Binding of heavy metals by oxidised kerogen in (palaeo)weathered black shales

Sub-aerial weathering of black shales drives the gradual leaching of sulphur- and organic-bound heavy metal elements, which are usually abundant in these rocks due to depositional conditions. The formation of oxygen functional groups in kerogen, however, can lead to an opposing mechanism - metal adsorption and binding, similar to a process common in soils. An increase in cation exchange capacity (CEC) measured previously using metal complexes on black shales oxidised under laboratory conditions implies that the same phenomenon may occur in a naturally oxidised black shale. This idea was tested on a unique, well-developed and -preserved Permian palaeoweathering profile containing two neighbouring but diverse black shales from the Devonian/Carboniferous boundary in the Holy Cross Mountains (Poland). In the studied black shale beds, the oxygen groups formed in kerogen in the partially-weathered zone were found to be responsible for significant changes in adsorption properties measured using hexamminecobalt(III) and Cu(II)‑triethylenetetramine cations, which are common probes for CEC. Compared to a pristine part of black shales, the partially weathered zone was depleted of total organic carbon (TOC), sulphur, and sulphur- and organic-bound metals, and highly enriched in Cu, which is generally present in low levels in the nascent shales. In the partially weathered zone, where TOC content is reduced, apparent CEC values surpass the CECs predicted from the contents and structures of clay minerals, and correlate linearly with the content of oxygen groups developed during weathering. The adsorption properties of carboxyl groups in the oxidised kerogen are suggested as being responsible for the syn- or post-weathering enrichment in Cu caused by the remobilisation of older Cu-sulphide ores present in the area. As opposed to natural weathering, aggressive oxidation, e.g. under laboratory conditions produces a high proportion of cross-linked oxygen groups that do not participate in metal cation adsorption. The CEC values of artificially oxidised samples reached a limit corresponding to those of naturally oxidised shales

Aromatic hydrocarbons from the Middle Jurassic fossil wood of the Polish Jura

Aromatic hydrocarbons are present in the fossil wood samples in relatively small amounts. In almost all of the tested samples the dominating aromatic hydrocarbon is perylene and its methyl and dimethyl derivatives. The most important biomarkers present in the aromatic fraction are dehydroabietane, siomonellite and retene, compounds characteristic for conifers. The distribution of discussed compounds is highly variable due to such early diagenetic processes affecting the wood as oxidation and the activity of microorganisms. MPI1 parameter values (methylphenanthrene index) for the majority of the samples are in the range of 0.1 to 0.5, which results in the highly variable values of Rc (converted value of vitrinite reflectance) ranging from 0.45 to 0.70%. Such values suggest that MPI1 parameter is not useful as maturity parameter in case of Middle Jurassic ore-bearing clays, even if measured strictly on terrestrial organic matter (OM). As a result of weathering processes (oxidation) the distribution of aromatic hydrocarbons changes. In the oxidized samples the amount of aromatic hydrocarbons, both polycyclic as well as aromatic biomarkers decreases

Lower Wenlock black shales in the northern Holy Cross Mountains, Poland: Sedimentary and geochemical controls on the Ireviken Event in a deep marine setting

The stratigraphic variability and geochemistry of Llandovery/Wenlock (L/W) Series boundary sediments in Poland reveals that hemipelagic sedimentation under an anoxic/euxinic water column was interrupted by low density bottom currents or detached diluted turbid layers that resulted in intermittent seafloor oxygenation. TOC values and inorganic proxies throughout the Wilków 1 borehole section suggest variable redox conditions. U/Mo ratios >1 throughout much of the Aeronian and Telychian Stages, together with an absence of pyrite framboids, suggests oxygenated conditions prevailed. However, elevated TOC near the Aeronian/Telychian boundary, together with increased U/Th and V/(V+Ni) ratios and populations of small pyrite framboids are consistent with the development of dysoxic/anoxic conditions at that time. U/Th, V/Cr and V/(V+Ni) ratios, as well as Uauthig and Mo concentrations suggest that during the Ireviken black shale (IBS) deposition, bottom-water conditions deteriorated from oxic during the Telychian to mostly suboxic/anoxic immediately prior to the L/W boundary, before a brief reoxygenation at the end of the IBS sedimentation in the Sheinwoodian Stage. Rapid fluctuations in U/Mo during the Ireviken Event (IE) are characteristic of fluctuating redox conditions that culminated in an anoxic/euxinic seafloor in the Sheinwoodian. Following IBS deposition, conditions once again became oxygen deficient with the development of a euxinic zone in the water column. The Aeronian to Sheinwoodian deep-water redox history was unstable, and rapid fluctuations of the chemocline across the L/W Series boundary probably contributed to the IE extinctions, which affected mainly pelagic and hemipelagic fauna

Anomalous Upper Devonian mercury enrichments : comparison of Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) and Atomic Absorption Spectrometry (AAS) analytical data

Mercury geochemistry is emerging recently as a hot topic in chemostratigraphical and facies research, owing to the diagnostic character of Hg enrichments as a proxy of volcanic activity (crucial in the context of assumed causal links between volcanic cataclysms and mass extinctions). Thus, as a prerequisite to such far-reaching interpretations, reliable analytical determinations of Hg concentrations are necessary. In conventionally performed analyses in sedimentary geochemistry, Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) is usually applied, as an analytical standard for trace elements, including Hg. However, with a detection limit (DL) of 10 ppb, such measured values have been questioned as a conclusive geochemical indicator of Hg anomalies, and, instead, far more accurate techniques, such as Atomic Absorption Spectrometry (AAS; DL = 0.2 ppb), are requested. As a preliminary test of this view, we present comparative analysis of 91 samples from three sections encompassing the key Frasnian-Famennian and Famennian-Tournaisian boundary intervals in Morocco (Lahmida), Germany (Kahlleite) and Uzbekistan (Novchomok), for which Hg concentrations were determined by both methods in the same samples. Despite some differences, especially at low Hg concentrations, both analytical methods reveal the same 12 extraordinarily enriched samples in excess of 1 ppm (with one exception, the determination error is <20%), as well as similar overall chemostratigraphic patterns characterized by a few prominent Hg spikes, with a top value of 5.8 ppm. The Hg concentrations determined by ICP-MS and AAS are significantly correlated, as high as r=0.98 (Novchomok), even if the first method reveals a general tendency toward slightly heightened values (by ~15 to 30% for medians). Therefore, ICP-MS results can conclusively be used in mercury chemostratigraphy in order to recognize extraordinary volcanic (or other) signals, at least in the Devonian geological record. False Hg anomalies were not generated by these conventional ICP-MS determinations

Application of organic environmental markers in the assessment of recent and fossil organic matter input in coal wastes and river sediments : a case study from the Upper Silesia Coal Basin (Poland)

Various types of coal waste material (fresh, self-heated, soil-covered) and river sediments polluted by coal dust were studied. Characteristic geochemical features of recent vegetation input in river sediments were identified, e.g. the dominance of n-alkanols and n-alkanoic acids over n-alkanes. In the river sediments, several coal-related compounds were also found, e.g. n-alkylbenzenes, acetophenone and methylated phenols. The occurrence of sterols, stanols, vanillin, and methylbenzoic, benzeneacetic, oxalic, succinic and levulinic acids in coal waste samples (with the exception of fresh coal wastes) may indicate primitive soil-forming processes related to vegetation and moss cover. These compounds were also commonly identified in river sediments. Their distribution, characteristic of extant (as opposed to fossil) organic matter, was confirmed by several applied geochemical ratios, such as the EOP index (even-over-odd predominance) of fatty acids, (Σn-alkanoic acid + Σlong chain n-alkanes)/Σshort chain n-alkanes or (Σn-alkanoic acids + Σn-alkanols)/Σn-alkanes and various CPI (carbon preference indexes of n-alkanes)

The characteristics of organic matter from the Triassic clays of NW margin of the Holy Cross Mts ( Poland) - preliminary report

The molecular character of organic matter in Triassic clays on the NW border of the Holy Cross Mts was determined in apolar, aromatic and polar fractions of extractable organic matter (OM) using GC-MS analysis. The contribution of terrestrial higher plants to the kerogene is revealed by the dominant presence of odd long-chain n-alkanes and by the occurrence of retene. Benzophenone, fluorenone, cyclopenta(def)phenanthrenone, antracenone and benzanthrone were among the compounds identified in polar fractions of the soluble organic matter. These commonly originate during strong oxidation of sedimentary organic matter. The aromatic fraction is characterised by the presence of phenyl derivatives (PhPAC) such as phenylnaphthalenes, terphenyls, phenyldibenzofurans and phenylphenanthrenes that are also products of the abiotic oxidation of organic matter. The oxidation processes that occurred during sedimentation and during early diagenesis resulted in a very significant decrease in the organic matter content in the clays. This decrease considerably influences their technological properties

Ordovician Jeleniów Claystone Formation of the Holy Cross Mountains, Poland - reconstruction of redox conditions using pyrite framboid study

The aim of this research is to reconstruct palaeoredox conditions during sedimentation of the Jeleniów Claystone Formation deposits, using framboid pyrite diameter measurements. Analysis of pyrite framboids diameter distribution is an effective method in the palaeoenvironmental interpretation which allow for a more detailed insight into the redox conditions, and thus the distinction between euxinic, dysoxic and anoxic conditions. Most of the samples is characterized by framboid indicators typical for anoxic/euxinic conditions in the water column, with average (mean) values ranging from 5.29 to 6.02 μm and quite low standard deviation (SD) values ranging from 1.49 to 3.0. The remaining samples have shown slightly higher values of framboid diameter typical for upper dysoxic conditions, with average values (6.37 to 7.20 μm) and low standard deviation (SD) values (1.88 to 2.88). From the depth of 75.5 m till the shallowest part of the Jeleniów Claystone Formation, two samples have been examined and no framboids has been detected. Because secondary weathering should be excluded, the lack of framboids possibly indicates oxic conditions in the water column. Oxic conditions continue within the Wólka Formation based on the lack of framboids in the ZB 51.6 sample

New constraints on the Middle Palaeozoic to enozoic burial and hermal history of the Holy ross Mts. (Central Poland) : results of numerical modelling

A 1-D burial-thermal modelling was performed using data from two borehole sections representative of the central part of the Holy Cross Mts. area. This area is located in the axial part of the Permian-Mesozoic Mid-Polish Trough that was inverted during the latest Cretaceous-Paleocene. The modelling involved different variants of restored stratigraphy of eroded Carboniferous to Cretaceous strata, whereas calibration was based on samples from cored Middle-Upper Devonian sediments. The modelling results are consistent with the assumption of a Variscan (Carboniferous-Early Permian) heat flow elevated up to 80 mWm-2, which is further confirmed by independent regional evidence. The zone of increased thermal maturity in the Devonian may be partly accounted for by a thicker Carboniferous section (by ca. 500 m) compared to previous estimates. Two variants of the post-Carboniferous geohistory were analysed. The variant of a thinner Permian-Mesozoic section, implying lower magnitude of the Late Cretaceous-Paleogene inversion, allows more realistic assumptions regarding heat flow distribution through time, including the possibility to incorporate an elevated Variscan heat flow. The alternative scenario, assuming deeper burial, generally lower heat flow and smaller Carboniferous thickness, is regarded as less probable. The accepted variant of the Permian-Mesozoic burial history implies that the total post-Carboniferous burial in the study area was on the order of 2000-2500 metres rather than 3000-3500 metres. The respective Upper Cretaceous thickness could have been 400 to 500 m instead of ca. 1000 m, whereas the Late Cretaceous Paleogene inversion more likely started in the Santonian than in the late Maastrichtian. Consequently, the preferred magnitude of total inversion was on the order of 2500 m

Vitrinite equivalent reflectance of Silurian black shales from the Holy Cross Mountains, Poland

A number of independent methods have been used to measure the thermal maturity of Silurian rocks from the Holy Cross Mountains in Poland. Black shales are characterized by diverse TOC values varying from 0.24-7.85%. Having calculated vitrinite equivalent reflectance using three different formulas, we propose that the most applicable values for the Silurian rocks are those based on Schmidt et al. (2015) equation. Based on this formula, the values range from % 0.71 VReqvVLR (the vitrinite equivalent reflectance of the vitrinite-like macerals) to % 1.96 VReqvVLR. Alternative, complementary methods including Rock Eval pyrolysis and parameters based on organic compounds (CPI, Pr/n-C17, Ph/n-C18, MPI1, and MDR) from extracts did not prove adequate as universal thermal maturity indicators. We have confirmed previous suggestions that Llandovery shales are the most likely Silurian source rocks for the generation of hydrocarbons in the HCM

New constraints on the Middle Palaeozoic to Cenozoic burial and thermal history of the Holy Cross Mts. (Central Poland): results from numerical modelling

A 1-D burial-thermal modelling was performed using data from two borehole sections representative of the central part of the Holy Cross Mts. area. This area is located in the axial part of the Permian-Mesozoic Mid-Polish Trough that was inverted during the latest Cretaceous-Paleocene. The modelling involved different variants of restored stratigraphy of eroded Carboniferous to Cretaceous strata, whereas calibration was based on samples from cored Middle-Upper Devonian sediments. The modelling results are consistent with the assumption of a Variscan (Carboniferous-Early Permian) heat flow elevated up to 80 mWm-2, which is further confirmed by independent regional evidence. The zone of increased thermal maturity in the Devonian may be partly accounted for by a thicker Carboniferous section (by ca. 500 m) compared to previous estimates. Two variants of the post-Carboniferous geohistory were analysed. The variant of a thinner Permian-Mesozoic section, implying lower magnitude of the Late Cretaceous-Paleogene inversion, allows more realistic assumptions regarding heat flow distribution through time, including the possibility to incorporate an elevated Variscan heat flow. The alternative scenario, assuming deeper burial, generally lower heat flow and smaller Carboniferous thickness, is regarded as less probable. The accepted variant of the Permian-Mesozoic burial history implies that the total post-Carboniferous burial in the study area was on the order of 2000-2500 metres rather than 3000-3500 metres. The respective Upper Cretaceous thickness could have been 400 to 500 m instead of ca. 1000 m, whereas the Late Cretaceous-Paleogene inversion more likely started in the Santonian than in the late Maastrichtian. Consequently, the preferred magnitude of total inversion was on the order of 2500 m