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Variation in stable carbon isotopes in organic matter from the Gunflint Iron Formation
Geochimica et Cosmwhimica Acta,1971. Vol. 41, pp. 425 to 430. Pergamon Press.Printedin GreatBritam
Variation in stable carbon isotopes in organic matter from the Gunflint Iron Formation ELSOS. BARCH~ORN Departments of Biology and of Geology, Harvard University, Cambridge, Massachusetts 02138, U.S.A. ANDREWH. KNOLL Department of Geological Sciences, Harvard University, Cambridge, Massachusetts 02138, U.S.A. HARRYDEMBICKI,JR, and WARRENG. MEINSCHEIN Department of Geology, Indiana University, Bloomington, Indiana 47401, U.S.A. (Received
4 May 1976; accepted
in revised form 8 October
1976)
Abstract-In order to examine possible variations in organic carbon isotopic ratios within a single Precambrian formation, the kerogen separated from 15 samples of the approximately 2000m.y. old Gunflint Iron Formation and the conformably overlying Rove Formation, representing a wide range
of lithologies and geographic localities, was isotopically analyzed. From the resulting data, four conclusions can be drawn: (1) 6r3C values of the shallow water algal chert facies are significantly more negative (- 25 to -30x,) than those of the deeper water chert-carbonate and taconite facies (- 15 to -20x,). Comparative data for modem marine algal mats shows a range of 6r3C values from -8.4 to -19y&, PDB. Values obtained for fresh water mats were slightly more negative. (2) These differences in isotopic ratios can be correlated with similar differences in preserved microbiotas. (3) Anthraxolite lenses are depleted in r3C relative to the reduced carbon in surrounding sediments. (4) The effect of Keweenawan diabase intrusions upon the carbon isotopic composition is pronounced, but limited to the immediate vicinity of the contact.
of the reduced carbon. This has been clearly demonstrated for recent environments (see DEGENS,1969, pp. RECOGNITIONof the fact that photoautotrophic 313-314 for summary graphs) and it seems reasonable organisms preferentially incorporate “C (relative to that ancient sediments, differentiated into facies, the other stable carbon isotope 13C) into photosynmight also contain evidence for environment-related thetic products (RANKAMA,1948; WICKMAN, 1952; variation in the isotopic composition of photosyntheCRAIG, 1953; PARK and EPSTEIN,1960) has provided tically fixed carbon. geochemists with a powerful means of investigating For this reason, we decided that analysis of a ancient rock sequences. Several workers (HOERING, number of samples from a single Precambrian forma1967; OEHLERet al., 1972; MCKIRDY and POWELL, tion might provide a useful complement to existing 1974; EICHMANN and SCHIDLOWSKI, 1975) have isotopic data. The Gunflint lron Formation, Ontario, measured the stable isotopic ratios of reduced carbon presents a virtually unique opportunity for such a in series of sediments ranging in age from approxistudy for several reasons: (1) it is yell dated geomately 3400m.y. B.P. to the present. These analyses chronometrically; (2) it contains quite varied litholohave produced a consistent body of data which suggies; (3) over most of the outcrop area, regional metagests that the process of photosynthesis is at least morphism is very low to negligible, and contact metaas old as the oldest unmetamorphosed strata of the morphism is restricted to the immediate vicinity of Swaziland Supergroup, South Africa (approx. isolated dikes and sills; (4) it is accessible in outcrop 3400 m.y. B.P.). over virtually its entire extent along strike and, hence, It is well-known, however, that the extent of isois well known stratigraphically; and (5) the formation topic fractionation accompanying photosynthesis contains a complex and highly diverse assemblage of differs appreciably among groups of organisms microfossils. The latter circumstance is of particular (SMITH,1972). Some of this variation is undoubtedly import because it makes possible a correlation a result of different physiological pathways of carbon between biological source material and observed isofixation (HATCH and SLACK, 1970; SMITH and topic ratios of the reduced carbon. EPSTEIN,1972), but other differences are environmental in nature (DEGENS, 1969; TROUGHTON, 1972). GENERAL GEOLOGY Because of this important environmental effect, the The regional geology of the Gunflint Iron Formation carbon isotopic ratios of organic matter in sediments (Fig. 1) has been known since the early work of BRODERICK might be expected to reflect, in part, the habitat of (1920) and GILL (1924) and was summarized in detail by the plant community responsible for the production GOODWIN (1956, 1960) and M~~REHOUSE (1960). Further INTRODUCTION
425
426
E. S. BARGHOORN, A. H. KNOLL,H. DEMBICKI,JR and W. G. MEINSCHEIN
After
Goodwin
Sample to
Table
‘54
numbers
refer
I Fig. 1. Location map of the Gunflint and Rove formations in Western Ontario.
information, especially with respect to the easternmost outcrops of the formation, was reported by BARCHO~RNand TYLER(1965). The cyclical nature of sedimentation in the Gunflint basin was clearly established by GOODWIN(1956) who defined upper and lower sedimentary members having remarkably similar facies sequences, each comprised of a basal algal chert followed by shale, chert-taconite, and chert-carbonate units. Each sequence documents a transgressiveregressive cycle. It is primarily from the study of the algal chert units, especially the lower chert, that the paleobiological significance of the formation was first established (TYLERand BARGHOORN, 1954). As a basin sequence the Gunflint Formation is both remarkably persistent and notably thin over its known extent of some 275 km from Gunflint Lake on the MinnesotaOntario border to its point of submergence beneath Lake Superior in the Rossport-Schreiber area of Ontario. A detailed petrological description of the Gunhint Formation is not pertinent to this paper, but it should be noted that the formation displays a very low degree of regional metamorphism (BARGHOORN and TYLER,1965) which decreases from southwest to northeast (G~XDWIN, 1956). Metamorphism is greatest in the area of Gunflint Lake where the sediments were intruded by the Duluth Gabbro. Eastward, metamorphic episodes are restricted to intrusions of diabase dikes and sills of Keweenawan age (112&1140 m.y. B.P.) which, within the outcrop area, are usually of minor consequence. SAMPLE SELECTION, DESCRIPTION, AND PREPARATION The collections upon which this study is based were made in July, 1974, at a series of localities along the strike of the Gunflint Formation. Samples were selected to represent as wide a range as possible of lithologies containing reduced carbon. In addition, especially carbon-rich (= 93.5% C) samples of anthraxolite from two widely separated occurrences in the lower Gunflint Formation were included. Finally, a highly carbonaceous black fissile shale from the conformably overlying Rove Formation was obtained for comparison. To test the effect of contact metamorphism on carbon isotopic composition a special suite of carbonaceous shale samples was collected from immediately adjacent to a 1 m thick diabase sill and at carefully determined short intervals up to 30cm from the contact. Sampling localities are indicated on the inset map in Fig. 1. Samples were treated with maximum care to exclude contamination by handling or any other extraneous carbon source before laboratory analysis. The chert and shale samples were crushed to a fine powder (~400 mesh) in an Angstrom disc mill and stored in acid-cleaned glass vials.
Analytical procedures were carried out at Indiana University. These involved initial exhaustive extraction of the powdered rocks with 2: iv/v benzene-methanol in a Soxhlet extractor. Carbonates and silicates were then removed by successive digestion with 1 N HCl and a solution of 5: 1 v/v HF-HCl. The resulting kerogen was separated from acid-resistant mineral grains by the chloroform sinkk float technique of MCKIRDYand POWELL(1974). Combustion and generation of CO1 were done using procedures described by RINALDI(1974). The CO2 was trapped in breakseal tubes and each sample transferred to a Varian GD-150 mass spectrometer for 6r3C analysis. i3C/izC ratios are reported in ¬ation, as per mil deviations from the “C/“C ratio of the PDB standard. 613C sample = (r3C/“C sample - ‘3C/12C standard) (i3C/“C
standard)
x 1000.
The results of the 6i3C analyses are shown in Table 1. DISCUSSION Before giving specific consideration to the tabulated data, a brief discussion of the organisms, and indeed the parts of organisms, which contributed to the Gunflint kerogen is relevant. The entire microbial assemblage responsible for the organic matter in these strata was prokaryotic (BARGHOORNand TYLER, 1965; CLOUD, 1965). Modern prokaryotic communities can be found in algal mats, and carbon isotopic compositions of the organic matter in a representative sample of these is given in Table 2. These values are in essential agreement with previously published figures for other mats (BEHRENS and FRISHMAN, 1971; CALDER and PARKER, 1973). Lipid fractions of these assemblages (where available) are depleted in i3C relative to whole-mat samples, a relationship which also holds for pure cultures of photoautotrophic and photoheterotrophic bacteria (Table 3; see also ABELSON and HOERING, 1961; DEGENS, 1969). (The relative i3C enrichment evidenced for those photoheterotrophs grown on succinate may reflect the utilization of portions of the succinate molecule which are preferentially enriched in i3C.) DEGENS (1969, p. 312) has tabulated data which shows the lipids to be among the most i3C-depleted carbon compounds in cells. A
Organic matter from the Gunflint Iron Formation Table 1. 613C values for organic carbon preserved in the Gunflint and Rove formations FXies
Lithology A. Gunflint Formation 1. Stromatolitic chat 2. Oolitic chert 3. Non-stromatolitic chert 4. Stromatolitic chert 5. Anthrax&e lin chertl 6. Anthraxolite (in cherti 7. Shale (above algal chert) 8. Chat 9. Chert 10. Shale (in cbert-tacomte) B. Rove Formation 11. Shale C. Lower Gunflint shale Intruded
Location
Lower algal chat Lower alaal chert Lower &al chat Lower algal chat Lower aleal chert Lower &al chert Shale Chert-carbonate Chert-taconite Chert-taconite
by a 1 m thick
Shale diabase sill At contact 2.5 cm below antact 7.5 cm below contact 30 cm below contact
Schreiber beach Pass Lake R. R. Bridee Kakabeka Falls Nolalu N&hl Schreiber beach Near Schreiber beach ‘Frustration Bav’ Kakabeka Fall; Kakabeka Falls
-24.96 -29.13 - 29.2’ -28.18 - 37.01 - 39.2 1 - 34.0 - 19.56 -15.73 -20.13
+ 0.08 + 0.08 2 0.08 & 0.05t + 0.12 IO.11 f 0.11
Pass Lake
-33.63
+ 0.07
- 12.4 -24.77 -29.87 -23.87
+ + ; +
Flint Flint Flint Flint
Quarry
Island Island Island Island
k 0.13 f 0.09
0.08 0.11 0.14 0.14
* Data from HOERING (1967). t Average of six shale samples in vertical sequence.
Table 2. 6i3C values for reduced carbon in modern blue-green algal mats (and a mat-derived laboratory culture) 6’T
whole
SOUrCe
lipid fraction (U
-21.1 - 18.5 -21.5
- 30.2
Average
- 14.6
- 22.2
Dominated Dominated $ Dominated 4 Dominated t
(1)
Rhodospirillum palusrris (H) Rhodospirillum rubrum (H) Rhodospirillum rubrum (H) Rhodopseudomonas sphoeroides (H) Chlorobium fhiosul/ataphilum (A) Chromarium sp. (A) Chlorooseudomonas ethvlica CA.H?) Rhodo,kwdomona.s g&tin& (H)
-11.7 - 15.7 -19.1 - 10.6 -x4 -x4 -11.1
by Entuphysalis. by Lyngbya and Entophysalis. by Schizothrix. by Lyngbya and Synechocystis.
Table 3. Si3C values of whole-organism
I. 2. 3. 4. 5. 6. 7. 8.
6°C
(Marine) 1. Hypersaline pool, Red Sea 2. Sumatidal mat. Caue Cod 3. I&tidal mat, Sha;k Bay, Austr.* 4. Intertidal mat, Shark Bay, Austr.t 5. Bahia Honda Bahamas 6. Submerged mat, Shark Bay, Austr.f 7. Crane Key, Florida (Fresh water) 8. Green Lake, New York 9. Fairy Meadows, Yellowstone Park 10. Laboratory mixed culture$
*
Organism
sample
(%)
_ _ __ - 14.2 _ _ _
Grown in air at 25°C.
and lipid fractions of photoautotrophic cultures)
c
m)
Source
Succinate Succinate Yeast extract SUCCitlZik
Atm. CO, Atm. CO2 ? Yeast extract
613C (? s0urce - 26.0 - 26.0 -21.0 - 26.0
_
-21.0
and photoheterotrophic
Whole sample wt (w) 20.86 5.95 3.53 39.96 2.31 4.35 5.52 17.99
bacteria (pure
Lipid fraction wt 6°C
(” .m )
-17.8 - 19.0 - 22.7 - 14.2 - 20.8 - 19.7 - 24.8 -21.2
0%) 15.10 4.33 1.74 1736 1.57 3.52 3.89 3.69
6°C (T&) -19.6 - 26.4 -26.1 - 16.9 - 20.7 - 26.4 - 27.6 -25.3
* (A) denotes a photoautotrophic bacterium. (H) denotes a photoheterotroph. Lipid extractions were made in all-glass systems using 3: 1 v/v benzene-methanol for 72 hr, solvent removed with N2 at 40°C. Each fraction washed three times in dilute HCI, followed by distilled water.
comparison of the 613C for Gunflint kerogen, modern mats, and the lipid portions of constituent microbes lends support to the hypothesis of MCKIRDY (1974) that degradationally resistant lipids may play an important role in kerogen forma:’ -r Recently PHILP and CALVIN (1976) have suggeste. Lthat kerogen may form from cell wall and sheath polysaccharides about
which lipids condense with time. This idea is also supported by our data as well as by observations of degraded algal cultures and fossil remains both of which indicate that degradationally resistant cyanophyte sheath material is preferentially preserved and hence likely to serve as a source for kerogen. With reference to the data presented in Table 1,
428
E. S. BARGHOORN, A. H. KNOLL,H. DEMBICKI,JR and W. G. MEINSCHEIN
one can make four statements, cussed below. They are:
each of which is dis-
(1) The basal Gunflint algal chert and shale facies are depleted in 13C relative to the chert-carbonate and taconite facies. (2) Differences in the 613C values between Gunflint facies correlate with marked differences in their biological source materials as evidenced by their respective microbiotas (Fig. 2). (3) The anthraxolites are anomolously “C-depleted relative to the kerogen of their encompassing cherts and shales. (4) The effects of igneous intrusion and concomitant thermal alteration are shown by a marked loss of “C at the contact. Variations in organic carbon isotopic ratios of kerogen from different Gunflint units appear to be facies-controlled and are not the products of metamorphism. Geographic variations of 6l 3C do not correlate with the pattern of regional metamorphism. Indeed, at a single locality, Kakebeka Falls, there is a marked contrast between 613C of the basal chert and the upper units. As shown below, the metamorphic effects of the Keweenawan sills are quite limited and can also be removed from consideration. Thus, we interpret the data to indicate original isotopic differences in the biological source material of the reduced carbon in the three environments. The range of 613C values obtained for the Gunflint Formation is approximately the same as that found for Precambrian sediments from around the world (HOERING, 1967; OEHLER et al., 1972; MCKIRDY and POWELL, 1974; EICHMANN and SCHIDLOWSKI, 1975). It indicates that considerations of facies are applicable to interpretation of the data from other Precambrian localities. The algal cherts of the lower Gunflint were deposited in a near-shore environment in a restricted, presumably marine, basin. Shallow water is indicated by the common presence of oolites and clasts or eroded algal mats, as well as by the stromatolites which often occur in profusion (AWRAMIK, 1973). The flora of this facies, as described in detail by BARGHOORN and TYLER (1965), is quite diverse, but the overwhelmingly dominant taxa are a slender filamentous blue-green alga, Gunjlintia, and a coccoid taxon (perhaps representing several cyanophytic species), Huroniospora (Fig. 2A). Although numerous other algae and bacteria occur in relatively small populations, the primary source of the organic carbon in the stromatolitic chert is these two organisms. In the non-stromatolitic lower chert at Kakabeka Falls, the problematical bacterium Kakabekia replaces Gunjintia as a major source of carbon. The Gunflint chert-carbonate facies and its lateral equivalent, the chert-taconite facies, were deposited in deeper water. Shallow water sedimentary features are absent, and the abundant siderite and pyrite of the cherttcarbonate unit have been interpreted to in-
dicate stagnant conditions in the central parts of a restricted basin (JAMES, 1954; GOODWIN, 1956). Further evidence of basin restriction is provided by halite and gypsum casts which occur in this facies (Table 1 (8, 9 and 10)). The flora here is dominated by a budding bacterium (Eoastrion) morphologically indistinguishable from the extant chemoheterotrophic genus Metallogenium (KLINE, in press) (Fig. 2B). Apparently planktonic coccoid blue-green algae (Fig. 2C) also can be found (KNOLL et al., in preparation), but they are not major components of the chert-carbonate flora, although these and other planktonic cyanophytes did provide a carbon source for the bottom community. Thus, the differing environments of the Gunflint basin supported distinct microbial communities which were responsible for the metabolism and different i3C isotopic fractionation of the organic matter preserved as kerogen. That is to say, the differences are primarily the direct result of different fractionation ratios in contrasting biological populations (Fig. 2) and not due to depositional environment or diagenetic history. In the lower algal cherts, atmospheric or dissolved COZ was fixed by blue-green algae and photosynthetic bacteria which in turn were metabolized by the other bacterial members of the assemblage with a net accumulation of lipid and sheath polysaccharide derivatives. Detrital organic matter provided carbon to the bottom-dwelling microbes of the chert-carbonate facies. Judging from the preserved fossils, it appears that this bacterial community was less diverse than the algal chert assemblages and, hence, may have been less versatile or efficient in its ability to metabolize the rain of organic matter from above. No microfossils are preserved in the carbonaceous shales, but we assume that the organic carbon isotopic ratios again reflect differences in environment and microbial communities. The anthraxolite is the most i3C-depleted material analyzed within the entire formation. Although the origin of this substance is poorly understood, its physical and chemical properties are well known. The material is a virtually pure carbon-hydrogen substance, containing only l-2’% oxygen. It is a completely amorphous solid, devoid of elastic constituents, exhibiting conchoidal fracture, and occurring in lenses up to 3 cm thick. It is suggested here that it originated as a petroleum or crude oil complex which migrated as liquid into fractures and sedimentary bedding planes where it polymerized in situ forming a solid, secondarily emplaced constituent of the sediment. If this model for the genesis of anthraxolite is correct, the depletion of 13C in this material may be attributable to the ‘cracking’ of “C-enriched petroleum-like materials to form alkene monomers which in turn would polymerize. As suggested by EICHMANN and SCHIDLOWSKI (1975), ‘chromatographic’ effects may also contribute to the isotopic fractionation of these originally mobile hydrocarbons. The effects of metamorphism on carbon in sedi-
Fig. 2. A. Stromatolitic flora in lower algal chert near Schreiber, Ontario. B. Bacterial bottom flora in chert-carbonate facies, ‘Frustration Bay’, Ontario. C. Apparently planktonic cyanophytes in chertcarbonate facies. ‘Frustration Bay’. Ontario. (Bar = ?Oprn in all cases.)
428
Organic matter from the Gunflint Iron Formation
ments have been considered by several workers (FRENCH, 1964; SILVERMAN,1964; MCKLRDY and POWELL, 1974). As mentioned above, Keweenawan diabase sills intruded the Gunflint Iron Formation some 800-900 m.y. after its deposition. Data in Table 1 suggest that this event caused profound but very localized alterations in the carbon isotopic ratios of kerogen. It is interesting, however, that even a relatively small intrusion can produce a sizable reduction in the relative abundance of “C. The pyrolytic effect apparently reflects loss of CH, from the carbon complexes in the shale at the time of intrusion. It would be of interest to examine more closely the possible changes in carbon isotopic ratios in Precambrian sediments which have undergone large scale thermal alteration such as might be induced by the injection of large granitic plutons. FRENCH(1964) examined the crystallinity of reduced carbon in sediments from the Biwabik Formation (the stratigraphic equivalent of the Gunflint Formation in Minnesota) which were intruded by the massive Duluth Gabbro, and he found that originally amorphous organic matter was altered to graphite throughout a zone extending nearly 3 km from contact. He did not determine 613C values. Alteration of isotopic values by regional metamorphism is germane to considerations of Precambrian sedimentary organic matter because it may explain carbon described by the relatively ‘%-enriched O~ZHLER et al. (1972) from the 3400m.y. old Theespruit Formation, South Africa (BROOKSet al., 1973; KARKICIANIS, 1975). Detailed examination of these older rocks is presently in progress. CONCLUSIONS To summarize, it is possible in certain circumstances to discern facies-related variations in stable carbon isotope ratios for reduced carbon in individual Precambrian formations. In the case of the approximately 2000 m.y. old Gunflint Iron Formation, a three-way correspondence among geological features (which provide information on depositional environments), microfossil populations, and organic carbon isotope ratios can be noted. The demonstration that not all kerogens are isotopically alike underscores the importance of facies data to the interpretation of t3C/1*C ratios of ancient organic matter. Although anthraxolite is an attractive substance for isotopic analysis, it is, in the Gunflint Formation at least, quite unrepresentative of the carbon isotope ratios of the shale and chert kerogen of the formation as a whole. Finally, as might have been predicted, the variation in carbon isotope ratios discernible in the modern world seems also to have been present some two billion years ago. ~ck~owledge~lents-We thank Drs. Rrcrrmn LEO, SOL SILVERMAN,and J. M. HAYESfor their invaluable assistance in obtaining the isotopic data used in this report. Drs. LYNN MARGULISand STJEPKOG~LUBIC provided most of the mats and cultures listed in Tables 2 and 3. The help 0.C.A. 4113-F
429
of Dr. STANLEYAWAMIK in the field is gratefully acknowledged. Also, we thank the reviewers of this paper whose many helpful criticisms measurably improved our report. Financial assistance was provided by NASA Grant NGL 22-007-069 and NSF Grant DES 73-06514 to E.S.B. and an NSF Graduate Fellowship to A.H.K.
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AWRAMIKS. M. (1973) The origin and evolution of stromatolites with special reference to the Gunflint Iron Formation. Unpublished Ph.D. thesis, Harvard University, 262 pp. BARGHWRNE. S. and TYLERS. A. (1965) Microorganisms from the Gunflint chert. Science 147, 563-577. BEHRENSE. W. and FRISHMANS. A. (1971) Stable carbon isotopes in blue-green algal mats. J. Geol. 79, 94100. BRODERICKT. M. (1920) Economic geology and stratigraphy of the Gunflint iron district, Minnesota. Econ. Geol. 15, 422452. BRINKS .I., MUIR M. D. and SHAW G. (1973) Chemistry and morphology of Precambrian organisms. Nature 244, 215-217. CALDERJ. A. and PARKERP. L. (1973) Geochemical implications of induced changes in “C fractionation by bluegreen algae. Geochim. Cosmochim. Acta 37, 133-140. CLOUD P. E. (1965) Significance of the Gunflint (Precambrian) microflora. Science 148, 27-34. CRAIG H. (1953) The geochemistry of stable carbon isotopes. Geochim. Cosmochim. Acta 3, 53-92. DEGENSE. T. (1969) Biogeochemistry of stable carbon isotopes. In Organic Geochemistry: Methods and Results, (editors G. Egfinton and M. T. .I. Murphy), pp. 304329. Springer. EICHMANN R. and SCHIDLOWSK~ M. (1975) Isotopic fractionation between coexisting organic carbon~arbonate pairs in Precambrian sediments. Geoch~m. Cusm~ch~m. Acta 39, 585-595.
FRENCHB. M. (1964) Graphitization of organic matter in a progressively metamorphosed Precambrian iron formation. Science 146, 917-918. GILL J. E. (1924) The Gunflint Iron-bearing Formation, Ontario. Summary Rep., Geol. Surv. Can. 1924, Part C, 28-88. GOODWIN A. M. (1956) Facies relations in the Gunflint Iron Formation. Econ. Geol. 51, 565-595. EDWIN A. M. (1960) The Gunflint Iron Formation of the Whitefish Lake area. Ont. Dept. Mines, Ann. Rep. 6!?(7), 41-63.
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sedimentary organic matter: new evidence from Australia and South Africa. Geology 2, 591-595. M~~REHOUSEW. W. (1960) Gunflint Iron Range in the vicinity of Port Arthur. Ont. Dept Mines, Ann. Rep. 69(7), l-40. OEHLERD. Z., SCHOPFJ. W. and KVENVOLDEN K. A. (1972) Carbon isotopic studies of organic matter in Precambrian rocks. Science 175, 12461248. PARK R. and EPSTEINS. (1960) Carbon isotopic fractionation during photosynthesis. Geochim. Cosmochim. Acta 21, llc126. PHILP R. P. and CALVIN(1976) Possible origin for insoluble organic (kerogen) debris in sediments from insoluble cell-wall materials of algae and bacteria. Nature 262, 134136. RANKAMAK. (1948) A note on the original isotopic composition of terrestrial carbon. J. Geol. 56, 199-209. RINADI G. G. L. (1974) Intramolecular distribution of
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Variation in stable carbon isotopes in organic matter from the Gunflint Iron Formation ELSOS. BARCH~ORN Departments of Biology and of Geology, Harvard University, Cambridge, Massachusetts 02138, U.S.A. ANDREWH. KNOLL Department of Geological Sciences, Harvard University, Cambridge, Massachusetts 02138, U.S.A. HARRYDEMBICKI,JR, and WARRENG. MEINSCHEIN Department of Geology, Indiana University, Bloomington, Indiana 47401, U.S.A. (Received
4 May 1976; accepted
in revised form 8 October
1976)
Abstract-In order to examine possible variations in organic carbon isotopic ratios within a single Precambrian formation, the kerogen separated from 15 samples of the approximately 2000m.y. old Gunflint Iron Formation and the conformably overlying Rove Formation, representing a wide range
of lithologies and geographic localities, was isotopically analyzed. From the resulting data, four conclusions can be drawn: (1) 6r3C values of the shallow water algal chert facies are significantly more negative (- 25 to -30x,) than those of the deeper water chert-carbonate and taconite facies (- 15 to -20x,). Comparative data for modem marine algal mats shows a range of 6r3C values from -8.4 to -19y&, PDB. Values obtained for fresh water mats were slightly more negative. (2) These differences in isotopic ratios can be correlated with similar differences in preserved microbiotas. (3) Anthraxolite lenses are depleted in r3C relative to the reduced carbon in surrounding sediments. (4) The effect of Keweenawan diabase intrusions upon the carbon isotopic composition is pronounced, but limited to the immediate vicinity of the contact.
of the reduced carbon. This has been clearly demonstrated for recent environments (see DEGENS,1969, pp. RECOGNITIONof the fact that photoautotrophic 313-314 for summary graphs) and it seems reasonable organisms preferentially incorporate “C (relative to that ancient sediments, differentiated into facies, the other stable carbon isotope 13C) into photosynmight also contain evidence for environment-related thetic products (RANKAMA,1948; WICKMAN, 1952; variation in the isotopic composition of photosyntheCRAIG, 1953; PARK and EPSTEIN,1960) has provided tically fixed carbon. geochemists with a powerful means of investigating For this reason, we decided that analysis of a ancient rock sequences. Several workers (HOERING, number of samples from a single Precambrian forma1967; OEHLERet al., 1972; MCKIRDY and POWELL, tion might provide a useful complement to existing 1974; EICHMANN and SCHIDLOWSKI, 1975) have isotopic data. The Gunflint lron Formation, Ontario, measured the stable isotopic ratios of reduced carbon presents a virtually unique opportunity for such a in series of sediments ranging in age from approxistudy for several reasons: (1) it is yell dated geomately 3400m.y. B.P. to the present. These analyses chronometrically; (2) it contains quite varied litholohave produced a consistent body of data which suggies; (3) over most of the outcrop area, regional metagests that the process of photosynthesis is at least morphism is very low to negligible, and contact metaas old as the oldest unmetamorphosed strata of the morphism is restricted to the immediate vicinity of Swaziland Supergroup, South Africa (approx. isolated dikes and sills; (4) it is accessible in outcrop 3400 m.y. B.P.). over virtually its entire extent along strike and, hence, It is well-known, however, that the extent of isois well known stratigraphically; and (5) the formation topic fractionation accompanying photosynthesis contains a complex and highly diverse assemblage of differs appreciably among groups of organisms microfossils. The latter circumstance is of particular (SMITH,1972). Some of this variation is undoubtedly import because it makes possible a correlation a result of different physiological pathways of carbon between biological source material and observed isofixation (HATCH and SLACK, 1970; SMITH and topic ratios of the reduced carbon. EPSTEIN,1972), but other differences are environmental in nature (DEGENS, 1969; TROUGHTON, 1972). GENERAL GEOLOGY Because of this important environmental effect, the The regional geology of the Gunflint Iron Formation carbon isotopic ratios of organic matter in sediments (Fig. 1) has been known since the early work of BRODERICK might be expected to reflect, in part, the habitat of (1920) and GILL (1924) and was summarized in detail by the plant community responsible for the production GOODWIN (1956, 1960) and M~~REHOUSE (1960). Further INTRODUCTION
425
426
E. S. BARGHOORN, A. H. KNOLL,H. DEMBICKI,JR and W. G. MEINSCHEIN
After
Goodwin
Sample to
Table
‘54
numbers
refer
I Fig. 1. Location map of the Gunflint and Rove formations in Western Ontario.
information, especially with respect to the easternmost outcrops of the formation, was reported by BARCHO~RNand TYLER(1965). The cyclical nature of sedimentation in the Gunflint basin was clearly established by GOODWIN(1956) who defined upper and lower sedimentary members having remarkably similar facies sequences, each comprised of a basal algal chert followed by shale, chert-taconite, and chert-carbonate units. Each sequence documents a transgressiveregressive cycle. It is primarily from the study of the algal chert units, especially the lower chert, that the paleobiological significance of the formation was first established (TYLERand BARGHOORN, 1954). As a basin sequence the Gunflint Formation is both remarkably persistent and notably thin over its known extent of some 275 km from Gunflint Lake on the MinnesotaOntario border to its point of submergence beneath Lake Superior in the Rossport-Schreiber area of Ontario. A detailed petrological description of the Gunhint Formation is not pertinent to this paper, but it should be noted that the formation displays a very low degree of regional metamorphism (BARGHOORN and TYLER,1965) which decreases from southwest to northeast (G~XDWIN, 1956). Metamorphism is greatest in the area of Gunflint Lake where the sediments were intruded by the Duluth Gabbro. Eastward, metamorphic episodes are restricted to intrusions of diabase dikes and sills of Keweenawan age (112&1140 m.y. B.P.) which, within the outcrop area, are usually of minor consequence. SAMPLE SELECTION, DESCRIPTION, AND PREPARATION The collections upon which this study is based were made in July, 1974, at a series of localities along the strike of the Gunflint Formation. Samples were selected to represent as wide a range as possible of lithologies containing reduced carbon. In addition, especially carbon-rich (= 93.5% C) samples of anthraxolite from two widely separated occurrences in the lower Gunflint Formation were included. Finally, a highly carbonaceous black fissile shale from the conformably overlying Rove Formation was obtained for comparison. To test the effect of contact metamorphism on carbon isotopic composition a special suite of carbonaceous shale samples was collected from immediately adjacent to a 1 m thick diabase sill and at carefully determined short intervals up to 30cm from the contact. Sampling localities are indicated on the inset map in Fig. 1. Samples were treated with maximum care to exclude contamination by handling or any other extraneous carbon source before laboratory analysis. The chert and shale samples were crushed to a fine powder (~400 mesh) in an Angstrom disc mill and stored in acid-cleaned glass vials.
Analytical procedures were carried out at Indiana University. These involved initial exhaustive extraction of the powdered rocks with 2: iv/v benzene-methanol in a Soxhlet extractor. Carbonates and silicates were then removed by successive digestion with 1 N HCl and a solution of 5: 1 v/v HF-HCl. The resulting kerogen was separated from acid-resistant mineral grains by the chloroform sinkk float technique of MCKIRDYand POWELL(1974). Combustion and generation of CO1 were done using procedures described by RINALDI(1974). The CO2 was trapped in breakseal tubes and each sample transferred to a Varian GD-150 mass spectrometer for 6r3C analysis. i3C/izC ratios are reported in ¬ation, as per mil deviations from the “C/“C ratio of the PDB standard. 613C sample = (r3C/“C sample - ‘3C/12C standard) (i3C/“C
standard)
x 1000.
The results of the 6i3C analyses are shown in Table 1. DISCUSSION Before giving specific consideration to the tabulated data, a brief discussion of the organisms, and indeed the parts of organisms, which contributed to the Gunflint kerogen is relevant. The entire microbial assemblage responsible for the organic matter in these strata was prokaryotic (BARGHOORNand TYLER, 1965; CLOUD, 1965). Modern prokaryotic communities can be found in algal mats, and carbon isotopic compositions of the organic matter in a representative sample of these is given in Table 2. These values are in essential agreement with previously published figures for other mats (BEHRENS and FRISHMAN, 1971; CALDER and PARKER, 1973). Lipid fractions of these assemblages (where available) are depleted in i3C relative to whole-mat samples, a relationship which also holds for pure cultures of photoautotrophic and photoheterotrophic bacteria (Table 3; see also ABELSON and HOERING, 1961; DEGENS, 1969). (The relative i3C enrichment evidenced for those photoheterotrophs grown on succinate may reflect the utilization of portions of the succinate molecule which are preferentially enriched in i3C.) DEGENS (1969, p. 312) has tabulated data which shows the lipids to be among the most i3C-depleted carbon compounds in cells. A
Organic matter from the Gunflint Iron Formation Table 1. 613C values for organic carbon preserved in the Gunflint and Rove formations FXies
Lithology A. Gunflint Formation 1. Stromatolitic chat 2. Oolitic chert 3. Non-stromatolitic chert 4. Stromatolitic chert 5. Anthrax&e lin chertl 6. Anthraxolite (in cherti 7. Shale (above algal chert) 8. Chat 9. Chert 10. Shale (in cbert-tacomte) B. Rove Formation 11. Shale C. Lower Gunflint shale Intruded
Location
Lower algal chat Lower alaal chert Lower &al chat Lower algal chat Lower aleal chert Lower &al chert Shale Chert-carbonate Chert-taconite Chert-taconite
by a 1 m thick
Shale diabase sill At contact 2.5 cm below antact 7.5 cm below contact 30 cm below contact
Schreiber beach Pass Lake R. R. Bridee Kakabeka Falls Nolalu N&hl Schreiber beach Near Schreiber beach ‘Frustration Bav’ Kakabeka Fall; Kakabeka Falls
-24.96 -29.13 - 29.2’ -28.18 - 37.01 - 39.2 1 - 34.0 - 19.56 -15.73 -20.13
+ 0.08 + 0.08 2 0.08 & 0.05t + 0.12 IO.11 f 0.11
Pass Lake
-33.63
+ 0.07
- 12.4 -24.77 -29.87 -23.87
+ + ; +
Flint Flint Flint Flint
Quarry
Island Island Island Island
k 0.13 f 0.09
0.08 0.11 0.14 0.14
* Data from HOERING (1967). t Average of six shale samples in vertical sequence.
Table 2. 6i3C values for reduced carbon in modern blue-green algal mats (and a mat-derived laboratory culture) 6’T
whole
SOUrCe
lipid fraction (U
-21.1 - 18.5 -21.5
- 30.2
Average
- 14.6
- 22.2
Dominated Dominated $ Dominated 4 Dominated t
(1)
Rhodospirillum palusrris (H) Rhodospirillum rubrum (H) Rhodospirillum rubrum (H) Rhodopseudomonas sphoeroides (H) Chlorobium fhiosul/ataphilum (A) Chromarium sp. (A) Chlorooseudomonas ethvlica CA.H?) Rhodo,kwdomona.s g&tin& (H)
-11.7 - 15.7 -19.1 - 10.6 -x4 -x4 -11.1
by Entuphysalis. by Lyngbya and Entophysalis. by Schizothrix. by Lyngbya and Synechocystis.
Table 3. Si3C values of whole-organism
I. 2. 3. 4. 5. 6. 7. 8.
6°C
(Marine) 1. Hypersaline pool, Red Sea 2. Sumatidal mat. Caue Cod 3. I&tidal mat, Sha;k Bay, Austr.* 4. Intertidal mat, Shark Bay, Austr.t 5. Bahia Honda Bahamas 6. Submerged mat, Shark Bay, Austr.f 7. Crane Key, Florida (Fresh water) 8. Green Lake, New York 9. Fairy Meadows, Yellowstone Park 10. Laboratory mixed culture$
*
Organism
sample
(%)
_ _ __ - 14.2 _ _ _
Grown in air at 25°C.
and lipid fractions of photoautotrophic cultures)
c
m)
Source
Succinate Succinate Yeast extract SUCCitlZik
Atm. CO, Atm. CO2 ? Yeast extract
613C (? s0urce - 26.0 - 26.0 -21.0 - 26.0
_
-21.0
and photoheterotrophic
Whole sample wt (w) 20.86 5.95 3.53 39.96 2.31 4.35 5.52 17.99
bacteria (pure
Lipid fraction wt 6°C
(” .m )
-17.8 - 19.0 - 22.7 - 14.2 - 20.8 - 19.7 - 24.8 -21.2
0%) 15.10 4.33 1.74 1736 1.57 3.52 3.89 3.69
6°C (T&) -19.6 - 26.4 -26.1 - 16.9 - 20.7 - 26.4 - 27.6 -25.3
* (A) denotes a photoautotrophic bacterium. (H) denotes a photoheterotroph. Lipid extractions were made in all-glass systems using 3: 1 v/v benzene-methanol for 72 hr, solvent removed with N2 at 40°C. Each fraction washed three times in dilute HCI, followed by distilled water.
comparison of the 613C for Gunflint kerogen, modern mats, and the lipid portions of constituent microbes lends support to the hypothesis of MCKIRDY (1974) that degradationally resistant lipids may play an important role in kerogen forma:’ -r Recently PHILP and CALVIN (1976) have suggeste. Lthat kerogen may form from cell wall and sheath polysaccharides about
which lipids condense with time. This idea is also supported by our data as well as by observations of degraded algal cultures and fossil remains both of which indicate that degradationally resistant cyanophyte sheath material is preferentially preserved and hence likely to serve as a source for kerogen. With reference to the data presented in Table 1,
428
E. S. BARGHOORN, A. H. KNOLL,H. DEMBICKI,JR and W. G. MEINSCHEIN
one can make four statements, cussed below. They are:
each of which is dis-
(1) The basal Gunflint algal chert and shale facies are depleted in 13C relative to the chert-carbonate and taconite facies. (2) Differences in the 613C values between Gunflint facies correlate with marked differences in their biological source materials as evidenced by their respective microbiotas (Fig. 2). (3) The anthraxolites are anomolously “C-depleted relative to the kerogen of their encompassing cherts and shales. (4) The effects of igneous intrusion and concomitant thermal alteration are shown by a marked loss of “C at the contact. Variations in organic carbon isotopic ratios of kerogen from different Gunflint units appear to be facies-controlled and are not the products of metamorphism. Geographic variations of 6l 3C do not correlate with the pattern of regional metamorphism. Indeed, at a single locality, Kakebeka Falls, there is a marked contrast between 613C of the basal chert and the upper units. As shown below, the metamorphic effects of the Keweenawan sills are quite limited and can also be removed from consideration. Thus, we interpret the data to indicate original isotopic differences in the biological source material of the reduced carbon in the three environments. The range of 613C values obtained for the Gunflint Formation is approximately the same as that found for Precambrian sediments from around the world (HOERING, 1967; OEHLER et al., 1972; MCKIRDY and POWELL, 1974; EICHMANN and SCHIDLOWSKI, 1975). It indicates that considerations of facies are applicable to interpretation of the data from other Precambrian localities. The algal cherts of the lower Gunflint were deposited in a near-shore environment in a restricted, presumably marine, basin. Shallow water is indicated by the common presence of oolites and clasts or eroded algal mats, as well as by the stromatolites which often occur in profusion (AWRAMIK, 1973). The flora of this facies, as described in detail by BARGHOORN and TYLER (1965), is quite diverse, but the overwhelmingly dominant taxa are a slender filamentous blue-green alga, Gunjlintia, and a coccoid taxon (perhaps representing several cyanophytic species), Huroniospora (Fig. 2A). Although numerous other algae and bacteria occur in relatively small populations, the primary source of the organic carbon in the stromatolitic chert is these two organisms. In the non-stromatolitic lower chert at Kakabeka Falls, the problematical bacterium Kakabekia replaces Gunjintia as a major source of carbon. The Gunflint chert-carbonate facies and its lateral equivalent, the chert-taconite facies, were deposited in deeper water. Shallow water sedimentary features are absent, and the abundant siderite and pyrite of the cherttcarbonate unit have been interpreted to in-
dicate stagnant conditions in the central parts of a restricted basin (JAMES, 1954; GOODWIN, 1956). Further evidence of basin restriction is provided by halite and gypsum casts which occur in this facies (Table 1 (8, 9 and 10)). The flora here is dominated by a budding bacterium (Eoastrion) morphologically indistinguishable from the extant chemoheterotrophic genus Metallogenium (KLINE, in press) (Fig. 2B). Apparently planktonic coccoid blue-green algae (Fig. 2C) also can be found (KNOLL et al., in preparation), but they are not major components of the chert-carbonate flora, although these and other planktonic cyanophytes did provide a carbon source for the bottom community. Thus, the differing environments of the Gunflint basin supported distinct microbial communities which were responsible for the metabolism and different i3C isotopic fractionation of the organic matter preserved as kerogen. That is to say, the differences are primarily the direct result of different fractionation ratios in contrasting biological populations (Fig. 2) and not due to depositional environment or diagenetic history. In the lower algal cherts, atmospheric or dissolved COZ was fixed by blue-green algae and photosynthetic bacteria which in turn were metabolized by the other bacterial members of the assemblage with a net accumulation of lipid and sheath polysaccharide derivatives. Detrital organic matter provided carbon to the bottom-dwelling microbes of the chert-carbonate facies. Judging from the preserved fossils, it appears that this bacterial community was less diverse than the algal chert assemblages and, hence, may have been less versatile or efficient in its ability to metabolize the rain of organic matter from above. No microfossils are preserved in the carbonaceous shales, but we assume that the organic carbon isotopic ratios again reflect differences in environment and microbial communities. The anthraxolite is the most i3C-depleted material analyzed within the entire formation. Although the origin of this substance is poorly understood, its physical and chemical properties are well known. The material is a virtually pure carbon-hydrogen substance, containing only l-2’% oxygen. It is a completely amorphous solid, devoid of elastic constituents, exhibiting conchoidal fracture, and occurring in lenses up to 3 cm thick. It is suggested here that it originated as a petroleum or crude oil complex which migrated as liquid into fractures and sedimentary bedding planes where it polymerized in situ forming a solid, secondarily emplaced constituent of the sediment. If this model for the genesis of anthraxolite is correct, the depletion of 13C in this material may be attributable to the ‘cracking’ of “C-enriched petroleum-like materials to form alkene monomers which in turn would polymerize. As suggested by EICHMANN and SCHIDLOWSKI (1975), ‘chromatographic’ effects may also contribute to the isotopic fractionation of these originally mobile hydrocarbons. The effects of metamorphism on carbon in sedi-
Fig. 2. A. Stromatolitic flora in lower algal chert near Schreiber, Ontario. B. Bacterial bottom flora in chert-carbonate facies, ‘Frustration Bay’, Ontario. C. Apparently planktonic cyanophytes in chertcarbonate facies. ‘Frustration Bay’. Ontario. (Bar = ?Oprn in all cases.)
428
Organic matter from the Gunflint Iron Formation
ments have been considered by several workers (FRENCH, 1964; SILVERMAN,1964; MCKLRDY and POWELL, 1974). As mentioned above, Keweenawan diabase sills intruded the Gunflint Iron Formation some 800-900 m.y. after its deposition. Data in Table 1 suggest that this event caused profound but very localized alterations in the carbon isotopic ratios of kerogen. It is interesting, however, that even a relatively small intrusion can produce a sizable reduction in the relative abundance of “C. The pyrolytic effect apparently reflects loss of CH, from the carbon complexes in the shale at the time of intrusion. It would be of interest to examine more closely the possible changes in carbon isotopic ratios in Precambrian sediments which have undergone large scale thermal alteration such as might be induced by the injection of large granitic plutons. FRENCH(1964) examined the crystallinity of reduced carbon in sediments from the Biwabik Formation (the stratigraphic equivalent of the Gunflint Formation in Minnesota) which were intruded by the massive Duluth Gabbro, and he found that originally amorphous organic matter was altered to graphite throughout a zone extending nearly 3 km from contact. He did not determine 613C values. Alteration of isotopic values by regional metamorphism is germane to considerations of Precambrian sedimentary organic matter because it may explain carbon described by the relatively ‘%-enriched O~ZHLER et al. (1972) from the 3400m.y. old Theespruit Formation, South Africa (BROOKSet al., 1973; KARKICIANIS, 1975). Detailed examination of these older rocks is presently in progress. CONCLUSIONS To summarize, it is possible in certain circumstances to discern facies-related variations in stable carbon isotope ratios for reduced carbon in individual Precambrian formations. In the case of the approximately 2000 m.y. old Gunflint Iron Formation, a three-way correspondence among geological features (which provide information on depositional environments), microfossil populations, and organic carbon isotope ratios can be noted. The demonstration that not all kerogens are isotopically alike underscores the importance of facies data to the interpretation of t3C/1*C ratios of ancient organic matter. Although anthraxolite is an attractive substance for isotopic analysis, it is, in the Gunflint Formation at least, quite unrepresentative of the carbon isotope ratios of the shale and chert kerogen of the formation as a whole. Finally, as might have been predicted, the variation in carbon isotope ratios discernible in the modern world seems also to have been present some two billion years ago. ~ck~owledge~lents-We thank Drs. Rrcrrmn LEO, SOL SILVERMAN,and J. M. HAYESfor their invaluable assistance in obtaining the isotopic data used in this report. Drs. LYNN MARGULISand STJEPKOG~LUBIC provided most of the mats and cultures listed in Tables 2 and 3. The help 0.C.A. 4113-F
429
of Dr. STANLEYAWAMIK in the field is gratefully acknowledged. Also, we thank the reviewers of this paper whose many helpful criticisms measurably improved our report. Financial assistance was provided by NASA Grant NGL 22-007-069 and NSF Grant DES 73-06514 to E.S.B. and an NSF Graduate Fellowship to A.H.K.
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