form in real time, and they can be easily destroyed. This is party because they can be destroyed by burrowing animals (which were not abundant during the time when wrinkle structures were very common), and partly because they are easily destroyed by stronger storms.
These structures only form on sandy and silty surfaces (i.e., grain sizes from ~ 0.1-0.25 mm).
AM: Where can ancient wrinkle structures found today?
Mariotti: Newfoundland (Canada), Death Valley (USA), Australia, India, and South Africa.
AM: Could you observe the formation of wrinkle structures in nature, or is it simply too difficult because they form so quickly (and are also quickly destroyed)?
Mariotti: We predict that wrinkles structures should form in water depth of about 10-100 m in the continental shelf, that is, in front of a beach facing the Ocean. Testing this mode would require us to dive, deliver some fragments of microbial mats, and wait for the right wave conditions to form the wrinkle structures. Alternatively the experiment might be attempted in water depth less than a meter in a protected environment, such as a sandy lagoon.
AM: Why were ancient wrinkle structures so much more prevalent (why were they not destroyed before being preserved in the rock record)?
Mariotti: Ancient wrinkle structures were more prevalent because microbial aggregates were more common in the past, especially during the time were only a small amount of grazers were present. However, it is also true that the growth of microbial mats on top of already formed wrinkle structures helped preserving them in the rock record, as microbial mats were more common in the past.
The study was supported in part by the NASA Astrobiology Institute and published in the journal Nature Geosciences.