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Introduction

Most marine massive Fe deposits formed 3.0−1.8 Ga, with a billion-year hiatus until a brief return around 0.8 Ga (Konhauser et al., 2017

Konhauser, K.O., Planavsky, N.J., Hardisty, D.S., Robbins, L.J., Warchola, T.J., Haugaard, R., Lalonde, S.V., Partin, C.A., Oonk, P.B.H., Tsikos, H., Lyons, T.W., Bekker, A., Johnson, C.M. (2017) Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history. Earth-Science Reviews 172, 140–177. https://doi.org/10.1016/j.earscirev.2017.06.012

). During this gap period, the iron formation (IF) shortage was thought to be a result of significantly decreased Fe²⁺ concentrations in the ocean, which was either substantially oxidised (Holland, 1990

Holland, H.D. (1990) Origins of breathable air. Nature 347, 17–17. https://doi.org/10.1038/347017a0

) or sulfurised (Canfield, 1998

Canfield, D.E. (1998) A new model for Proterozoic ocean chemistry. Nature 396, 450–453. https://doi.org/10.1038/24839

). Recent studies provide increasing evidence that ferruginous water was still widespread in the deep ocean until 0.58 Ga (Canfield et al., 2008

Canfield, D.E., Poulton, S.W., Knoll, A.H., Narbonne, G.M., Ross, G., Goldberg, T., Strauss, H. (2008) Ferruginous conditions dominated later neoproterozoic deep-water chemistry. Science 321, 949–952. https://doi.org/10.1126/science.1154499

). Although atmospheric oxygen levels are still controversial in the time window of 1.8−0.8 Ga (Planavsky et al., 2014

Planavsky, N.J., Reinhard, C.T., Wang, X.L., Thomson, D., McGoldrick, P., Rainbird, R.H., Johnson, T., Fischer, W.W., Lyons, T.W. (2014) Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals. Science 346, 635–638. https://doi.org/10.1126/science.1258410

; Zhang et al., 2016

Zhang, S., Wang, X., Wang, H., Bjerrum, C.J., Hammarlund, E.U., Costa, M.M., Connelly, J.N., Zhang, B., Su, J., Canfield, D.E. (2016) Sufficient oxygen for animal respiration 1,400 million years ago. Proceedings of the National Academy of Sciences 113, 1731–1736. https://doi.org/10.1073/pnas.1523449113

, 2021

Zhang, S., Wang, H., Wang, X., Ye, Y. (2021) The Mesoproterozoic Oxygenation Event. Science China: Earth Science 64, 2043–2068. https://doi.org/10.1007/s11430-020-9825-x

; Canfield et al., 2021

Canfield, D.E., van Zuilen, M.A., Nabhan, S., Bjerrum, C.J., Zhang, S.C., Wang, H.J., Wang, X.M. (2021) Petrographic carbon in ancient sediments constrains Proterozoic Era atmospheric oxygen levels. Proceedings of the National Academy of Sciences 118, e2101544118. https://doi.org/10.1073/pnas.2101544118

), it is believed that oxygen was deficient compared to the last 0.7 Ga.

Thus, although IF deposition is limited from 1.8−0.8 Ga, there are recent reports of significant ferrous carbonate deposition generating siderite-dominated IFs, such as the ∼1.40 Ga Xiamaling (XML) IF in North China (Canfield et al., 2018

Canfield, D.E., Zhang, S.C., Wang, H.J., Wang, X.M., Zhao, W.Z., Su, J., Bjerrum, C.J., Haxen, E.R., Hammarlund, E.U. (2018) A Mesoproterozoic Iron Formation. Proceedings of the National Academy of Sciences 115, E3895–E3904. https://doi.org/10.1073/pnas.1720529115

; Tang etal., 2018

Tang, D., Shi, X., Jiang, G., Wu, T., Ma, J., Zhou, X. (2018) Stratiform siderites from the Mesoproterozoic Xiamaling Formation in North China: Genesis and environmental implications. Gondwana Research 58, 1–15. https://doi.org/10.1016/j.gr.2018.01.013

) and the ∼1.33 Ga Jingtieshan IF in Qilian (Yang et al., 2018

Yang, X., Zhang, Z., Santosh, M., Duan, S., Liang, T. (2018) Anoxic to suboxic Mesoproterozoic ocean: Evidence from iron isotope and geochemistry of siderite in the Banded Iron Formations from North Qilian, NW China. Precambrian Research 307, 115–124. https://doi.org/10.1016/j.precamres.2018.01.007

). Siderite deposits are also found in the ∼1.45 Ga Sherwin ironstone in Australia (Planavsky et al., 2014

Planavsky, N.J., Reinhard, C.T., Wang, X.L., Thomson, D., McGoldrick, P., Rainbird, R.H., Johnson, T., Fischer, W.W., Lyons, T.W. (2014) Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals. Science 346, 635–638. https://doi.org/10.1126/science.1258410

). To further explore the mechanisms of siderite-dominated IF deposition and the associated marine Fe and C cycles, we analysed the contents and isotopic compositions of Fe and C of the XML IF. Combined with the reported data in Canfield et al. (2018)

Canfield, D.E., Zhang, S.C., Wang, H.J., Wang, X.M., Zhao, W.Z., Su, J., Bjerrum, C.J., Haxen, E.R., Hammarlund, E.U. (2018) A Mesoproterozoic Iron Formation. Proceedings of the National Academy of Sciences 115, E3895–E3904. https://doi.org/10.1073/pnas.1720529115

, our results show evidence for hydrothermal Fe²⁺ input during the deposition of XML IF, where the Fe and C cycles were tightly coupled.

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Geological Background and Samples

Samples were obtained from a core in the Xiahuayuan region located in the Yanliao Basin, which was an offshore basin in North China during the breakup of the Columbia supercontinent (Fig. 1a–c). The XML IF is found over 45 m of stratigraphic thickness and is defined as unit 5, which can be further divided into the lower part with greyish-black mudstone and the upper part with green sandy mudstone (Fig. 1d). Centimetre- to decimetre-sized laminar and nodular siderites are embedded in the clastic rocks (Fig. 1e–h), indicating a pore-water diagenetic environment. Detailed information of the geological background is in the Supplementary Information (SI).

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Methods and Results

As the normal Fe extraction protocol in Poulton and Canfield (2005)

Poulton, S.W., Canfield, D.E. (2005) Development of a sequential extraction procedure for iron: implications for iron partitioning in continentally derived particulates. Chemical Geology 214, 209–221. https://doi.org/10.1016/j.chemgeo.2004.09.003

proved ineffective at removing siderite from our IF samples, an acid extraction method was used. We used ferrozine to determine the contents of acid-soluble Fe²⁺, and used ferrozine with the addition of hydroxylamine to determine the sum of Fe²⁺ and Fe³⁺. On separate samples, the Cr-reduction method was used to quantify the Fe bound in pyrite (Fe_py). Iron isotope values (δ⁵⁶Fe) from whole rock samples were determined and expressed as the ⁵⁶Fe/⁵⁴Fe ratio relative to IRMM-014. Contents of total organic carbon (TOC), elements and main diagenetic minerals of whole rock samples were also determined. The isotopic composition of carbonate (δ¹³C_carb) of the IF samples and the isotopic composition of organic carbon (δ¹³C_org) of the clastic samples were determined and expressed relative to VPDB. Detailed methods are in the SI.

Geochemical data of the IF samples (defined as samples with total Fe (Fe_T) > 15 %) and the interbedded clastic rocks are illustrated in Figure 2 and Table S-1. As has been observed in the upper part (discussed in Canfield et al., 2018

Canfield, D.E., Zhang, S.C., Wang, H.J., Wang, X.M., Zhao, W.Z., Su, J., Bjerrum, C.J., Haxen, E.R., Hammarlund, E.U. (2018) A Mesoproterozoic Iron Formation. Proceedings of the National Academy of Sciences 115, E3895–E3904. https://doi.org/10.1073/pnas.1720529115

), siderite is the dominant diagenetic mineral of the lower IF samples (Table S-1). The lower IF samples have extremely high Fe_T/Al ratios (mostly >15) and Fe_T/Ti ratios (mostly >200), high Fe²⁺/Fe_T ratios (>0.8), extremely low Fe³⁺/Fe_T ratios (<0.02) and Fe_py/Fe_T ratios (<0.01), and negative δ⁵⁶Fe values (from −0.53 ‰ to −0.34 ‰). In contrast, the interbedded clastic rocks of the lower IF have low Fe_T/Al ratios (∼0.46) and Fe_T/Ti ratios (mostly from 4 to 10), medium Fe²⁺/Fe_T ratios (from 0.17 to 0.74), low Fe³⁺/Fe_T ratios (<0.1), medium Fe_py/Fe_T ratios (from 0.01 to 0.3), and positive δ⁵⁶Fe values (mostly from +0.19 ‰ to +0.72 ‰). The upper IF samples also have high Fe_T/Al ratios (mostly from 3 to 15) and Fe_T/Ti ratios (mostly from 30 to 200), high Fe²⁺/Fe_T ratios (>0.8), low Fe³⁺/Fe_T ratios (<0.15), extremely low Fe_py/Fe_T ratios (<0.01), and more negative δ⁵⁶Fe values (from −1.20 ‰ to −0.32 ‰) than those of the lower IF samples. The interbedded clastic rocks of the upper IF have medium Fe_T/Al ratios (from 0.5 to 2.7) and Fe_T/Ti ratios (mostly from 10 to 30), medium Fe²⁺/Fe_T ratios (from 0.46 to 0.83), low Fe³⁺/Fe_T ratios (<0.15), extremely low Fe_py/Fe_T ratios (<0.01), and fluctuating δ⁵⁶Fe values (from −0.71 ‰ to +0.38 ‰). The sedimentary rocks of the lower part (including the IF samples) have higher TOC values (mostly from 0.3 % to 1.0 %) than those of the upper part (mostly <0.3 %). The δ¹³C_org values of most samples were between −33 ‰ and −32 ‰, while the δ¹³C_carb values of the IF samples fluctuated from −27 ‰ to −8 ‰.

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Depositional Redox Environments

We could not use standard Fe speciation protocols on the siderite-rich samples, but high Fe_T/Al ratios (Lyons and Severmann, 2006

Lyons, T.W., Severmann, S. (2006) A critical look at iron paleoredox proxies: New insights from modern euxinic marine basins. Geochimica et Cosmochimica Acta 70, 5698–5722. https://doi.org/10.1016/j.gca.2006.08.021

) and high Fe_T/Ti ratios (Dauphas et al., 2004

Dauphas, N., Van Zuilen, M., Wadhwa, M., Davis, A.M, Marty, B.J., Philip, E. (2004) Clues from Fe isotope variations on the origin of early Archean BIFs from Greenland. Science 306, 2077–2080. https://doi.org/10.1126/science.1104639

), combined with high Fe²⁺/Fe_T ratios (>0.8) and extremely low Fe_py/Fe_T ratios (<0.01) confirmed ferruginous conditions during the IF deposition, consistent with the previous redox-sensitive element proxies from Canfield et al. (2018)

Canfield, D.E., Zhang, S.C., Wang, H.J., Wang, X.M., Zhao, W.Z., Su, J., Bjerrum, C.J., Haxen, E.R., Hammarlund, E.U. (2018) A Mesoproterozoic Iron Formation. Proceedings of the National Academy of Sciences 115, E3895–E3904. https://doi.org/10.1073/pnas.1720529115

and Tang et al. (2018)

Tang, D., Shi, X., Jiang, G., Wu, T., Ma, J., Zhou, X. (2018) Stratiform siderites from the Mesoproterozoic Xiamaling Formation in North China: Genesis and environmental implications. Gondwana Research 58, 1–15. https://doi.org/10.1016/j.gr.2018.01.013

. As we did not use the standard Fe speciation protocol, water column redox conditions in the clastic rocks could only be evaluated where the Fe_T/Al and Fe_T/Ti ratios well exceeded the crustal average values (∼0.5 and ∼10, respectively, Rudnick and Gao, 2003

Rudnick, R.L., Gao, S. (2003) Composition of the continental crust. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Elsevier−Pergamon, Oxford, UK, 1–64. https://doi.org/10.1016/B0-08-043751-6/03016-4

). For the clastic rocks of the upper part, high Fe_T/Al ratios (0.5−2.7), high Fe_T/Ti ratios (10−30) and extremely low sulfurised Fe ratios (Fe_py/Fe_T < 0.01) indicate ferruginous deposition conditions. For clastic rocks of the lower part, the Fe_T/Al and Fe_T/Ti ratios hover around the crustal average values, and the sulfurised Fe ratios range from 0.01 to 0.3, so the redox conditions during deposition are still uncertain. However, a euxinic depositional environment can be excluded, as it needs a degree of iron pyritisation (DOP) of at least 0.4 (Raiswell and Canfield, 2012

Raiswell, R., Canfield, D.E (2012) The Iron Biogeochemical Cycle Past and Present. Geochemical Perspectives 1, 1–220. https://doi.org/10.7185/geochempersp.1.1

).

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Sources of the Iron

Positive Eu anomalies (Eu/Eu* ratios 1.0−1.4) for all IF samples (Fig. 2) indicate a hydrothermal impact on the water chemistry during the IF formation, and hydrothermal Fe²⁺ may have been the most important source of Fe²⁺ to the Precambrian ocean (Johnson et al., 2020

Johnson, C., Beard, B., Weyer, S. (2020) Iron Geochemistry: An Isotopic Perspective. Springer Nature Press, Switzerland AG. https://doi.org/10.1007/978-3-030-33828-2

). During the deposition of the XML IF, a hydrothermal Fe²⁺ source may be associated with the breakup of the supercontinent Columbia (Tang et al., 2018

Tang, D., Shi, X., Jiang, G., Wu, T., Ma, J., Zhou, X. (2018) Stratiform siderites from the Mesoproterozoic Xiamaling Formation in North China: Genesis and environmental implications. Gondwana Research 58, 1–15. https://doi.org/10.1016/j.gr.2018.01.013

). However, the positive Eu anomalies of the XML IF are rather muted when compared with the ∼1.3 Ga Jingtieshan IF (Eu/Eu* ratios 4.4–6.5) (Yang et al., 2018

Yang, X., Zhang, Z., Santosh, M., Duan, S., Liang, T. (2018) Anoxic to suboxic Mesoproterozoic ocean: Evidence from iron isotope and geochemistry of siderite in the Banded Iron Formations from North Qilian, NW China. Precambrian Research 307, 115–124. https://doi.org/10.1016/j.precamres.2018.01.007

), so the XML IF may be rather distal from the hydrothermal source.

Another source of Fe²⁺ to seawater is released from clastic particulates depositing in anoxic waters on the shelf (Raiswell and Canfield, 2012

Raiswell, R., Canfield, D.E (2012) The Iron Biogeochemical Cycle Past and Present. Geochemical Perspectives 1, 1–220. https://doi.org/10.7185/geochempersp.1.1

). Indeed, a benthic clastic source of dissolved Fe²⁺ likely supplies most Fe that accumulates as reactive Fe in anoxic depositional environments today, as revealed through sequential sediment extractions (Raiswell and Canfield, 2012

Raiswell, R., Canfield, D.E (2012) The Iron Biogeochemical Cycle Past and Present. Geochemical Perspectives 1, 1–220. https://doi.org/10.7185/geochempersp.1.1

).

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Iron Isotope Fractionation Models

Formation of the XML IF has been suggested to result from three steps (Canfield et al., 2018

Canfield, D.E., Zhang, S.C., Wang, H.J., Wang, X.M., Zhao, W.Z., Su, J., Bjerrum, C.J., Haxen, E.R., Hammarlund, E.U. (2018) A Mesoproterozoic Iron Formation. Proceedings of the National Academy of Sciences 115, E3895–E3904. https://doi.org/10.1073/pnas.1720529115

), including: 1) oxidation of seawater Fe²⁺ to Fe-oxyhydroxides (Fe(OH)₃), likely at the chemocline of the ancient marine basin, 2) reduction of Fe(OH)₃ to diagenetic Fe²⁺ in the water column and sediments, and 3) crystallisation of Fe²⁺ as siderite. Whole rock δ⁵⁶Fe values would be affected by the initial δ⁵⁶Fe of seawater Fe²⁺ and the net isotopic fractionation between products and reactants (Δ⁵⁶Fe_{product-reactant}) in the above steps. These steps are presented in Figure 3 and are reviewed in more detail in Johnson et al. (2020)

Johnson, C., Beard, B., Weyer, S. (2020) Iron Geochemistry: An Isotopic Perspective. Springer Nature Press, Switzerland AG. https://doi.org/10.1007/978-3-030-33828-2

as well as the SI.

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To briefly summarise, the oxidation of Fe²⁺ generates ⁵⁶Fe-enriched Fe(OH)₃ (Fe³⁺) compared to the Fe²⁺ source. With the subsequent reduction of this Fe³⁺, and generation of diagenetic Fe²⁺, both the Fe²⁺ product and the residual Fe³⁺ will be progressively ⁵⁶Fe-enriched. Due to the Δ⁵⁶Fe_{Fe(II)-Fe(III)} of diagenetic Fe²⁺ and residual Fe³⁺, whole rock δ⁵⁶Fe values in clastic rocks with residual Fe³⁺ are generally heavier than the Fe²⁺ in siderite alone. For the higher molar ratio of Fe²⁺ and bicarbonate (HCO₃⁻) as products during step 2 (4∶1) than as reactants in step 3 (1∶1), excessive diagenetic Fe²⁺ diffuse upwards and/or insufficient HCO₃⁻ diffuse downwards during step 3. Because of the different isotopic trends of diagenetic Fe²⁺ and HCO₃⁻, the δ⁵⁶Fe and δ¹³C_carb of siderite depend on the position of its precipitation. The genetic model shown in Figure 3 is illustrative only, not quantitative, but the trends in δ⁵⁶Fe are similar to those reproduced in the open system diffusion/reaction model (Johnson et al., 2020

Johnson, C., Beard, B., Weyer, S. (2020) Iron Geochemistry: An Isotopic Perspective. Springer Nature Press, Switzerland AG. https://doi.org/10.1007/978-3-030-33828-2

).

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Dynamic Iron Cycle in the 1.4 Ga Marine Environment

The δ⁵⁶Fe values of both lower and upper IF samples are significantly reduced compared to those of the clastic rocks in the same intervals (Fig. 2). With an average Fe content in the upper crust of 3.6 % (Rudnick and Gao, 2003

Rudnick, R.L., Gao, S. (2003) Composition of the continental crust. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Elsevier−Pergamon, Oxford, UK, 1–64. https://doi.org/10.1016/B0-08-043751-6/03016-4

), and for siderite of 48 %, rocks in the lower part are consistent with the mixing of a clastic end-member with a δ⁵⁶Fe value of +0.6 ‰ and a diagenetic siderite end member with a δ⁵⁶Fe value of −0.4 ‰ (R² = 0.895, Fig. 4a), which is close to the measured δ⁵⁶Fe values (from −0.53 ‰ to −0.34 ‰) of IF samples. However, for the upper part, the optimal δ⁵⁶Fe value of the diagenetic siderite was −1.1 ‰, with a δ⁵⁶Fe value of the clastic end member also set to +0.6 ‰ (Fig. 4a). Thus, the circumstances of IF formation in the lower and upper parts were clearly different. Details of the calculation method are in the SI.

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Moreover, the δ¹³C_carb values of IF samples are much more ¹³C-depleted than the Mesoproterozoic seawater HCO₃⁻ (∼0 ‰, Saltzman and Thomas, 2012

Saltzman, M.R., Thomas, E. (2012) Carbon Isotope Stratigraphy. In: Gradstein, F.M., Ogg, J.G., Ogg, M.S.G. (Eds.) The Geologic Time Scale. Elsevier, Amsterdam, The Netherlands, 207–232. https://doi.org/10.1016/B978-0-444-59425-9.00011-1

), supporting the idea that the siderites most likely precipitated in diagenetic sediment environments with varied and limited exchange with the overlying waters. The δ¹³C_carb values of siderites from the lower part are from −15 ‰ to −12 ‰, and thus more ¹³C-enriched than most samples from the upper part (Fig. 2). This difference in δ¹³C_carb values would suggest that the siderite from the lower part precipitated closer to the water–sediment interface, with a greater seawater HCO₃⁻ contribution, when compared to the siderites from the upper part (Fig. 3). Following this logic, if we assume that the patterns of water column δ⁵⁶Fe values remained similar during all of the XML IF formation, the siderites from the lower part should be more ⁵⁶Fe-depleted than those from the upper part (Fig. 3). However, the elevated δ⁵⁶Fe values (from −1.1 ‰ to −0.4 ‰) contradict this hypothesis and indicate more ⁵⁶Fe-enriched water in the Yanliao Basin during the IF formation in the lower part.

We propose that the lower part of the XML IF received an enhanced input of hydrothermal fluids with δ⁵⁶Fe values of 0 ‰. The hydrothermal fluids with elevated Fe²⁺ concentrations (German et al., 1990

German, C.R., Klinkhammer, G.P., Edmond, J.M., Mitra, A., Elderfield, H. (1990) Hydrothermal scavenging of rare-earth elements in the ocean. Nature 345, 516–518. https://doi.org/10.1038/345516a0

) generated higher Fe²⁺ concentrations and elevated δ⁵⁶Fe values in the bottom waters of the Yanliao Basin. Higher Fe²⁺ concentrations would have resulted in siderite precipitation with lower HCO₃⁻ concentrations and closer to the sediment–water interface, consistent with heavier δ¹³C_carb values of siderites from the lower part. Higher Fe²⁺ concentrations could have also elevated the rates of Fe-associated primary production, leading to higher values of TOC (Fig. 2).

Generally, the positive Eu anomalies of the XML IF can be taken to indicate hydrothermal contributions. We also note that the positive Eu anomalies are relatively small and of the same magnitude in both the upper and lower IFs. Overall, the positive Eu anomalies in the IFs (Fig. 4b) do support a hydrothermal source of Fe²⁺ to the Yanliao Basin. This conclusion is consistent with the enhanced hydrothermal Fe²⁺ supply that we argue impacted both the δ⁵⁶Fe and δ¹³C_carb values of the lower XML IF.

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Implications on Marine Iron and Carbon Cycles

The XML IF was formed within a ferruginous marine basin with an active iron cycle that was intimately coupled to the C cycle. However, there are differences in the dynamics of the Fe and C cycles in the upper and lower parts of the sedimentary sequence where the IFs are formed. In particular, an enhanced hydrothermal Fe²⁺ contribution elevated the concentrations of Fe²⁺ in the water column and the rates of Fe associated with the C cycle in the lower IF, and should be a key controlling factor in regulating the rates of Fe associated with marine primary production during the siderite-dominated XML IF formation.

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Acknowledgements

We thank Zhihong Li, Heidi Jensen and Iben Rosendahl for the experimental analysis. The research is financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA14010101), National Key Research and Development Program of China (2017YFC0603101), National Natural Science Foundation of China (41872125, 42102146) and Villum Foundation (16518).

Editor: Claudine Stirling

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References

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Supplementary Information

The Supplementary Information includes:

Geological Background

Methods

Table S-1

Figures S-1 to S-4

Supplementary Information References

Download Table S-1 (Excel).

Download the Supplementary Information (PDF).