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Sial A.N.,Federal University of Pernambuco | Campos M.S.,Varzea Grande University | Gaucher C.,University of the Republic of Uruguay | Frei R.,Copenhagen University | And 7 more authors.
Journal of South American Earth Sciences | Year: 2015

The Jucurutu Formation in the Seridó Belt, northeastern Brazil, encompasses fine-to coarse-grained amphibolite-facies marbles, locally with cross-bedding and stromatolites. Banded iron formations (BIF) at three localities in this belt comprise itabirites (actinolite- or cummingtonite-itabirite and quartz-hematite itabirite) and iron ores, which are overlain by marbles of the Jucurutu Formation. Diamictites of uncertain stratigraphic position in the Seridó Belt exhibit gneiss and quartzite clasts up to 0.6m long and a fine-grained metapelitic matrix. The C-isotope stratigraphic pathways for the Jucurutu Formation show negative δ13C values at the base of the formation followed upsection by positive values. At the Ferro do Bonito iron Mine, values as low as-12‰ in carbonates just above the contact with underlying BIF are followed by values of ca.-5‰ and by positive values up section (+4 to+10‰). δ13C values for carbonates of the Jucurutu Formation deposited on top of BIFs at the Riacho Fundo, São João do Sabugi, and Serra da Formiga sections are all positive, the contact surfaces between carbonates and BIFs being covered by weathered material. The 87Sr/86Sr values for carbonate rocks of the Jucurutu Formation are mostly in the 0.7074-0.7075 interval, a ratio commonly observed in the late Cryogenian to the early Ediacaran. The studied BIFs exhibit Cr concentrations one magnitude higher than usual and largely unfractionated Cr isotope composition, similar to average magmatic values. They are characterized by δ53Cr values between-0.42 and-0.12‰, by Ce anomalies (Ce/Ce*) ranging from-0.4 to 0.7, by positive Eu anomalies, and by super-chondritic Y/Ho ratios (>20). The combination of geochemical parameters and association with mafic rocks resemble Algoma-type BIFs. These BIFs possibly formed proximally to hydrothermal vents, in anoxic and acidic deep waters, whereby Fe and Cr were leached from mafic or ultramafic rocks. High concentrations of Cr in the BIF with a magmatic inventory isotope composition, together with pronounced positive Eu anomalies, fit a rift depositional scenario, characterized by narrow basins and strong hydrothermal input, in which anoxic (ferruginous) conditions prevailed. This is substantiated by the occurrence of positive Ce anomalies. Their deposition was followed by carbonates which show "normal" Cr concentrations with slightly positively fractionated δ53Cr values. These carbonates may represent post-glacial cap carbonate sequences deposited in an anoxic to slightly oxic shallow marine environment, as suggested by Ce/Ce* versus Pr/Pr* and Ce/Ce* versus Nd cross-relationships. © 2015 Elsevier Ltd.

Sial A.N.,Federal University of Pernambuco | Chen J.,Chinese Academy of science | Lacerda L.D.,Federal University of Ceará | Peralta S.,National University of San Juan | And 9 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2014

There is a renewed interest in volcanism as the major trigger for dramatic climatic changes at the Cretaceous-Paleogene transition (KTB), which were accompanied by a decrease in biodiversity and mass extinction. We have used Hg contents as proxy for volcanic activity at the classical localities of Gubbio (Italy) and Stevns Klint (Denmark) where the KTB layer is easily recognizable, and at a near-complete succession exposed at the Bajada del Jagüel locality in the Neuquén Basin, Argentina. These three localities display similar δ13Ccarb trends with markedly negative excursion at the KTB layer. Bulk-rock oxygen isotopes yielded similar pathways across the KTB layers in these localities and, if considered near-primary, the negative δ18O excursion at the KTB in Gubbio and Bajada del Jagüel suggest warming temperatures during this transition, whereas the negative excursion immediately followed by positive one at Stevns Klint points to a cycle of warm followed by colder climate. At Stevns Klint, Hg contents reach 250ngg-1 within the KTB layer (Fiskeler Member) and 45ng.g-1 at 1.5m above that, while within the Scaglia Rossa Formation at Gubbio, three Hg peaks across the KTB are observed, one of them within the KTB layer (5.3ngg-1). Hg shows several peaks across the KTB in the Neuquén Basin, with up to 400ngg-1 in the Jagüel Formation. The phenomena that caused dramatic changes at the KTB probably expelled huge amounts of Hg into the atmosphere as recorded by these high Hg levels. A co-variation between Hg and Al2O3 in the studied sections suggest that Hg is adsorbed onto clays. Hg concentrations and also Hg isotopes are perhaps a powerful tool in the assessment of the role of volcanic activity during extreme climatic and biotic events, and in assessing the role of meteorite impact versus volcanism as the predominant cause of past global catastrophes and mass extinction. © 2014 Elsevier B.V.

Fru E.C.,Swedish Museum of Natural History | Fru E.C.,Nordic Center for Earth Evolution Nord | Ivarsson M.,Swedish Museum of Natural History | Ivarsson M.,Nordic Center for Earth Evolution Nord | And 9 more authors.
Nature Communications | Year: 2013

Debates on the formation of banded iron formations in ancient ferruginous oceans are dominated by a dichotomy between abiotic and biotic iron cycling. This is fuelled by difficulties in unravelling the exact processes involved in their formation. Here we provide fossil environmental evidence for anoxygenic photoferrotrophic deposition of analogue banded iron rocks in shallow marine waters associated with an Early Quaternary hydrothermal vent field on Milos Island, Greece. Trace metal, major and rare earth elemental compositions suggest that the deposited rocks closely resemble banded iron formations of Precambrian origin. Well-preserved microbial fossils in combination with chemical data imply that band formation was linked to periodic massive encrustation of anoxygenic phototrophic biofilms by iron oxyhydroxide alternating with abiotic silica precipitation. The data implicate cyclic anoxygenic photoferrotrophy and their fossilization mechanisms in the construction of microskeletal fabrics that result in the formation of characteristic banded iron formation bands of varying silica and iron oxide ratios. © 2013 Macmillan Publishers Limited. All rights reserved.

McCarthy F.M.,Brock University | Katz M.E.,Rensselaer Polytechnic Institute | Kotthoff U.,University of Hamburg | Browning J.V.,Rutgers University | And 7 more authors.
Geosphere | Year: 2013

Integrated Ocean Drilling Program Expedition 313 recovered Miocene sequences at Holes M0027A and M0029A on the New Jersey shallow shelf that contain a characteristic acid-resistant organic component. The palynofacies within each sequence refl ects variations in terrigenous versus authigenic fl ux through the Miocene that are associated with sea-level change. Very high ratios of terrigenous versus marine palynomorphs and of oxidation-resistant versus susceptible dinocysts are associated with seismic sequence boundaries, consistent with their interpretation as sequence-bounding unconformities generated at times of low sea level. Comparison of palynological distance from shoreline estimates with paleodepth estimates derived from foraminiferal data allows relative sea level to be reconstructed at both sites. Ages assigned using dinocyst biostratigraphy are consistent with other chronostratigraphic indicators allowing sequence boundaries to be correlated with Miocene oxygen isotope (Mi) events. Paleoclimatic evidence from the pollen record supports the global climate changes seen in the oxygen isotope data. Although chronological control is relatively crude, Milankovitch-scale periodicity is suggested for parasequences visible in thick sequences deposited in relatively deep water where substantial accommodation existed, such as during the early Langhian at Site 29 (Middle Miocene Climatic Optimum). Palyno logical analysis thus supports the long-held hypothesis that glacioeustasy is a dominant process controlling the architecture of continental margins.

Affaton P.,Aix - Marseille University | Kalsbeek F.,Geological Survey of Denmark | Boudzoumou F.,Marien Ngouabi University | Trompette R.,Rue Pascal | And 3 more authors.
Precambrian Research | Year: 2016

The African part of the Neoproterozoic Araçuai-West Congo orogen forms a more than 1400. km long belt, running from Gabon in the north to Angola in the south, parallel to the South Atlantic coast. It consists of the West Congo fold belt in the west and a largely undeformed sedimentary foreland basin in the east. In the Republic of Congo (Congo Brazzaville) these are known as, respectively, the 'Mayombe fold belt' and the 'Niari basin'. The Mayombe fold belt is largely made up of strongly deformed metasedimentary rocks of the Mayombe Supergroup and its reworked granitoid basement complex. The Niari basin comprises the sedimentary strata of the West Congo Supergroup, which contain a unit of diamictite and associated cap-carbonate, related to the Marinoan (ca. 635. Ma) glaciation. In this paper detrital zircon U-Pb data are employed to investigate the relationships between the Mayombe and West Congo Supergroups in Congo Brazzaville, as well as the relationships with similar stratigraphic units in the Democratic Republic of Congo (DR Congo). Unfortunately, major differences in stratigraphical nomenclature exist between Congo Brazzaville and DR Congo which, together with geological differences, complicate comparisons. The terminology used in this paper follows the Congo Brazzaville tradition.U-Pb data have been obtained by LA-ICP-MS on 1674 zircon grains from samples of the Mayombe and West Congo Supergroups (seven and eight samples, respectively). Two of the samples have Palaeoproterozoic ages. The other samples are of Neoproterozoic age. Zircons from the latter fall into four main age groups: Group 1, 500-800. Ma, forming 8% of the total population; Group 2, 900-1200. Ma (29%); Group 3, 1800-2300. Ma (19%); and Group 4, 2500-3100. Ma (31%). Only ca. 13% of the zircons fall outside of these groups. Most zircons of Group 1 (600-800. Ma) are interpreted to have been derived from late Neoproterozoic magmatic arc rocks and syn- to post-collisional granites of the Araçuai orogen in Brazil, since rocks of these ages are rare in the Congo craton to the east. Most of Group 2 zircons have probably been derived from early Neoproterozoic (1000-900. Ma) rhyolitic and granitic rocks exposed in DR Congo. The Palaeoproterozoic and Archaean zircons of Groups 3 and 4 may have been derived both from eastern and western source regions.None of the stratigraphical units beneath the Marinoan diamictite contain the 500-800. Ma zircons of Group 1. This is interpreted to indicate deposition of these units during the earlier Neoproterozoic rift and drift phases in the history of the Araçuai-West Congo orogen, while the strata that do contain Group 1 zircons were deposited during the collisional phases and post-collisional uplift. A significant hiatus is likely to be present beneath the Marinoan diamictite. Based on these results, a revision of the stratigraphy of the Mayombe and West Congo Supergroups is recommended.Three granitoid rocks and two metasedimentary samples from the reworked basement complex have also been investigated, yielding Palaeoproterozoic ages, 2050-2100. Ma for the granites, and Palaeoproterozoic and Archaean detrital zircon ages for the metasedimentary rocks.Sm-Nd isotope data have been obtained on a number of (meta-)argilitic and siltstone samples. All samples yielded Palaeoproterozoic model ages, indicating mixing of sediments of different ages, as also observed in the zircon age distributions. © 2015 Elsevier B.V.

Rodler A.S.,Copenhagen University | Rodler A.S.,Nordic Center for Earth Evolution Nord | Frei R.,Copenhagen University | Frei R.,Nordic Center for Earth Evolution Nord | And 3 more authors.
Precambrian Research | Year: 2016

Chromium isotopes constitute a powerful paleoenvironmental tracer recording fluctuations of atmospheric oxygenation and continental weathering thus facilitating the reconstruction of the redox state of ancient seawater. We use the δ53Cr signature coupled with REE+Y patterns and redox-sensitive trace elements to monitor environmental changes recorded by marine carbonates of the Otavi Group, Namibia. These carbonates were deposited in a platform and foreslope setting in subtropical latitudes during the Neoproterozoic and comprise the transition from a marine depositional setting through glaciation into a postglacial environment in four stages. Preglacial carbonates (stage 1) yield positively fractionated δ53Cr values, increased U and Mn concentrations, indicative of mobilization during oxidative terrestrial weathering and stabilization in oxic surface waters. Carbonates deposited just before the Ghaub diamictites (stage 2) record δ53Cr values (>+0.4‰) comparable to modern seawater and negative Ce anomalies (∼0.7) characteristic for oxygenated seawater. We interpret this as a pulse of intense oxidative weathering shortly before the advance of the glaciers. Marginal shale contamination persists in carbonates of both sections and is slightly elevated during the glacial aftermath; Cr is vulnerable towards detrital contamination. Early postglacial cap dolostones (stage 3) were influenced by enhanced detrital contamination potentially supplied by freshwater particulate load, which was then drastically reduced in the overlying postglacial limestones in the upper Maieberg Fm (stage 4) where near-preglacial δ53Cr values are reached again. REE+Y patterns along with Eu and Ce anomalies record a transformation from a marine, slightly anoxic and stratified water column with distal hydrothermal influence to a freshwater-influenced depositional environment with decreased hydrothermal activity and fluctuating oxic surface water conditions after glacial retreat. Here, we demonstrate that carbonate δ53Cr signatures are sensitive to changes in continental weathering balanced between detrital contamination and oxidative weathering on land and are capable of tracing fluctuating redox conditions prior and after one of the major syn-Marinoan glaciations. © 2016 Elsevier B.V.

Rodler A.S.,Copenhagen University | Rodler A.S.,Nordic Center for Earth Evolution Nord | Hohl S.V.,Free University of Berlin | Guo Q.,CAS Beijing Institute of Geographic Sciences and Nature Resources Research | And 2 more authors.
Chemical Geology | Year: 2016

The Yangtze Platform in South China is considered a key site for studying Neoproterozoic ocean oxygenation. Comprehensive Cr concentration and isotopic signatures are presented from cap dolostones of the Doushantuo Formation from a deep water and a shallow marine carbonate section, that have previously been used to constrain the redox evolution of Ediacaran seawater in space and time employing major and trace element abundances as well as Sr-, O- and C-isotope compositions. The majority of the studied samples yield elevated Cr concentrations and negatively fractionated δ53Cr signals due to (i) detrital contamination consistent with Cr signatures of other post-Marinoan cap dolostones, (ii) diagenetic carbonate alteration and post-depositional remobilization of Cr. Al concentrations were used as indicator of detrital contamination and to calculate authigenic Cr concentration and isotope signatures when the detrital contribution of Cr was <40%, which generally resulted in slightly lower Cr concentrations and slightly higher Cr isotopic signals in these samples. Diagenetic alteration of a majority of the studied cap dolostones is evident from low Sr concentrations and light δ18Ocarb signals that correlate with low Cr concentrations and negatively fractionated δ53Cr values. Post-depositional remobilization of Cr results in the loss of heavy Cr isotopes and consequently light Cr isotopes are retained during precipitation of secondary carbonates. However, some samples might still show pristine Ediacaran seawater signals. Two samples pertaining to the deep water cap dolostone section at Yanwutan and one sample pertaining to the shallow water section at Jijiawan yield slightly positively fractionated δ53Cr signals, particularly when corrected to presumably authigenic signals (δ53Crauth). These values are consistent with those from other Neoproterozoic carbonate sections deposited in an oxygenated environment. This short sequence of positively fractionated δ53Crauth is concurrent with a cap dolostone 87Sr/86Sr signal close to Ediacaran seawater values with relatively high Sr and Cr concentrations, with decreased detrital contamination and with a pronounced negative Ce anomaly. They indicate either light pulses of enhanced oxidative weathering or, inferred from a few least-altered samples, prevailing oxic conditions during cap dolostone deposition. The Cr isotopic composition of Ediacaran cap dolostones pertaining to the Doushantuo Formation reveals fine-scale fluctuations in post-glacial detrital contamination and diagenesis balanced against a clear signal of oxidative weathering. Cr isotopes have the potential to further enhance our understanding of Neoproterozoic weathering regimes and past ocean oxygenation, however, before the Cr isotope system can be faithfully applied to delineate changes in redox conditions recorded in ancient marine carbonates, possible diagenetic alteration of the Cr signal as well as Cr contribution from detrital contamination need to be taken into consideration. © 2016 Elsevier B.V.

Dossing L.N.,Copenhagen University | Dossing L.N.,Nordic Center for Earth Evolution Nord | Dideriksen K.,Copenhagen University | Dideriksen K.,University of California at Berkeley | And 3 more authors.
Chemical Geology | Year: 2011

Stable chromium (Cr) isotopes can be used as a tracer for changing redox conditions in modern marine systems and in the geological record. We have investigated isotope fractionation during reduction of Cr(VI) aq by Fe(II) aq. Reduction of Cr(VI) aq by Fe(II) aq in batch experiments leads to significant removal of lighter Cr isotopes from solution. Assuming Rayleigh distillation, the fractionation factor α=0.9964 agrees with published results for Cr(VI) reduction by magnetite. In experiments designed to mimic natural aqueous systems, Fe(II) aq was added at constant rate at pH 6.8 and 8.1. Cr isotope composition of solids and solution displays Rayleigh distillation with α=0.9985. Chemical composition, electron microscopy, X-ray diffraction and Cr isotope composition agree with a model where Cr(VI) aq reduction occurs through two mechanisms: (a) homogeneous Cr(VI) aq reduction leads to the formation of unstable Cr(VI)-bearing green rust and results in Cr isotope fractionation of a magnitude similar to that observed during reduction by Fe(II) aq in our batch experiments, (b) intercalated Cr(VI) is reduced by the Fe(II) in the green rust, resulting in negligible isotope fractionation. The original Cr isotope composition of the reacted Cr(VI) aq is preserved if >90% of Cr(VI) aq is reduced by Fe(II) aq, which supports previous observations that under pH conditions relevant to surface seawater, Cr isotopes in Fe-rich chemical precipitates can provide important information for the interpretation of the paleo-redox state of the seawater from which the sediments precipitated. © 2011 Elsevier B.V.

Dahl T.W.,Nordic Center for Earth Evolution Nord | Dahl T.W.,University of Southern Denmark | Canfield D.E.,Nordic Center for Earth Evolution Nord | Canfield D.E.,University of Southern Denmark | And 7 more authors.
Earth and Planetary Science Letters | Year: 2011

The Ediacaran appearance of large animals, including motile bilaterians, is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances (pO2). To date, direct evidence for low oxygen in pre-Ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins, but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time. Because the abundance and isotopic composition of molybdenum (Mo) in organic-rich euxinic sediments can vary in response to changes in global redox conditions, Mo geochemistry provides independent constraints on the global evolution of well-oxygenated environments. Here, we establish a theoretical framework to access global marine Mo cycle in the past from the abundance and isotope composition of ancient seawater. Further, we investigate the ~750Ma Walcott Member of the Chuar Group, Grand Canyon, which accumulated in a rift basin with open connection to the ocean. Iron speciation data from upper Walcott shales indicate that local bottom waters were anoxic and sulfidic, consistent with their high organic content (up to 20wt.%). Similar facies in Phanerozoic successions contain high concentrations of redox-sensitive metals, but in the Walcott Member, abundances of Mo and U, as well as Mo/TOC (~0.5ppm/wt.%) are low δ98Mo values also fall well below modern equivalents (0.99±0.13‰ versus ~2.35‰ today). These signatures are consistent with model predictions where sulfidic waters cover ~1-4% of the global continental shelf area, corresponding to a ~400-800 fold increase compared to the modern ocean. Therefore, our results suggest globally expansive sulfidic water masses in mid-Neoproterozoic oceans, bridging a nearly 700million-year gap in previous Mo data. We propose that anoxic and sulfidic (euxinic) conditions governed Mo cycling in the oceans even as ferruginous subsurface waters re-appeared 800-750Ma, and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic O2 level, consistent with the hypothesis that pO2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the Ediacaran Period. © 2011 Elsevier B.V.

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