Formation of iron sulfide at faecal pellets and other microniches within suboxic surface sediment.

Faecal pellet deposition and bioturbation may lead to heterogeneously distributed particles of localized highly reactive organic matter (microniches) being present below the oxygen penetration depth. Where O2, NO3-, and Fe/Mn oxyhydroxides become depleted within these microniches or where they exist in zones of sulfate reduction, significant localized peaks in sulfide concentration can occur. These discrete zones of sulfide evolution can cause formation of iron sulfides that would not be predicted by analysis of the ‘bulk’ sediment. Using a reaction-transport model developed specifically for investigating spherical microniches, and incorporating 3D diffusion, we investigated how the rate constants of organic matter (OM) degradation, particle porosity and niche lifetime, affect dissolved sulfide and iron concentrations, and formation of iron sulfide at such niches. For all of the modelled scenarios the saturation index for iron sulfide is positive, indicating favourable conditions for FeS precipitation in all niches. Those simulations within the microniche lifetime range of 2.5 to 5 days gave comparable concentration ratios of sulfide to iron in solution within the niche to experimentally observed values. Our model results provide insight into the mechanisms of preservation of OM, including soft tissue, in the paleo record, by predicting the conditions that result in preferential deposition of precipitates at the edge of microniches. Decreases in porosity, shorter niche lifetimes and increases in OM degradation rate constants, all tend to increase the likelihood that FeS precipitation will preferentially occur at the edges of a niche, rather than uniformly throughout the niche volume

Geochemistry of reduced inorganic sulfur, reactive iron, and organic carbon in fluvial and marine surface sediment in the Laizhou Bay region, China

Understanding the geochemical cycling of sulfur in sediments is important because it can have implications for both modern environments (e.g., deterioration of water quality) and interpretation of the ancient past (e.g., sediment C/S ratios can be used as indicators of palaeodepositional environment). This study investigates the geochemical characteristics of sulfur, iron, and organic carbon in fluvial and coastal surface sediments of the Laizhou Bay region, China. A total of 63 sediment samples were taken across the whole Laizhou Bay marine region and the 14 major tidal rivers draining into it. Acid volatile sulfur, chromium (II)-reducible sulfur and elemental sulfur, total organic carbon, and total nitrogen were present in higher concentrations in the fluvial sediment than in the marine sediment of Laizhou Bay. The composition of reduced inorganic sulfur in surface sediments was dominated by acid volatile sulfur and chromium (II)-reducible sulfur. In fluvial sediments, sulfate reduction and formation of reduced inorganic sulfur were controlled by TOC and reactive iron synchronously. High C/S ratios in the marine sediments indicate that the diagenetic processes in Laizhou Bay have been affected by rapid deposition of sediment from the Yellow River in recent decades

A Rezi Rzt-1 fúrás felső-triász képződményeinek szervetlen geokémiai jellemzői (Kösseni Formáció, Keszthelyi-hegység): őskörnyezeti tanulmány | Inorganic geochemical characteristics of the Upper Triassic rocks from borehole Rezi Rzt–1 (Kössen Formation, Keszthely Mountains, Hungary): a palaeoenvironmental study

Tanulmányunkban a Dunántúli-középhegységből származó nori–rhaeti bitumenes márgák (Kösseni Formáció) szervetlen geokémiai adatait használtuk fel az üledékképződés őskörnyezeti viszonyainak jellemzésére. A Rezi Rzt–1 fúrás (Keszthelyi-hegység) maganyagának röntgen fluoreszcens és atomabszorpciós spektrometriás vizsgálati eredményeit, valamint korábban publikált összes szerves szén (TOC) adatokat felhasználva a fő- és nyomelemek eloszlását, a kén, a vas, továbbá bizonyos redoxérzékeny nyomelemek dúsulási viszonyainak összefüggéseit hasonlítottunk össze. Az átlagos archaikum utáni ausztráliai agyagpala („post-Archean Australian average shale” – PAAS) összetételhez képest, amely a finomszemcsés üledékes kőzetek geokémiai vizsgálatakor leggyakrabban használt viszonyítási alap, az Rzt–1 fúrásban a Kösseni Formáció alsó (201,0–265,0 m) és középső (73,0–201,0 m) szakaszát képviselő minták fő- és nyomelem dúsulási tényezői hasonlóak. A TOC–S diagram adatai között kitűnő korreláció (r=0,82) mutatható ki. A C–S–Fe kapcsolatrendszer (TOC–S diagram, Fe(tot)–TOC–S diagram, DOP [„degree of pyritization” – a piritesedés mértéke] viszonyszámok) és a redox-érzékeny fémek (Cu, Ni, Pb, Zn, Cr és Co) Al-ra normált arányai alapján azonban a Kösseni Formáció alsó és középső szakasza változó oxigén-ellátottságú aljzatvízben halmozódott fel. A Kösseni Formáció alsó szakaszának kialakulásakor erősen rétegzett vízoszloppal és tartós anoxikus környezeti feltételekkel számolhatunk. A középső szakasz alsó részének képződése idején váltakozó diszoxikus, illetve anoxikus redox őskörnyezet lehetett. A középső szakasz legfelső részének lerakódásakor valószínűleg diszoxikus–oxikus (a normál tengerihez hasonló) paleoredox környezet állhatott fenn. | In this study, inorganic geochemical data were used to elucidate palaeodepositional environments for Norian–Rhaetian bituminous marls (Kössen Formation) in Transdanubian Range, Hungary. Using X-ray fluorescence and atomic absorbance spectrometric analyses and previously published total organic carbon (TOC) data of the core samples from borehole Rezi Rzt–1 (Keszthely Mountains), major and traceelement patterns, sulphur (S), carbon (C) and selected redox-sensitive trace-element relationships were compared. Relative to the post-Archean Australian average shale (PAAS), which provides a consistent normalising scheme for geochemistry of fine-grained sedimentary rocks, major and trace element enrichment factors both of the lower (201.0–265.0 m) and middle interval (73.0–201.0 m) of Kössen Formation from Rzt–1 core generally show similar distribution patterns. In TOC–S crossplot, the strong positive correlation (r=0.82) may indicate a C-limited marin depositional system. C–S–Fe relationships (e.g. TOC–S plot, Fe(tot)–TOC–S ternary diagram and DOP [degree of pyritization] values) and Alnormalised redox-sensitive trace metal distributions (Cu, Ni, Pb, Zn, Cr and Co), however, suggest that the lower and middle interval of Kössen Formation accumulated under variable bottom-water conditions. Persistently anoxic conditions with a strongly stratified water column prevailed during accumulation of the lower interval of Kössen Formation. Palaeoredox conditions may have been intermittently dysoxic and anoxic during deposition of the lower part of the Kössen middle interval; and it is likely that conditions ranged from dysoxic to oxic, possibly being close to normal marine conditions at times during accumulation of the uppermost part of the Kössen middle interval

Pyritized radiolarians from the Mid-Cretaceous deposits of the Pieniny Klippen Belt : a model of pyritization in an anoxic environment

Excellently preserved, pyritized radiolarian skeletons have been found within the Upper Cenomanian deposits in the Pieniny Klippen Belt (PKB-Carpathians, Poland). On the basis of a study of their chemical composition, structure of replacing skeletons and exceptional preservation of all morphological details, we propose a new model where the pyritization process took place not in sediment but while the radiolarian skeletons were suspended in the anoxic water column. The radiolarians rich in organic matter, sinking through the upper (iron rich) part of an anoxic water column, became the sites of organic matter decomposition and enhanced bacterial sulphate reduction. Dissolved iron in this zone diffused into the radiolarians and precipitated as iron sulphides replacing the opaline skeletons. This process was controlled by the rates of opal dissolution and of bacterial sulphate reduction, and the availability of dissolved iron. The preservation of radiolarians in the Upper Cenomanian deposits from different depth sub basins of the PKB was compared. We found that the extent of pyritization and preservation of radiolarian skeletons may be dependent on the depth of the basin and the position of the oxic-anoxic interface

Investigations on the Peach 4 Debrite, a Late Pleistocene Mass Movement on the Northwest British Continental Margin

The Peach 4 debrite is the most recent in a series of large scale Pleistocene MTDs within the Barra fan on the northwest British continental margin. Geophysical data indicate that Peach 4 was formed through a combination of blocky and muddy debris flows and affects an area of ~ 700 km2. BGS core sample 56 -10 36, located directly over the Peach 4 debrite, provides a minimum age of 14.68 ka cal BP for the last major failure. An upwards fining turbidite sequence in BGS core sample 56 -10 239 is associ-ated with increased As and S concentrations, indicators of diagenetic pyrite which forms under anoxic conditions. It is proposed that As and S concentrations may pro-vide a method of distinguishing between contourite and turbidite sedimentation, though further research is required

Depositional environment and source rock potential of Cenomanian and Turonian sedimentary rocks of the Tarfaya Basin, Southwest Morocco

Detailed organic and inorganic geochemical analyses were used to assess the depositional environment and source rock potential of the Cenomanian and Turonian oil shale deposits in the Tarfaya Basin. This study is based on core samples from the Tarfaya Sondage-4 well that penetrated over 300m of Mid Cretaceous organic matter-rich deposits. A total of 242 samples were analyzed for total organic and inorganic carbon and selected samples for total sulfur and major elements as well as for organic petrology, Rock-Eval pyrolysis, Curie-Point-pyrolysis-gaschromatography-Mass-Spectrometry and molecular geochemistry of solvent extracts. Based on major elements the lower Cenomanian differs from the other intervals by higher silicate and lower carbonate contents. Moreover, the molecular geochemistry suggests anoxic bottom marine water conditions during the Cenomanian-Turonian Boundary Event (CTBE; Oceanic Anoxic Event 2: OAE2). As a proxy for the Sorg/Corg ratio, the ratio total thiophenes/total benzenes compounds was calculated from pyrolysate compositions. The results suggest that Sorg/ Corg is low in the lower Cenomanian, moderate in the upper Cenomanian, very high in the CTBE (CenomanianTuronian Boundary Event) and high in the Turonian samples. Rock-Eval data reveal that the lower Cenomanian is a moderately organic carbon-rich source rock with good potential to generate oil and gas upon thermal maturation. On the other hand, the samples from the upper Cenomanian to Turonian exhibit higher organic carbon content and can be classified as oil-prone source rocks. Based on Tmax data, all rocks are thermally immature. The microscopic investigations suggest dominance of submicroscopic organic matter in all samples and different contents of bituminite and alginite. The lower Cenomanian samples have little visible organic matter and no bituminite. The upper Cenomanian and CTBE samples are poor in bituminite and have rare visible organic matter, whereas the Turonian samples change from bituminite-fair to bituminite-rich and to higher percentages of visible organic matter towards the younger interval. These differences in the organic matter type are attributed to i) early diagenetic kerogen sulfurization and ii) the upwelling depositional environment. Moreover, kerogen sulfurization was controlled by the relationship between carbonate, iron and sulfur as well as the organic matter. Thus, the organic carbon-rich deposits can be grouped into: i) low Sorg and moderately organic matter-rich oil prone source rocks, ii) moderate Sorg and organic-carbon-rich oil prone source rocks, iii) high Sorg and organic carbon-rich oil prone source rocks and iv) very high Sorg and organic carbon-rich oil prone source rocks, the latter representing the CTBE interval. Types 2 to 4 will generate sulfur-rich petroleum upon maturation or artificial oil shale retorting. This integrated organic and inorganic approach sheds light on the various processes leading to the development of the world-class oil shales deposited through the Cenomanian to Turonian. In addition, this study shows how the changes in the depositional environment might have controlled kerogen sulfurization and organic matter preservation and structure. This detailed approach provides a better understanding on source rock development during the Cenomanian to Turonian in a global context, as many of the geochemical features were identified worldwide for deposits related to OAE2

Depositional environment and source rock potential of Cenomanian and Turonian sedimentary rocks of the Tarfaya Basin, Southwest Morocco

Detailed organic and inorganic geochemical analyses were used to assess the depositional environment and source rock potential of the Upper Albian to Turonian oil shale deposits in the Tarfaya Basin. This study is based on core samples from the Tarfaya Sondage-4 well that penetrated over 300m of Mid Cretaceous organic matter-rich deposits. A total of 242 samples were analyzed for total organic and inorganic carbon and selected samples for total sulfur and major elements as well as for organic petrology, Rock-Eval pyrolysis, Curie-Point-pyrolysis-gas-chromatography-Mass-Spectrometry and molecular geochemistry of solvent extracts. Based on major elements the Albian and Lower Cenomanian differ from the other intervals by higher silicate and lower carbonate contents. Moreover, the molecular geochemistry suggests marine anoxic bottom water conditions during the Cenomanian-Turonian boundary event (CTBE; oceanic anoxic event 2: OAE2). As a proxy for the Sorg/Corg ratio, the ratio total thiophenes/total benzenes compounds was calculated from pyrolysate compositions. The results suggest that Sorg/Corg is low in the Albian, moderate in the Cenomanian, very high in the CTBE and high in the Turonian samples.  Rock-Eval data reveal that the Albian is a moderately organic carbon-rich source rock with good potential to generate oil and gas upon thermal maturation. On the other hand, the samples from the Cenomanian to the Turonian exhibit higher organic carbon contents and can be classified as oil-prone source rocks. Based on Tmax data, all rocks are thermally immature.The microscopic investigations suggest dominance of submicroscopic organic matter in all samples and different contents of bituminite and alginite. The Albian samples have little visible organic matter and no bituminite. The Cenomanian and CTBE samples are poor in bituminite and have rare visible organic matter, whereas the Turonian samples change from bituminite-fair to bituminite-rich and to higher percentages of visible organic matter towards the younger interval. These differences in the organic matter type are attributed to 1) early diagenetic kerogen sulfurization and 2) the upwelling depositional environment. Moreover, kerogen sulfurization was controlled by the relationship between carbonate, iron and sulfur as well as the organic matter. Thus, the organic carbon-rich deposits can be grouped into: 1) low Sorg and moderately organic matter-rich oil prone source rocks, 2) moderate Sorg and organic-carbon-rich oil prone source rocks, 3) high Sorg and organic carbon-rich oil prone source rocks and 4) very high Sorg and organic carbon-rich oil prone source rocks, the latter being represented by the CTBE. Types 2 to 4 will generate sulfur-rich petroleum upon maturation or artificial oil shale retorting

Developing proxies to constrain redox gradients in terminal Ediacaran oceans

There is a long-standing interest in the relationship between the rise of early Metazoans and changes in the redox structure of the oceans. As such, there is a need for reliable geochemical proxy archives that record palaeo-redox. Before we can use proxies in deep time we must understand their application in modern environments, and ensure pristine seawater signals can be extracted effectively. We investigate the sulfur cycle in the modern ocean, using new data from minor sulfur isotopes to constrain the proportional pyrite burial flux - a key control on atmospheric oxygen regulation through time - to between 20 and 35%. Ce anomalies in rare earth element patterns record redox information, and we develop the leaching methods for extracting pristine signals from carbonates. We suggest that a partial leach in nitric acid reduces the risk of contamination. We apply multiple redox proxies (Fe-speciation, TOC, carbon isotopes, CAS-pyrpaired sulfur isotopes and Ce anomalies) to terminal Ediacaran carbonates from the Nama Group, Namibia, to reconstruct the redox structure of the Nama Group and its relationship to the distribution of biomineralising Metazoans. We generate a holistic redox model that distinguishes between anoxic, intermediate and fully oxygenated waters, including identification of manganous conditions using novel observations of positive Ce anomalies. We distinguish between spatial and temporal variability in redox using nine sections from variable relative water depths. Dynamic redox conditions are associated with small, monospecific communities of Metazoans in short-lived horizons. Metazoans are largely absent from low oxygen manganous waters, whereas fully oxic waters host large, complex Metazoan communities. We suggest that redox exerted an important control on the ecological structure of terminal Ediacaran Metazoan communities

Iron limitation in the Western Interior Seaway during the Late Cretaceous OAE 3 and its role in phosphorus recycling and enhanced organic matter preservation

The sedimentary record of the Coniacian–Santonian Oceanic Anoxic Event 3 (OAE 3) in the North American Western Interior Seaway is characterized by a prolonged period of enhanced organic carbon (OC) burial. This study investigates the role of Fe in enhancing organic matter preservation and maintaining elevated primary productivity to sustain black shale deposition within the Coniacian–Santonian-aged Niobrara Formation in the USGS #1 Portland core. Iron speciation results indicate the development of a reactive Fe limitation coeval with reduced bioturbation and increased organic matter preservation, suggesting that decreased sulfide buffering by reactive Fe may have promoted enhanced organic matter preservation at the onset of OAE 3. An Fe limitation would also provide a feedback mechanism to sustain elevated primary productivity through enhanced phosphorus recycling. Additionally our results demonstrate inconsistencies between Fe-based and trace metal redox reconstructions. Iron indices from the Portland core indicate a single stepwise change, whereas the trace metal redox proxies indicate fluctuating redox conditions during and after OAE 3. Using Fe speciation to reconstruct past redox conditions may be complicated by a number of factors, including Fe sequestration in diagenetic carbonate phases and efficient sedimentary pyrite formation in a system with limited Fe supply and high levels of export production