2015 Annual Science Report
University of Wisconsin Reporting | JAN 2015 – DEC 2015
Project 3G: A 3,400 Ma-Old Shallow Water Anaerobic Sulfuretum Evidences the Anoxic Archean Atmosphere
Carbonaceous cherts of the ~3430 Ma Strelley Pool Formation contain innumerable “swirls” of fossilized sulfuretum bacteria encompassing quartz-replaced anhydrite nodules intermixed with layered assemblages of phototrophic filamentous fossil microbes. The geologic setting of the fossil-hosting unit, the preservation of the sulfuretum swirls adpressed to quartz pseudomorphs of precipitated anhydrite or gypsum, and the lack of physical disruption of the assemblage document its near-surface quiescent marine environment. The anaerobic physiology of the sulfuretum microbes indicates that Earth’s surface was anoxic. This exceedingly ancient biota is therefore interpreted to be composed of anaerobic H2S-producing sulfuretum microbes and H2S-using anoxygenic phototrophic bacteria. As such, this first-identified fossil microbial consortium provides firm evidence of the anoxia of Earth’s early environment.
Carbonaceous cherts of the ~3430 Ma Strelley Pool Formation (Anchor Ridge locality, Panorama greenstone belt, Western Australia) contain thousands of “swirls” of randomly oriented sulfuretum fossil bacteria encompassing quartz-replaced anhydrite nodules intermixed with layered assemblages of phototrophic filamentous fossilized microbes. Their distinctive swirled fabric, identical to that of recently described Mid-Precambrian sulfuretums and the anaerobic sub-surface of modern counterparts (Schopf et al., 2015), as well as the morphology of their component microorganisms and their enclosure of small ellipsoidal pseudomorphs after anhydrite, identifies them as a sulfuretum.
However, and unlike the two deep-sea mid-Precambrian sulfuretums communities recently described (Schopf et al., 2015), this Early Archean sulfuretum is preserved in a shallow water evaporite in which the preserved microbes are closely intermixed with (quartz-replaced) anhydrite and/or gypsum laths and rosettes. Both the sulfuretum swirls and the evaporitic pseudomorphs are outlined by copious amounts of syngenetic fine-grained pyrite. This evaporite-precipitating very shallow water environment must have been anoxic – evidenced by the copious pyrite and by the strictly anaerobic physiology of such sulfuretum bacteria. Thus, at ~3430 Ma, Earth’s surface environment was anoxic.
In addition to the cobweb sulfuretum swirls, the chert contains well-defined layered microbial fabric. Notably, the swirls and layered fabric are closely intermixed. The layered fabric indicates that its filamentous components were phototrophs; the intimate association of the swirls and layers indicates that microbes in both shared the same oxygen-relations; and because the sulfuretum microbes are anaerobes, this must have been true also of the phototrophs. Thus, the layered fabric must have been produced by anoxygenic photosynthetic bacteria (e.g., early-evolved Chloroflexaleans) rather than O2-producing cyanobacteria, or perhaps, by facultatively anoxygenic cyanobacteria (cf. modern Oscillatoria liminetica). In either case, the phototrophs would have consumed the sulfuretum-generated H2S to form a microbial consortium.
Examples of the cobweb sulfuretum swirls are shown in Figures 1 (outlined by authigenic pyrite) and 2 (cellularly three-dimensionally permineralized); Figures 3 through 6 illustrate representative carbonaceous filaments preserved in the sulfuretum swirls and the layered phototrophic fabric.
Insert Figues1 and 2, and 3-6 here
This study provides promise of documenting:
(1) An exceedingly old (~3430 Ma) fossil assemblage
(2) The 1st firmly established Archean sulfuretum
(3) The 1st evidence of anoxygenic Photosynthetic Bacteria in geologic record
(4) The 1st microbial consortium reported from geological record
(5) Strong paleobiologically based evidence of 3430 Ma anoxic Earth
As such, it appears to be of pivotal importance, providing a major advance in efforts to unravel the earliest history of life. Moreover, given the widespread occurrence of gypsiferous units on Mars, evidence of sulfate-/sulfide-metabolizing UV-resistant assemblages such as this might also be preserved in Mars rocks.
PROJECT INVESTIGATORS:James Schopf
PROJECT MEMBERS:Anatoliy Kudryavtsev
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere.
Co-evolution of microbial communities
Adaptation and evolution of life beyond Earth
Biosignatures to be sought in nearby planetary systems