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The Life and Times of Banded Iron Formations S RESEARCH FOCUS RESEARCH FOCUS: The life and times of banded iron formations Albertus J.B. Smith PPM Research Group and DST-NRF Centre of Excellence for Integrated Mineral and Energy Resource Analysis, Department of Geology, University of Johannesburg, 2006 Auckland Park, South Africa Banded iron formations (BIFs) have been at the center of many debates al. tested this hypothesis on photoferrotrophs to see if precipitating fer- in geology, especially regarding the early (i.e., Archean and Paleoprotero- ric oxyhydroxides can act as sunscreen. By growing bacterial cultures in zoic) Earth and its surface environments. BIFs are chemical sedimentary the presence and absence of UV light and ferrous iron, respectively, they rocks that have an anomalously high iron content (>15 wt% Fe) and typi- conclusively showed the following: UV radiation negatively affects the cally contain layers of chert (Klein, 2005). BIFs have potential value as growth of bacterial cultures in the absence of ferrous iron, and in the pres- proxies for marine chemistry (e.g., Viehmann et al., 2015) and life (e.g., ence of precipitating ferric oxyhydroxide nanoparticles, the UV radiation Konhauser et al., 2002) at the time of their deposition. Detailed reviews of had little effect on bacterial growth. This positive result provides evidence their characteristics and proposed depositional models are readily avail- on how iron-oxidizing bacterial colonies could have survived in the shal- able (e.g., Klein, 2005; Beukes and Gutzmer, 2008; Bekker et al., 2010). low ocean of the early Earth. It should be noted that the authors state that What stands out of these characteristics are their limited occurrence in the “cells…are in close proximity to the produced nanoparticular miner- time (ca. 3.8–1.8 Ga and ca. 750 Ma) and evolving stratigraphic settings als, although the cell surfaces remain mostly free from precipitates.” This through time. The deposition of glacially associated BIFs (ca. 750 Ma) would suggest that further research is required to determine the suspen- has been satisfactorily explained as being caused by the Snowball Earth sion dynamics of the ferric oxyhydroxide nanoparticles and their longer- event (Klein and Beukes, 1993; Klein 2005). However, the deposition of term efficiency as sunscreen in an open marine environment. the older greenstone belt–hosted (ca. 3.8–2.5 Ga) and craton margin (ca. However, one should always proceed with caution when interpreting 3.0–1.8 Ga) BIFs, especially before the Great Oxidation Event (GOE; ca. the precipitation mechanisms in BIFs. For example, some pre-GOE chem- 2.45–2.32 Ga; Bekker et al., 2004; Holland, 2005), is more challenging ical sedimentary sequences show evidence for primary to early diagenetic to explain. hematite deposited below wave base, as well as BIF deposition associated The first challenge in pre-GOE BIF deposition relates to the transport with deepwater, stromatolite-free carbonates, suggesting deposition below of dissolved ferrous iron (Fe2+) in the ocean. In contrast to today’s highly the photic zone (e.g., Beukes and Gutzmer, 2008; Hoashi et al., 2009; oxygenated oceans, dissolved ferrous iron transport would have been pos- Smith at el., 2013). This implies that the ferrous iron never reached the sible prior to the GOE, when Earth’s surface environments were generally photic zone, excluding photoferrotrophs. However, increasing evidence considered to be anoxic (Holland, 2005). In addition, a greater abundance for “whiffs of oxygen” in pre-GOE oceans (e.g., Anbar et al., 2007; Crowe of large hydrothermal plumes added large amounts of ferrous iron to the et al., 2013; Planavsky et al., 2014; Stüeken et al., 2015) suggests micro- early oceans (Bekker et al., 2010). The next challenge related to BIFs is oxic chemolithoautotrophs, that do not require sunlight and therefore did explaining the mechanisms that precipitated the iron. Although this could not need protection from UV radiation, could have played a major role in have been a non-redox process in some instances (e.g., Rasmussen et al., BIF deposition,. 2015), the general consensus is that the iron was originally precipitated as The research done on the role of bacteria in the deposition of BIFs ferric (Fe3+) oxyhydroxides following oxidation of the ferrous iron (e.g., illustrates an important point: one should always exercise great caution Bekker et al., 2010; Smith et al., 2013). Oxidation mechanisms that have when studying any BIF occurrence, as their characteristics and deposi- been proposed include: UV photo-oxidation (Cairns-Smith, 1978); free tional settings vary. This makes a “one-size-fits-all” depositional model oxygen formed by photosynthetic bacteria (Klein and Beukes, 1993); unlikely. However, Gauger et al. should be commended on proving a hy- and iron-oxidizing bacteria (i.e., anoxic photoferrotrophs or micro-oxic pothesis on what was likely a significant BIF depositional mechanism. chemilithoautotrophs; Konhauser et al., 2002). With the UV photo-oxi- Besides their importance with regard to life on early Earth, BIFs are dation of iron being shown to be ineffective (Konhauser et al., 2007), the also of great economic importance, as the majority of super-large iron ore importance of what BIFs can tell us about life on early Earth has been deposits formed from precursor BIFs. These high-grade (>60 wt% Fe) coming to the forefront. ores consist mostly of hematite with significant goethite. Recent ad- The hypothesis of the precipitation of iron in BIFs by iron-oxidizing vances have been made in dating the hematite using (U-Th)/ 21Ne, bacteria has been gaining popularity in the last decade as it can explain the (U-Th)/He, and 4He/3He radiometric techniques. Being able to time ore- oxidation of iron in completely anoxic environments. This hypothesis is forming events is of great value, as it can then be correlated to regional supported by laboratory studies on photoferrotrophs (e.g., Kappler et al., geological events, leading to the identification of larger-scale ore-form- 2005), as well as the fact that the carbonates in BIFs have depleted d13C ing events. In this issue of Geology, Farley and McKeon (2015, p. 1083) values, suggesting the source was organic carbon related to the bacterial used (U-Th)/21Ne hematite dating to delineate two ore-forming events at life (Beukes and Gutzmer, 2008; Bekker et al., 2010). Photoferrotrophs re- ca. 772 and 453 Ma, respectively, in the Gogebic iron range, Michigan, quire sunlight, implying that they lived in the photic zone of early Earth’s USA. What was interesting to note, was that these ages occur at times of oceans. However, the pre-GOE Earth lacked an ozone layer, as indicated tectonic quiescence in the region. Furthermore, the authors used 4He/3He by sulfur mass independent fractionation (SMIF; Farquhar et al., 2000; spectra inverse modeling to determine the cooling history of the ores, Pavlov and Kasting, 2002). The UV radiation would therefore have had which was then used to interpret depth of ore formation, erosional rates, catastrophic effects on any unprotected bacterial cells living in the photic and the unroofing associated with Pleistocene glaciation. From these zone. This problem lies at the heart of the paper by Gauger et al. (2015) conclusions, one can see the following contributions of this study: the in this issue of Geology (p. 1067). It has been proposed that planktonic need to identify fluid-mobilizing events in the region to account for ore life could have used solutes in the water column as sunscreen against formation between ca. 800 and 400 Ma, and a better understanding of UV radiation (see Gauger et al., 2015, and references therein). Gauger et the landscape evolution of the region during the Phanerozoic. GEOLOGY, December 2015; v. 43; no. 12; p. 1111–1112 | doi:10.1130/focus122015.1 GEOLOGY© 2015 Geological | Volume Society 43 | ofNumber America. 12 For | www.gsapubs.orgpermission to copy, contact [email protected]. 1111 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/12/1111/3548226/1111.pdf by guest on 28 September 2021 The methodology, results, and conclusions by Farley and McKeon Crowe, S.A., Døssing, L.N., Beukes, N.J., Bau, M., Kruger, S.J., Frei, R., and open many avenues for investigation into BIF-hosted iron ore regions. Canfield, D.E., 2013, Atmospheric oxygenation three billion years ago: Na- For example, the supergene-enriched iron ores from the Griqualand West ture, v. 501, p. 535–538, doi:10.1038/nature12426. Farley, K.A., and McKeon, R., 2015, Radiometric dating and temperature history region in South Africa are associated with a regional erosional unconfor- of banded iron formation-associated hematite, Gogebic iron range, Michi- mity that transects numerous units within the Transvaal Supergroup (Van gan, USA: Geology, v. 43, p. 1083–1086, doi:10.1130/G37190.1. Schalkwyk and Beukes, 1986). The Transvaal Supergroup is of great geo- Farquhar, J., Bao, H., and Thiemens, M., 2000, Atmospheric influence of Earth’s logical significance, as it marks, among other paleoenvironmental events, earliest sulfur cycle: Science, v. 289, p. 756–758, doi:10.1126/science .289 .5480.756. the GOE (e.g., Bekker et al., 2004). Determining the depositional age of Gauger, T., Konhauser, K., and Kappler, A., 2015, Protection of phototrophic the Transvaal Supergroup has been problematic due to contradicting ages iron(II)-oxidizing bacteria from UV irradiation by biogenic iron(III) miner- determined for volcanic units, dikes, and carbonates (e.g., Cornell et al., als: Implications for early Archean banded iron formation: Geology, v. 43, 1996; Bau et al., 1999; Wiggering and Beukes, 1990; Kampmann et al., p. 1067–1070, doi:10.1130/G37095.1. 2015). Determining the age of the iron ores, and therefore also the age of Hoashi, M., Bevacqua, D.C., Otake, T., Watanabe, Y., Hickman, A.H., Utsu- nomiya, S., and Ohmoto, H., 2009, Primary haematite formation in an oxy- the regional unconformity, would provide further geochronological con- genated sea 3.46 billion years ago: Nature Geoscience, v.
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  • Banded Iron Formations
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