Microbially Induced Sedimentary Structures from the 2.9 Ga old Pongola and Witwatersrand Supergroups, South Africa

Old Dominion University Research Foundation, Norfolk VA

Investigators

Abstract

EARTH'S EARLIEST BACTERIA IN A SANDY MARINE ENVIRONMENT: FOSSIL MICROORGANISMS FROM THE 2.9 BILLION YEARS OLD PONGOLA AND WITWATERSRAND SUPERGROUPS, SOUTH AFRICA PI: Nora Noffke; Department of Ocean, Earth & Atmospheric Sciences; 4600 Elkhorn Avenue; Norfolk, VA 23529; nnoffke@odu.edu. Bacteria and cyanobacteria form organic, carpet-like layers that cover large areas of coastal sediments. Such layers have been termed "microbial mats". In sandy marine sediments, microbial mats cause very characteristic sedimentary structures. These structures have been termed "microbially induced sedimentary structures - MISS", and they are known not only from modern marine environments, but from sandy sediments of all Earth ages. The fossil MISS include significant information about the ancient, mat-forming microorganisms, and therefore they also constitute a significant tool in deciphering the strange world of the earliest Earth, the Archean. Whereas many studies on Archean marine deposits focus on carbonate rocks or cherts, the PI follows a new approach by investigating sandstone successions in order to detect MISS. Therefore, the proposed study opens a new window in understanding Earth's earliest life and environments. A pilot study conducted by the PI in the Pongola Supergroup, South Africa, detected microbially induced sedimentary structures that provide evidence for the existence of filamentous bacteria forming microbial mats in 2.9 billion year old shelf environments - the oldest-known occurrence of microbial mats in siliciclastic (sandy) rocks (Noffke, Hazen & Nhleko, 2003). The structures include body fossils that resemble the cells of modern cyanobacteria, chloroflexi, or sulfur-oxidizing bacteria. The body fossils could indeed hint on the oldest cyanobacteria. Mineralogical, geochemical, and isotopic analyses are consistent with a biological origin of the structures. The follow-up investigations will elucidate the paleoenvironmental conditions that permit the development of microbial mats in the Archean. A detailed survey of MISS-bearing rock strata will document the distribution and variation of the ancient microbial mats. It will show, how different microbial mat types reflect the extreme life conditions characteristic for the Archean Earth. In addition, the study will set up a catalogue of MISS in order to compare the structures with those of younger Earth ages. The PI will organize a reference repository of MISS at Old Dominion University, which would serve the national and international geoscientific community for research and teaching.