MODERN TERRESTRIAL SEDIMENTARY BIOSTRUCTURES AND THEIR FOSSIL ANALOGS IN MESOPROTEROZOIC SUBAERIAL DEPOSITS

Abstract

Microbial communities developing on modern clastic sedimentary surfaces of arid lands are dominated by phototrophic microorganisms that form a variety of characteristic “microbially induced sedimentary structures” (MISS) through their interactions with detrital sedimentary grains, aided by secretions of extracellular polymeric substances and other organic materials. In this study, we describe modern MISS from unvegetated arid topsoils and compare them with fossil MISS found within decimeter- to meter-thick sedimentary sequences of Mesoproterozoic siliciclastic outcrops of the Dripping Spring Quartzite formation of the Apache Group in central Arizona, USA. These sequences contain numerous bedding plane exposures with desiccation surfaces including polygonal cracks, curls, and chips. Repetition of these structures within stratigraphic sequences indicates recurring episodes of subaerial exposure. Some of these MISS contain cellular microfossils that exhibit morphological adaptations for surviving desiccation. The strong similarities between modern and ancient MISS in this study provide additional criteria for recognizing morphological biosignatures of terrestrial microbial communities in ancient deposits. Our results provide compelling evidence for the presence of land-based microbial communities by the Mesoproterozoic (∼1200 Ma). The association of MISS features further suggests that the primary producers that had colonized Mesoproterozoic land surfaces were likely desiccation-adapted photosynthetic microbes, similar to modern desert soil crust communities.