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The Gunflint Microbiota* Precambrian Research, 5 (1977) 121--142 121 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands THE GUNFLINT MICROBIOTA* STANLEY M. AWRAMIK and ELSO S. BARGHOORN Department of Geological Sciences, University of California, Santa Barbara, Calif. 93106 (U.S.A.) The Biological Laboratories, Harvard University, Cambridge, Mass. 02138 (U.S.A.) ABSTRACT Awramik, S.M. and Barghoorn, E.S., 1977. The Gunflint microbiota. Precambrian Res., 5: 121--142. The microbiota of the Gunflint Iron Formation (~2 Ga old) is sufficiently great in diversity as to represent a "benchmark" in the level of evolution at a time only somewhat less than intermediate between the origin of the earth and the present. To date, thirty entities from these ~ 2 Ga old microfossiliferous cherts have been de- scribed and all but two systematically categorized. From our continuing detailed study of the Gunflint microbiota (ESB for over 20 years) and, in light of our recent investigations on blue-green algal cell degradation, we conclude that: (1) A considerable number of the taxa systematically described are either of doubtful biological origin, doubtful taxonomic assignment, and/or morohologically indistinguishable from previously described Gunflint microorganisms. (2) The microbiota is wholly prokaryotic. At present, we recognize sixteen taxa falling within three categories: (1) blue-green algae (6 taxa; e.g. Gunflintia minuta); (2) budding bacteria (4 taxa; e.g. Eoastrion simplex); and (3) unknown affinities (6 taxa; e.g. Eosphaera tyleri). Organisms of undoubted euka- ryotic affinities have yet to be found in the Gunflint. The Gunfiint assemblage includes a high percentage of morphologic entities of obscure taxonomic position. Recently, Walter (1975) and Knoll and Barghoorn (1975) reported Gunfiint-type micro- biotas of approximately the same age as the Gunflint from two localities in Australia. The dominant morphotypes of the Gunflint microbiota appear to be cosmopolitan and the striking similarity of the three assemblages may strengthen the potential of ancient micro- biotas for use in Precambrian biostratigraphy. INTRODUCTION Through a remarkable combination of biological, sedimentological ~and geo- chemical events there is preserved in black cherts of the Gunflint Iron Forma- tion an assemblage of microorganisms of immense significance in the known Precambrian record of life. The Gunflint microbiota is sufficiently diverse as to represent a "benchmark" in the level of evolution at a time only somewhat less than intermediate between the origin of the earth and the present. *Contribution No. 64 of the Biogeology Clean Laboratory, University of California, Santa Barbara, Calif. 93106, U.S.A. 122 The potential importance of the Gunflint in the emerging field of Precam- brian paleobiology was first noted in a brief preliminary paper by Tyler and Barghoorn (1954) which was expanded in a subsequent report (Barghoorn and Tyler, 1965) and by other authors (Cloud, 1965; Cloud and Hagan, 1965; Deflandre, 1968; Licari and Cloud, 1968; Hofmann, 1969; 1971; Edhorn, 1973; Darby, 1974; Ka~mierczak, 1976; Tapman, 1976). To date thirty morphotypes from these ~2.109 year old microfossiliferous cherts have been described and all but two systematically categorized (Table I; the two bacterial morphotypes reported in Schopf et al. (1965) have not been classified). From our continuing detailed study of the microbiota (ESB for over twenty years) and, in light of our recent investigations on blue-green algal cell degradation (Awramik et al., 1972; Knoll and Barghoorn, 1975) we consider that (1) many of the described taxa are of doubtful biological origin, doubtful taxonomic assignment, or vaguely described (Table I), and most important, (2) we interpret the microbiota as wholly prokaryotic. At present, we recognize sixteen taxa, six of which are new and here described; one emended; the nine remaining taxa are retained, but we report additional data based on new observations of Gunflintia and Huroniospora. These six- teen taxa fall within three categories: (1) blue-green algae, (2) budding bac- teria, and (3) unknown affinities (Table II). The Gunflint type of microbiota is now represented by three other locali- ties of approximately the same age: (1) the Duck Creek Dolomite, northwes- tern Australia (Knoll and Barghoorn, 1975); (2) the Frere Formation in the Nabberu Basin, Western Australia (Walter et al., 1976); and (3) the Belcher Islands, N.W.T., Canada (Hofmann and Jackson, 1969; Hofmann, 1976}. All but the Belcher Islands microbiota share many common elements (except certain rare and bizarre morphotypes at present known only from the Gun- flint, most of which are presented here for the first time). Based on prelimin- ary reports, the Duck Creek and Frere microbiotas are dominated by forms also abundant in the Gunflint. It thus appears that the Gunflint microbiota was cosmopolitan and represents an accurate representation of the level of evolution some 2 Ga ago. GEOLOGIC SETTING AND LOCALITIES The Gunflint Iron Formation is exposed in almost continuous outcrop from west of Gunflint Lake to Thunder Bay some 180 km to the east and continues eastward in isolated exposures on the north shore of Lake Superior, an additional distance of some 120 km, to an area just west of Schreiber, Ontario, Canada (Fig.l). The geologic setting of the Gunflint has been described in detail (Goodwin, 1960; Moorhouse, 1960; Barghoorn and Tyler, 1965; Cloud, 1965; Hofmann, 1969; Morey, 1973) and only new information on a previously unreported locality need be presented here. The microfossils described in this report were discovered in cherts from 123 TABLE I Taxa reported from the Gunflint Iron Formation and their provisional status Taxon Accepted Taxon revised here or additional studies needed (see below) 1. Anabaenidium barghoornii Edhorn 1973 2. Animikiea septata Barghoorn 1965 3. Archaeorestis schreiberensis Barghoorn 1965 × 4. Chlamydomonopsis primordialis Edhorn 1973 5. Cumulusphaera lamellosa Edhorn 1973 6. Entosphaeroides amplus Barghoorn 1965 7. Eoastrion bifurcatum Barghoorn 1965 8. Eoastrion simplex Barghoorn 1965 5 9. Eomicrhystridium aremoricanum Deflandre 1968 × 10. Eosphaera tyleri Barghoorn 1968 × 11. Glenobotrydion aenigmatis Schopf 1968 (see Edhorn 1973) 12. Gunflintia grandis Barghoorn 1965 × 13. Gunflintia minuta Barghoorn 1965 × 14. Huroniospora macroreticulata Barghoorn 1965 15. Huroniospora microreticulata Barghoorn 1965 16. Huroniospora psilata Barghoorn 1965 17. Kakabekia umbellata Barghoorn 1965 × 18. Palaeoanacystis irregularis Edhorn 2 19. Palaeorivularia ontarica Korde 1958 (see Oehler 1976) 6 20. Palaeoscytonema moorhousi Edhorn 1973 2 21. Palaeospiralina minuta Edhorn 1973 2 22. Palaeospiralis canadensis Edhorn 1973 2 23. Palaeospirulina arcuata Edhorn 1973 2 24. Primorivularia thunderbayensis Edhorn 1973 2 25. "Schizothrix" atavia Edhorn 1973 2 26. Sphaerophycus gigas Edhorn 1973 2 27. Veryhachium? sp. Hofmann 1971 ?3 28. Menneria levis Lopukhin, 1971 (see also Lopukhin, 1975) 4 1. Revised or emended description herein. 2. Poor preservation, insufficient data, and in the light of degradation studies of Recent algae, probably related to better preserved taxa. 3. Biogenicity uncertain. 4. Not found in thin section. Additional data needed. 5. See Kline 1975 and in preparation. 6. Abiogenic. 7. Internal bodies may be degraded trichome cells, endospores, or prokaryotic guests. two localities (see Fig.lB); (1) the black stromatolitic cherts from the Schreiber Beach locality (Barghoorn and Tyler, 1965; Cloud, 1965) located along the north shore of Lake Superior (Fig.2). This area is approximately 6.4 km west of Schreiber Beach proper; and (2) non-stromatolitic, thinly bedded, gray to blue-gray cherts from a new microfossiliferous locality, 124 TABLE II Affinities of Gunflint morphotypes Blue-green algae X Gunflintia minuta tricnome X Gunflintia grandis trichome × Animikiea septata sheath × Huroniospora spp. coccoid-solitary × Corymbococcus hodgkissii coccoid-colonial aggregates × Enterosphaeroides amplus ?sheath Budding bacteria Eoastrion simplex Archaeorestis schreiberensis Archaeorestis magna (?) Kakabekia umbellata Unassignable affinities x Eomicrhystridium aremoricanum X Eosphaera tyleri × Xenothrix inconcreta × Thymos halis × Exochobrachium triangulum Galaxiopsis melanocentra × Found almost exclusively within stromatolitic laminae. 1 kilometer west of the mouth of the Blende River, along the north shore of Lake Superior west of the Sibley Peninsula (Fig.3). The new locality, which we informally call "Frustration Bay", occurs in the Upper Gunflint member west of the Blende River along the north shore of Lake Superior. Rock types consist of gray to blue-gray finely laminated cherts with subordinate black layers (Fig.2) interbedded with ferruginous car- bonates. The cherts are non-stromatolitic and frequently are brecciated within a ferruginous carbonate matrix. The Upper Gunflint member is very complex with extensive lateral variations in facies (Moorhouse, 1960). Poor exposure and lack of lateral continuity at this locality make detailed field relations diffi- cult to interpret. AGE OF THE GUNFLINT The Gunflint Iron Formation has a generally accepted age of approximately 2 Ga (Barghoorn and Tyler, 1965; Cloud, 1965). Though this 2 Ga old Animikie age is widely accepted, radiogenic ages range from 1685 + 24 Ma (a minimum age; Faure and Kovach, 1969) to 1900+200 Ma (Hurley et al., 1962). 125 N t Ontario ' sil reiber ~~...~~ ~;...:~:~i..'L~ii!'~!'i/'a'~':
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