Modern Invertebrates Mining for Oxygen Is a Recapitulation of Earliest Behaviors

Trace fossils represent the behaviors of animals that lived on or within sediments. In marine settings, activities such as foraging and ‘mining' for food, seeking shelter and avoiding predation, locomotion, and stowing of waste are the most commonly inferred behaviors. The mining of oxygen-rich sediment associated with photosynthetic biomats has not been explored. Studies of modern chironomid larvae, however, shed light on this behavior.

Seasonally sampled cores of burrowed sediment containing Chironomid larvae were collected from Cooking Lake, Alberta and analyzed to: (1) establish their typical burrow architecture; (2) record micrometer-scale geochemical profiles throughout a seasonal cycle; and (3) link changing geochemical conditions to changing burrowing behaviors. We observed that the larvae lived in soft, water-saturated sediment, maintained by open burrows accreted by the animal's mucous. Chironomid-larvae burrows were Y-shaped. The larvae used a porpoising behavior to exploit food in suboxic and anoxic sediment. The geochemical analyses revealed that H2S was present in the pore waters to within 1.5 mm of the sediment-water interface during the summer, but in the winter, the H2S front extended upwards into the water column due to the cessation of algal/cyanobacterial activity. Burrowing behavior was not linked to seasonal changes in the sediment chemistry. This is due to the ability of Chironomid larvae to exploit oxygen oases within the sediment.

We explore whether similar behavior may have existed in the earliest evolved animals. The most primitive of them comprise of simple undermat tunnels in Cambrian strata. Consideration of these various burrow morphologies lead us to conclude that in some settings oxygen mining was an important behavior to invertebrate survival, perhaps as early as the late Edicaran. Indeed, the common co-occurrence of various trace fossils and ancient biomats can be best explained by the animal expoitation of oxygen-producing biomats.