Microbially generated bubbles preserved by biofilms in a siliciclastic sedimentary flat

Abstract

Modern marine microbial mats in siliciclastic settings are dominated by cyanobacteria and other microbes with significant roles in photosynthesis, such as pennate diatoms. Diatoms create a biofilm in the topmost layers, cutting off the incident radiation to the underlying mat of thickly packed cyanobacterial filaments. The thickness and cell density of these biofilms rich in exopolymeric substances (EPS) are related to hydration. Furthermore, biofilms may play a role in the entrapment of metabolic gases, the formation of bubbles and ultimately their preservation. This paper stems from field observations on the formation and early preservation of microbially generated bubbles in a siliciclastic sedimentary flat in Argentina (Paso Seco, 40°S; 62°W) under specific environmental conditions. On the one hand, when the flat is flooded by a stagnant layer of seawater, large-sized diatoms (Nitzschia sigma) secrete significant amounts of EPS and increase biomass by cell division, creating a ca.1to 2 cm thick biofilm that limits diffusion of gases and traps O2 bubbles. On the other hand, when the flat dries up after flooding, a biofilm with a ‘papyrus’-like texture is formed and deposited in layers on top of the sedimentary flat. This dry biofilm of coriaceous texture has high organic matter and chlorophyll a contents. Although they are very subtle structures, trapped, EPS-bound bubbles can withstand surface water currents and remain as permanent features, even when the biofilm dehydrates. As such, they have been found preserved in siltstones and claystones at a coastal palaeoichnological site in Argentina (Pehuen-Co). This study posits that bubble preservation is promoted by the mucilaginous-mineral matrix that becomes progressively desiccated, a finding that is of interest to sedimentologists and palaeobiologists.