Ferruginous Conditions Research Articles

A continental-weathering control on orbitally driven redox-nutrient cycling during Cretaceous Oceanic Anoxic Event 2

The Cretaceous period (~145–65 m.y. ago) was characterized by intervals of enhanced organic carbon burial associated with increased primary production under greenhouse conditions. The global consequences of these perturbations, oceanic anoxic events (OAEs), lasted up to 1 m.y., but short-term nutrient and climatic controls on widespread anoxia are poorly understood. Here, we present a high-resolution reconstruction of oceanic redox and nutrient cycling as recorded in subtropical shelf sediments from Tarfaya, Morocco, spanning the initiation of OAE2. Iron-sulfur systematics and biomarker evidence demonstrate previously undescribed redox cyclicity on orbital time scales, from sulfidic to anoxic ferruginous (Fe-rich) water-column conditions. Bulk geochemical data and sulfur isotope modeling suggest that ferruginous conditions were not a consequence of nutrient or sulfate limitation, despite overall low sulfate concentrations in the proto–North Atlantic. Instead, fluctuations in the weathering influxes of sulfur and reactive iron, linked to a dynamic hydrological cycle, likely drove the redox cyclicity. Despite the potential for elevated phosphorus burial in association with Fe oxides under ferruginous conditions on the Tarfaya shelf, porewater sulfide generation drove extensive phosphorus recycling back to the water column, thus maintaining widespread open-ocean anoxia.

Geological characteristics and age of the Dahongliutan Fe-ore deposit in the Western Kunlun orogenic belt, Xinjiang, northwestern China

The large-scale Dahongliutan hematite-rich Fe deposit in the Tianshuihai terrane (Western Kunlun orogenic belt), Xinjiang, northwestern China, contains proven Fe ore reserves exceeding 100Mt at an average grade of 35wt.% Fe(t) (maximum 53wt.%). The deposit is hosted in the Neoproterozoic Tianshuihai Group, a suite of neritic, siliciclastic and sedimentary carbonate rocks subjected to greenschist-facies metamorphism. The Fe orebodies are laminate, stratoid, lenticular and interlayered with marbles and schists. Ore minerals mainly contain hematite, with minor siderite and limonite and rare sulfides, whereas gangue minerals mainly comprise quartz, dolomite, ankerite, calcite, muscovite and chloritoid. Most Fe ores display lepidoblastic, blastopsammitic, crystalloblastic and metasomatic textures with massive or banded structures. Based on mineral assemblages, the Fe ores are divided into four types, namely: (1) Quartz–hematite; (2) Quartz–dolomite–calcite–muscovite–hematite; (3) Quartz–ankerite–hematite; (4) Quartz–siderite–hematite.Zircon U–Pb analyses on the marbles and schists yielded a Neoproterozoic age (Ediacaran; 593±7Ma), whereas those on the ore-intruding medium-grained porphyritic biotite-quartz monzonite, medium to fine grained biotite-quartz monzonite and metagabbro yielded Cambrian–Early Ordovician ages (i.e., 484.5±2.7, 527.9±2.8 and 532.3±3.1Ma), which altogether constrain the ore formation to the Late Neoproterozoic to Early Cambrian (532–593Ma). In addition, the 163 detrital zircon analyses on the Dahongliutan marble and schist define five major age populations, namely: 2561–2329, 2076–1644, 1164–899, 869–722 and 696–593Ma. Provenance studies show that the detritals at Dahongliutan were mainly sourced from the North Kunlun and Tianshuihai terranes.Comparison with typical BIF deposits in the world, we infer that the deposit belongs to Neoproterozoic (Ediacaran) BIF-related sedimentary metamorphic type iron deposit and resembles Superior-type BIF, probably reflecting the recurrence of anoxic ferruginous conditions in the Neoproterozoic–Early Cambrian deep sea.

A global transition to ferruginous conditions in the early Neoproterozoic oceans

Eukaryotic life expanded during the Proterozoic eon1, 2.5 to 0.542 billion years ago, against a background of fluctuating ocean chemistry2, 3, 4. After about 1.8 billion years ago, the global ocean is thought to have been characterized by oxygenated surface waters, with anoxic and sulphidic waters in middle depths along productive continental margins and anoxic and iron-containing (ferruginous) deeper waters5, 6, 7. The spatial extent of sulphidic waters probably varied through time5, 6, but this surface-to-deep redox structure is suggested to have persisted until the first Neoproterozoic glaciation about 717 million years ago8, 9, 10, 11. Here we report an analysis of ocean redox conditions throughout the Proterozoic using new and existing iron speciation and sulphur isotope data from multiple cores and outcrops. We find a global transition from sulphidic to ferruginous mid-depth waters in the earliest Neoproterozoic, coincident with the amalgamation of the supercontinent Rodinia at low latitudes. We suggest that ferruginous conditions were initiated by an increase in the oceanic influx of highly reactive iron relative to sulphate, driven by a change in weathering regime and the uptake of sulphate by extensive continental evaporites on Rodinia. We propose that this transition essentially detoxified ocean margin settings, allowing for expanded opportunities for eukaryote diversification following a prolonged evolutionary stasis before one billion years ago.

Reconstruction of early Cambrian ocean chemistry from Mo isotopes

The Neoproterozoic–Cambrian transition was a key time interval in the history of the Earth, especially for variations in oceanic and atmospheric chemical composition. However, two conflicting views exist concerning the nature of ocean chemistry across the Precambrian–Cambrian boundary. Abundant geochemical evidence suggests that oceanic basins were fully oxygenated by the late Ediacaran, while other studies provide seemingly conflicting evidence for anoxic deep waters, with ferruginous conditions [Fe(II)-enriched] persisting into the Cambrian. Here, two early Cambrian sedimentary platform and shelf-slope sections in South China were investigated to trace early Cambrian ocean chemistry from Mo isotopes. The results reveal that early Cambrian sediments deposited under oxic to anoxic/euxinic conditions have δ98/95Mo values ranging from −0.28‰ to 2.29‰, which suggests that early Cambrian seawater may have had δ98/95Mo values of at least 2.29‰, similar to modern oceans. The heaviest and relatively homogeneous δ98/95Mo values were recorded in siltstone samples formed under completely oxic conditions, which is considered that Mn oxide-free shuttling was responsible for such heavy δ98/95Mo value. Further, combined with Fe species data and the accumulation extent of Mo and U, the variation of δ98/95Mo values in the two studied sections demonstrate a redox-stratified ocean with completely oxic shallow water and predominantly anoxic (even euxinic) deeper water having developed early on, which eventually became completely oxygenated. This suggests that oceanic circulation at the time became reorganized, and such changes in oceanic chemistry may have been responsible for triggering the “Cambrian Explosion” of biological diversity.

High methylmercury production under ferruginous conditions in sediments impacted by sewage treatment plant discharges

Sewage treatment plants (STPs) are important point sources of mercury (Hg) to the environment. STPs are also significant sources of iron when hydrated ferric oxide (HFO) is used as a dephosphatation agent during water purification. In this study, we combined geochemical and microbiological characterization with Hg speciation and sediment amendments to evaluate the impact of STP's effluents on monomethylmercury (MMHg) production. The highest in-situ Hg methylation was found close to the discharge pipe in subsurface sediments enriched with Hg, organic matter, and iron. There, ferruginous conditions were prevailing with high concentrations of dissolved Fe2+ and virtually no free sulfide in the porewater. Sediment incubations demonstrated that the high MMHg production close to the discharge was controlled by low demethylation yields. Inhibition of dissimilatory sulfate reduction with molybdate led to increased iron reduction rates and Hg-methylation, suggesting that sulfate-reducing bacteria (SRB) may not have been the main Hg methylators under these conditions. However, Hg methylation in sediments amended with amorphous Fe(III)-oxides was only slightly higher than control conditions. Thus, in addition to iron-reducing bacteria, other non-SRB most likely contributed to Hg methylation. Overall, this study highlights that sediments impacted by STP discharges can become local hot-spots for Hg methylation due to the combined inputs of i) Hg, ii) organic matter, which fuels bacterial activities and iii) iron, which keeps porewater sulfide concentration low and hence Hg bioavailable.

Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age

Research Article| April 01, 2015 Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age Birger Rasmussen; Birger Rasmussen * 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia *E-mail: B.Rasmussen@curtin.edu.au Search for other works by this author on: GSW Google Scholar Bryan Krapež; Bryan Krapež 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia Search for other works by this author on: GSW Google Scholar Janet R. Muhling; Janet R. Muhling 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia2Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Stirling Highway, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Alexandra Suvorova Alexandra Suvorova 2Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Stirling Highway, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Geology (2015) 43 (4): 303–306. https://doi.org/10.1130/G36309.1 Article history received: 26 Sep 2014 rev-recd: 04 Jan 2015 accepted: 22 Jan 2015 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Birger Rasmussen, Bryan Krapež, Janet R. Muhling, Alexandra Suvorova; Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age. Geology 2015;; 43 (4): 303–306. doi: https://doi.org/10.1130/G36309.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The early ocean was characterized by anoxic, iron-rich (ferruginous) conditions before the rise of atmospheric oxygen ∼2.45 b.y. ago. A proxy for ferruginous conditions in the ancient ocean is the deposition of banded iron formations (BIFs), which are iron- and silica-rich chemical sediments whose constituents were largely derived from seawater. Although experiments simulating ancient ocean chemistry support the rapid growth of iron-silicate phases, the main iron precipitates are hypothesized to have been ferric oxyhydroxides. The paradox between the prevailing reducing conditions and the deposition of oxidized iron phases is explained by biologically mediated oxidation in the water column. New high-resolution microscopy of BIFs and shales throughout the 2.63–2.45 b.y. old Hamersley Group, Australia, reveals the presence of vast quantities of nanometer-sized iron-silicate particles in laminated chert. The nanoparticles are finely disseminated in early diagenetic chert and locally define sedimentary lamination, indicating that they represent relicts of the original sediments. By inference from experimental studies simulating the composition of the early Precambrian ocean, we suggest that the nanoparticles precipitated from anoxic seawater enriched in silica and dissolved iron, and were silicified upon deposition. The prevalence of iron-silicate nanoparticles implies that they were pervasive background precipitates in ferruginous, silica-enriched oceans, forming the primary sediments of BIFs during periods of enhanced submarine mafic volcanism. Our results imply that silicate precipitation was a major sink of seawater iron and silica before the Great Oxidation Event and, because of the reactivity of nanoparticle surfaces, may also have influenced the transport and geochemical cycling of trace metals and nutrients. Our hypothesis that the basic building blocks of BIFs were predominantly iron-silicate muds rather than iron oxides and/or hydroxides may lead to new insights into seawater chemistry on the early Earth and the role of biology in the deposition of BIFs. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Selenium isotopes trace anoxic and ferruginous seawater conditions in the Early Cambrian

Selenium (Se) isotopes can yield substantial isotopic fractionation (up to 20‰) confirmed by experiments and field investigations, depending on various biotic or abiotic redox transformations. Therefore, it is expected that redox changes in the ancient oceans would induce significant isotopic fractionation, and the Se isotopic signatures recorded in old sedimentary rocks might provide new insight into how the redox state of the ancient ocean has evolved. However, previous studies have shown that Se is slightly enriched in the lighter isotope relative to the bulk earth values in most deposited conditions (oxic, anoxic, and even sulfidic). Here, our results reveal that ferruginous conditions can result in excessive accumulation of Se in sediments with an elevated Se/S ratio and significant isotope fractionation (about 6‰), which leads us to propose that Se isotopes are an appropriate geochemical proxy to trace unique oceanic conditions over times. Accordingly, Se isotopic variations measured in three Early Cambrian formations in southern China suggest that anoxic waters with ferruginous conditions must have been present in early Cambrian ocean along the eastern margin of the Yangtze platform, and oceanic circulation was stepwise reorganized. This may have triggered biological diversification from the Ediacaran to the Early Cambrian.

Deep-water anoxygenic photosythesis in a ferruginous chemocline.

Ferruginous Lake Matano, Indonesia hosts one of the deepest anoxygenic photosynthetic communities on Earth. This community is dominated by low-light adapted, BChl e-synthesizing green sulfur bacteria (GSB), which comprise ~25% of the microbial community immediately below the oxic-anoxic boundary (OAB; 115-120 m in 2010). The size of this community is dependent on the mixing regime within the lake and the depth of the OAB-at ~117 m, the GSB live near their low-light limit. Slow growth and C-fixation rates suggest that the Lake Matano GSB can be supported by sulfide even though it only accumulates to scarcely detectable (low μm to nm) concentrations. A model laboratory strain (Chlorobaculum tepidum) is indeed able to access HS- for oxidation at nm concentrations. Furthermore, the GSB in Lake Matano possess a full complement of S-oxidizing genes. Together, this physiological and genetic information suggests that deep-water GSB can be supported by a S-cycle, even under ferruginous conditions. The constraints we place on the metabolic capacity and physiology of GSB have important geobiological implications. Biomarkers diagnostic of GSB would be a good proxy for anoxic conditions but could not discriminate between euxinic and ferruginous states, and though GSB biomarkers could indicate a substantial GSB community, such a community may exist with very little metabolic activity. The light requirements of GSB indicate that at light levels comparable to those in the OAB of Lake Matano or the Black Sea, GSB would have contributed little to global ocean primary production, nutrient cycling, and banded iron formation (BIF) deposition in the Precambrian. Before the proliferation of oxygenic photosynthesis, shallower OABs and lower light absorption in the ocean's surface waters would have permitted greater light availability to GSB, potentially leading to a greater role for GSB in global biogeochemical cycles.

Oxygenation of Ediacaran Ocean recorded by iron isotopes

The increase in atmospheric oxygen during the late Neoproterozoic Era (ca. 800–542Ma) may have stimulated the oxygenation of the deep oceans and the evolution of macroscopic multicellular organisms. However, the mechanism and magnitude of Neoproterozoic oxygenation remain uncertain. We present Fe isotopes, Fe species and other geochemical data for two sections of the Doushantuo Formation (ca. 635–551Ma) deposited after the Nantuo glacial episode in the Yangtze Gorge area, South China. It is highlighted that highly positive δ56Fe values reflect a lower oxidation rate of Fe(II)aq under ferruginous conditions, and in turn near zero δ56Fe values indicate oxidizing conditions. Our study suggests that during the deposition of the bottom of Member II of the Doushantuo Formation the shallow seawater was oxic, but the deep water was characterized by ferruginous conditions, which is consistent with a redox chemical stratification model. Subsequent anoxic conditions under shallow seawater, represented by positive δ56Fe and negative δ13Ccarb excursions, should be ascribed to the upwelling of Fe(II)aq and dissolved organic carbon (DOC)-rich anoxic deep seawater. The oxidation of Fe (II)aq and DOC-rich anoxic deep seawater upon mixing with oxic shallow water provides an innovative explanation for the well-known negative δ13Ccarb excursions (ENC2) and positive δ56Fe excursions in the middle of Doushantuo Formation. Meanwhile, the upwelling Fe (II)aq- and DOC-rich anoxic deep seawater could increase photosynthetic activity. The balance between oxygen consumption and production was most important criteria for the oxygenation of Early Ediacaran Ocean and diversity of eukaryotic organisms.

Thermodynamic modeling of iron and trace metal solubility and speciation under sulfidic and ferruginous conditions in full scale continuous stirred tank biogas reactors

We investigated the equilibrium chemistry and chemical speciation of S, Fe and metals (Co, Ni, Cu, Zn, Cd, and Pb) in eight full scale Continuous Stirred Tank Biogas Reactors (CSTBR). Five reactors were digesting mixtures of different organic wastes (referred to as Co-Digester; CD) and three were digesting Sewage Sludge (SS). Iron was continuously added to the CD reactors to remove sulfide produced during anaerobic digestion and SS was rich in Fe, amended for phosphate removal in wastewater treatment plants prior to anaerobic digestion. As a consequence of different S:Fe molar ratios (0.3–2.8), ferruginous (Fe(II)-dominated) conditions prevailed in SS reactors and sulfidic (S(-II)-dominated) conditions in CD reactors. In all reactors, the chemical speciation of S, as determined by S K-edge X-ray Absorption Near-Edge Structure spectroscopy, was dominated by FeS(s). Reduced organic S forms, consisting of RSH (thiol) and RSR (organic sulfide), were the second most abundant S species. Zero-valent S (elemental S, polysulfides, and possible traces of pyrite) was detected in all reactors, ranging between 6% and 26% of total S, with the highest proportion formed under ferruginous conditions. Thermodynamic modeling suggested that Fe in the aqueous phase was dominated by Fe(II)-thiol complexes under sulfidic conditions (CD reactors) and by Fe(II)-phosphate complexes under ferruginous conditions (SS reactors). Thiols, representing organic functional groups, and sulfide complexes were the major aqueous species of Co(II), Ni(II), Cd(II) and Pb(II) under sulfidic conditions. Under ferruginous conditions thiol complexes were still important, but carbonate and phosphate complexes in particular dominated the aqueous phase speciation of Co(II) and Ni(II). The aqueous phase speciation of Zn and Cu was dominated by Zn(II)-sulfide and Cu(I)-polysulfide complexes, respectively. The results highlights the importance of S:Fe molar ratio as a regulating factor for the chemical speciation of metals in biogas reactors which in turn is important for microbial trace metal uptake and growth as well as potential metal toxicity. Both these aspects are critical for a successful performance of biogas production process.

Bitumen II from the Paleoproterozoic Here’s Your Chance Pb/Zn/Ag deposit: Implications for the analysis of depositional environment and thermal maturity of hydrothermally-altered sediments

The formation of sedimentary exhalative (SEDEX) Pb/Zn deposits is linked to ocean euxinia, but recent evidence suggests that ferruginous conditions may have dominated the deep ocean during the Middle Proterozoic, a maximum period for SEDEX distribution. Biomarkers of sulfate-reducing and sulfide-oxidising bacteria are valuable indicators of euxinic conditions in such settings. Organic matter (OM) from SEDEX deposits is often affected by alteration and/or migration, but OM entrapped within the kerogen/mineral matrix (Bitumen II) may be less affected than the freely-extractable OM (Bitumen I). We analysed Bitumen II from the Paleoproterozoic Here’s Your Chance (HYC) Pb/Zn/Ag deposit to find evidence of euxinic conditions in the depositional environment. n-Alkane distributions in Bitumen II are markedly distinct from previously-reported Bitumen I. Bitumen II contains long-chain n-alkanes (up to C36 or C38) and a strong even-over-odd distribution in a number of samples, which are 4‰ to 7‰ depleted in 13C compared to n-alkanes in Bitumen I and verified as indigenous by comparison with δ13C of isolated kerogen. These features are interpreted as evidence of sulfate-reducing and sulfide-oxidising bacteria, confirming that HYC was deposited under euxinic conditions. Bitumen II has the potential to reveal information from OM that is degraded and/or overprinted in Bitumen I. Commonly-used methylphenanthrene maturity ratios give conflicting information as to the relative maturity of Bitumens I and II. Bitumen I contains a far higher proportion of methylated phenanthrenes than Bitumen II. As Bitumen II is sequestered within the kerogen/mineral matrix it may have restricted access to the ‘methyl pool’ of organic compounds that can donate methyl groups to aromatic hydrocarbons. Parameters that include both phenanthrene and methylphenanthrenes do not appear suitable to compare the maturity of Bitumens I and II; hence there is no clear evidence that Bitumen II is of lower thermal maturity than Bitumen I.

Phosphorus sources for phosphatic Cambrian carbonates

The fossilization of organic remains and shell material by calcium phosphate minerals provides an illuminating, but time-bounded, window into Ediacaran–Cambrian animal evolution. For reasons that remain unknown, phosphatic fossil preservation declined significantly through Cambrian Series 2. Here, we investigate the phosphorus (P) sources for phosphatic Cambrian carbonates, presenting sedimentological, petrographic, and geochemical data from the Cambrian Series 2–3 Thorntonia Limestone, Australia, some of the youngest Cambrian strata to display exceptional phosphatic preservation of small shelly fossils. We find that within Thorntonia sediments, phosphate was remobilized by organic decay and bacterial iron reduction, with subsequent reprecipitation largely as apatite within the interiors of small shelly fossils. We discuss the merits of bioclastic-derived, organic matter–bound, or iron-bound P as potential sources to these strata. Petrographic observations suggest that the dissolution of phosphatic skeletal material did not provide the P for fossil preservation. In contrast, high organic carbon contents imply significant organic fluxes of P to Thorntonia sediments. Sedimentology and iron-speciation data indicate that phosphorus enrichment occurred during times of expanded anoxic, ferruginous conditions in subsurface water masses, suggesting that phosphorus adsorption to iron minerals precipitating from the water column provided a second significant P source to Thorntonia sediments. Simple stoichiometric models suggest that, by themselves, neither organic carbon burial nor an iron shuttle can account for the observed phosphorus enrichment. Thus, we infer that both processes were necessary for the observed phosphorus enrichment and subsequent fossil preservation in the Thorntonia Limestone.

Biomineralization of iron-phosphates in the water column of Lake Pavin (Massif Central, France)

The availabilities of iron and phosphorus have considerably impacted biological productivity in past and present natural aquatic environments, and therefore have been key regulators of climate changes over geological time scales. Microbial organisms are known to play important roles in reactions that drive Fe and P cycling at redox interfaces in Earth’s surface environments. Here we study the depth variations of Fe and P speciation in Lake Pavin (Massif Central, France), a deep (bottom depth ∼92m) and permanently stratified lake with anoxic and ferruginous conditions in the water column below ∼60m depth. We particularly focus on the potential roles of microbes on Fe and P transformations and traces left by these processes in the sedimenting particular matter.Bulk chemical analyses, powder X-ray diffraction and X-ray absorption spectroscopy (XAS) at the Fe K-edge were performed to characterize the mineralogy and Fe oxidation state of solid particles at different depths in the water column and in sediments deposited at the bottom of the lake. Fe is mainly hosted by Fe(III)-(oxyhydr)oxides and phyllosilicates in the shallower oxygenated water column of the lake (25m). The amount of Fe in suspended matter increases with depth, and an additional amorphous Fe(II)–Fe(III)-phosphate phase is detected close to the chemocline (at 56m depth), while vivianite (an Fe(II)-phosphate with a formula of Fe(II)3(PO4)2·8(H2O)) becomes dominant in the deeper anoxic water (67m and 86m depths). Fe-(oxyhydr)oxides are preserved down to these depths in the water column suggesting that Fe-reduction has little impact on the particulate Fe budget over the monimolimnion. These Fe-(oxyhydr)oxides undergo reductive dissolution at the surface of the sediments, where vivianite is the main Fe-bearing phase. These results are confirmed by imaging at the micrometer and nano-scales using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and synchrotron-based scanning transmission X-ray microscopy (STXM), as well as high spatial and energy resolution XANES spectra at the Fe L2,3-edges and the C K-edge. Moreover, polyphosphate-accumulating microorganisms were observed by microscopy and chemical imaging at all depths except in the deeper part of the lake (86m), and some Fe was associated with these polyphosphate inclusions below 56m. Finally, the Fe-phosphates encrust microbial bodies, suggesting that microbial activities might be involved in the formation of these phases. Based on comparisons with laboratory experiments, we propose that Fe-oxidation (in contrast to Fe-reduction) may play a role in the precipitation of Fe-phosphates in the water column of Lake Pavin. Polyphosphate-accumulating microorganisms could also be involved in Fe-phosphate formation in the lake, for instance by increasing dissolved phosphate concentrations in the monimolimnion.

∼750 Ma banded iron formation from the Arabian-Nubian Shield—Implications for understanding neoproterozoic tectonics, volcanism, and climate change

Neoproterozoic Banded Iron Formation (BIF) from Sawawin, NW Saudi Arabia and the Central Eastern Desert of Egypt define the 200×100km Arabian-Nubian Shield (ANS) BIF basin. ANS BIF formed ∼750Ma, prior to the Sturtian glacial episode (which began ∼716Ma). BIF deposition occurred in a marine basin associated with arc/backarc basin volcanism and immature clastic sedimentation. Beds are composed of alternating iron- and silica-rich laminae, which may reflect seasonal changes in deposition of Fe vs. Si. Fe-rich layers are dominantly composed of primary fine-grained hematite “dust” and minor apatite, with abundant secondary magnetite. Rapid deposition is revealed by: (1) major and trace element data indicating that ANS-BIF are very pure (<20% detrital input) chemical sediments in spite of being deposited in a basin with high sedimentation rates, and (2) pervasive evidence for soft-sediment deformation, suggesting that rapid sedimentation of dense, weak materials resulted in slumping. Nd and Pb isotopic compositions are predominantly mantle-like, indicating the dominance of hydrothermal sources or weathering of juvenile ANS crust for these elements. REE data show HREE-enriched patterns typical of modern seawater, with small positive Eu and small negative Ce anomalies. Low abundances of transition elements that are commonly abundant in proximal hydrothermal deposits of modern oceans may indicate that ANS-BIF formed at some distance from hydrothermal vents, or may reflect prior sulfide scavenging by euxinic and sulfidic deep ocean waters. REE data and Zn/Co share characteristics of both modern seawater and hydrothermal vent fluids suggesting derivation from a mixture of shallow suboxic seawater with a dilute, low-T hydrothermal vent fluid. Considered in conjunction with BIF of similar age on other paleocontinents, these observations support that rapid and widespread re-oxygenation of Fe+2 in previously anoxic or suboxic seawater led to rapid precipitation of hematite “dust” and BIF deposition ∼750Ma. Sulfate limitation or diminished bacterial sulfate reduction, required to form BIF instead of pyrite, may reflect large-scale glaciation but evidence for deep ferruginous conditions prior to Cryogenian glaciations suggests than any scenario that substantially reduced continental weathering (i.e., hard snowball or slushball) could have primed the oceans for BIF deposition. The likely short duration of ANS BIF deposition (a few to 10s of kyr) and apparent timing well before the “Sturtian” glaciation suggest that conditions favoring Neoproterozoic BIF formation could have existed over an extended period (10s of myrs) and that Neoproterozoic Oxidation Event began during the “Kaigas-Sturtian” time frame.

Deposition of 1.88-billion-year-old Iron Formations as a Consequence of Rapid Crustal Growth

The reappearance of major iron formations at 1.88 billion years ago (after the rise in atmospheric oxygen) is puzzling because their deposition requires anoxic and iron-rich sea water, but is here explained as a consequence of major mantle activity and rapid crustal growth at that time. Ocean-floor iron formations are deposited when the oceans are depleted in oxygen, so the rise of atmospheric oxygen by around 2.32 billion years ago is thought to have been responsible for their decline. Their sudden reappearance 500 million years later, in an oxygenated atmospheric environment, remains a mystery. Most of these 1.88-billion-year-old iron formations are found in North America, but new geochronology data reported here show that major iron formations were also deposited in Australia at about the same time, suggesting that they reflect the chemistry and redox state of the global ocean. The authors propose that enhanced volcanic and hydrothermal activity released vast volumes of iron, changing ocean chemistry and depositing major iron formations. The decline of igneous activity led to the disappearance of iron formations. Iron formations are chemical sedimentary rocks comprising layers of iron-rich and silica-rich minerals whose deposition requires anoxic and iron-rich (ferruginous) sea water. Their demise after the rise in atmospheric oxygen by 2.32 billion years (Gyr) ago1 has been attributed to the removal of dissolved iron through progressive oxidation2 or sulphidation3,4 of the deep ocean. Therefore, a sudden return of voluminous iron formations nearly 500 million years later poses an apparent conundrum3,5. Most late Palaeoproterozoic iron formations are about 1.88 Gyr old6,7,8 and occur in the Superior region of North America5,9,10. Major iron formations are also preserved in Australia, but these were apparently deposited11 after the transition to a sulphidic ocean at 1.84 Gyr ago that should have terminated iron formation deposition4, implying that they reflect local marine conditions5,12. Here we date zircons in tuff layers to show that iron formations in the Frere Formation of Western Australia are about 1.88 Gyr old, indicating that the deposition of iron formations from two disparate cratons was coeval and probably reflects global ocean chemistry. The sudden reappearance of major iron formations at 1.88 Gyr ago—contemporaneous with peaks in global mafic–ultramafic magmatism13,14, juvenile continental and oceanic crust formation15,16, mantle depletion17,18 and volcanogenic massive sulphide formation5,19—suggests deposition of iron formations as a consequence of major mantle activity and rapid crustal growth5,10,15,20. Our findings support the idea that enhanced submarine volcanism and hydrothermal activity linked to a peak in mantle melting released large volumes of ferrous iron and other reductants that overwhelmed the sulphate and oxygen reservoirs of the ocean, decoupling atmospheric and seawater redox states, and causing the return of widespread ferruginous conditions. Iron formations formed on clastic-starved coastal shelves where dissolved iron upwelled and mixed with oxygenated surface water. The disappearance of iron formations after this event may reflect waning mafic–ultramafic magmatism and a diminished flux of hydrothermal iron relative to seawater oxidants.

Deposition of 1.88-billion-year-old iron formations as a consequence of rapid crustal growth

Iron formations are chemical sedimentary rocks comprising layers of iron-rich and silica-rich minerals whose deposition requires anoxic and iron-rich (ferruginous) sea water. Their demise after the rise in atmospheric oxygen by 2.32 billion years (Gyr) ago has been attributed to the removal of dissolved iron through progressive oxidation or sulphidation of the deep ocean. Therefore, a sudden return of voluminous iron formations nearly 500 million years later poses an apparent conundrum. Most late Palaeoproterozoic iron formations are about 1.88 Gyr old and occur in the Superior region of North America. Major iron formations are also preserved in Australia, but these were apparently deposited after the transition to a sulphidic ocean at 1.84 Gyr ago that should have terminated iron formation deposition, implying that they reflect local marine conditions. Here we date zircons in tuff layers to show that iron formations in the Frere Formation of Western Australia are about 1.88 Gyr old, indicating that the deposition of iron formations from two disparate cratons was coeval and probably reflects global ocean chemistry. The sudden reappearance of major iron formations at 1.88 Gyr ago--contemporaneous with peaks in global mafic-ultramafic magmatism, juvenile continental and oceanic crust formation, mantle depletion and volcanogenic massive sulphide formation--suggests deposition of iron formations as a consequence of major mantle activity and rapid crustal growth. Our findings support the idea that enhanced submarine volcanism and hydrothermal activity linked to a peak in mantle melting released large volumes of ferrous iron and other reductants that overwhelmed the sulphate and oxygen reservoirs of the ocean, decoupling atmospheric and seawater redox states, and causing the return of widespread ferruginous conditions. Iron formations formed on clastic-starved coastal shelves where dissolved iron upwelled and mixed with oxygenated surface water. The disappearance of iron formations after this event may reflect waning mafic-ultramafic magmatism and a diminished flux of hydrothermal iron relative to seawater oxidants.

Progressive oxidation of anoxic and ferruginous deep-water during deposition of the terminal Ediacaran Laobao Formation in South China

The Laobao Formation, a black chert sequence, was deposited in deep-water in the terminal Ediacaran Nanhua Basin. The chert samples have finely laminated fabric, and have no distinct recrystallization or metasomatic relict texture. There is an extremely limited detrital component and no obvious hydrothermal influence. Therefore, the chert appears to be an authigenic deep-water precipitate. The trace elements in the chert were mainly derived from seawater and thus could be a relatively unique proxy for late Precambrian seawater conditions. Redox-sensitive trace elements and relevant ratios were used to reconstruct the depositional environment of the chert sequence. Combined with iron species results of the Laobao Formation, enrichments in redox-sensitive trace elements (U, V, Mo), depleted Mn, very low Th/U ratios (<2), high V/Sc (>7.9) and V/(V+Ni) (>0.6) ratios suggest an overall anoxic and ferruginous conditions over the deposition of the chert. The pyrite sulfur isotope compositions are between −15.4‰ and 11.0‰, and show a decreasing trend toward the top of the section, possibly implying sulfate concentration gradually increased in the terminal Ediacaran Nanhua Basin, which is consistent with implication of their pyrite framboid diameter characteristics. Our data confirm that deep-water in the Nanhua Basin was still anoxic and ferruginous, and it was progressively oxidized during the terminal Ediacaran period, which could create necessary conditions for the emergence and rapid evolution of multicellular metazoans.

An Ediacaran iron formation: New evidence for ferruginous late Neoproterozoic seawater

It is widely believed that the Earth's oceans became increasingly oxygenated during the late Neoproterozoic, most notably after the end of the Marinoan glaciation approximately ∼635 million years ago. However, recent geochemical analyses from various Ediacaran sediments suggest that some deep ocean basins were instead anoxic and ferruginous [Fe(II)-enriched] throughout the Ediacaran and possibly into the Cambrian, suggesting a more complex global redox structure than previously thought. Acceptance of this idea has been hindered by an apparent absence of Ediacaran iron formations (IF); chemical sediments that were common under such Fe-rich conditions in earlier Archaean-Palaeoproterozoic oceans. Here, we report detailed sedimentological, stratigraphic, petrographic and geochemical data from an Ediacaran IF and associated rocks, including ‘iron-rich’ black shales, siltstones and chert deposits of the Arroyo del Soldado Group in Uruguay. The Arroyo del Soldado Group is a mixed siliciclastic-carbonate succession interrupted by two major episodes of basin flooding and sediment starvation characterized by the deposition of IF and cherts. On the basis of stacking patterns, two stratigraphic sequences are identified within the group. Sequence A consists of continental shelf siliciclastic facies (Yerbal Formation) grading upward into a carbonate ramp system (Polanco Limestone Formation). Separated by a major erosional unconformity, sequence B is composed of coarse-grained alluvial conglomerates and sandstones (Barriga Negra Formation) capped by finer marine siliciclastics (Cerro Espeulitas Formation). The Yerbal and the Cerro Espuelitas formations, which constitute the main subject of this study, are characterized by retrogradational stacking patterns with fining-upward units deposited during transgressive system tracts. Significantly, the top of both units are characterized by distinct fine-grained siliciclastics and chemical sediments, consisting of banded siltstones, cherts and IF for the Yerbal Formation, and ‘iron-rich’ shales interbedded with black shales that pass upwards into thick chert deposits for the Cerro Espuelitas Formation. Significantly, the Yerbal Formation is a pre-Gaskiers glaciation deposit (ca. 600–590 Ma), while the Cerro Espuelitas Formation is post-Gaskiers (ca. 570–560 Ma) in age, providing insight into the palaeoceanographic conditions immediately before and after the last major Precambrian glacial event. The IF and cherts have coherent rare earth element and yttrium (REY) patterns and display the essential shale-normalized characteristics of marine precipitates. Rare earth element and yttrium signatures and mixing calculations show that they differ from Archaean and Palaeoproteorzoic IF in that high-temperature hydrothermal input did not influence their chemistry. Instead, we suggest that low-temperature hydrothermal input may account for the geochemical signatures displayed. By analyzing the REY signature of chemical precipitates (cherts and IF) and evaluating the redox chemistry of the marine water column within a sedimentological and sequence stratigraphic framework, our results confirm that ferruginous conditions dominated the pre-Gaskiers but also the post-glaciation deep-water chemistry, and that global ocean oxygenation may not have occurred well into the upper Ediacaran or even during the Cambrian, as recently proposed by Canfield et al. (2008).

Petrology and geochemistry of the banded iron formation (BIF) of Wadi Karim and Um Anab, Eastern Desert, Egypt: Implications for the origin of Neoproterozoic BIF

Banded iron formation (BIF) is exposed among the Precambrian rocks in the Wadi Karim and Um Anab areas in the Eastern Desert of Egypt. The BIF conformably alternates with Neoproterozoic arc metavolcanic rocks, which comprise metabasalts and mafic to intermediate metapyroclastic rocks. The BIF of Wadi Karim belongs to the oxide and mixed carbonate-oxide facies, while that of Um Anab belongs to the oxide facies only. Iron bands are generally composed of iron-rich mesobands rhythmically alternating with jasper, chert or carbonate mesobands as in the Wadi Karim area, or chert mesobands as in the Um Anab area. The BIF of Wadi Karim is composed essentially of magnetite, hematite and microcrystalline quartz, in addition to ankerite in the carbonate-bearing bands, while that of Um Anab is composed of magnetite and microcrystalline quartz. The positive correlation between Al 2O 3 and TiO 2 in the studied BIFs indicates that these chemical sediments incorporate minor detrital components. The limited areal extent of the studied BIF and its association with metavolcanic rocks are features of Algoma-type BIF of Archean greenstone belts. The spatial and temporal association between the BIF and the arc metavolcanic rocks indicates a genetic relationship between volcanic activity and BIF. The REY patterns of the studied BIFs suggest their precipitation after mixing of reduced bottom water carrying hydrothermal component with oxidized seawater in variable proportions. The absence of strong positive Eu anomaly, which characterizes BIF of Archean greenstone belts, in the BIF of the Eastern Desert may be attributed to contribution of Fe and Si through low-temperature hydrothermal solutions and to a high oxygen level of the atmosphere in the Neoproterozoic. It is proposed that the deposition occurred in an oceanic island arc setting, probably intra-arc or back-arc basins. In contrast with the majority of the Neoproterozoic iron formations (IFs), the studied BIFs are intimately associated with volcanic rocks and show no direct evidence of forming in response to glaciation. Consequently we attribute their formation to volcanic activity accompanying the break up of Rodinia rather than to snowball Earth condition. Thus, the Neoproterozoic IFs can be subdivided into Algoma and Rapitan-types. The Algoma-type, of volcanic association, is represented by African and Arabian Shield IFs, whereas the Rapitan-type, of glacial association, comprises deposits from all continents. The Wadi Karim and Um Anab BIFs constitute an additional evidence for the widespread return of IFs in the Cryogenian and Ediacaran after disappearance for ∼1 Ga reflecting the recurrence of anoxic ferruginous conditions in the Neoproterozoic deep sea.

Ferruginous Conditions: A Dominant Feature of the Ocean through Earth's History

The reconstruction of oceanic paleoredox conditions on Earth is essential for investigating links between biospheric oxygenation and major periods of biological innovation and extinction, and for unravelling feedback mechanisms associated with paleoenvironmental change. The occurrence of anoxic, iron-rich (ferruginous) oceanic conditions often goes unrecognized, but refined techniques are currently providing evidence to suggest that ferruginous deep-ocean conditions were likely dominant throughout much of Earth's history. The prevalence of this redox state suggests that a detailed appraisal of the influence of ferruginous conditions on the evolution of biogeochemical cycles, climate, and the biosphere is increasingly required.