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A newly located exposure of the Niveau Thomel, an organic-rich level at the Cenomanian-Tur-onian boundary, provides a highly expanded record of Oceanic Anoxic Event (OAE) 2, except for the lower relatively condensed glauconite-rich part of the section. The new locality, close to Barreme in the Vocontian Basin, SE France, is developed in deep-water hemi-pelagic facies (shales, marls, marly limestones, variably enriched in organic matter) and provides an improved understanding of palaeoceanographic events associated with OAE 2. Investigation of the biostratigraphy (nannofossils and planktonic foraminifera), organic and inorganic geochemistry (bulk carbonate δ 18 O, total organic carbon (TOC), bulk organic, biomarker-specific and carbonate δ13C, major and trace elements, and Rock-Eval data) has allowed characterization of the sediments in great detail. The combined study further constrains the detailed relationship between bio-and chemostratigraphy (particularly with respect to the details of the well-displayed positive carbon-isotope excursion) for this interval. The section also provides new evidence, in the form of a positive oxygen-isotope excursion and an offset between carbonate and organic-carbon carbon-isotope records, which confirms the importance of cooling accompanied by a drop in dissolved CO 2 in near-surface waters during the Plenus Cold Event that characterized the early part of OAE 2. Evidence for increased oxygenation of bottom waters, together with elevated concentrations of redox-sensitive and chalcophilic elements registered elsewhere through the level of the Plenus Cold Event, may be reflected in enhanced concentrations of iron (in glauconite) and nickel in coeval strata from the Clot Chevalier section. © 2018 Gebrüder Borntraeger, Stuttgart, Germany.

Allanite-bearing black coastal sands of Kavala (N. Greece) were studied using a combination of single-crystal XRD, EMPA, μ-XRF, bulk ICP-MS, LA-ICP-MS, μ-XANES and γ-ray spectrometry. The sands are rich in REE (ΣREE + Y: 4010 to 10,810 mg/kg), Th (236–1205 mg/kg) and other critical metals such as Nb, Ta and Co. The main REE- and Th-hosts are allanite and titanite. The allanite unit cell parameters were calculated whereas its formula was found to be (REE0.470Ca1.499Th0.031) (Ti0.031Fe+3 1.089Al1.880) (Si2.906Al0.094O12) (OH). The μ-XANES spectra showed that LREE are present in trivalent oxidation state. Analyses of the non-magnetic sand fractions showed higher LREE (12,470 mg/kg) due to accumulation of allanite. The materials showed elevated radioactivity ranging from 885 ± 13 to 3467 ± 20 Bq/kg. The obtained results provide new insights on the provenance of the sands, the abundance of REE, Th, and other immobile elements, and offer new clues for potential exploration and exploitation. © 2019 Elsevier GmbH

Rare earth element (REE) abundances in iron formations (IFs) provide key evidence to track the redox evolution of the Precambrian oceans. Deriving information about processes and sources from IFs is complicated by the variable roles of diagenetic dissolution and precipitation that modify primary depositional signals in the rocks. Iron formations are typically mineralogically heterogeneous with varying amounts of carbonate (ankerite + siderite), oxide (predominantly magnetite) and silicate (chert and Fe-silicates). To date, little is known about the respective contributions of these fractions to the bulk-rock REE signal, and which fraction most accurately records primary seawater signals. Here we evaluate an optimized sequential extraction scheme for REE analysis in the ca. 2.46 Ga Asbestos Hills IFs from the Griqualand-West Basin in South Africa. These rocks were deposited immediately before the Great Oxidation Event and are potentially an important archive for large-scale redox changes in the (near) surface environment. To fully evaluate our method, we measured lanthanide elements plus ytrrium (REY) in individual minerals, in bulk rock samples and in three separate fractions using acetate, oxalate and total digestion sequential extractions. The extraction techniques were then verified using a mass balance approach, the first time that this has been comprehensively applied to IFs. Similarities between the carbonate fraction data and modern seawater, coupled with strong measurement reproducibility and systematic stratigraphic variability suggests that carbonate most closely tracks the primary seawater composition. The Asbestos Hills IFs show distinctive REY signatures with strongly variable (Yb/Pr)SN ratios, positive La anomalies, positive and constant Y/Ho, weakly positive Eu anomalies and a lack of Ce anomalies in their carbonate fraction. Stratigraphic variations in these parameters suggest deposition in a stratified fully anoxic ocean, which became shallower over time. The silicate fraction was influenced by various amounts of allochthonous material (detrital and volcanic ash), although some minerals including greenalite, minnesotaite and stilpnomelane may have tracked seawater REE variability. The oxide fraction was found to be the most REY-depleted and therefore susceptible to allochthonous influences as well as diagenetic controls. It shows a systematic MREE enrichment that has not been described in IFs before, but has been seen in sub-oxic, ferrous environments. An increase in the ratio of carbonate to oxide minerals ongoing upwards through the Asbestos Hills IF, from deep to shallow water conditions, most likely reflected an increase in dissimilatory iron reduction close to the margin of the anoxic basin.

Iron formations (IFs) are important geochemical repositories that provide constraints on atmospheric and ocean chemistry, prior to and during the onset of the Great Oxidation Event. Trace metal abundances and their Mo-Cr-U isotopic ratios have been widely used for investigating ocean redox processes through the Archean and Paleoproterozoic. Mineralogically, IFs consist of three main Fe-bearing fractions: (1) Fe-Ca-Mg-Mn carbonates, (2) magnetite and/or hematite and (3) Fe-silicates. These fractions are typically fine-grained on a sub-μm scale and their co-occurrence in varying amounts means that bulk-rock or microanalytical geochemical and stable isotope data can be influenced by cryptic changes in mineralogy. Fraction specific geochemical analysis has the potential to resolve mineralogical controls and reveal diagenetic versus primary precipitative controls on IF mineralogy. Here we adapt an existing sequential extraction scheme for Fe-phases (Poulton and Canfield, 2005) to the high Fe-content in IF and the specific three-fraction mineralogy. We optimized the scheme for magnetite-dominated Archean IFs using samples from the hematite-poor Asbestos Hills Subgroup IF, Transvaal Supergroup, South Africa. Previously commonly-used hydroxylamine-HCl and dithionite leaches were omitted since ferric oxides are quantitatively insignificant in these IF samples. The acetate leach was tested at variable temperatures, reaction times and under different atmospheres in order to ensure that all micro-crystalline Fe-carbonates were effectively dissolved, resulting in an optimum extraction for 48 h at 50 °C under anoxic conditions. The dissolution of magnetite by NH4-oxalate was also tested, resulting in an optimum extraction for 24 h under an ambient atmosphere. Finally, a HF-HClO4-HNO3 leach was used to dissolve the residual silicate fraction which has to date not been considered in detail in IF. Accuracy of the extraction technique was generally excellent, as verified using 1) elemental recoveries, 2) comparison of major and trace element distributions against mineralogy and 3) comparison to results from microanalytical techniques. This study focuses on the distribution of three frequently used geochemical proxies in IF; U, Mo and Cr. Molybdenum abundances in the Kuruman and Griquatown IF are low and show an apparent correlation with mineralogical variability, as determined by the sequential extraction. This suggests that changes in bulk-rock mineralogy, rather than redox chemistry might significantly affect Mo stable isotopes. For Cr, a minor bulk-rock stratigraphic increase can be related to the oxide and silicate fraction. However, a positive relationship with Zr indicates that this was also controlled by detrital or volcanic ash input. Uranium is predominantly bound to the silicate fraction and shows clear correlations with Zr and Sc implying detrital reworking under anoxic conditions. The discrepant behaviour of these three proxies indicate that mineralogy should be taken into account when interpreting heterogeneous bulk-rock samples and that fraction specific techniques will provide new insights into the evolution of atmosphere and ocean chemistry.

We presented new C and O isotope data of rockforming calcite in terrestrial carbonate deposits from Neogene basins of Attica (Greece), coupled with standard mineralogical and bulk geochemical results. Whereas both isotope datasets [δ18O from -8.99 to -3.20‰(VPDB); δ13C from -8.17 to +1.40‰(VPDB)] could be interpreted in principle as indicative of a meteoric origin, the clear lack of a statistical correlation between them suggests diverse sources for the isotopic variation of the two elements. On the basis of broad correlations between lower carbon isotope data with increasing Fe and bulk organic carbon, we interpreted the light carbon isotope signatures and As enrichments as both derived mainly from a depositional process involving increased supply of metals and organic carbon to the original basins. Periodically augmented biological production and aerobic cycling of organic matter in the ambient lake waters, would have led to the precipitation of isotopically light calcite in concert with elevated fluxes of As-bearing iron oxy-hydroxide and organic matter to the initial terrestrial carbonate sediment. The terrestrial carbonate deposits of Attica therefore represented effective secondary storage reservoirs of elevated As from the adjacent mineralized hinterland; hence these and similar deposits in the region ought to be regarded as key geological candidates for anomalous supply of As to local soils, groundwater and related human activities. © 2017 E. E. Kampouroglou et al.

New biomarker evidence is presented that constrains the conditions of deposition and preservation of sediments rich in extractable organic matter from the Aptian-Albian sedimentary record of the Ionian Zone in NW Greece. Analyses were specifically carried out on total lipid extracts from 12 organic rich samples from the Vigla Shale Member, two of which represent the regional record for the early Albian Paquier Event of OAE1b. In the aliphatic fractions, compound groups such as n-alkanes, isoprenoid alkanes, steroids, hopanoids and terpenoids were recognized, pointing to a mixed origin from algae and prokaryotes with an additional measurable contribution from terrigenous plant matter. These compounds suggest deposition of the primary organic sediments under conditions of decreased bottom water oxygen concentrations. Mass spectral evidence from the aromatic fractions of most samples reveal the variable presence of diagenetic and catagenetic derivatives of carotenoid pigments (including some with bound sulfur) that are characteristic of photoautotrophic sulfur bacteria. The presence of specific biomarkers originating from photosynthetic anaerobic microorganisms, similar to those in ancient and contemporary euxinic basins such as the Black Sea, suggest that photic zone euxinic conditions were intermittently operative in the Aptian-Albian stages of the Ionian Basin, but were not exclusively characteristic of the black shale representing the Paquier Event itself. © 2013 Elsevier Ltd.

Organic carbon-rich shales from localities in England, Italy, and Morocco, which formed during the Cenomanian-Turonian oceanic anoxic event (OAE), have been examined for their total organic carbon (TOC) values together with their carbon, nitrogen, and iron isotope ratios. Carbon isotope stratigraphy (δ13 Corg and δ13 Ccarb) allows accurate recognition of the strata that record the oceanic anoxic event, in some cases allowing characterization of isotopic species before, during, and after the OAE. Within the black shales formed during the OAE, relatively heavy nitrogen isotope ratios, which correlate positively with TOC, suggest nitrate reduction (leading ultimately to denitrification and/or anaerobic ammonium oxidation). Black shales deposited before the onset of the OAE in Italy have unusually low bulk δ57 Fe values, unlike those found in the black shale (Livello Bonarelli) deposited during the oceanic anoxic event itself: These latter conform to the Phanerozoic norm for organic-rich sediments. Pyrite formation in the pre-OAE black shales has apparently taken place via dissimilatory iron reduction (DIR), within the sediment, a suboxic process that causes an approximately -2‰ fractionation between a lithogenic Fe(III)oxide source and Fe(II)aq. In contrast, bacterial sulfate reduction (BSR), at least partly in the water column, characterized the OAE itself and was accompanied by only minor, iron isotope fractionation. This change in the manner of pyrite formation is reflected in a decrease in the average pyrite framboid diameter from ∼10 to ∼7 μm. The gradual, albeit irregular increase in Fe isotope values during the OAE, as recorded in the Italian section, is taken to demonstrate limited isotopic evolution of the dissolved iron pool, consequent upon ongoing water column precipitation of pyrite under euxinic conditions. Given that evidence exists for both nitrate and sulfate reduction during the OAE, it is evident that redox conditions in the water column were highly variable, in both time and space. Copyright 2007 by the American Geophysical Union.

Integrated biostratigraphic (planktonic foraminifera, calcareous nannofossils), chemostratigraphic (bulk C and O isotopes) and compound-specific organic geochemical studies of a mid-Cretaceous pelagic carbonate - black shale succession of the Ionian Zone (western Greece), provide the first evidence for the Cenomanian-Turonian oceanic anoxic event (OAE2, 'Bonarelli' event) in mainland Greece. The event is manifested by the occurrence of a relatively thin (35 cm), yet exceptionally organic carbon-rich (44.5 wt% TOC), carbonate-free black shale, near the Cenomanian-Turonian boundary within the Vigla limestone formation (Berriasian-Turonian). Compared to the 'Bonarelli' black-shale interval from the type locality of OAE2 in Marche-Umbria, Italy, this black shale exhibits greatly reduced stratigraphic thickness, coupled with a considerable relative enrichment in TOC. Isotopically, enriched δ13C values for both bulk organic matter (-22.2‰) and specific organic compounds are up to 5‰ higher than those of underlying organic-rich strata of the Aptian-lower Albian Vigla Shale member, and thus compare very well with similar values of Cenomanian-Turonian black shale occurrences elsewhere. The relative predominance of bacterial hopanoids in the saturated, apolar lipid fraction of the OAE2 black shale of the Ionian Zone supports recent findings suggesting the abundance of N2-fixing cyanobacteria in Cretaceous oceans during the Cenomanian-Turonian and early Aptian oceanic anoxic events. © Springer-Verlag 2006.

Στην παρούσα εργασία εξετάζεται η χρήση των σταθερών ισοτόπων του άνθρακα και του οξυγόνου, στην μελέτη παγκόσμιων παλαιοωκεανογραφικών μεταβολών, με ιδιαίτερη αναφορά στα ωκεάνια ανοξικά επεισόδια (OAEs). Η ανάλυση των σταθερών ισοτόπων εφαρμόστηκε στα Κρητιδικά ιζήματα των ζωνών Ιόνιας και Πίνδου της Δυτικής Ελλάδας. Στην Ιόνια ζώνη τα σταθερά ισότοπα του άνθρακα και του οξυγόνου, σε συνδυασμό με τα βιοστρωματογραφικά στοιχεία, καταγράφουν την παλαιοπεριβαλλοντική μεταβολή που αντιστοιχεί στα ανοξικά επεισόδια Bonarelli (Κενομάνιο/Τουρώνιο, ΟΑΕ2) και Paquier (Κατ. Άλβιο OAE1b). Στη ζώνη Πίνδου, εντός των Κρητιδικών ιζημάτων, εντοπίζονται δύο πλούσια σε οργανικό υλικό επίπεδα. Σύμφωνα με την βιοστρωματογραφική και ισοτοπική ανάλυση, το πρώτο αντιστοιχεί σε ΟΑΕ Σαντόνιας ηλικίας. Αυτό το τοπικό ωκεάνιο ανοξικό επεισόδιο αναφέρεται για πρώτη φορά. Το δεύτερο επίπεδο, ηλικίας Άπτιο - Άλβιο, πιθανά συσχετίζεται με κάποιο από τα επεισόδια Paquier (OAE1b) και Selli (OAE1a), τα οποία στην Ελλάδα ήταν μέχρι σήμερα γνωστά μόνο στην Ιόνια ζώνη., In the present paper we examine the use of carbon and oxygen stable isotopes in the study of global palaeoceanographic changes, with special reference to the oceanic anoxic events (OAEs). The analysis of stable isotopes was applied to the examination of Cretaceous sediments from the Ionian and Pindos zones of Western Greece. In the Ionian zone the carbon and oxygen stable isotopes, combined with biostratigraphic data, record the palaeoenvironmental change corresponding to the anoxic events Bonarelli (Cenomanian/Turonian, OAE2) and Paquier (Lower Albian, OAE1b). In the Pindos zone, within the Cretaceous sediments, we observed two organic-carbon-rich levels. According to the biostratigraphic and isotopie analysis, the first level corresponds to an OAE of Santonian age. This local oceanic anoxic event is described for the first time. The second level, Aptian - Albian age, possibly correlates to either the Paquier event (OAE 1b) or the Selli event (OAE 1a), which in Greece were until now known only in the Ionian zone.

High-resolution geochemical records from a depth transect through the Cenomanian/Turonian (C/T) Tarfaya Basin (northwest African Shelf) reveal high-amplitude fluctuations in accumulation rates of organic carbon (OC), redox-sensitive and sulphide-forming trace metals, and biomarkers indicative of photic zone euxinia. These fluctuations are in general coeval and thus imply a strong relationship of OC burial and water column redox conditions. The pacing and regularity of the records and the absence of a prominent continental signature suggest a dynamic depositional setting linked to orbital and higher-frequency forcing. Determining the dominant frequency depends on the definition of the most pronounced oceanic anoxic event (OAE2) and its duration. We propose that eccentricity is the main forcing factor at Tarfaya and controlled fluctuations in wind-driven upwelling of nutrient-rich, oxygen-depleted intermediate waters from the adjacent Atlantic and the periodic development of photic zone and bottom water euxinia on the mid- Cretaceous northwest African shelf. Accumulation records clearly identify the basin center as the primary site of sediment deposition with highest temporal variability and an up to six-fold increase in OC burial from ∼2 g/m² · yr prior to the OAE2 to ∼12 g/m² · yr during the OAE2. Photic zone and bottom water euxinia alternated with periods of greater oxygenation of the water column in response to climate forcing. Mass balance calculations imply that ∼2% of the overall global excess OC burial associated with the OAE2 was deposited in the Tarfaya Basin, an area that represented only ∼0.05% of the total global C/T ocean floor. In fact, the lateral extent of similar black shales along the African continental margin indicates that this part of the ocean contributed significantly to the global increase in organic carbon burial during the OAE2.

We present new, detailed carbon-isotope records for bulk carbonate, total organic carbon (TOC) and phytane from three key sections spanning the Cenomanian-Turonian boundary interval (Eastbourne, England; Gubbio, Italy; Tarfaya, Morocco), with the purpose of establishing a common chemostratigraphic framework for Oceanic Anoxic Event (OAE) 2. Isotope curves from all localities are characterized by a positive carbon-isotope excursion of c. 4‰ for TOC and phytane and c. 2.5‰ for carbonate, although diagenetic overprinting appears to have obliterated the primary carbonate carbon-isotope signal in at least part of the Tarfaya section. Stratigraphically, peak ä13C values for all components are followed by intervals of high, near-constant ä13C in the form of an isotopic plateau. Recognition of an unambiguous return to background ä13C values above the plateau is, however, contentious in all sections, hence no firm chemostratigraphic marker for the end-point of the positive isotopic excursion can be established. The stratigraphically consistent first appearance of the calcareous nannofossil Quadrum gartneri at or near the Cenomanian-Turonian boundary as established by ammonite stratigraphy, in conjunction with the end of the ä13C maximum characteristic of the isotopic plateau, provides a potentially powerful tool for delimiting the stratigraphic extent and duration of OAE 2. This Oceanic Anoxic Event is demonstrated to be largely, if not wholly, confined to the latest part of the Cenomanian stage

We present new stable (C, O) isotopic, biostratigraphic and organic geochemical data for the Vigla Shale Member of the Ionian Zone in NW Greece, in order to characterize organic carbon-rich strata that potentially record the impact of Cretaceous Oceanic Anoxic Events (OAEs). In a section exposed near Gotzikas (NW Epirus), we sampled a number of decimetre-thick, organic carbon-rich units enclosed within marly, locally silicified, Vigla Limestone (Berriasian-Turonian). All these units are characterized by largely comparable bulk geochemical characteristics, indicating a common marine origin and low thermal maturity. However, the stratigraphically highest of these black shales is further distinguished by its much higher total organic-carbon (TOC) content (28.9 wt%) and Hydrogen Index (HI) (529), and much enriched δ13Corg value (-22.1‰). Planktonic foraminiferal and calcareous nannofossil biostratigraphy indicate a lower to middle Albian age for the strata immediately above, and a lower Aptian age for the strata below, the uppermost black shale. In terms of molecular organic geochemistry, the latter black shale is also relatively enriched in specific isoprenoidal compounds (especially monocyclic isoprenoids), whose isotopic values are as high as - 15‰, indicating a substantial archaeal contribution to the organic matter. The striking similarities between the molecular signatures of the uppermost Vigla black shale and coeval organic-rich strata from SE France and the North Atlantic (ODP Site 1049C) indicate that this level constitutes a record of the Paquier Event (OAE 1b). © 2004 Cambridge University Press.