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Abstract

Figures and Tables

Supplementary Figures and Tables

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Introduction

The nitrogen isotope compositions (d¹⁵N values) of organic matter (OM) found in ancient sedimentary rocks are used as proxies for reconstructing the microbial nitrogen cycle and the palaeo-oceanic environment (e.g., Beaumont and Robert, 1999

Beaumont, V., Robert, F. (1999) Nitrogen isotope ratios of kerogens in Precambrian cherts: a record of the evolution of atmosphere chemistry? Precambrian Research 96, 63–82.

, Shen et al., 2006

Shen, Y., Pinti, D.L., Hashizume, K. (2006) Biogeochemical cycles of sulfur and nitrogen in the Archean ocean and atmosphere. In: Benn, K., Mareschal, J.C., Condie, K. (Eds.) Archean Geodynamics and Environments, Geophyscal Monograph Series 164, 305–320.

; Ader et al., 2016

Ader, M., Thomazo, C., Sansjofre, P., Busigny, V., Papineau, D., Laffont, R., Cartigny P., Halverson, G.P. (2016) Interpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: assumptions and perspectives. Chemical Geology 429, 93–10.

; Stüeken et al., 2016

Stüeken, E.E., Kipp, M.A., Koehler, M.C., Buick, R. (2016) The evolution of Earth’s biogeochemical nitrogen cycle. Earth-Science Reviews 160, 220–239.

). OM in sedimentary rock is considered to be a complex mixture of several organisms, however, they are poorly understood. In the stepwise combustion method, components hosted by different carriers, which are intimately mixed in a sample and cannot be separated by other physical methods, can be resolved based on the combustion temperature. A preliminary study was performed using kerogen samples extracted from two rock samples from the Gunflint Formation, indicating temperature-dependent d¹⁵N heterogeneities within each kerogen sample (Ishida et al., 2012

Ishida, A., Hashizume, K., Kakegawa, T. (2012) Stepwise combustion analyses of distinct nitrogen isotopic compositions on Paleoproterozoic organic matter, Geochemical Journal 46, 249–253.

). In this study, we further examined 11 samples in search of internal d¹⁵N heterogeneities in the Gunflint kerogen.

The Gunflint Formation, belonging to the Animikie Group, is located on the northwestern shore of Lake Superior, Ontario, Canada (Goodwin, 1956

Goodwin, A.M. (1956) Facies relations in the Gunflint iron formation. Economic Geology 51, 565-595.

). The age of the Gunflint Formation is estimated as 1878 ± 1.3 Ma by U-Pb dating (Fralick et al., 2002

Fralick, P., Davis, D.W., Kissin, S.A. (2002) The age of Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Science 39, 1085–1091.

). An evolution model for the Animikie Basin has been proposed by, e.g., Schulz and Cannon (2007)

Schulz, K.J., Cannon, W.F. (2007) The Penokean orogeny in the Lake Superior region. Precambrian Research 157, 4–25.

. The tectonic evolution in this region leads to transition from foreland-type volcanism to the back-arc type volcanism, resulting in active supply of detrital material from the continent. In the present study, samples were collected covering the variation representative of shallow to deep lithology. Lithology and whole rock major and trace element geochemistry, which were consistent with previous studies, are summarised in Supplementary Information.

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Stepwise Nitrogen Analysis of Kerogen

The stepwise combustion method discriminates organic nitrogen fractions with different chemical natures. The analytical protocols and performances are described in Ishida et al. (2012)

Ishida, A., Hashizume, K., Kakegawa, T. (2012) Stepwise combustion analyses of distinct nitrogen isotopic compositions on Paleoproterozoic organic matter, Geochemical Journal 46, 249–253.

, and references therein. Kerogen samples were combusted in an oxygen atmosphere with temperatures from 450 ºC to 1100 ºC. No isotope measurement was performed at 450 ºC, where surficial contamination, such as adsorbed nitrogen and/or ammonium ions, dominates. Empirically two distinct nitrogen isotope plateaus appear in kerogen analyses: one appears at temperatures between 475 ºC and 575 ºC (lower temperature fraction: LT-fraction), and the other appears at temperatures more than 575 ºC (higher temperature fraction: HT-fraction) (Ishida et al., 2012

Ishida, A., Hashizume, K., Kakegawa, T. (2012) Stepwise combustion analyses of distinct nitrogen isotopic compositions on Paleoproterozoic organic matter, Geochemical Journal 46, 249–253.

). Between 475 ºC and 575 ºC, combustion temperatures were increased in 25 ºC steps to precisely resolve LT- and HT-fractions. Beyond this temperature range, one batch of combustion was performed at 1100 ºC for analyses of the HT-fraction. Besides the stepwise combustion, d¹⁵N values of bulk kerogen were also determined.

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Nitrogen Isotope Heterogeneity in Gunflint Kerogen

The d¹⁵N values of the bulk kerogen samples, ranging between +2.9 and +8.0 ‰, are consistent with previous reports (e.g., Beaumont and Robert, 1999

Beaumont, V., Robert, F. (1999) Nitrogen isotope ratios of kerogens in Precambrian cherts: a record of the evolution of atmosphere chemistry? Precambrian Research 96, 63–82.

; Godfrey et al., 2013

Godfrey, L.V., Poulton, S.W., Bebout, G.E., Fralick, P.W. (2013) Stability of the nitrogen cycle during development of sulfidic water in the redox-stratified late Paleoproterozoic Ocean. Geology 41, 655–658.

). The isotope shift by metamorphism can induce preferential release of ¹⁴N from the OM. However, the low metamorphic grade of the Gunflint Formation (below greenschist facies) cannot be responsible for a significant fractionation, up to 5 ‰ (e.g., Haendal et al., 1986

Haendal, D., Muhle, K., Nitzsche, H., Stiehl, G., Wand, U. (1986) Isotopic variations of the fixed nitrogen in metamorphic rocks. Geochimica et Cosmochimica Acta 50, 749–758.

, Ader et al., 2016

Ader, M., Thomazo, C., Sansjofre, P., Busigny, V., Papineau, D., Laffont, R., Cartigny P., Halverson, G.P. (2016) Interpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: assumptions and perspectives. Chemical Geology 429, 93–10.

). The lack of systematic correlation between d¹⁵N values and C/N ratios among kerogen samples expected by the metamorphism (e.g., Haendal et al., 1986

Haendal, D., Muhle, K., Nitzsche, H., Stiehl, G., Wand, U. (1986) Isotopic variations of the fixed nitrogen in metamorphic rocks. Geochimica et Cosmochimica Acta 50, 749–758.

) supports the above argument (Fig. S-3).

The modern marine biosphere involves a nitrogen biological cycle that consists mainly of fixation (N₂?NH₄⁺), nitrification (NH₄⁺?NO₂^-?NO₃^-), and denitrification (NO₃^-, NO₂^-?N₂). When biological N₂ fixation is dominant, the d¹⁵N values of OM tend to be close to the atmospheric value (0 ‰), because of minimal isotopic fractionation expected in fixation. However, when nitrification and denitrification are coupled with fixation, the d¹⁵N value of OM may grow to positive values. This isotopic shift is considered as evidence of isotopic fractionation driven by biological nitrification and denitrification (e.g., Shen et al., 2006

Shen, Y., Pinti, D.L., Hashizume, K. (2006) Biogeochemical cycles of sulfur and nitrogen in the Archean ocean and atmosphere. In: Benn, K., Mareschal, J.C., Condie, K. (Eds.) Archean Geodynamics and Environments, Geophyscal Monograph Series 164, 305–320.

; Brandes et al., 2007

Brandes, J.A., Devol, A.H., Deutsch, C. (2007) New Developments in the Marine Nitrogen Cycle. Chemical Reviews 107, 577–589.

; Sigman et al., 2009

Sigman, D.M., Karsh, K.L., Casciotti, K.L. (2009) Ocean process tracers: nitrogen isotopes in the ocean. In: Steele, J.H., Thorpe, S.A., Turekian, K.K. (Eds.) Encyclopedia of Ocean Science. Elsevier, Amsterdam, 4138–4153.

; Hashizume et al., 2016

Hashizume, K., Pinti, D.L., Orberger, B., Cloquet, C., Jayananda, M., Soyama, H. (2016) A biological switch at the ocean surface as a cause of laminations in a Precambrian iron formation. Earth Planetary Science Letters 446, 27–36.

). The d¹⁵N values of OM can vary depending on the balance between the rate of N₂ fixation and the rates of nitrification/denitrification (e.g., Lehmann, et al., 2002

Lehmann, M.R., Bemasconi, S.M., Barbieri, A., McKenzie, J.A. (2002) Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis. Geochimica et Cosmochimica Acta 66, 3573–3584.

, Pinti and Hashizume, 2011

Pinti, D.L., Hashizume, K. (2011) Early life record from nitrogen isotopes. In: Golding, S., Glikson, M. (Eds.) Earliest Life on Earth: Habitats, Environments and Methods of Detection. Springer, Netherlands, 183–205.

). For example, Papineau et al. (2013)

Papineau, D., Purohit, R., Fogel, M.L., Shields-Zuo, G.A. (2013) High phosphate availability as a possible cause for massive cyanobacterial production of oxygen in the Paleoproterozoic atmosphere. Earth and Planetary Science Letters 362, 225–236.

reported that the d¹⁵N values of OM in Palaeoproterozoic Indian sedimentary formations were approximately 0 ‰. These values were interpreted to be the result of a system dominated by N₂ fixation associated with cyanobacterial blooms. In contrast, our data show much more positive d¹⁵N values for the bulk kerogen samples, reaching values as high as +8 ‰. Such positive values suggest that nitrite and nitrate were available in the Gunflint ocean. Kerogen from Palaeoproterozoic samples, except for phosphatic shales, also exhibit positive values, up to +12 ‰ (e.g., Beaumont and Robert, 1999

Beaumont, V., Robert, F. (1999) Nitrogen isotope ratios of kerogens in Precambrian cherts: a record of the evolution of atmosphere chemistry? Precambrian Research 96, 63–82.

; Papineau et al., 2009

Papineau, D., Purohit, R., Goldberg, T., Pi, D., Shields, G.A., Bhu, H., Steele, A., Fogel, M.L. (2009) High primary productivity and nitrogen cycling after the Paleoproterozoic phosphogenic event in the Aravalli Supergroup, India. Precambrian Research 171, 37–56.

, Godfrey et al., 2013

Godfrey, L.V., Poulton, S.W., Bebout, G.E., Fralick, P.W. (2013) Stability of the nitrogen cycle during development of sulfidic water in the redox-stratified late Paleoproterozoic Ocean. Geology 41, 655–658.

, Stüeken et al., 2016

Stüeken, E.E., Kipp, M.A., Koehler, M.C., Buick, R. (2016) The evolution of Earth’s biogeochemical nitrogen cycle. Earth-Science Reviews 160, 220–239.

). To operate such a biological nitrogen cycle, oxygenated surface water is required at this time.

Stepwise combustion analyses of d¹⁵N were performed on kerogen samples mainly from the Current River section (see Supplementary Information and Table S-2). In this section, the lithology changed from shale (#0714, #0715) to carbonate-sand zone (#0711 to #0703) to arkosic sandstone (#0701, #0702). Such lithological changes represent a local marine regression. Figure 1 shows the results of the stepwise combustion analyses performed on 13 kerogen samples in our study. The average d¹⁵N values of the LT-fractions and the HT-fractions (d¹⁵N_LT and d¹⁵N_HT, respectively) were calculated using the d¹⁵N values and the released amounts for the relevant temperature steps. Some samples exhibited higher d¹⁵N_HT than d¹⁵N_LT, while other samples exhibited the opposite trend. The ranges of the d¹⁵N values for both fractions are similar to the overall range of bulk kerogen d¹⁵N values. In most samples, the HT-fractions have lower amounts of nitrogen than those of the LT-fractions. The differences in the combustion temperature possibly correspond to differences in the carbon structure for fractions of kerogen that co-exist in OM (Ishida et al., 2012

Ishida, A., Hashizume, K., Kakegawa, T. (2012) Stepwise combustion analyses of distinct nitrogen isotopic compositions on Paleoproterozoic organic matter, Geochemical Journal 46, 249–253.

).

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The nitrogen released in the HT-fraction possibly has stronger bonds with carbon in the form of graphitised kerogen, compared to the LT-fraction. Ishida et al. (2012)

Ishida, A., Hashizume, K., Kakegawa, T. (2012) Stepwise combustion analyses of distinct nitrogen isotopic compositions on Paleoproterozoic organic matter, Geochemical Journal 46, 249–253.

concluded that the HT-fraction might not be a metamorphosed portion of OM with the same origin as the LT-fraction, since the HT-fraction does not always show higher d¹⁵N values than the LT-fraction. Because the chemical structures of kerogen are known to be heterogeneous, some components of kerogen are easy to graphitise but the others are non-graphitised (e.g., Bustin et al., 1995

Bustin, R.M., Ross, J.V. Rouzaud, J.N. (1995) Mechanisms of graphite formation from kerogen: Experimental evidence. International Journal of Coal Geology 28, 1–36.

; Beyssac et al., 2002

Beyssac, O., Rouzaud, J.N., Goffe’, B., Brunet, F., Chopin, C. (2002) Graphitization in a high-pressure, low-temperature metamorphic gradient: a Raman microspectroscopy and HRTEM study. Contributions to Mineralogy and Petrology 143, 19–31.

). We interpret the OM observed in the HT-fraction to represent the residue of graphitised parts of kerogen. Such graphitisation could be promoted by the heterotrophic microbial degradation during diagenesis. Thus, we interpret the nitrogen in the LT-fractions to represent more indigenous nitrogen isotope compositions in the ecosystem at that time.

A positive correlation was observed between the d¹⁵N_LT and the Pr/Sm ratios of bulk rock (Fig. 2a). The Pr/Sm ratios among our samples are interpreted to represent the contribution of continental inputs to the ocean, supported by the negative correlation with Eu/Eu* (Bolhar et al., 2005

Bolhar, R., Van Krenendonk, M., Kamber, B.S. (2005) A trace element study of siderite-jasper banded iron formation in the 3.45 Ga Warrawoona Group, Pilbara Craton-Formation from hydrothermal fluids and shallow seawater. Precambrian Research 137, 93–114.

) (Fig. 3a), and positive correlations with abundances of elements that represent detrital inputs (Fig. 3b,c). The d¹⁵N_HT showed a poor correlation with the Pr/Sm ratios (Fig. 2b). The variable d¹⁵N_LT values in a positive range and their correlation to the Pr/Sm ratios could be caused by intense nitrification/denitrification when the nutrient supply from the land enhanced.

section	sample ID	litho facies	bulk rock						kerogen
			Total C	TOC	Ccarb	Spyr	Pr/Sm	Eu/Eu*	C	N	H	S	C/N	d13C	d34S (pyr)	d15N	±
			wt. %				(PAAS normalised)1		wt. %				atom	‰
Pass Lake Road	0716	Chert	0.1	0.0	0.1	0.0	1.22	0.87	n.a.	n.a.	n.a.	n.a.	-	n.a.	n.a.	n.a.	n.a.
	0718	Carbonate	6.6	0.0	6.6	0.0	0.61	2.22	n.a.	n.a.	n.a.	n.a.	-	n.a.	n.a.	n.a.	n.a.
	0719	Silicified carbonate	3.1	0.0	3.1	0.0	0.66	2.24	n.a.	n.a.	n.a.	n.a.	-	n.a.	n.a.	n.a.	n.a.
	0720	Silicified carbonate	3.7	0.1	3.6	0.0	0.67	2.18	n.a.	n.a.	n.a.	n.a.	-	n.a.	n.a.	n.a.	n.a.
	0721	Taconite	3.0	0.2	2.9	0.6	0.82	1.78	16.8	0.2	0.3	32.6	109.6	-32.6	3.0	4.7	0.60
	0722	Chert (BIF)	0.2	0.0	0.2	0.0	0.88	1.73	n.a.	n.a.	n.a.	n.a.	-	n.a.	n.a.	6.8	1.00
	0723	Carbonate (BIF)	5.2	0.1	5.1	0.1	0.82	1.41	n.a.	n.a.	n.a.	n.a.	-	-25.1	n.a.	5.4	0.40
Blend River North	0301	Black shale	0.3	0.3	0.0	0.0	1.76	1.25	70.7	1.2	3.5	5.6	67.4	-31.6	2.1	2.9	0.70
	0302	Black shale	0.8	0.3	0.5	0.2	0.49	1.48	53.1	0.6	2.4	18.2	96.2	-31.6	4.7	n.a.	n.a.
	0303	Ferruginous shale	4.1	0.3	3.8	0.1	0.76	1.54	69.4	1.2	3.2	9.6	67.6	-31.3	7.1	n.a.	n.a.
	0304	Black shale	1.1	0.9	0.1	0.0	1.43	1.19	73.1	1.1	3.2	3.2	75.5	-32.1	8.6	5.9	0.70
	0305	Carbonate	9.1	0.8	8.3	0.4	0.69	1.59	49.8	0.9	2.0	18.8	64.7	-31.9	8.2	6.5	0.70
	0306	Ferruginous carbonate	10.4	0.3	10.2	0.1	0.78	1.32	60.5	1.1	2.9	13.3	63.9	-29.8	n.a.	n.a.	n.a.
	0307	Silicified carbonate	5.0	0.1	4.9	0.0	0.78	1.54	n.a.	n.a.	n.a.	n.a.	-	n.a.	n.a.	n.a.	n.a.
	0308	Carbonate/shale	4.0	0.0	3.9	0.0	0.83	1.18	44.1	0.0	0.0	15.2	-	n.a.	n.a.	n.a.	n.a.
	0309	Ferruginous carbonate	9.9	0.2	9.7	0.0	0.83	1.27	76.0	1.4	3.5	5.7	65.6	n.a.	n.a.	n.a.	n.a.
	0310	Chert	3.2	0.1	3.1	0.0	0.83	1.29	n.a.	n.a.	n.a.	n.a.	-	n.a.	n.a.	n.a.	n.a.
	0311	Chert	1.9	0.0	1.8	0.0	0.68	0.72	60.3	0.7	2.2	8.5	94.9	n.a.	n.a.	n.a.	n.a.
Current River	0701	Carbonate-rich arkosic sandstone	4.7	0.5	4.2	0.0	0.86	1.28	67.3	0.9	2.2	5.6	91.2	-33.0	26.9	5.6	0.80
	0702	Arkosic sandstone	0.8	0.2	0.6	0.0	1.87	1.11	56.6	0.6	3.2	2.5	101.7	-32.2	8.3	6.7	0.70
	0703	Sandy carbonate	4.2	0.2	4.0	0.0	0.99	1.24	82.7	1.1	3.7	2.8	88.5	-32.3	10.9	7.1	0.50
	0704	Sandy carbonate	5.9	0.2	5.7	0.0	0.98	1.18	82.9	1.0	3.8	3.2	98.4	-32.5	11.2	4.9	1.00
	0705	Sandy carbonate	4.4	0.1	4.3	0.0	1.23	1.42	52.6	0.7	2.2	14.8	86.8	-31.6	9.7	6.0	0.70
	0706	Sandy carbonate	5.4	0.3	5.1	0.2	1.03	1.52	47.8	0.6	2.0	19.6	87.6	-32.6	15.1	6.9	0.60
	0707	Sandy carbonate	2.9	0.2	2.8	0.0	0.92	1.18	66.1	0.8	2.8	10.2	98.3	-31.9	5.8	5.9	0.80
	0708	Sandy carbonate	7.1	0.3	6.7	0.0	0.76	1.76	85.7	1.0	4.1	1.6	98.8	-32.9	16.0	6.9	0.50
	0709	Laminated carbonate with detrital quartz	1.8	0.2	1.6	0.1	1.03	1.31	59.9	0.8	2.5	14.2	86.7	-32.4	14.8	7.3	0.60
	0710	Laminated carbonate with detrital quartz	0.3	0.1	0.2	0.0	1.18	1.48	72.1	1.0	4.4	5.9	80.3	-32.0	-	3.2	0.60
	0711	Massive carbonate	9.0	0.1	8.9	0.1	0.93	1.63	54.3	0.8	2.9	14.6	80.9	-32.6	13.4	8.0	0.40
	0712	Laminated carbonate with detrital quartz	0.7	0.3	0.4	0.1	1.16	1.63	66.0	1.0	3.4	9.4	78.3	-32.5	11.3	7.2	0.30
	0713	Carbonate shale (phosphorous)	6.2	0.2	6.0	0.1	1.02	1.68	43.2	0.6	1.8	20.7	90.1	-32.4	9.2	6.4	0.60
	0714	Black shale	4.5	0.9	3.7	0.1	1.42	1.22	68.6	1.0	3.0	4.8	78.2	-33.6	9.3	4.8	0.90
	0715	Black shale	0.2	0.3	0.0	0.0	1.20	1.28	8.2	0.2	1.2	1.6	61.3	-32.2	11.5	5.9	0.70
Telly Fox	0724	Carbonate	9.7	0.3	9.3	0.2	0.93	1.53	37.8	0.6	1.7	25.4	77.5	-31.4	-1.1	n.a.	n.a.
	0725	Silicified shale	0.2	0.2	0.0	0.4	0.63	1.23	n.a.	n.a.	n.a.	n.a.	-	-31.5	20.7	5.5	0.70
	0726	Carbonate/shale	0.3	0.4	0.0	0.2	0.71	1.40	60.1	0.9	2.9	12.5	78.6	-31.6	10.8	4.5	0.30
	0729	Silicified carbonate	0.3	0.1	0.3	0.1	0.24	0.72	n.a.	n.a.	n.a.	n.a.	-	-28.9	n.a.	4.3	0.30
	0730	Silicified carbonate	0.3	0.1	0.2	0.0	0.20	0.99	n.a.	n.a.	n.a.	n.a.	-	n.a.	n.a.	n.a.	n.a.
Kakabeka Fall	0601	Shale (BIF)	7.3	0.8	6.6	0.2	0.92	1.67	64.8	1.2	2.8	9.9	62.5	-31.3	5.9	6.3	0.80
	0602	Chert (BIF)	0.3	0.1	0.2	0.0	0.41	0.87	57.5	1.0	2.5	8.5	68.2	-29.7	n.a.	n.a.	n.a.
	0603	Shale (BIF)	5.5	0.4	5.1	0.1	0.84	1.46	60.6	1.0	2.0	12.2	70.8	-30.9	6.1	n.a.	n.a.
	0604	Ferruginous chert	2.7	0.1	2.6	0.1	1.16	1.37	46.4	0.9	1.6	15.6	60.3	-27.8	n.a.	n.a.	n.a.
	0605	Ferruginous chert	0.0	0.0	0.0	0.0	0.75	0.72	n.a.	n.a.	n.a.	n.a.	-	n.a.	n.a.	n.a.	n.a.

Analytical error of d¹³C and d³⁴S are within 0.2 ‰.
Underlined values: below instumental reproducibility (0.1 %), n.a.: not analysed because of low amount of organic matter.
¹ Details are described in Tables S-1 and S-4.

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Palaeo-ecosystem in the Gunflint Ocean

The carbon isotope compositions (d¹³C values) of the examined kerogen samples, ranging from -33.6 to -25.1 ‰, are consistent with the carbon isotope values fixed by cyanobacteria (e.g., Schidlowski, 1988

Schidlowski, M. (1988) A 3,800-million-year isotopic record of life from carbon in sedimentary rocks. Nature 333, 313–318.

; House et al., 2000

House, H.C., Schopf, J.W., McKeegan, K.D., Coath, C.D., Harrison, T.M., Stetter, K.O. (2000) Carbon isotopic composition of individual Precambrian microossils. Geology 28, 707–710.

). The d¹³C of kerogen in the sedimentary rocks, in general, corresponds to the values of contemporary primary producers. The presence of oxygenic primary producers in the Gunflint ocean is consistent with the biological nitrogen cycle proposed in the present study, and with the mineralogical and trace element geochemistry (see Supplementary Information).

The OM sinking through the water column would consume the dissolved oxygen, producing a temporary or partial sub-oxic zone in the Gunflint ocean, similar to a red tide event in the modern ocean (Minagawa and Wada, 1986

Minagawa, M., Wada, E. (1986) Nitrogen isotope ratios of red tide organisms in the east china sea: A characterization of biological nitrogen fixation. Marine Chemistry 19, 245–259.

). In the water column with spatially variable redox conditions ranging from oxic to anoxic, nitrification/denitrification processes could be enhanced, thereby inducing ¹⁵N enrichment in the OM (e.g., Shen et al., 2006

Shen, Y., Pinti, D.L., Hashizume, K. (2006) Biogeochemical cycles of sulfur and nitrogen in the Archean ocean and atmosphere. In: Benn, K., Mareschal, J.C., Condie, K. (Eds.) Archean Geodynamics and Environments, Geophyscal Monograph Series 164, 305–320.

; Sigman et al., 2009

Sigman, D.M., Karsh, K.L., Casciotti, K.L. (2009) Ocean process tracers: nitrogen isotopes in the ocean. In: Steele, J.H., Thorpe, S.A., Turekian, K.K. (Eds.) Encyclopedia of Ocean Science. Elsevier, Amsterdam, 4138–4153.

). One of the triggers of red tide in the modern ocean environment is considered to be an increased amount of nutrient supply to the ocean from the land (e.g., Taylor et al., 1995

Taylor, D., Nixon, S., Granger, S., Buckley, B. (1995) Nutrient limitation and the eutrophication of coastal lagoons. Marine Ecology Progress Series 127, 235–244.

; Valiela et al., 1997

Valiela, I., McClelland, J., Hauxwell, J., Behr, P.J., Hersh, D., Foreman, K. (1997) Macroalgal blooms in shallow estuaries: Controls and ecophysiological and ecosystem consequences. Limnology and Oceanography 42, 1105–1118.

; Hauxwell et al., 1998

Hauxwell, J., McClelland, J., Behr, P.J., Valiela, I. (1998) Relative importance of grazing and nutrient controls of macroalgal biomass in three temperate shallow estuaries. Estuaries 21, 347–360.

). The rapid growth of continents in the Animikie basin enhanced the supply of elements from the terrestrial region to the Gunflint ocean. Our data suggest that increased continental input brought more phosphorus and trace metal elements to the sedimentary environment (Fig. S-3, Table S-5), rather than modern-like nutrients such as dissolved OM, which are less likely at this time. The increased flux of such elements into the Gunflint ocean would promote the high production and nitrogen cycling in the biosphere of the shallow ocean (e.g., Papineau et al., 2009

Papineau, D., Purohit, R., Goldberg, T., Pi, D., Shields, G.A., Bhu, H., Steele, A., Fogel, M.L. (2009) High primary productivity and nitrogen cycling after the Paleoproterozoic phosphogenic event in the Aravalli Supergroup, India. Precambrian Research 171, 37–56.

; Edwards et al., 2012

Edwards, C.T., Pufahl, P.K., Hiatt, E.E., Kyser, T.K. (2012) Paleoenvironmental and taphonomic controls on the occurrence of Paleoproterozoic microbial communities in the 1.88 Ga Ferriman Group, Labrador Trough, Canada. Precambrian Research 212–213, 91–106.

; Stüeken et al., 2015

Stüeken, E.E., Buick, R., Guy, B.M., Koehler, M.C. (2015) Isotopic evidence for biological nitrogen fixation by molybdenum-nitrogenase from 3.2 Gyr. Nature 520, 666–674.

), as represented by the positive correlation between Pr/Sm ratios and d¹⁵N_LT of kerogen observed in this study.

The active production of OM may have facilitated the formation of the organic-rich sediments as well. The high flux of OM would keep the interior of sediments under suboxic to anoxic conditions, where the activity of sulphate-reducing bacteria was promoted. Such conditions would produce the heavier d³⁴S values in the final products, such as pyrite (see Table 1 and Supplementary Information). A local involvement of methanogens and methanotrophs in such a benthic or subsurface environment is also plausible and is supported by the carbon isotope study of microfossils (House et al., 2000

House, H.C., Schopf, J.W., McKeegan, K.D., Coath, C.D., Harrison, T.M., Stetter, K.O. (2000) Carbon isotopic composition of individual Precambrian microossils. Geology 28, 707–710.

). The primary OM precipitated to the sea floor would be decomposed by those heterotrophic bacteria, resulting in nitrogen isotope heterogeneities in the kerogen left in the sediments. The combined approaches of stepwise nitrogen isotope analyses and multiple geochemical analyses in this study can be used as a proxy to understand the evolution of biosphere and ocean environment in the Precambrian age.

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Acknowledgements

Constructive reviews by Dr. Stüeken and an anonymous reviewer greatly helped to improve the quality of the article. This research was supported by Grant-in-Aid for JSPS (DC1) to A.I. (21-2645), and by KAKENHI to T.K. (24403013, 15H021440), and K.H. (JP17340168).

Editor: Bruce Watson

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References

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Supplementary Information

1. The Gunflint Formation

The Gunflint Formation is located on the north-western shore of Lake Superior, Ontario, Canada (Fig. S-1). Together with the Rove Formation, the Gunflint Formation belongs to the Animikie Group (Goodwin, 1956). The Animikie Group unconformably overlies the Archean Superior craton, which consists of the accretion of volcanic and sedimentary rocks originally formed by the Algoman orogeny (e.g., Ojakangas et al., 2001). The Gunflint Formation sedimentation began with quartz sand in a tidal environment and was often associated with stromatolites. Owing to water depth fluctuations, the sedimentary rocks transitioned to chert carbonate, iron formations, or taconitic carbonate. The Gunflint Formation is overlain by the shale-dominated Rove Formation. In particular, laterally uniform banded iron formations (BIFs) and iron-rich layers formed from Ontario to Minnesota in the same basin (Schulz and Cannon, 2007).

The sedimentary rocks of the Animikie Group formed in close association with the Penokean orogenic events, which involved rifting, submarine hydrothermal activity, subduction, accretion of arc volcanic and magmatic rocks, and the oldest Palaeoproterozoic accretionary orogeny (e.g., Blackwelder, 1914; Hoffman, 1988; Ojakangas et al., 2001). The Animikie Basin, in particular during the depositional period of the Gunflint Formation, is interpreted to have formed in a back-arc basin (Hemming et al., 1995; Van Wyck and Johnson, 1997). Submarine volcanism caused black smoker-type submarine hydrothermal activity in deep parts of the Animikie back-arc basin, forming Kuroko-type volcanogenic massive sulphide deposits (De Matties, 1994). At the end of arc/back-arc activity, arc-related crustal blocks (Pembine-Wausau Terrane) collided with the Superior craton. This tectonic event changed the back-arc basin into a foreland basin (Ojakangas et al., 2001). The protoliths of the sedimentary rocks in the Gunflint Formation in the Animikie Group also changed rapidly, from Archean rocks and arc-type volcanic rocks to Penokean fold-and-thrust belt rocks (Schulz and Cannon, 2007). These tectonic settings supplied large quantities of diverse detrital materials to the Gunflint ocean, resulting in a unique and complex sedimentary environment with radical tectonically driven changes in sea level (e.g., Pufahl and Fralick, 2000; Fralick et al., 2017). The upper part of the Gunflint Formation contains evidence of a potential meteorite impact, such as tektite, accretionary lapilli tuff and stormy sediments. These features were caused by the Sudbury meteorite, which fell at 1850 ± 1 Ma (Addison et al., 2005). This age is consistent with the last deposition period of the Gunflint Formation. An ash layer near the base of the Rove Formation yielded a U-Pb zircon age of 1836 ± 5 Ma (Addison et al., 2005), and a volcanic ash layer in the Gunflint Formation yielded an U/Pb zircon age of 1878 ± 1.3 Ma (Fralick et al., 2002). Both radiogenic ages constrain upper and lower age limits of the Gunflint Formation.

The Gunflint Formation in Canada is one of the most well-known Palaeoproterozoic sedimentary formations that has not suffered high-grade metamorphism (e.g., Floran and Papike, 1975; Fralick et al., 2002; Alleon et al., 2016). The first report of Precambrian microfossils was from silicified stromatolites in this formation (Tyler and Barghoon, 1954). Following this find, several geochemical analyses have been performed on these microstructures and kerogen (e.g., House et al., 2000; Godfrey et al., 2013; Wacey et al. 2013, 2016; Williford et al., 2013). Geochemical studies on the organic matter in the Gunflint Formation have also been performed but only a few have focused on the nitrogen isotope compositions of the organic matter (e.g., Beaumont and Robert, 1999; Ishida et al., 2012; Godfrey et al., 2013). The sedimentary rocks of the Gunflint Formation were deposited in a tectonically active foreland or back-arc basin (e.g., Ojakangas, 1994; Fralick et al., 2002). In these environments, marine transgressions and regressions occurred frequently, and weathering inputs from terrestrial regions intensified, often in the same sedimentary basin.

Studied samples were collected from the following 6 sections (Fig. S-1): (1) Pass Lake section, (2) Blende Lake North section, (3) Current River section, (4) Terry Fox section, (5) Kakabeka Falls section and (6) Highway 590 east section. The lithology, mineralogy and chemistry of each section are described in Section 4. The occurrence of pyrobitumen was reported in the Lake Superior region (Mancuso, 1989), but we did not include samples with pyrobitumen in the following chemical and isotope analyses. Sampling was performed avoiding and removing the weathering surface carefully.

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2. Methods

Mineralogical observations
Thin sections of the samples were observed under both a petrographic microscope (Olympus, BX50) and a scanning electron microscope with an energy-dispersive X-ray spectrometer (SEM/EDX) to determine the mineral assemblages and chemical compositions. A portion of the extracted kerogen was observed under a transmission electron microscope (TEM; JEOL, JEM-2010) to determine the carbon structures. The kerogen sample was mounted on a copper grid with a support membrane. The primary TEM electron beam was accelerated to 200 keV.

Elemental concentrations
Kerogen samples were extracted following the procedures of Vandenbroucke (2003). Detailed procedure is described in Ishida et al. (2012). Analyses of C, H, S and N concentrations were performed on both the extracted kerogen and bulk sample powder using an elemental analyser (EA; CarloErba EA1108). Approximately 3 milligrams of kerogen and 20 milligrams of powdered bulk rock samples were combusted. The reproducibility and accuracy of the analyses are within 0.1 % for each element.

The chemical compositions of 43 representative bulk rock samples were analysed using an inductivity coupled plasma mass spectrometer (ICP-MS). Approximately 100 grams of whole rock material was homogeneously powdered, and 20 grams of powder was analysed. The analyses were performed by Activation Laboratories Ltd., Canada. The concentrations of major and trace elements, including rare earth elements (REEs), were measured. The values of Post-Archean Australian Shale (PAAS; Pourmand et al., 2012) were used as the reference compositions for normalising the sample values.

Carbon and sulphur isotopic composition analyses
Carbon and sulphur stable isotope compositions were determined using an EA and a MAT252-ConfloIII system. Samples were combusted in the EA. The generated CO₂ or SO₂ gases were carried into the IRMS by a helium flow. The carbon isotope compositions are expressed relative to the international Pee Dee Belemnite standard (PDB; ¹³C/¹²C = 0.011237) in the standard delta (d) notation as follows:

d¹³C = [(¹³C/¹²C)_sample/(¹³C/¹²C)_PDB - 1 ] × 1000

The sulphur isotopic compositions are expressed relative to the international standard Canyon Diablo Troilite (CDT; ³⁴S/³²S=0.045005) in the standard delta (d) notation as follows:

d³⁴S = [(³⁴S/³²S)_sample/(³⁴S/³²S)_CDT - 1] × 1000

The reproducibility and accuracy of these analyses are within 0.2 ‰ (1s).

Nitrogen isotope analyses of kerogen
The nitrogen isotope compositions were determined by applying both bulk and stepwise combustion methods. Because of the low concentrations of nitrogen in the kerogen, a high-sensitivity method using a static-type mass spectrometer was required. The Balzers QMG420 mass spectrometer at Osaka University, Japan, was used. The analytical protocols and performances are described in detail in Ishida et al. (2012) and references therein. For the bulk kerogen analysis, samples were heated in an oxygen atmosphere at 1200 ºC for 90 minutes to extract all gases at one time. In the stepwise analysis, the combustion involved 6 or 7 temperature steps, typically at 450, 475, 500, 525, 550, 575 and 1100 ºC. These temperature steps were chosen according to the results of Ishida et al. (2012) to capture the observed shift in the ¹⁵N/¹⁴N ratios at 575 to 600 ºC in examined kerogen samples. No measurement was performed at the first 450 ºC step, where surficial contamination, such as adsorbed nitrogen and/or ammonium ions, can dominate the sample gas. Samples were then combusted from 475 ºC to 575 ºC at 25 ºC steps to observe the shift in d¹⁵N values across the low and high temperature ranges. Samples were finally combusted at 1100 ºC to confirm the complete release of nitrogen from the samples. The nitrogen isotope compositions are expressed relative to atmospheric N₂ (¹⁵N/¹⁴N = 0.00367) by the standard delta (d) notation as follows:

d¹⁵N = [(¹⁵N/¹⁴N)_sample/(¹⁵N/¹⁴N)_air - 1 ] × 1000

The d¹⁵N values and the amounts of released N and C for each temperature fraction were measured. The typical reproducibility (1s) of the isotope ratio for 1 ng of standard air is 0.7–0.9 ‰ (Ishida et al., 2012). The blank level of nitrogen was usually negligible (typically <0.01 ng at <1000 ºC) compared to the amount released from organic matter.

Unless otherwise noted, the error bars in this study represent 1s errors, and the elemental ratios represent compositional ratios.

3. Lithology, Mineralogy, and Whole Rock Chemistry

The data of bulk chemical compositions in the following sections are summarised in Table S-1.

Pass Lake section
Alternating layers of carbonate and sandstone, banded chert/carbonate, and black shale were observed (48.60289ºN, 88.779709ºW to 48.593987ºN, 88.768745ºW). In the southern part, banded chert and carbonate cropped out with alternations of iron-rich and silica-rich layers on the scale of a few centimetres. The iron-rich layers consisted of iron oxide with minor euhedral siderite (Fig. S-2b). Thin black shale layers were occasionally intercalated in iron layers. In the black shale, euhedral to semi-euhedral pyrite crystals were the major iron-bearing minerals. The SiO₂ concentrations ranged from 54.2 to 97.2 % for the banded chert/carbonate and 8.3 to 26.3 % for the black shale layer. Carbonate, taconite and tuffaceous shale were dominant in the northern part (closer to Highway 11). These layers were stratigraphically lower than the banded chert/carbonate (Goodwin, 1956). In the banded carbonate section, dolomitic massive carbonate and granular chert layers were present in alternating layers. Most parts of the granular chert layer were silicified, and the components were composed of microcrystalline quartz. The silicification had resulted in variable SiO₂ concentrations in the carbonate samples, ranging from 2.8 to 73.6 %. Well-preserved oolite was found in the microcrystalline quartz matrix (Fig. S-2a, sample #0721). The diameter of each grain was several hundreds of µm. The core material of the ooids was typically fragmented quartz, with occasional carbonate and minor pyrite. These core materials were concentrically covered by ferrodolomitic carbonate. The chemical compositions of the dolomitic carbonate were analysed by SEM/EDS. In general, their compositions were similar to the ferrodolomitic massive carbonate observed in this section. Diagenetic pyrite crystals were commonly observed throughout all examined layers. The Fe₂O₃ concentrations of the silicified carbonate layers ranged from 2.8 to 45.2 %.

Blende Lake North section
The outcrop was a few hundred metres northwest of the Pass Lake section (48.59729ºN, 88.790367ºW). It consisted of black shale with minor beds of ash layers and ferruginous chert. The chert layers contained haematitic iron oxide. The black shale layers contained alternations of tuff-rich layers and friable organic/clay-rich layers. High Fe₂O₃ contents (up to 45 %) were observed in samples #0301 to #0311. High Al₂O₃, TiO₂ and K₂O contents were observed in the black shale layers (samples #0301 to #0304), indicating the presence of abundant clay.

Current River section
The Current River section was a continuous exposure of the Gunflint Formation along a riverbed. The examined section ranged from Trowbridge Falls (48.489132°N, 89.188187°W) to the end of the North Current River (48.490452°N, 89.194267°W). The lithologies changed gradually from a lower black shale-dominated zone to an upper carbonate-sand zone (sample #0701 to #0712). The carbonate-sand zone consisted of alternating layers of carbonate and taconitic or cherty sandstone. Some carbonate layers consisted of millimetre-scale laminated and brecciated structures. The cherty sandstone consisted of a microcrystalline quartz matrix and coarser quartz grains with minor grains of carbonate. Euhedral to semi-euhedral ferrodolomite crystals were observed in the laminated layer with detrital quartz (sample #0704; Fig. S-2d). The massive carbonate layers often contained reworked veins associated with brecciation. In the middle layers of this section, shale and carbonate were observed. Layers of organic matter contained detrital lithic fragments, such as quartz and clay minerals (sample #0709; Fig. S-2d). Phosphoritic pebbles were found together with shale pebbles in the lowermost layer of carbonate. Phosphoritic pebbles consisted of calcium apatite, iron oxide, carbonate, quartz, pyrite, clay minerals and organic matter (sample #0713; Fig. S-2e). Framboidal pyrite crystals were observed around the phosphoritic pebbles but not inside of the pebbles (Fig. S-2e, 48.489521°N, 89.188225°W). The framboidal pyrites were approximately 10 µm in diameter and contain less than 1 wt. % arsenic, whereas the euhedral pyrite did not contain arsenic. In the lower layers, black shale was dominant and consisted of fine grains of detrital quartz, illite, chlorite, and organic matter with minor carbonate grains (sample #0715; Fig. S-2f). Silicification was significant in all samples, with variable SiO₂ concentrations ranging from 15.2 to 74.9 %. Overall, the Fe₂O₃ concentrations ranged from 8.9 to 30.3 %, suggesting high Fe fluxes during deposition.

Terry Fox section
The Terry Fox section covered the Gunflint and Rove Formations (48.484425°N, 89.167976°W). The stratigraphic top part of the Gunflint Formation was exposed here and consisted of dolomite to ferrodolomite layers and a granular chert layer (Fig. S-2g). The dolomitic layers consisted of euhedral dolomite and anhedral quartz with minor organic matter with euhedral pyrite crystals (Fig. S-2i). The chert layer consisted of microcrystalline quartz grains and a quartz matrix (Fig. S-2h). A Logan diabase sill formed a cliff exposure of the Rove Formation (Floran and Papike, 1975). The Fe₂O₃ concentrations (6.2 to 13 %) in the carbonate samples were less than those of other carbonates. Silicification was also significant, and the SiO₂ concentrations were variable (18.3 to 96.7 %).

Kakabeka Falls and Highway 590 east sections
Banded cherts with iron-rich and carbonaceous layers were observed near the parking area at the park entrance (48.40279°N, 89.623459°W). Boulders that were plausibly derived from this outcrop were found outside of the park and were analysed in this study. The iron-rich layers consisted of iron oxide, minor euhedral siderite, quartz, chlorite and organic matter (Fig. S-2j). Those samples had high Fe₂O₃ concentrations of up to 40 %. The chert layer consisted of equigranular quartz and features high SiO₂ concentrations (72.9 to 96.6 %). The P₂O₅ concentrations of the iron-rich layers were higher than those of the chert layers. At the intersection of Highways 11 and 590, silicified stromatolites, a chert layer and a minor black shale layer were observed (48.404877°N, 89.632538°W). The black shale layer consisted of fine grains of detrital quartz and organic matter with minor pyrite grains.

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sample ID	SiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5
	wt. %
0716	97.74	0.24	0.85	0.009	0.02	0.15	0.08	0.12	0.009	0.02
0718	43.08	0.19	6.18	1.341	7.98	15.83	0.11	0.05	0.005	0.01
0719	74.27	0.13	2.88	0.537	3.91	7.57	0.09	0.04	0.001	0.01
0720	68.08	0.13	5.16	0.715	4.34	8.71	0.08	0.07	0.005	0.01
0721	73.66	0.11	4.98	0.766	3.06	6.87	0.01	0.01	0.005	0.01
0722	97.15	0.31	0.69	0.017	0.01	1.07	0.05	0.04	0.001	0.02
0723	54.19	0.11	21.39	1.388	3.55	2.93	0.01	0.06	0.009	0.53
0301	63.37	14.67	8.31	0.082	1.89	0.18	0.11	5.74	1.167	0.09
0302	48.44	15.94	15.22	0.23	2.1	0.99	0.08	8.14	2.523	0.34
0303	45.29	5.22	26.32	1.17	3.42	1.93	0.02	0.5	0.255	0.1
0304	55.12	14.02	15.48	0.212	2.66	0.57	0.07	3.6	0.659	0.06
0305	16.74	1.27	28.04	2.83	6.82	12.75	0.13	0.06	0.012	1.23
0306	2.80	0.94	43.48	3.567	6.96	6.75	0.24	0.06	0.006	0.33
0307	51.43	0.04	24.87	1.602	2.45	2.58	0.02	0.01	0.003	0.75
0308	62.17	0.22	17.06	1.31	2.54	2.68	0.2	0.04	0.007	0.15
0309	5.04	1.43	45.22	3.325	6.88	5.49	0.01	0.01	0.003	0.34
0310	70.31	0.08	16.22	0.9	1.74	1.1	0.01	0.01	0.003	0.16
0311	82.50	0.1	9.69	0.594	0.98	0.56	0.22	0.04	0.007	0.07
0701	50.14	2.22	25.99	1.067	2.93	1.41	0.02	0.02	0.091	0.13
0702	71.07	5.23	14.79	0.107	2.66	1.19	0.04	0.06	0.222	0.04
0703	52.52	2.73	21.16	0.613	4.07	4.36	0.08	0.03	0.132	0.03
0704	41.28	1.47	27.22	0.836	4.29	4.85	0.12	0.04	0.05	0.07
0705	44.56	3.06	30.28	0.668	3.55	1.29	0.03	0.03	0.124	0.13
0706	45.52	2.04	13.18	0.438	5.39	12.47	0.01	0.07	0.07	0.09
0707	62.39	2.93	16.18	0.463	3.19	4.05	0.02	0.01	0.225	0.03
0708	26.51	2.98	29.33	0.87	6.96	8.26	0.09	0.06	0.083	0.09
0709	65.35	4.7	19.08	0.232	2.74	0.74	0.12	0.19	0.2	0.11
0710	74.91	5.75	10.75	0.03	1.83	0.47	0.1	1.22	0.355	0.07
0711	15.22	2.16	15.78	0.747	9.15	22.39	0.12	0.3	0.113	0.02
0712	75.36	5.53	10.47	0.071	1.7	0.66	0.07	1.73	0.29	0.05
0713	39.03	1.46	18.8	0.792	5.68	10.88	0.09	0.12	0.055	0.45
0714	50.18	4.99	23.59	0.458	3.53	2.95	0.03	0.62	0.164	0.06
0715	69.70	11.04	8.86	0.01	1.72	0.14	0.01	5.22	0.641	0.08
0724	18.30	0.29	11.3	1.524	10.85	22.79	0.03	0.01	0.009	0.54
0725	76.26	5.49	12.97	0.023	2.67	0.18	0.06	0.07	0.139	0.08
0726	44.25	1.91	6.22	0.484	8.91	14.81	0.06	0.55	0.065	0.03
0729	96.74	0.16	1.29	0.024	0.1	1.08	0.45	0.06	0.007	0.02
0730	96.60	0.13	0.68	0.017	0.01	0.69	0.14	0.03	0.005	0.03
0601	27.53	2.95	39.9	0.46	5.3	1.43	0.14	0.14	0.053	0.33
0602	96.59	0.34	1.79	0.019	0.08	0.03	0.13	0.08	0.009	0.02
0603	47.39	0.64	30.08	0.534	2.58	1.58	0.13	0.11	0.026	0.36
0604	72.91	0.31	16.58	0.352	1.12	0.78	0.04	0.06	0.01	0.17
0605	84.92	0.09	13.26	0.014	0.06	0.01	0.08	0.01	0.005	0.02

sample ID	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y	Ho	Er	Tm	Yb	Lu	SREE	Pr/Sm	Eu/Eu*	Ce/Ce*	Pr/Pr*
	ppm																(PAAS normalised)
0716	1	3.2	0.18	0.6	0.1	0.05	0.1*	0.1*	0.1*	2*	0.1*	0.1*	0.05*	0.1*	0.04*	5.13	1.22	0.87	1.81	0.68
0718	8.5	18.3	2.15	9.3	2.4	1.04	2.7	0.4	2.5	18	0.5	1.2	0.14	0.7	0.08	67.91	0.61	2.22	1.03	0.93
0719	5.7	11.5	1.27	5.1	1.3	0.55	1.5	0.2	1.2	7	0.2	0.6	0.08	0.4	0.05	36.65	0.66	2.24	1.03	0.94
0720	5.1	10	1.19	4.7	1.2	0.51	1.3	0.2	1.5	9	0.3	0.9	0.12	0.6	0.08	36.7	0.67	2.18	0.98	0.98
0721	3.1	6.1	0.85	2.7	0.7	0.31	0.9	0.2	1	7	0.2	0.7	0.1	0.5	0.07	24.43	0.82	1.78	0.90	1.18
0722	1.9	4.1	0.39	1.5	0.3	0.14	0.4	0.1*	0.3	2*	0.1*	0.2	0.05*	0.1*	0.04*	9.23	0.88	1.73	1.15	0.89
0723	4.9	11.9	1.09	4.1	0.9	0.28	1.1	0.2	0.9	7	0.2	0.5	0.07	0.3	0.04*	33.44	0.82	1.41	1.24	0.88
0301	47.5	80.7	6.49	17.2	2.5	0.6	2	0.4	2.5	16	0.5	1.7	0.27	1.9	0.27	180.53	1.76	1.25	1.07	0.93
0302	16.9	50.9	6.47	31.4	8.9	2.37	8.4	1.2	6.5	33	1.3	3.6	0.55	3.5	0.5	175.49	0.49	1.48	1.13	0.90
0303	12.1	26.6	2.79	11.2	2.5	0.67	2.3	0.3	1.8	11	0.3	1	0.14	0.9	0.13	73.73	0.76	1.54	1.10	0.91
0304	26.6	42.7	3.37	9.8	1.6	0.39	1.4	0.3	2	14	0.4	1.4	0.23	1.5	0.22	105.91	1.43	1.19	1.04	0.89
0305	21.8	62.5	8.43	36.6	8.3	2.36	8.1	1.1	5.7	36	1	2.5	0.29	1.5	0.2	196.38	0.69	1.59	1.07	0.98
0306	7.9	17.2	1.62	6.1	1.4	0.4	1.5	0.3	1.6	11	0.3	0.9	0.13	0.7	0.1	51.15	0.78	1.32	1.16	0.89
0307	3.3	7.7	0.69	2.6	0.6	0.18	0.6	0.1*	0.5	5	0.1*	0.3	0.05*	0.1*	0.04*	21.47	0.78	1.54	1.23	0.87
0308	2.5	5.7	0.49	1.7	0.4	0.11	0.4	0.1*	0.4	3	0.1*	0.3	0.05*	0.3	0.04*	15.3	0.83	1.18	1.24	0.88
0309	10.5	24.3	2.32	8.6	1.9	0.46	1.9	0.3	2	15	0.4	1.2	0.16	1	0.14	70.18	0.83	1.27	1.19	0.90
0310	2.3	5.6	0.49	1.8	0.4	0.12	0.4	0.1*	0.3	3	0.1*	0.2	0.05*	0.1*	0.04*	14.61	0.83	1.29	1.27	0.86
0311	1	2.5	0.2	0.7	0.2	0.05	0.1*	0.1*	0.1*	2*	0.1*	0.1*	0.05*	0.1*	0.04*	4.65	0.68	0.72	1.34	0.84
0701	2.9	8	0.76	2.9	0.6	0.15	0.5	0.1*	0.4	3	0.1*	0.3	0.05*	0.2	0.04*	19.71	0.86	1.28	1.30	0.89
0702	15.1	27.8	1.65	4.1	0.6	0.13	0.3	0.1*	0.4	3	0.1*	0.3	0.05*	0.3	0.05	53.73	1.87	1.11	1.26	0.77
0703	3.4	8.5	0.73	2.4	0.5	0.13	0.4	0.1*	0.4	3	0.1*	0.3	0.05*	0.3	0.04*	20.06	0.99	1.24	1.30	0.90
0704	2.7	6.5	0.58	1.9	0.4	0.11	0.3	0.1*	0.3	2*	0.1*	0.2	0.05*	0.1*	0.04*	12.99	0.98	1.18	1.25	0.92
0705	8.1	17.8	1.45	4.6	0.8	0.2	0.7	0.1*	0.4	3	0.1*	0.2	0.05*	0.2	0.04*	37.45	1.23	1.42	1.24	0.88
0706	7.8	16.5	1.52	5.1	1	0.25	0.8	0.1*	0.6	4	0.1*	0.3	0.05*	0.3	0.04*	38.17	1.03	1.52	1.15	0.93
0707	3.4	7.8	0.54	2	0.4	0.11	0.4	0.1*	0.5	4	0.1*	0.3	0.05*	0.3	0.05	19.8	0.92	1.18	1.36	0.75
0708	4.1	10.6	1.12	4.4	1	0.29	1.1	0.1*	0.7	5	0.1*	0.3	0.05*	0.2	0.04*	28.81	0.76	1.76	1.19	0.93
0709	5.9	14.2	1.37	4.6	0.9	0.2	0.7	0.1*	0.4	2*	0.1*	0.3	0.05*	0.2	0.04*	28.77	1.03	1.31	1.20	0.95
0710	6.8	14	1.22	3.8	0.7	0.19	0.5	0.1*	0.5	3	0.1*	0.7	0.05*	0.3	0.05	31.76	1.18	1.48	1.16	0.92
0711	10.1	21.2	1.92	6.6	1.4	0.42	1.3	0.2	1.3	12	0.3	0.7	0.09	0.5	0.07	58.1	0.93	1.63	1.16	0.91
0712	5.6	11.6	1.03	3.2	0.6	0.19	0.5	0.1*	0.5	4	0.1*	0.3	0.05*	0.3	0.04*	27.82	1.16	1.63	1.16	0.93
0713	17.4	38.1	3.16	10.5	2.1	0.65	2	0.3	1.6	13	0.3	0.8	0.11	0.6	0.08	90.7	1.02	1.68	1.23	0.87
0714	18.2	30.4	2.51	7.3	1.2	0.23	0.8	0.1*	0.4	2*	0.1*	0.3	0.05*	0.2	0.04*	61.54	1.42	1.22	1.05	0.92
0715	12.7	22.6	2.13	6.7	1.2	0.3	0.9	0.2	0.9	7	0.2	0.7	0.12	0.8	0.13	56.58	1.20	1.28	1.03	0.96
0724	8.1	17.7	1.51	5.2	1.1	0.34	1.4	0.2	1.4	12	0.3	0.8	0.1	0.6	0.07	50.82	0.93	1.53	1.21	0.88
0725	0.8	2.4	0.28	1.3	0.3	0.1	0.4	0.1*	0.6	2*	0.1*	0.4	0.06	0.3	0.05	6.99	0.63	1.23	1.19	0.89
0726	3.8	8	0.84	3.3	0.8	0.26	0.9	0.2	1	7	0.2	0.6	0.08	0.4	0.06	27.44	0.71	1.40	1.08	0.92
0729	0.4	0.6*	0.07	0.5	0.2	0.05	0.2	0.1*	0.2	2*	0.1*	0.1*	0.05*	0.2	0.04*	1.82	0.24	0.72	0.86	0.68
0730	0.3*	0.6*	0.09	0.7	0.3	0.08	0.3	0.1*	0.2	2*	0.1*	0.1*	0.05*	0.2	0.04*	1.87	0.20	0.99	0.87	0.69
0601	7.8	17.5	1.76	6.3	1.3	0.41	1.5	0.2	1.4	10	0.3	0.8	0.1	0.6	0.09	50.06	0.92	1.67	1.14	0.94
0602	0.3*	0.6*	0.06	0.2*	0.1	0.05	0.1*	0.1*	0.1	2*	0.1*	0.1*	0.05*	0.1*	0.04*	0.31	0.41	0.87	1.08	0.97
0603	11.3	30.2	2.71	10.3	2.2	0.45	2.5	0.1*	0.7	15	0.1*	0.4	0.06	0.4	0.12	76.34	0.84	1.46	1.31	0.86
0604	4	11.6	1.03	3.2	0.6	0.16	0.6	0.1*	0.7	4	0.1*	0.4	0.06	0.4	0.05	26.8	1.16	1.37	1.37	0.93
0605	1.3	3.9	0.22	0.9	0.2	0.05	0.1*	0.1*	0.3	2*	0.1*	0.2	0.05*	0.2	0.04*	7.27	0.75	0.72	1.74	0.63

sample ID	Sc	As	Hf	Th	U	Zr	Cr	Co	Cd	Cu	Ni	Zn	Ag	Pb	V	Ba	Sr	Ga	Ge	Rb	Mo
	ppm
0716	1*	5	0.2*	0.3	0.1*	12	20*	1*	0.5	5	1*	1*	0.3*	5*	6	31	5	1*	4	3	2*
0718	1*	5	0.2*	0.1*	0.3	4	20*	1*	0.6	4	3	2	0.9	5*	32	15	12	1*	1*	2*	2*
0719	1*	5	0.2*	0.1*	0.4	9	20*	1*	0.5	9	3	1*	0.6	5*	28	20	10	1*	2	2*	2*
0720	1*	5	0.2*	0.1*	0.1*	5	20*	1*	0.6	7	4	49	0.5	14	16	15	12	1*	1*	2*	2*
0721	1*	43	0.2*	0.1*	0.4	4	20*	9	0.5	11	5	10	0.5	6	11	24	8	1*	2	2*	2*
0722	1*	5	0.2*	0.1*	0.3	4	20*	1*	0.5	8	3	3	0.3*	5*	5	114	11	1*	1*	2*	2*
0723	1*	5	0.2*	0.1*	0.5	4	20*	3	1.7	6	7	4	0.7	7	6	21	17	1*	1*	2*	2*
0301	19	5	4.4	9.8	2.4	169	150	8	0.7	127	29	18	0.3*	8	142	466	33	23	4	190	2*
0302	23	6	5.3	7.2	3.3	175	70	28	1.1	208	64	19	0.5	10	340	1620	18	17	3	11	27
0303	1*	14	0.8	2.7	1.7	33	40	6	0.5	3	1*	1*	0.3*	5*	56	64	11	13	3	18	2*
0304	15	5	3	7.8	3.1	124	70	18	1.3	22	44	23	0.4	5*	131	333	20	23	4	124	4
0305	2	16	0.2*	0.3	3	4	20*	4	1.6	6	15	6	1.3	13	72	19	65	4	1*	2*	2*
0306	1*	9	0.2*	0.1*	2.3	4	20*	4	2.1	11	13	8	1.9	12	76	15	25	3	1*	2*	2*
0307	1*	5	0.2*	0.1*	0.2	4	20*	2	1.6	3	2	4	0.7	8	5	10	16	1*	1*	2*	2*
0308	1*	5	0.2*	0.1*	0.2	4	20*	2	1.4	6	5	4	0.7	5*	5	22	14	1*	2	2*	2*
0309	2	5	0.2*	0.2	0.9	4	20*	4	2.2	5	9	6	1.5	12	28	11	25	3	1*	2*	2*
0310	1*	5	0.2*	0.1*	0.1*	4	20*	2	1.3	5	3	4	0.5	6	5	13	17	1*	2	2*	2*
0311	1*	5	0.2*	0.1*	0.1*	4	20*	3	0.8	9	4	3	0.3*	5*	5	14	5	1*	1*	2*	2*
0701	3	5	0.6	1.9	0.2	37	20*	6	1.8	1*	9	7	0.9	7	24	21	11	5	2	2*	2*
0702	4	5	1.6	3.6	0.5	74	40	9	1.2	9	15	52	0.3*	5*	47	25	7	12	6	5	2*
0703	3	5	0.8	2.4	0.5	40	20*	6	1.7	6	9	10	0.4	5*	24	18	13	7	3	2*	2*
0704	1*	5	0.4	1	0.3	19	20*	6	1.8	4	8	6	0.8	9	10	14	14	4	2	2*	2*
0705	4	5	1	2.8	0.4	53	20*	12	1.8	3	11	10	0.5	8	29	23	9	8	3	2*	2*
0706	3	15	0.5	1.4	0.8	20	20*	21	1.1	17	13	8	0.5	12	25	18	45	4	2	2*	2*
0707	4	5	1.4	3.3	0.6	59	40	6	1.4	4	11	10	0.4	5	29	29	21	7	4	2*	2*
0708	6	5	0.4	1.9	0.4	24	20*	7	1.9	5	12	10	0.6	6	27	11	14	6	3	2*	2*
0709	4	5	1.3	4.1	0.9	60	30	10	1.6	18	15	13	0.4	5*	34	42	9	9	3	5	2*
0710	3	5	2.1	5.3	1	100	40	9	1	21	19	14	0.3*	5*	42	131	13	12	5	22	2*
0711	6	5	0.8	1.9	0.8	36	20*	1*	1.1	4	10	7	0.6	7	31	37	51	6	1*	6	2*
0712	3	5	2	3.5	1.4	78	50	6	0.9	13	17	13	0.3*	5*	54	167	14	13	5	35	2*
0713	3	22	0.9	1.5	1.7	37	20*	19	1.3	9	15	5	0.8	11	18	19	24	5	2	2*	2*
0714	14	5	0.7	2.4	1.2	33	20*	11	1.7	16	21	11	0.7	7	84	74	16	11	4	14	2*
0715	11	5	3.3	9.7	1.7	133	80	6	0.7	21	28	18	0.3*	5*	87	493	28	13	4	122	2*
0724	1	16	0.2*	0.2	1.3	4	20*	1*	1.2	5	8	2	0.7	6	21	15	42	1*	1*	2*	2*
0725	4	38	0.8	2	0.8	35	20*	20	1	14	49	17	0.5	16	45	18	13	13	4	2*	2*
0726	4	5	0.4	1	1.2	23	20*	1*	0.5	9	10	5	0.3*	5*	46	62	56	3	1*	11	2*
0729	1*	14	0.2*	0.1*	0.6	4	20*	3	0.5	8	4	21	0.3*	5*	6	16	6	1*	2	2*	2*
0730	1*	5	0.3	0.1*	0.3	4	20*	1*	0.5	6	2	1*	0.3*	5*	5	8	3	1*	2	2*	2*
0601	4	12	0.3	0.8	1.5	11	20*	4	2.1	7	18	19	0.5	15	60	35	23	5	2	3	2*
0602	1*	5	0.2*	0.2	0.1*	5	20*	3	0.5	13	3	47	0.3*	9	5	76	14	1*	6	3	2*
0603	1*	9	0.2*	0.6	0.3	11	20*	4	1.9	9	9	24	0.4	12	13	29	28	1*	2	2*	2*
0604	1*	14	2.7	0.3	0.3	99	20*	3	1.3	11	7	27	0.3*	6	5	15	12	2	5	2*	2*
0605	1*	2	0.2*	0.1*	0.1*	4	20*	3	1.1	7	4	2	0.3*	5*	9	5	2	1*	3	2*	2*

* : below detection limit

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4. Chemical and Light Element Isotope Compositions of Bulk Rock and Kerogen

Light element concentrations in bulk rock and kerogen
The total organic carbon (TOC), carbonate carbon (C_carb), total carbon (TC), total nitrogen (TN), and pyrite sulphur (S_pyr) concentrations in the bulk rock samples are summarised in Table 1. The TOC and S_pyr concentrations ranged from 0.0 to 8.9 wt. % and from 0.0 to 1.1 wt. %, respectively. The TN concentrations of all bulk rock samples were below detection limit (<0.1 wt. %). In Figure S-3a, the TOC concentrations were plotted against the S_pyr concentrations. Generally, the shallower lithofacies, such as the stromatolite and sandstone facies, exhibited low TOC and S_pyr concentrations, whereas the deeper lithofacies, such as the carbonate and shale facies, exhibited a range of concentrations.

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The hydrogen, carbon, nitrogen and sulphur concentrations of the extracted kerogens were also summarised in Table 1. The observed H, C, and N concentrations in the kerogen samples ranged from 0.0 to 4.4 wt. %, 8.2 to 89 wt. %, and 0.0 to 1.4 wt. %, respectively. The predominant portion of the sulphur in Table 1 appears to be primarily associated with pyrite and to a lesser degree with organic sulphur as pyrite could not be removed from the kerogen by our procedure. The correlation between the H/C and N/C ratios for the examined kerogen samples compared with a previous data set of kerogen samples from the Palaeoproterozoic to Mesoproterozoic (Schopf and Klein, 1992), is shown in Figure S-4. The previous data plots showed a graphitising trend in the N/C and H/C ratios related to the differences in metamorphic grade of each geological section. The N/C and H/C ratios observed in this diagram indicate that the kerogen examined in this study was moderately mature, which was expected based on the metamorphic grade of the Gunflint Formation, which was as low as greenschist facies (Floran and Papike, 1975, Easton et al., 2000).

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Carbon and sulphur isotope compositions
The d¹³C values of the kerogen samples and d³⁴S values of pyrite are shown in Table 1. The d¹³C values ranged from -33.6 to -25.1 ‰. In general, the d¹³C values of the ferruginous or chert samples tend to be heavier than those of shale and carbonate samples. Only one outlier value, -25.1 ‰, was observed in sideritic BIF (sample #0723) from the Pass Lake section. No correlation was observed between the d¹³C and TOC values (Fig. S-3b). The d¹³C values of Fe-limestones and siderites, ranging from -6.0 to 0.5 ‰ with mean value of -3.3 ‰, were summarised by the previous studies (e.g., Winter and Knauth, 1992, Shield and Veizer, 2002, Ohmoto et al., 2004). These values of dissolved carbon are responsible for the source of organic carbon.

The d³⁴S values of pyrite ranged from -1.1 to +26.9 ‰ with a mean value of +9.9 ‰ (Table 1). The highest value of +26.9 ‰ was obtained from a sample of the carbonate-sand zone from the Current River section (sample #0701). The lowest value of -1.1 ‰ was obtained from an iron-rich layer in a BIF sample from the Pass Lake section (sample #0724).

5. Nitrogen Isotope Compositions and TEM Observations

The bulk d¹⁵N values of kerogen, ranging from +2.9 to +8.0 ‰, are summarised in Table 1. The heaviest value of +8.0 ‰ was observed in the kerogen from a carbonate-sand zone (sample #0711), whereas the lowest value of +2.9 ‰ was observed in the black shale (sample #0301). Generally, there are no lithofacies-dependent variations of d¹⁵N values.

Figure 1 (in the main text) shows the results of the stepwise combustion analyses performed on 11 kerogen samples in this study (Table S-2). The data from the 2 samples analysed by Ishida et al. (2012) were replotted and were included in Figure 1 for comparison. The nitrogen release amounts, the d¹⁵N values for each temperature step, and the average d¹⁵N values for the lower (500–575 ºC) and higher (600–1100 ºC) temperature fractions are shown. The average d¹⁵N values of the lower and higher-temperature fractions were calculated using the d¹⁵N values and the release amounts for the relevant temperature steps. Some samples exhibited higher d¹⁵N values for the higher-temperature fraction than for the lower-temperature fraction, while other samples exhibited the opposite trend, similar to the situation observed in Ishida et al. (2012). The release amounts of carbon appear to be fairly well correlated with those of nitrogen, suggesting that the nitrogen was hosted by carbon structures (Ishida et al., 2012).

In Figure 1, we set the boundary between the higher and lower-temperature fractions at 575 ºC, the temperature at which the d¹⁵N shift is most likely to occur. However, this boundary may vary within a range of ±25 ºC among the samples, according to the previous observations by Ishida et al. (2012). However, the overall trend of the average d¹⁵N values among the samples seems to be insensitive to the exact boundary temperatures adopted for the statistics. To illustrate this pattern, in Figure S-5 and Table S-3, we prepared another version of the average d¹⁵N values for the lower and higher temperature ranges. In this version, we determined the boundary of the lower- and the higher-fractions from the stepwise d¹⁵N profiles. Figure S-6a and b also demonstrate that the correlation between the d¹⁵N_LT and the Pr/Sm ratios is the same, while no correlation was observed between the d¹⁵N_HT and the Pr/Sm ratios. Thus, there is no significant difference between these two types of deviations. Figure S-6c demonstrates that the relationship between the d¹⁵N values of bulk kerogen (d¹⁵N_bulk) and the Pr/Sm, showing that there is no correlation between them.

Temp	C	H	N	C/N	d15N	±
°C	ppm			(atomic)	‰
0701 (0.047 mg)
500	82705	69	81	1193	-5.6	0.7
525	85022	121	84	1180	5.1	0.8
550	125467	221	72	2040	4.9	0.9
575	164260	212	389	493	6.6	0.7
1100	348763	436	3547	115	5.8	0.8
total	806217	1059	4172
average				225	5.6	0.8
0703 (0.226 mg)
475	79345	2544	54	1724	0.5	1.8
500	92155	1235	82	1312	3.8	0.4
525	151538	1392	215	822	5.9	0.6
550	188591	1150	466	472	6.6	0.4
575	218032	939	823	309	7.0	0.7
1100	135681	576	1141	139	7.9	0.5
total	865342	7835	2780
average				363	7.1	0.5
0705 (0.286 mg)
450	pre-combustion
475	115150	182	61	2200	5.9	1.2
500	141665	245	lost
525	46160	359	200	269	6.9	1.0
550	10286	559	388	31	5.8	1.4
575	5026	844	1445	4	8.4	1.7
1100	34316	2720	2177	18	7.3	1.6
total	352604	4908	-
average				902	5.4	1.4
0706 (0.555 mg)
475	40784	22	59	803	0.1	0.8
500	91937	655	217	495	5.0	0.4
525	110533	829	401	322	7.6	0.4
550	89845	573	356	295	7.9	0.4
575	25852	147	37	804	6.4	0.5
1100	22105	474	42	617	8.4	0.5
total	381056	2701	1112
average				400	6.9	0.4
0709 (0.067 mg)
500	180948	197	480	440	2.9	0.9
525	172407	137	1058	190	7.4	0.6
550	89034	50	409	254	8.7	0.4
575	30518	54	42	850	5.6	0.5
1100	53034	154	59	1045	4.4	0.7
total	525940	593	2049
average				299	7.3	0.6
0711 (0.412 mg)
450	pre-combustion
475	60541	93	126	803	7.2	0.5
500	99552	477	317	495	7.8	0.3
525	122193	545	756	322	6.7	0.4
550	125328	578	1018	295	8.8	0.3
575	13805	265	75	804	8.1	0.4
1100	lost
total	421419	1958	2291
average				215	8.0	0.4
0712 (0.072 mg)
500	192440	1022	544	412	4.6	0.8
525	263813	374	2211	139	6.2	0.7
550	139411	250	1212	134	8.7	0.5
575	15570	70	54	339	7.7	0.7
1100	40299	229	21	2254	4.7	1.2
total	651534	1944	4041
average				188	7.2	0.3
0713 (0.065 mg)
500	157778	365	280	656	5.0	0.7
525	116425	210	196	691	7.6	0.5
550	24157	42	25	1108	6.6	0.6
575	6824	17	8	944	5.9	1.3
1100	29652	98	24	1457	2.7	1.3
total	334837	731	535
average				731	6.4	0.6
0714 (0.611 mg)
450	pre-combustion
475	96101	762	115	973	2.9	0.7
500	85257	837	122	814	4.4	0.8
525	103927	626	145	834	4.9	0.8
550	135435	770	231	683	4.5	1.0
575	117687	504	258	533	6.1	1.1
1100	24930	144	202	144	5.8	0.8
total	563338	3644	1074
average				612	4.8	0.9
0721 (0.153 mg)
500	54442	17	33	1923	2.1	0.6
525	57008	15	62	1079	5.5	0.6
550	29440	6	19	1785	5.6	0.7
575	5187	6	3	1856	lost
1100	32579	24	15	2568	6.8	0.8
total	178657	67	132
average				1579	4.7	0.6
0723 (0.412 mg)
450	pre-combustion
475	28654	68	94	357	4.6	0.3
500	29798	76	138	252	5.2	0.5
525	34056	68	246	161	6.6	0.5
550	38036	77	360	123	5.1	0.3
575	38036	43	402	110	5.4	0.3
1100	45206	166	30	1739	6.6	0.4
total	213785	498	1271
average				196	5.4	0.4

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Sample ID	Litho facies	d15N (‰): divided by temperature				d15N (‰): divided by d15N values
		d15NLT	±	d15NHT	±	temperature range	d15NLT	±	temperature range	d15NHT	±
0701	Carbonate sandstone	4.6	0.7	5.8	0.8	500-550 °C	5.0	0.8	575-1100 °C	5.9	0.8
0703	Carbonate sandstone	6.5	0.6	7.9	0.5	500-575 °C	6.5	0.6	1100 °C	7.9	0.5
0704	Carbonate sandstone	5.9	0.8	5.5	0.8	500-575 °C	5.9	0.8	600-650 °C	5.2	0.8
0705	Carbonate sandstone	7.8	1.6	7.3	1.6	500-575 °C	7.8	1.6	1100 °C	7.3	1.6
0706	Carbonate sandstone	7.1	0.4	8.4	0.5	500-550 °C	7.1	0.4	575-1100 °C	7.4	0.5
0708	Carbonate sandstone	4.3	1.0	7.1	0.8	500-600 °C	5.0	1.0	625-1100 °C	7.3	0.8
0709	Carbonate sandstone	6.5	0.6	4.4	0.7	500-575 °C	6.5	0.6	1100 °C	4.9	0.7
0711	Carbonate sandstone	7.9	0.4	-	-	500-550 °C	7.9	0.4	-	-	-
0712	Carbonate sandstone	6.8	0.7	4.7	1.2	500-550 °C	6.8	0.7	575-1100 °C	6.9	0.8
0713	Shale (phosphorous)	6.1	0.6	2.7	1.3	500-525 °C	6.1	0.6	550-1100 °C	4.9	1.0
0714	Black shale	5.1	1.0	5.8	0.8	500-550 °C	4.6	0.9	575-1100 °C	6.0	0.9
0721	Taconite	4.4	0.6	6.8	0.8	500-550 °C	4.4	0.6	1100 °C	6.8	0.8
0723	Carbonate BIF	5.5	0.4	6.6	0.4	500-575 °C	5.5	0.4	1100 °C	6.6	0.4

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The TEM observation of kerogen supports the heterogeneous carbon structure (Fig. S-7). Two types of carbon structures were observed in kerogen: disordered and folding structure, and well-ordered graphane-stacking structures. The differences in host organic matter would be responsible for the heterogeneity of nitrogen isotope compositions.

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6. Tectonic and Oceanic Environment in the Animikie Basin

Continental and hydrothermal fluxes
The samples examined in the present study include both siliciclastic and chemical sedimentary rocks. Both types contain variable amounts of detrital minerals. The contribution of detrital minerals to the bulk rock chemistry can be evaluated using major and trace elements. For example, K and Ti are correlated with Al, suggesting K and Ti were also introduced into the sediments by detrital clay and oxide minerals. V, Th, Sc, and Ni show positive correlations with Zr in all examined samples, suggesting that V, Th, and Sc are also associated with detrital minerals, most likely oxides (Fig. S-8). The carbonate samples contain appreciable amounts of Al, K and Ti, suggesting that large amounts of weathering products were added to the shallow part of the Animikie basin. The effects of detrital materials are also observed in the REE patterns of these samples. For example, a portion of the carbonate samples (e.g., samples #0702 and #0705) from the Current River section exhibit enrichment in light REEs (LREEs) relative to heavy REEs (HREEs), comparable to the patterns of the shales (e.g., samples #0301 and #0304). These trends suggest a significant input of continental detritus in these carbonate samples (Fig. S-9, Tables S-1, S-4).

The concentrations of Fe were generally high in carbonate samples (Fig. S-8b), indicating that anoxic submarine hydrothermal plumes supplied dissolved Fe²⁺ to the shallow ocean (e.g., Simonson, 1985; Derry and Jacobsen, 1990; Isley, 1995; Carrigan and Cameron, 1991; Fralick et al., 2002; Poulton et al., 2010). In addition, most samples exhibit positive Eu anomalies, supporting the above model for carbonate precipitation under the influence of submarine hydrothermal plumes (e.g., Derry and Jacobsen, 1990; Poulton et al., 2010).

The carbonates observed at the Terry Fox section represent the uppermost section of the Gunflint Formation. They are depleted in light REEs (LREEs) compared to heavy REEs (HREEs) (Fig. S-9d). These patterns apparently resemble the REE pattern of the contemporary ocean (Johannesson et al., 2006 and references therein). The Al and Zr concentrations are much lower in this section. Furthermore, the positive Eu anomaly is less clear in these samples, which also have lower Fe concentrations. These observations suggest that submarine hydrothermal and the continental fluxes were comparatively reduced during the deposition of the carbonate in the Terry Fox section.

The REE patterns of shale samples are shown in Figure S-9. Samples #0301 and #0304 are enriched in LREEs and feature flat MREE to HREE patterns. This type is the most dominant among the examined shale samples. Their protoliths were most likely enriched in LREEs relative to PAAS, thereby indicating a greater contribution from felsic to intermediate source rocks. Shale samples from the Terry Fox section (samples #0725 and #0726) and one sample from the Blend Lake North section (sample #0302) show LREE-depleted and flat MREE and HREE patterns. Such a pattern is explained by greater contributions from mafic components, including basaltic tuffs.

An evolution model for the Animikie Basin has been proposed by previous investigators (e.g., Schulz and Cannon, 2007). This model was based on the geology and chronology of the Marquette Range Supergroup in the Wisconsin and Michigan area. The proposed model can be applied to the Gunflint Formation because of its chronological and lithological correlation to the Marquette sections. At ca. 1875 Ma, an arc started to collide with an embayment along the southern edge of the Superior craton, forming a broad and shallow basin in which northerly-derived siliciclastic sediments were deposited on Archean crust. After collision, the northward subduction led to a period of back-arc extension, generating submarine volcanism and the release of hydrothermal plumes in the Animikie Basin. As a result, laterally extensive BIFs and other Fe-rich rocks formed in the Animikie basin. During this period of arc collision and back-arc extension, the Superior craton, which has an intermediate crustal composition, was the main source of detrital material. By 1850 Ma, subduction had ceased, and the Sudbury impact event had occurred. As a result, a foreland basin replaced the back-arc basin, and more detrital material was supplied from “old” portions of the arc, which had more mafic tholeiitic volcanics. These tholeiitic volcanics have different REE patterns than the Superior craton. Thus, the change in sediment source is recorded in the LREE-depleted characteristics of shales in the upper part of the Gunflint Formation. This tectonic history is consistent with the observed LREE-enriched patterns in a majority of the shales (Fig. S-9).

sample ID	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y	Ho	Er	Tm	Yb	Lu
	(PAAS normalised)
0716	0.022	0.036	0.018	0.016	0.015	0.041	0.017*	0.112*	0.019*	0.073*	0.095*	0.033*	0.111*	0.033*	0.091*
0718	0.191	0.207	0.212	0.249	0.349	0.856	0.447	0.449	0.469	0.659	0.475	0.390	0.310	0.232	0.182
0719	0.128	0.130	0.125	0.137	0.189	0.453	0.248	0.224	0.225	0.256	0.190	0.195	0.177	0.133	0.114
0720	0.114	0.113	0.117	0.126	0.174	0.420	0.215	0.224	0.282	0.330	0.285	0.293	0.266	0.199	0.182
0721	0.070	0.069	0.084	0.072	0.102	0.255	0.149	0.224	0.188	0.256	0.190	0.228	0.222	0.166	0.160
0722	0.043	0.046	0.038	0.040	0.044	0.115	0.066	0.112*	0.056	0.073*	0.095*	0.065	0.111*	0.033*	0.091*
0723	0.110	0.135	0.107	0.110	0.131	0.230	0.182	0.224	0.169	0.256	0.190	0.163	0.155	0.100	0.091*
0301	1.066	0.914	0.639	0.461	0.363	0.494	0.331	0.449	0.469	0.586	0.475	0.553	0.599	0.631	0.616
0302	0.379	0.577	0.637	0.841	1.293	1.951	1.390	1.346	1.221	1.208	1.235	1.171	1.220	1.162	1.140
0303	0.272	0.301	0.275	0.300	0.363	0.551	0.381	0.337	0.338	0.403	0.285	0.325	0.310	0.299	0.296
0304	0.597	0.484	0.332	0.263	0.232	0.321	0.232	0.337	0.376	0.513	0.380	0.455	0.510	0.498	0.502
0305	0.489	0.708	0.831	0.981	1.206	1.942	1.340	1.234	1.070	1.318	0.950	0.813	0.643	0.498	0.456
0306	0.177	0.195	0.160	0.163	0.203	0.329	0.248	0.337	0.300	0.403	0.285	0.293	0.288	0.232	0.228
0307	0.074	0.087	0.068	0.070	0.087	0.148	0.099	0.112*	0.094	0.183	0.095*	0.098	0.111*	0.033*	0.091*
0308	0.056	0.065	0.048	0.046	0.058	0.091	0.066	0.112*	0.075	0.110	0.095*	0.098	0.111*	0.100	0.091*
0309	0.236	0.275	0.229	0.230	0.276	0.379	0.314	0.337	0.376	0.549	0.380	0.390	0.355	0.332	0.319
0310	0.052	0.063	0.048	0.048	0.058	0.099	0.066	0.112*	0.056	0.110	0.095*	0.065	0.111*	0.033*	0.091*
0311	0.022	0.028	0.020	0.019	0.029	0.041	0.017*	0.112*	0.019*	0.073*	0.095*	0.033*	0.111*	0.033*	0.091*
0701	0.065	0.091	0.075	0.078	0.087	0.123	0.083	0.112*	0.075	0.110	0.095*	0.098	0.111*	0.066	0.091*
0702	0.339	0.315	0.163	0.110	0.087	0.107	0.050	0.112*	0.075	0.110	0.095*	0.098	0.111*	0.100	0.114
0703	0.076	0.096	0.072	0.064	0.073	0.107	0.066	0.112*	0.075	0.110	0.095*	0.098	0.111*	0.100	0.091*
0704	0.061	0.074	0.057	0.051	0.058	0.091	0.050	0.112*	0.056	0.073*	0.095*	0.065	0.111*	0.033*	0.091*
0705	0.182	0.202	0.143	0.123	0.116	0.165	0.116	0.112*	0.075	0.110	0.095*	0.065	0.111*	0.066	0.091*
0706	0.175	0.187	0.150	0.137	0.145	0.206	0.132	0.112*	0.113	0.146	0.095*	0.098	0.111*	0.100	0.091*
0707	0.076	0.088	0.053	0.054	0.058	0.091	0.066	0.112*	0.094	0.146	0.095*	0.098	0.111*	0.100	0.114
0708	0.092	0.120	0.110	0.118	0.145	0.239	0.182	0.112*	0.131	0.183	0.095*	0.098	0.111*	0.066	0.091*
0709	0.132	0.161	0.135	0.123	0.131	0.165	0.116	0.112*	0.075	0.073*	0.095*	0.098	0.111*	0.066	0.091*
0710	0.153	0.159	0.120	0.102	0.102	0.156	0.083	0.112*	0.094	0.110	0.095*	0.228	0.111*	0.100	0.114
0711	0.227	0.240	0.189	0.177	0.203	0.346	0.215	0.224	0.244	0.439	0.285	0.228	0.200	0.166	0.160
0712	0.126	0.131	0.101	0.086	0.087	0.156	0.083	0.112*	0.094	0.146	0.095*	0.098	0.111*	0.100	0.091*
0713	0.390	0.432	0.311	0.281	0.305	0.535	0.331	0.337	0.300	0.476	0.285	0.260	0.244	0.199	0.182
0714	0.408	0.344	0.247	0.196	0.174	0.189	0.132	0.112*	0.075	0.073*	0.095*	0.098	0.111*	0.066	0.091*
0715	0.285	0.256	0.210	0.180	0.174	0.247	0.149	0.224	0.169	0.256	0.190	0.228	0.266	0.266	0.296
0724	0.182	0.201	0.149	0.139	0.160	0.280	0.232	0.224	0.263	0.439	0.285	0.260	0.222	0.199	0.160
0725	0.018	0.027	0.028	0.035	0.044	0.082	0.066	0.112*	0.113	0.073*	0.095*	0.130	0.133	0.100	0.114
0726	0.085	0.091	0.083	0.088	0.116	0.214	0.149	0.224	0.188	0.256	0.190	0.195	0.177	0.133	0.137
0729	0.009	0.007*	0.007*	0.013	0.029	0.041	0.033	0.112*	0.038	0.073*	0.095*	0.033*	0.111*	0.066	0.091*
0730	0.007*	0.007*	0.009	0.019	0.044	0.066	0.050	0.112*	0.038	0.073*	0.095*	0.033*	0.111*	0.066	0.091*
0601	0.175	0.198	0.173	0.169	0.189	0.337	0.248	0.224	0.263	0.366	0.285	0.260	0.222	0.199	0.205
0602	0.007*	0.007*	0.006	0.005*	0.015	0.041	0.017*	0.112*	0.019*	0.073*	0.095*	0.033*	0.111*	0.033*	0.091*
0603	0.254	0.342	0.267	0.276	0.320	0.370	0.414	0.112*	0.131	0.549	0.095*	0.130	0.133	0.133	0.274
0604	0.090	0.131	0.101	0.086	0.087	0.132	0.099	0.112*	0.131	0.146	0.095*	0.130	0.133	0.133	0.114
0605	0.029	0.044	0.022	0.024	0.029	0.041	0.017*	0.112*	0.019*	0.073*	0.095*	0.065	0.111*	0.066	0.091*
PAAS values	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y	Ho	Er	Tm	Yb	Lu
	44.56	88.25	10.15	37.32	6.884	1.215	6.043	0.8914	5.325	27.31	1.053	3.075	0.451	3.012	0.4386

* below detection limit
PAAS values were referred from Pourmand et al. (2012)

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REE behaviour and implications for the redox conditions of the ocean
Redox stratification of Palaeoproterozoic oceans has been proposed by previous investigators (e.g., Canfield et al., 1996; Anbar and Knoll, 2002; Poulton et al., 2004, 2010; Hiatt et al., 2015). However, it is unclear whether the shallow part of the Gunflint ocean was widely oxic or anoxic (e.g., Poulton et al., 2004, 2010; Rasmussen et al., 2012; Hiatt et al., 2015).

The mineralogy and chemistry of our samples suggest widespread oxic shallow ocean conditions during the deposition of the Gunflint Formation. Grains of microscopic haematite are common in carbonate oolite (Pass Lake section, Fig. S-2a), and these grains are protected by a ferrodolomite matrix. Therefore, these haematite grains are not modern weathering products. The Ce/Ce* anomaly observed in the REE data further supports the argument of an oxic environment in the Gunflint sedimentary basin. The negative correlation observed between Ce/Ce* and Pr/Pr* in the examined samples (Fig. S-10) indicates that the observed Ce anomaly is a real trend, as opposed to false trends that may occur because of La anomalies (Bolhar et al., 2005). These observations are consistent with the redox-stratified model suggested by previous investigators (Planavsky et al., 2009).

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The occurrence of phosphorite supports the existence of a redox boundary in the water column (sample #0713). An Fe-pump model has been proposed by previous investigators to explain the initial concentration of phosphates in the Animikie sedimentary rocks (e.g., Bjerrum and Canfield, 1996; Edwards, et al., 2012; Hiatt et al., 2015). In this model, dissolved phosphorous was adsorbed on host Fe-hydroxides in the vicinity of the redox boundary and then deposited in the sediment. The presence of Fe-hydroxides is the key for the Fe-pump model, and oxic surface water is required to form Fe-hydroxides (Nelson et al., 2010). Later diagenetic processes re-mobilised and concentrated the phosphate, forming pristine phosphorite in the Gunflint sediments (Hiatt et al., 2015). Flooding processes brecciated and transported the pristine phosphorites to the deeper sedimentary basin, allowing re-deposition of granular and rounded phosphorites, as seen in Figure S-2e. Arsenic is only detected in the framboidal pyrite around the phosphorites, and other euhedral pyrites, which occur in the carbonate shale hosting phosphorites, do not contain As. Arsenic compounds can be precipitated in sediments associated with Fe-hydroxides and phosphorus because the chemical behaviour of As resembles that of Fe (O’Day, 2006). These local concentrations of As likely originated from submarine hydrothermal activity and were concentrated in the sediments by the Fe-pump process. Although the formation process of framboidal pyrite in the examined samples remains unclear, Fe-hydroxides can act as a precursor (Morse and Wang, 1997, Ohfuji and Rickard, 2005). In modern environments, phosphorus is released from terrestrial rocks by weathering and transported into the oceans by rivers. This phosphorous is then fixed into marine sediments through biological activities (Paytan and McLaughlin, 2007). In Palaeoproterozoic redox-stratified oceans, the adsorption of phosphorous to Fe-hydroxides might have played a more important role in the accumulation of phosphorous in marine sediments (e.g., Papineau et al., 2009, 2013; Edwards et al., 2012). In the present study, we could not observe such an occurrence of phosphorite except for sample #0713. We interpreted that the most of P-related minerals, which originally existed in the Gunflint Formation, were already silicified and lost.

The correlation coefficient values between the Pr/Sm ratio and the major and trace element concentrations, and d¹⁵N values among examined samples, are summarised in Table S-5. This table shows that some bioessential trace metals, such as Mo, V, and Ni, have positive correlations with the Pr/Sm ratios, suggesting that the increment of continental input supplied them to the ocean.

Major elements		Trace elements		REEs		d15N values
SiO2	0.23	Sc	0.67	La	0.62	d15NLT	0.56
Al2O3	0.68	As	-0.23	Ce	0.49	d15NHT	0.03
Fe2O3	-0.10	Hf	0.74	Pr	0.25	d15Nbulk	<0.01
MnO	-0.43	Ta	0.67	Nd	0.04
MgO	-0.36	Th	0.69	Sm	-0.12
CaO	-0.36	U	0.29	Eu	-0.23
Na2O	-0.13	Zr	0.76	Gd	-0.23
K2O	0.56	Cr	0.64	Tb	-0.15
TiO2	0.68	Co	0.37	Dy	-0.15
P2O5	-0.23	Cd	0.01	Y	-0.16
		Cu	0.56	Ho	-0.11
		Ni	0.42	Er	-0.01
		Zn	0.48	Tm	0.11
		Ag	-0.36	Yb	0.23
		Pb	-0.27	Lu	0.27
		V	0.53
		Ba	0.55
		Sr	-0.10
		Ga	0.69
		Ge	0.71
		Rb	0.59
		Mo	0.23

The components which have positive correlation coefficiencies are displayed in bold.

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Availability of sulphate
Previous investigators suggested that the sulphate concentration in the Animikie Basin was very low and that the Gunflint Formation records a global transition into a sulfidic ocean in the Palaeoproterozoic (e.g., Poulton et al., 2004, 2010; Johnston et al., 2006). However, some investigators questioned the suitability of using the Gunflint Formation to discuss a global transition into a sulphidic ocean and the availability of sulphate in the Animikie Basin (Pufahl et al., 2010). These arguments are based on C (TOC) and S (S_pyr) concentrations (i.e. S_pyr/TOC ratios) of sedimentary rocks, d³⁴S values of pyrite, or Fe-speciation analyses in the Gunflint, Rove, and Michigamme Formations. In particular, the S_pyr/TOC ratios in Gunflint Formation are the key issue in this discussion.

The TOC values in the present study are compatible with those of Poulton et al. (2004). In contrast, the S_pyr concentrations are much higher than those of Poulton et al. (2004). The S_pyr concentrations by Poulton et al. (2004) are less than 0.19 wt. % S, whereas those of the present study are up to 0.58 wt. % S. In particular, samples #0721 and #0725 feature high S_pyr values, but their TOC values are similar to those of the other samples. Sample #0725 is composed of Fe-rich dolomite. The pyrite crystals found in this sample are closely associated with laminated organic matter (Fig. S-2i). The absence of hydrothermal alteration and the generally high d³⁴S values suggest that these pyrite crystals are products of microbial sulphate reduction. Other carbonate samples with moderate TOC values also have microscopic pyrite in organic-rich laminae that yield moderate S_pyr concentrations (up to 0.1 wt. % S). The high S_pyr and low TOC concentrations in sample #0725 are explained by the difference in the digestibility of organic matter by sulphate-reducing bacteria. Microbial sulphate reduction generally proceeds through the following reaction:

SO₄^2- + 2CH₂O-R = 2HCO₃^- + H₂S + R

where R is the residual, non-digestible organic matter included in the TOC. If a large portion of the sedimentary organic matter is digestible by sulphate reducers, large amounts of S_pyr will be generated, even if the concentrations of TOC, represented in Reaction S-1 as CH₂O-R, are not similarly high (Berner and Raiswell, 1984, Raiswell and Berner, 1986).

The S_pyr/TOC ratio of the oolitic sample (sample #0721) was also high. If the pyrite crystals were formed by reaction (1), the original amount of organic matter must have been more than 10 times higher than the observed amount. The N/C ratio of the extracted kerogen from the oolite (N/C = 0.009) is much lower than the values of other kerogen samples examined in this study and in the literature (e.g., Alleon et al., 2016). These facts imply that the Gunflint oolitic samples were originally rich in organic matter, and the organic matter was degraded by sulphate-reducing bacteria during diagenesis. The study by Wacey et al. (2013) supports the above model arguing for the active digestion of organic matter by sulphate-reducing bacteria in the Gunflint sediments.

The scattered relationship between S_pyr and TOC concentrations among all the examined samples shows a weak correlation (Fig. S-3a). The overall S_pyr/TOC ratio for the Gunflint samples is 0.26 ± 0.08. This ratio is much higher than that of 0.04 obtained by Poulton et al. (2004). The samples examined by Poulton et al. (2004) were mainly carbonate or sandstone, whereas our samples contain black shales and/or thin shale laminae in BIF, and carbonates. Pufahl et al. (2010) also found that deep-water shale in the Michigamme Formation contains abundant pyrite and that deltaic sediments, such as sandstone, do not contain appreciable amounts of pyrite because of the introduction of more fresh river water in the shallow coastal environments. On the other hand, the oolitic samples and carbonates in the present study have appreciable amounts of pyrite, despite their deposition in shallow or low-stand environments. The Animikie Basin experienced several sea level transgression/regression cycles; thus, the occasional mixing between sulphate-rich seawater with sulphate-poor fresh water is possible. This explains the diversity of S_pyr concentrations in the literature.

Supplementary Information References