Precambrian Iron Formations Research Articles

Large-scale Mesoproterozoic iron formations in northwestern China

Iron formations (IFs) represent an economically important iron-rich rock type in the Precambrian geologic record. It is generally assumed that after ca. 1.85 Ga, large-scale IFs were not deposited for approximately 1.1 Gy. However, substantial IFs, including over 15 large- to small-scale iron deposits, have been identified recently in Northwest China, and they would constitute the largest Mesoproterozoic IFs globally if their Mesoproterozoic ages can be confirmed. Field observations reveal that these iron deposits are dominantly stratiform and stratoid within the thick clastic-carbonate and volcanic-sedimentary succession. The ores mainly comprise Fe-rich (specularite, hematite, and siderite) and Si-rich (jasper and quartz) minerals with banded or massive structures. Geochemical investigations of typical iron ores show high SiO2 and TFe2O3 and relatively low Al2O3 and TiO2 contents, indicating that they are primary chemical sediments with high hydrothermal and low detrital contributions. These features suggest that these iron deposits in the NQOB are typical IFs, which resemble the Archean and Paleoproterozoic IFs worldwide. New zircon U-Pb dating and lithostratigraphic correlations indicate that these IFs were deposited at ca. 1.1 Ga. The sizable ore reserves and Mesoproterozoic ages confirm that these IFs are the largest reported IFs for the billion years between 1.8 and 0.8 Ga. The large-scale Mesoproterozoic-aged IFs help fill the longest gap in Earth history where the IFs were thought absent from the geologic record and provide an unusual perspective to understand the reappearance mechanism of Precambrian IFs.

Hydrothermal vent fluid-seawater mixing and the origins of Archean iron formation

Precambrian iron formation provides valuable windows onto ancient marine environments, but exactly how these Fe- and Si-rich chemical sedimentary rocks formed have remained a matter of debate. Traditional models envisage Fe(III)-oxide formation through chemical and/or biological oxidation of soluble Fe2+, but sedimentological and textural data show that, throughout much of Archean stratigraphy, the finest-grained Fe(III)-oxides, previously interpreted to reflect primary components, were largely derived through the oxidation of the Fe(II)-silicate mineral greenalite. These observations have formed the basis for an alternative model where Fe2+ and SiO2(aq) combined to precipitate greenalite upon the mixing of hydrothermal fluids with ambient seawater. All models for iron formation genesis invoke hydrothermal alteration of seafloor basalt as the principal source of soluble Fe2+, and its subsequent transport in an anoxic deep ocean. However, the processes and products associated with the venting of near-axis hydrothermal fluids into Precambrian seawater, and implications for Archean iron formation, have not been investigated.In order to test models for Archean iron formation genesis, we used reaction path models to investigate the interactions between ancient hydrothermal vent fluids and anoxic seawater. We assembled available thermodynamic data and executed reaction path models to examine: (1) the equilibration between anoxic SO4-free seawater and basalt/gabbro at subseafloor hydrothermal conditions (400oC and 400-500 bar), (2) the cooling and decompression of these fluids during their ascent through the oceanic crust, and (3) their subsequent mixing with anoxic SO4-free seawater. Our results confirm previous suggestions that the effective lack of SO4 in Archean seawater would have allowed fayalite-magnetite-pyrrhotite-quartz equilibria to buffer fluid chemistry, in turn leading to reaction zone fluids characterised by high Fe/H2S ratios. Cooling and decompression of these reducing fluids upon ascent, and further reaction with mafic wall rock, leads to significant Fe-chlorite and quartz precipitation with little to no sulfide precipitation. As these vent fluids mix with cold seawater directly, or with conductively-heated seawater, mineral assemblages are dominated by pyrite, greenalite, and/or siderite. These results were obtained with no constraints imposed by Archean iron formation, aside from the exclusion of minnesotaite, a demonstrably secondary mineral.The quantity of hydrothermally-derived Fe2+ partitioned into Fe(II)-bearing minerals upon mixing (96-99% of total aqueous Fe), the relative proportion of greenalite precipitated, and its further concentration via plume-driven transport (by virtue of its low density), together imply that the mixing between hydrothermal vent fluids and anoxic seawater may have served as a principal mechanism for generating a large proportion of the mineral mass now preserved as Archean iron formation. These results reconcile long-standing sedimentological and geochemical observations indicating that Archean banded iron formations record a strong hydrothermal influence, and suggest that these unique deposits have the potential to constrain the nature of seafloor-hosted hydrothermal systems on the ancient Earth, and their chemical interactions with seawater.

Study on Iron Source and Original Mineral Assemblage of Iron Formation in Paleoproterozoic Wuzhiling Formation in North China

The Precambrian iron formation is related to the evolution of the earth's environment and is an important record of the evolution of ancient ocean, ancient environment and atmospheric conditions. The Wuzhiling Formation iron formation at the Songshan boundary is one of the newly identified granular iron formations (GIF). Through the study of mineral characteristics and mineral morphology of iron formation in Wuzhiling Formation, it is found that its material source may be hydrothermal fluid; the original sediments may be hematite particles and 'green rust'.

Chemical extraction of iron from carbonate in banded iron formations for isotope analysis

The iron isotope composition of iron-bearing carbonates is commonly used to obtain insights into ancient environmental conditions. However, it is often challenging to target only Fe‑carbonates (e.g. siderite and ankerite) from samples containing a variety of other Fe-bearing minerals, such as observed in Precambrian iron formations. Chemical extraction (i.e. leaching) methods of Fe‑carbonates could be an alternative to in-situ measurements and/or micro-drilling techniques applied to isotopic studies. Yet, only a few studies have looked at the effects of leaching carbonates (e.g. partial and/or total dissolution) on their Fe isotope composition. Here, we tested several leaching protocols, using 5 to 20% acetic acid (HAc) and 0.4 M HCl, on a siderite standard and three natural samples, including an iron formation, Fe-rich and Fe-poor carbonates. We showed that carbonate mineralogy has a strong control on how much of each mineral phase was being dissolved, and that variations in HAc concentration from 5% to 20% are less likely to change how much siderite dissolves (e.g. ∼30% dissolution) under a 12 h period at room temperature. Importantly, the Fe isotope composition of partially dissolved siderite had indistinguishable values within error from the whole-rock composition (i.e. complete dissolution) as shown with HAc and HCl attacks. Carbonates from the three natural samples were almost completely dissolved under the same protocol with 5 to 20% HAc, while 0.4 M HCl attacks dissolved additional mineralogical phases, which might contribute to the Fe leachate. Moreover, the iron isotope composition of carbonate leachates was preserved without generating anomalous results. Hence, weak chemical leaches represent a reliable tool to study Fe isotopic composition of carbonate to understand how the Fe cycle was operating throughout Earth's history.

Carbon isotope stratigraphy of Precambrian iron formations and possible significance for the early biological pump

The origin of Precambrian iron-formations (IF) remains contentious, particularly with respect to the mineralogy of primary precipitates and the exact processes and conditions leading to their formation. Despite the uncertainties, prevailing hypotheses range from biological precipitation of ferrihydrite to abiotic water-column formation of greenalite. By contrast, iron carbonate minerals (siderite, ankerite) in IF have traditionally been attributed to diagenetic origins based on textural and isotopic relationships. Recent studies on IF from the Neoarchaean-Paleoproterozoic Transvaal Supergroup of South Africa have revealed evidence for apparently primary, low-δ13C, Fe/Mn-bearing Mg calcite as precursor to iron carbonate formation and as a potentially underestimated pathway of isotopically light carbon burial during IF deposition. Here, we present whole-rock δ13C data and carbonate-specific geochemical analyses for samples from five drill cores that capture the entire stratigraphic extent of the Kuruman and Griquatown IF of the Transvaal Supergroup. Our results demonstrate remarkable consistency in stratigraphic profiles among the locations for the trends and magnitudes of bulk δ13C values that are independent of paragenetic association, modal mineralogy, and chemical composition of the bulk carbonate fraction of each sample. We interpret these records as resulting from water-column abiotic carbonate formation that was accompanied by kinetic isotopic effects associated with fluctuating conditions (pH, alkalinity) controlling carbonate supersaturation in ambient seawater. Although our interpretation provides strong support for abiotic, anoxic models for IF genesis prior to the Great Oxidation Event (GOE), it does not entirely preclude additional biological mechanisms of primary ferric oxyhydroxide formation and its possible role in an early biological pump.

Anoxygenic photosynthesis linked to Neoarchean iron formations in Carajás (Brazil).

Microbial activity is often invoked as a direct or indirect contributor to the precipitation of ancient chemical sedimentary rocks such as Precambrian iron formations (IFs). Determining a specific metabolic pathway from the geological record remains a challenge, however, due to a lack of constraints on the initial conditions and microbially induced redox reactions involved in the formation of iron oxides. Thus, there is ongoing debate concerning the role of photoferrotrophy, that is the process by which inorganic carbon is fixed into organic matter using light as an energy source and Fe(II) as an electron donor, in the deposition of IFs. Here, we examine ~2.74-Ga-old Neoarchean IFs and associated carbonates from the Carajás Mineral Province, Brazil, to reconstruct redox conditions and to infer the oxidizing mechanism that allowed one of the world's largest iron deposits to form. The absence of cerium (Ce) anomalies reveals that conditions were pervasively anoxic during IF deposition, while unprecedented europium (Eu) anomalies imply that Fe was supplied by intense hydrothermal activity. A positive and homogeneous Fe isotopic signal in space and time in these IFs indicates a low degree of partial oxidation of Fe(II), which, combined with the presence of 13 C-depleted organic matter, points to a photoautotrophic metabolic driver. Collectively, our results argue in favor of reducing conditions during IF deposition and suggest anoxygenic photosynthesis as the most plausible mechanism responsible for Fe oxidation in the Carajás Basin.

The Yunmengshan iron formation at the end of the Paleoproterozoic era

Precambrian iron formations (IF) carry rich information on the interplay between the geosphere and the early biospheres. Here, we report mineralogical and petrologic characterizations of the Yunmengshan IF deposited at ~1.7 Ga, the transition between Paleoproterozoic and Mesoproterozoic eras. The IF contains granular hematite with evident oncoidal structures and lacks laminated structures, which are distinct to the typical banded iron formation (BIF) or granular iron formation (GIF) deposited early in the marine environments with high silica content. Based on observations of clastic sedimentary structures around the Yunmengshan IF, including parallel bedding, cross-bedding, asymmetric ripple mark, and mud crack, we deduce that the Yunmengshan IF was mainly deposited in a supratidal to intertidal zone. Electron microscopic observations show two types of hematite euhedral crystals: laminar hematite of 200–500 nm thick and 3-5 μm in diameter and granular hematite of 200–300 nm in diameter randomly distributed in the matrix. Stilpnomelane and minnesotaite, as Fe2+-silicates in the typical Archean- and the early Paleoproterozoic BIF, are identified but with extremely low abundance. The morphology and paragenetic association of these minerals imply that the Yunmengshan IF was probably deposited in an aquatic environment with low Si concentration. These results indicate that the Yunmengshan IF represents a transitional type of iron deposition between BIF/GIF and ironstone in the geological history of the iron cycle on the surface of Earth.

Insights into the processes and controls on the absolute abundance and distribution of manganese in Precambrian iron formations

The role of manganese as redox tracer in the context of iron formations (IF) has received renewed attention in recent years. The utility of several redox-sensitive trace metal proxies in IF (e.g., Mo) is based on the premise that simultaneously elevated manganese concentrations in the form of primary Mn(IV) oxides, would have controlled the redox behaviour and stable isotope variations of such elements during primary deposition and diagenesis. Recent research on the mineral fraction-specific geochemistry of Paleoproterozoic IF from South Africa (Griquatown and Kuruman Iron Formations) reports no preserved Mn(IV) oxides and a strong affinity of manganese (>90%) to partition into carbonate minerals as Mn(II). In this study, we exploit further this relationship by examining in detail the mineral-specific abundance and distribution of manganese across the same IF succession, with main emphasis on the Mn-enriched upper 120 m of the Griquatown IF. Dominant carbonate minerals include ankerite, siderite and occasional calcite, with MnO contents across all three minerals ranging from less than 1 to well over 10 wt%. Co-existing ankerite and siderite on the scale of individual thin sections record very similar MnO (and MgO) contents at statistically very low variance. Calcite is only observed as relic phase showing apparent textural evidence for replacement by ankerite and siderite. The calcite is Mg-rich (up to 15 wt% MgO) and contains the highest MnO contents out of the entire dataset obtained. Variability in the MnO content of calcite is also high and anti-correlates strongly with CaO content. Ankerite and siderite co-existing with calcite have similar MnO and MgO concentrations, consistent with conservative transfer during replacement reactions and only net addition of Fe(II) from pore fluids. For one of the calcite-containing samples, all three carbonate minerals were also analysed sequentially for carbon isotopes and yielded very similar δ13C values (−6.7 to −6.0‰). We interpret the above features as indicative of primary calcite being the dominant mode of manganese acquisition in the initial sediment. Diagenetic replacement reactions then homogeneously reworked Mn (along with Mg and Ca) in a largely conservative fashion into diagenetic ankerite and siderite. Our interpretation questions IF models that assume high primary Mn(IV) oxide deposition and its bacterially-mediated diagenetic reconstitution in IF during transient oxygenation events (oxygen “whiffs”); instead, it lends support to emerging ideas for primary water-column processes leading to the formation and precipitation of reduced Fe(+Mn) mineral species as precursors to IF.

Fate of organic compounds during transformation of ferrihydrite in iron formations

The absence of organic compounds from Precambrian iron formations (IF) challenges the hypothesis of the biogenic origin of IF. Here we address the fate of adsorbed organic compounds during transformation from ferrihydrite to hematite. We determined the binding energy between hematite and common molecular terminations found in extracellular polymeric substances and biofilms: carboxylic, hydroxyl and phosphate functional groups. We found that the bond between hematite and hydroxyl groups is approximately 2 times stronger than the bond between hematite-carboxyl and -phosphate groups. We transformed synthetic ferrihydrite to hematite at 90 oC in presence of glycerol, which has a high density of hydroxyl groups and measured the amount of mineral associated glycerol before and after the transformation. We show that the transformation releases glycerol highlighting that organic compounds adsorbed at precursor ferrihydrite could be desorbed during the process of IF sedimentation and diagenesis. Our results suggest that the absence of organic compounds in IF should not be used as evidence against their biogenic origin.

A critical evaluation of copper isotopes in Precambrian Iron Formations as a paleoceanographic proxy

Trace metals in Iron Formations (IF) have been widely used as proxies for oceanic redox processes and oxygen evolution leading to the Great Oxidation Event (GOE). Copper has hitherto received comparatively little attention, with a single study reporting variation in δ65Cu between pre- and post-GOE black shales. This is attributed to postulated isotope fractionation effects in the pre-GOE ocean during widespread iron oxide deposition in IF. Here we focus on the application of Cu isotopes in two classic IF-containing sequences of the Neoarchean-Paleoproterozoic, namely the Hamersley (Australia) and Transvaal (South Africa) Supergroups. We specifically targeted the oxide-rich Joffre and Kuruman IF, the carbonate-rich Griquatown IF, and the Mn-rich Hotazel IF, which collectively record over 100 Ma of sustained IF deposition in the pre-GOE ocean. The aim was to assess the utility of Cu isotopes in IF as a paleoceanographic proxy in view of existing oxygen evolution models. Iron formation Cu concentrations are low compared to average crust and modern oceanic Fe-Mn oxide deposits, with average values between 1 and 5 ppm for all four IF data sets. Copper concentrations show no systematic variability with mineralogy, no statistical correlation with bulk Fe and Mn contents, good correlations with Ti (rCu-Ti = 0.73) for the Joffre data set and with Cr, Ni, and V for the South African data sets, and shale-like Cu/Ti ratios. Isotopic results show statistically invariant average δ65Cu values very close to 0‰ for all but the Joffre IF which has a marginally negative average δ65Cu value (−0.24 ± 0.22‰). Combined with other trace transition metal systematics, the Cu isotope data point to a Cu source that was controlled largely by inputs of fine volcanic-derived particles (ash), thus placing limitations on its utility as a paleoceanographic redox proxy during IF genesis.

Shallow-marine ironstones formed by microaerophilic iron-oxidizing bacteria in terminal Paleoproterozoic

The microbial origin of Precambrian iron formations is debated due to the lack of direct fossil evidence. In order to reveal the genesis of ironstones under low-oxygen levels, integrative studies of sedimentology, petrography, mineralogy, and geochemistry were conducted on the intertidal to shallow subtidal ooidal and stromatolitic ironstones from the terminal Paleoproterozoic Chuanlinggou Formation (ca. 1.65–1.64 Ga) of North China, using microscopy, SEM, EDS, ICP-OES, ICP-MS and MC-ICP-MS techniques. Mineralogical study shows that the Fe-rich mineral is predominantly hematite that resulted from dehydration of amorphous Fe-oxyhydroxide during diagenesis. Petrographic observation indicates that the iron was oxidized and precipitated from seawater rather than sourced from terrestrial detritus. Basinward increases of the ironstone abundance, Eu anomalies (from 1.39 to 1.56) and δ56Fe values of the ironstones (from +0.5‰ to +1.0‰) suggest that the iron was mainly sourced from seafloor hydrothermal fluids, and partially oxidized and precipitated in shallow subtidal to intertidal environments. The common existence of Fe-oxide coated sheaths, spiral stalks, residual extracellular polymer substances (EPS) and other biogenic fabrics indicates that microaerophilic iron-oxidizing bacteria (FeOB) may have played an important role in precipitating the Chuanlinggou ironstones. The extremely low oxygen concentrations implied by the proliferation of microaerophilic FeOB in the shallow waters, the weak positive Ce anomalies (0.94–1.12) and low Mn concentrations in the ironstones are broadly consistent with the previous result of a Cr isotope study. Thus the establishment of a microaerophilic FeOB genetic model for the widespread Chuanlinggou ironstones in North China provides new insight into the origin of Precambrian iron formations and the redox evolution of ocean-atmosphere systems during the “Boring Billion”.

Widespread and Persistent Deposition of Iron Formations for Two Billion Years

AbstractComposed of chemical precipitates rich in iron and silica, Precambrian iron formations from marine sedimentary records may reveal biogeochemical processes over the first half of Earth history. The limited record of early Archean rock suggests that preservation biases the iron formation record. Like ophiolites, which provide a sparse record of past ocean floor, iron formations deposited on oceanic crust ought to also be rare and preserved only when accreted onto cratons. To correct for potential preservation bias, we scaled masses of iron formations to the areal extent of basement rock of similar age and found that the resultant record is consistent with persistent deposition of iron formations across much of the deep ocean for two billion years. Widespread and long‐term iron formations imply that ferrous iron was available in the deep ocean for billions of years and that the requisite (bio)geochemical mechanisms to produce iron formations were present by 3.8 Ga.

Earth\u2019s youngest banded iron formation implies ferruginous conditions in the Early Cambrian ocean

It has been proposed that anoxic and iron-rich (ferruginous) marine conditions were common through most of Earth history. This view represents a major shift in our understanding of the evolution of marine chemistry. However, thus far, evidence for ferruginous conditions comes predominantly from Fe-speciation data. Given debate over these records, new evidence for Fe-rich marine conditions is a requisite if we are to shift our view regarding evolution of the marine redox landscape. Here we present strong evidence for ferruginous conditions by describing a suite of Fe-rich chemical sedimentary rocks—banded iron formation (BIF)—-deposited during the Early Cambrian in western China. Specifically, we provide new U-Pb geochronological data that confirm a depositional age of ca. 527 Ma for this unit, as well as rare earth element (REE) data are consistent with anoxic deposition. Similar to many Algoma-type Precambrian iron formations, these Early Cambrian sediments precipitated in a back-arc rift basin setting, where hydrothermally sourced iron drove the deposition of a BIF-like protolith, the youngest ever reported of regional extent without direct links to volcanogenic massive sulphide (VMS) deposits. Their presence indicates that marine environments were still characterized by chemical- and redox-stratification, thus supporting the view that—despite a dearth of modern marine analogues—ferruginous conditions continued to locally be a feature of early Phanerozoic seawater.

Oxic Fe(III) reduction could have generated Fe(II) in the photic zone of Precambrian seawater

Many marine Precambrian iron formations (IF) record deep anoxic seawater enriched in Fe(II) (i.e. ferruginous) overlain by mildly oxygenated surface water. This is reflected by iron-rich sediments forming in deep basins, and relatively iron-poor sediments forming in shallow, sunlit waters. Such an iron gradient is often interpreted as a redox interface where dissolved Fe(II) was oxidized and precipitated as Fe(III)-bearing minerals. As such, sedimentary iron enrichments are proxy to the progressive oxidation of the oceans through geological time. However, this interpretation is founded on the assumption that Fe(II) could not persist within an oxygenated water column. Here, we cultivated cyanobacteria in an illuminated column supplied with Fe(II)-rich seawater medium in a laboratory-scale analog of a continental margin supporting IF deposition. We first observed Fe(II) oxidation with oxygen, then biologically-mediated reduction of Fe(III) (oxyhydr)oxides, which maintained a pool of Fe(II) in the presence of oxygen. Such steady-state iron redox cycling may have maintained dissolved, and hence mobile Fe(II) in oxygenated seawater above ferruginous deep basins such as those inferred for many Precambrian IF.

Hydrothermal oxidation in the Biwabik Iron Formation, MN, USA

Precambrian iron formations throughout the world, notably in Australia, Brazil, and South Africa, show evidence of hypogene (≥ 110 °C, mostly > 250 °C) oxidation, alteration, and silica dissolution as a result of tectonic or magmatic activity. Although hydrothermal oxidation has been proposed for the prototype Lake Superior-type iron formation, the Biwabik Iron Formation in Minnesota (USA), it has not been documented there. By examining oxidized and unoxidized Biwabik Iron Formation in three mines, including material from high-angle faults that are associated with oxidation, we document an early hypogene oxidation event (~ 175 °C) involving medium-salinity aqueous fluids (8.4 ± 4.9 wt% NaCl equiv) that infiltrated iron formation along high-angle faults. At the Hibbing Taconite Mine, hydrothermal fluids oxidized iron carbonates and silicates near faults, producing goethite ± quartz. In contrast with much of the oxidized iron ores on the Mesabi Range, silica was not removed but rather recrystallized during this event, perhaps lying in a rock-dominated system at low cumulative fluid flux. During the hydrothermal oxidation event in the Hibbing Taconite deposit, quartz-filled microfractures and irregular inclusions commonly formed in coarse variably oxidized magnetite, currently the ore mineral: these inclusions degrade the ore by introducing excess silica in magnetic concentrate. Hydrothermal oxidation at Hibbing Taconite Mine is overprinted by later, relatively minor supergene oxidation both along faults and near the surface, which locally dissolved quartz. At the Fayal Reserve Mine, widespread silicate and carbonate gangue dissolution and iron oxidation was followed by precipitation of pyrite, Mn-siderite, apatite, and other minerals in void spaces, which prevented post-oxidation compaction and significant volume loss in the sampled rocks. Although definitive temperature data for this assemblage are needed, the weight of evidence indicates that this mineralization is hypogene. The association of oxidation with epithermal conditions constrains the oxidation and subsequent mineralization to have taken place during the Precambrian, the only time when these rocks would have experienced the necessary temperatures. The mineralization at Fayal Reserve shows little supergene overprint: pyrite is largely unoxidized. Hydrothermal oxidation in both mines was likely produced by basinal fluids that were expelled during the 1.83–1.87 Ga Penokean Orogeny, and mixing with meteoric fluids along faults, although a 1.1 Ga rift-related fluid flow event is also possible. Later supergene overprinting of the iron formation was minor.

Microbial diversity and iron oxidation at Okuoku-hachikurou Onsen, a Japanese hot spring analog of Precambrian iron formations.

Banded iron formations (BIFs) are rock deposits common in the Archean and Paleoproterozoic (and regionally Neoproterozoic) sedimentary successions. Multiple hypotheses for their deposition exist, principally invoking the precipitation of iron via the metabolic activities of oxygenic, photoferrotrophic, and/or aerobic iron-oxidizing bacteria. Some isolated environments support chemistry and mineralogy analogous to processes involved in BIF deposition, and their study can aid in untangling the factors that lead to iron precipitation. One such process analog system occurs at Okuoku-hachikurou (OHK) Onsen in Akita Prefecture, Japan. OHK is an iron- and CO2 -rich, circumneutral hot spring that produces a range of precipitated mineral textures containing fine laminae of aragonite and iron oxides that resemble BIF fabrics. Here, we have performed 16S rRNA gene amplicon sequencing of microbial communities across the range of microenvironments in OHK to describe the microbial diversity present and to gain insight into the cycling of iron, oxygen, and carbon in this ecosystem. These analyses suggest that productivity at OHK is based on aerobic iron-oxidizing Gallionellaceae. In contrast to other BIF analog sites, Cyanobacteria, anoxygenic phototrophs, and iron-reducing micro-organisms are present at only low abundances. These observations support a hypothesis where low growth yields and the high stoichiometry of iron oxidized per carbon fixed by aerobic iron-oxidizing chemoautotrophs like Gallionellaceae result in accumulation of iron oxide phases without stoichiometric buildup of organic matter. This system supports little dissimilatory iron reduction, further setting OHK apart from other process analog sites where iron oxidation is primarily driven by phototrophic organisms. This positions OHK as a study area where the controls on primary productivity in iron-rich environments can be further elucidated. When compared with geological data, the metabolisms and mineralogy at OHK are most similar to specific BIF occurrences deposited after the Great Oxygenation Event, and generally discordant with those that accumulated before it.

Fe isotope fractionation during Fe(II) oxidation by the marine photoferrotroph Rhodovulum iodosum in the presence of Si – Implications for Precambrian iron formation deposition

The iron (Fe) isotopic composition of Precambrian iron formations (IFs), besides providing geological context through its mineralogical properties, was suggested to function as a biosignature that can be used to infer a potential microbial role in the formation of the deposited Fe minerals. Anoxygenic phototrophic Fe(II)-oxidizing bacteria (photoferrotrophs), capable of oxidizing Fe(II) anoxically using light energy, were potentially involved in Fe(II) oxidation in anoxic or suboxic Precambrian oceans. The effect of Si on Fe isotopic fractionation between aqueous Fe(II) and Fe–Si-co-precipitates has been investigated before. However, it is currently unknown how stable Fe isotopes are fractionated during enzymatic Fe(II) oxidation under marine hydrogeochemical conditions, and particularly how the presence of Si affects the Fe isotope composition and the isotopic exchange among different Fe phases. We therefore studied Fe isotope fractionation during Fe(II) oxidation by the marine photoferrotroph Rhodovulum iodosum in simulated Precambrian seawater amended with 1mM dissolved Si. Our results show that the change in the Fe isotope compositions over time for both the initial aqueous Fe(II) (Feaq) and the Fe(III) precipitates (Feppt) follow a Rayleigh distillation model. Moreover, the fractionation (ε56Feppt-aq) determined independently from either δ56Feaq or δ56Feppt data resulted in a value of 2.3±0.3 (2SD, N=6). This value differs from the fractionation factor determined previously for Fe(II) oxidation by R. iodosum in the absence of Si, where the fractionation calculated from δ56Feaq (i.e. 0.96–1.18) was different from that calculated from δ56Feppt (1.96–1.98). This difference was attributed to isotopic exchange processes with soluble and sorbed Fe species. The present study suggests that Si present in Precambrian oceans retards Fe isotopic exchange, likely through combined effects of complexation of dissolved Fe species by Si and sorption of Si to Fe(III) minerals, thus lowering sorption of Fe(II) to the Fe(III) minerals, which is necessary for isotopic exchange. In summary our data suggests that Si in ancient oceans played a key role for the Fe isotope composition of Fe(III) minerals that were deposited by photoferrotrophic iron oxidation in Precambrian oceans by minimizing subsequent isotope exchange and recrystallization processes with aqueous Fe(II).

A biological switch at the ocean surface as a cause of laminations in a Precambrian iron formation

Banded iron formations (BIFs) exhibit alternating silica- and iron-rich laminae, potentially reflecting the dynamics of the paleo-environments in which they were formed, although the exact mechanism remains unclear. Here the formation of a 2.7–2.9 Ga BIF from Dharwar Craton, India, is deciphered by analyzing the inter-band variations of the redox-sensitive isotope biomarkers, 15N/14N and 56Fe/54Fe. Organic matter with δ15N values as high as +12.0±0.8‰ appears to be trapped in silica. Iron oxides exhibit systematically positive δ56Fe values, ranging between +0.80±0.05‰ and +1.67±0.02‰. Compared to the iron-rich bands, silica-rich bands, which show higher δ56Fe values, exhibit an order of magnitude higher concentrations of 15N-rich organic nitrogen, normalized by the abundances of its host silica. The presence of 15N-rich organic matter may imply the emergence of a modern-like biological nitrogen cycle that requires the formation of oxidized nitrogen compounds. The higher concentration of 15N-rich organic nitrogen for the silica-rich bands possibly suggests that the photosynthetic activity was higher during the formation periods of these bands. The heavier iron isotope compositions of the silica-rich bands cannot be explained alone by iron oxidation through probable pathways. The relative 56Fe-enrichment in silica-rich bands is explained here by the progressive dissolution of iron oxides to the ocean, through iron reduction by 15N-rich organic matter actively produced at the ocean surface. The formation of iron-rich bands possibly corresponds to periods of reduced biological productivity, when precipitated iron was not effectively dissolved to the ocean. The observed shift in the organic concentration between Fe- and Si-rich bands could be the switch that triggered the BIF laminations. This shift could conceivably represent periodic fluctuations in the oxygen generation, which possibly occurred over periods of millennia, at the dawn of the Great Oxidation Event.

From Precambrian Iron-Formation to Terraforming Mars: The JIMES Expedition to Santorini

ABSTRACTThe iron embayments at Santorini, Greece, have long been considered by geologists to be the most useful modern environment for understanding variables related to precipitation of Precambrian iron-formation. To help understand the rock record, the embayments were studied almost monthly for a year to assess seasonal variations in iron bacteria and diatoms along with mineralogy, weather, water chemistry, and ecology. Unidentified red rods dominated and accounted for most ferrihydrite production. Diatom abundance was seasonal, including Parlibellus delognei which produces molecular oxygen within iron-coated sheaths. The gross structures of the microbial iron precipitates were in the form of rods, spheres, and braids. Speculations resulting from our observations suggest that life's origin could have been intimately related to chemical/physical processes occurring where volcanic sources discharged iron through highly porous siliceous substrates and into the primitive ocean. The diverse community also provides a potentially useful ecosystem for Mars terraforming experiments.

Precambrian iron formations from the Cauvery Suture Zone, Southern India: Implications for sub-marine hydrothermal origin in Neoarchean and Neoproterozoic convergent margin settings

Thick horizons of iron formations including Banded Iron Formations (BIFs) and Banded Silicate Formations (BSFs) occur as E–W trending bands in the eastern part of Cauvery Suture Zone (CSZ) in the Sothern Granulite Terrane of India. Some of these occur in close association with the Neoarchean-Neoproterozoic suprasubduction zone complexes, where as some others are associated with metamorphosed accretionary sequences including pyroxene granulites and other high grade rocks. The iron formations are highly deformed and metamorphosed under amphibolite to granulite facies conditions and are composed of quartz–magnetite–hematite–goethite–garnet–pyrite together with grunerite and pyroxene. Here we report the geochemical characteristics of twenty representative samples from the iron formations that reveal a widely varying composition with Fe2O3(t) (22–65 wt.% as total iron) total- Fe2O3/TiO2 (205–6532), MnO/TiO2 (0.25–12.66) and SiO2 (33–85 wt.%), broadly representing the two types of iron formations. These formations also show very low Al/(Al+Fe+Mn) ratio (0.001–0.01), Al2O3 (0.07–0.76wt.%), Al2O3/TiO2 ratio (2.7–21), MgO (0.01–4.41wt.%), CaO (0.1–1.24wt.%), Na2O (0.01–0.05wt.%) and K2O (0.01wt.%) together with low total REE (3.38–31.63ppm). The trace and REE elemental distributions show wide variation with high Ni (274ppm), and Zn contents (up to 87ppm) when compared to mafic volcanics of the adjoining areas. Tectonic discrimination plots indicate that the iron formations of the Cauvery Suture Zone are of hydrothermal origin. Their chondrite normalized patterns show slight positive Eu anomaly (Eu/Eu*=up to 1.77) and relatively less fractionation of REE with slight LREE enrichment compared to HREE. However, the PAAS (Post Archean Average of Australian Sediments) normalized REE patterns display significant positive Eu anomaly (Eu/Eu* up to 2.32) with well represented negative Ce anomalies (Ce/Ce*=0.66–1.28). The above results together with petrological characteristics and available geochronology of the associated lithologies suggest that the iron formations can be correlated to Algoma-type. The Fe and Si were largely supplied by medium to high temperature sub-marine hydrothermal systems in Neoarchean and Neoproterozoic convergent margin settings.