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Pyritization: a palaeoenvironmental and redox proxy reevaluated

Profile image of Philippe Van CappellenPhilippe Van Cappellen

2003, Estuarine, Coastal and Shelf Science

https://doi.org/10.1016/S0272-7714(03)00058-1
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Abstract

Pyrite as a proxy for the redox states of bottom waters was evaluated in the context of modern salt marsh sediments. Two sites, 40 m apart, with very different physicochemical properties were sampled seasonally. The sites differed from each other in (1) the amount of tidal flushing, (2) vegetation, (3) redox condition below the sediment-water interface, (4) carbon content and (5) solid phase iron oxyhydroxide and iron sulfide content of the sediment. The calculated degree of pyritization (DOP) varies between 0.44 and 1.0, depending on the site and the season. Different DOP values were obtained for the same samples when different techniques were applied for extraction of reactive iron. Framboidal pyrite was dominant; however, other textural forms of pyrite were also observed. The size distribution of the individual pyrite crystals and the framboids was fairly narrow; with the majority of them having diameters <10 lm. Despite deposition taking place in aerobic waters (fully oxygenated waters) at the sampled sites, both the DOP and pyrite textures indicated dysoxic (waters with low oxygen concentrations) to euxinic (waters with no oxygen present, and H 2 S may be continually or intermittently present) bottom waters. These results indicate that caution must be applied when using DOP and pyrite morphologies as proxies for interpreting the redox conditions of different depositional environments.

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    Zsolt A Berner, Utz Kramar

    Sedimentology, 2013

    Authigenic pyrite grains from a section of the Lower Toarcian Posidonia Shale were analysed for their trace-element contents and sulphur-isotope compositions. The resulting data are used to evaluate the relationship between depositional conditions and pyrite trace-element composition. By using factor analysis, trace-elements in pyrite may be assigned to four groups: (i) heavy metals (including Cu, Ni, Co, Pb, Bi and Tl); (ii) oxyanionic elements (As, Mo and Sb); (iii) elements partitioned in sub-microscopic sphalerite inclusions (Zn and Cd); and (iv) elements related to organic or silicate impurities (Ga and V). Results indicate that trace-element contents in pyrite depend on the site and mechanism of pyrite formation, with characteristic features being observed for diagenetic and syngenetic pyrites. Diagenetic pyrite formed within anoxic sediments generally has a high heavy metals content, and the degree of pyritization of these elements increases with increasing oxygen deficiency, similar to the degree of pyritization of reactive Fe. The highest gradient in the increase of the degree of trace element pyritization with bottom-water oxygenation was found for the elements Ni < Cu < Mo = As < Tl. In contrast, syngenetic pyrite formed within a euxinic water column typically is enriched in As, Mo and Sb, but is low in heavy metals, and the geochemical variation reflects changes in sea water composition.

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    Redox conditions during sedimentation of the Middle Jurassic (Upper Bajocian–Bathonian) clays of the Polish Jura (south-central Poland)
    Patrycja Szczepanik

    Facies, 2009

    Depositional redox conditions of the uppermost Bajocian-Bathonian (Middle Jurassic) ore-bearing clays of the Gnaszyn/Kawodrza area in the Polish Jura have been determined using an integrated geochemical (Th/U and U/Th ratios, degree of pyritisation (DOP), sulphur stable isotopes, biomarker analysis) and petrographic approach (measurements of pyrite framboid diameters, and microfacies analysis). The Th/U and U/Th ratios indicate that oxic conditions prevailed on the sea-Xoor during this interval, and 34 S isotopes suggest open-system conditions. DOP values, however, are rather scattered, and may reXect oxic, dysoxic, or anoxic conditions. We consider that the DOP values result from reducing conditions within the sediment and the chemistry of the pore-waters, rather than true sea-Xoor redox conditions. Pyrite framboid populations also indicate that dysoxic conditions prevailed within the sediment, beneath an oxygenated water column. Biomarker data did not provide any evidence of water column stratiWcation or anoxia during sedimentation of the Middle Jurassic clays.

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    Fate of radium on the discharge of oil and gas produced water to the marine environment
    faraaz ahmad

    Chemosphere, 2021

    h i g h l i g h t s Selected field marine sediments in the vicinity of a wastewater discharge site contained elevated levels of Ra and Ba. Ra speciation and fate was determined via heavy liquid extractions, mixing experiments and geochemical modelling. Barite particles isolated from field sediments contained measurable 226 Ra activity confirming the radiobarite fate pathway. Synthetic/field produced waters were mixed with seawaters to successfully mimick the formation of (radio)barite particles. Ra uptake experiments showed Ra primarily exists in the form of radiostrontiobarite following produced water discharge.

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