Magdolna Hetényi

Carbon isotope anomaly and other geochemical changes at the Triassic-Jurassic boundary from a marine section in Hungary

Most mass extinctions are linked with carbon isotope excursions, implying that biotic crises are coupled with changes in the global carbon cycle. The isotopic evolution during the end-Triassic extinction is far less documented than that for the other major Phanerozoic extinctions. Here we report a sharp and short-lived 23.5‰ excursion in carbon isotope values for carbonate (d 13 Ccarb) corresponding to a 22‰ excursion in the isotopic composition of marine organic matter (d 13 Corg) and other geochemical

CHEMICAL STRUCTURE OF THE ORGANIC MATTER IN A PLIOCENE MAAR-TYPE SHALE : IMPLICATED BOTRYOCOCCUS RACE STRAINS AND FORMATION PATHWAYS

Abstract A maar-type Pliocene oil shale from Pula (Hungary) was examined by Scanning and Transmission Electron Microscopy (SEM, TEM) bitumen analysis, spectroscopic analyses (FTIR, solid state 13 C NMR) and pyrolytic analyses of insoluble material. The investigated samples, corresponding to the “massive section” of Pula deposit, exhibit TOC values ranging from 18 to 42%, and Rock-Eval analysis indicated that such samples are comprised of low maturity type I kerogen. Electron Microscopy observations confirmed that recognizable microfossils in this organic matter-rich material almost exclusively correspond to Botryococcus braunii colonies, the morphology of which is perfectly retained. Identification of bitumen and pyrolysate constituents (hydrocarbons, ketones, fatty acids), along with morphological and spectroscopic features, indicate (1) that B. braunii provided a major input to the most organic matter-rich section of Pula oil shale; (2) that both the n -alkadienes-producing and the lycopadiene-producing strains ( A and L , respectively) were present; (3) that the selective preservation of the insoluble and non-hydrolysable macromolecules building up B. braunii outer walls was by far the main process in the formation of this material; (4) that such a process, along with B. braunii prolific growth in the favourable environment that occurred in the crater lake, accounts for the very high TOC and oil potential of Pula deposit; and (5) that the condensation of high molecular weight ether lipids from the A race and of some bacterial lipids also likely contributed to Pula kerogen formation.

Preservation of algaenan and proteinaceous material during the oxic decay of Botryococcus braunii as revealed by pyrolysis-gas chromatography/mass spectrometry and 13C NMR spectroscopy

Botryococcus braunii cells were grown until the late-stationary phase of growth and subsequently decomposed under oxic conditions for 201 days using a microbial consortium obtained from a freshwater lake.Degradation exhibited multi-G model kinetics, with a ‘labile’ fraction lost at a rate two to three times slower than those observed for the degradation of other previously studied phytoplankton, and a ‘refractory’ fraction lost even more slowly.Scanning electron microscopy of the 201-day detritus, as previously seen for the kerogen, indicates the preservation of cell wall material with loss of intracellular contents.Detrital samples analyzed by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and solid-state ramp-CPMAS 13 C NMR, however, indicates the preservation of highly aliphatic material, algaenan, as well as ‘intrinsically labile’ proteinaceous components.These results further support the encapsulation hypothesis that proteins may be sterically protected from enzymatic attack via intimate associations with refractory, macromolecular organic matter. # 2003 Elsevier Science Ltd.All rights reserved.

Hydrogen index as reflecting intensity of sulphidic diagenesis in non-bioturbated, shaly sediments

Abstract Based on 13 published porewater H 2 S and sulphate profiles the amount of H 2 S escaping from non-bioturbated shales varies between some few % to 45% of the amount of bacterially generated H 2 S. This finding permits calculation of the original organic carbon (TOC or ) content of immature nonbioturbated shales using TOC and sulphur content data. In two immature non-bioturbated sequences from Hungary (Toarcian and Oligocene) the first-order correlation between HI and TOC/TOC or was found to be stronger than that between HI and TOC, indicating that sulphate reduction was the leading process both in decrease in TOC content and degradation of kerogen source potential.

Estimation of primary productivity in the Toarcian tethys- A novel approach based on TOC, reduced sulphur and manganese contents

Abstract About 60 samples from the non-bioturbated Urkut Manganese Ore Formation (Bakony Mountains, Hungary), representing the early Toarcian anoxic event, were studied for: (1) TOC content, Rock-Eval parameters and kerogen δ 13 C; (2) S content and δ 34 S and (3) Al, Ca, Mn, Si and Ti contents. Using the example of the formation, a novel approach for estimating paleoproductivity is presented, taking into account reduced sulphur and manganese contents, which reflect the amount of organic carbon lost during early burial. Drastic changes in Mn content (between 0.7% and 37%) during deposition of the formation resulted in very variable early diagenetic processes, with sulphate reduction dominating in low-Mn sediments, and Mn reduction dominating in high-Mn sediments. The amounts of reduced sulphur and manganese allowed calculation of the original organic carbon (TOC or ) content throughout the formation. Based on stratigraphic variations in chemical composition, changes in the rate of sedimentation were assessed for the three main members of the formation. Next, using the formula describing the relationship between measured productivity, rate of sedimentation and carbon flux reaching the sea-floor, developed by Suess (1980) , past productivity was calculated. The values obtained, lower than those prevailing in present-day upwelling systems, are probably underestimated because of uncertainties of chronostratigraphic and water-depth data. However, relative differences between productivity values obtained for the three members are considered as realistic and suggest an increase in productivity during deposition of the formation. This increase is not related to stratigraphic variations in Mn content and is accompanied by an increase in the growth of calcareous plankton.

Paleoenvironments, source rock potential and thermal maturity of the Upper Benue rift basins, Nigeria: implications for hydrocarbon exploration

Abstract The Upper Benue rift comprising the Gongola and Yola Basins in Nigeria consist of the Aptian-Albian Bima Formation, the Yolde Formation (Cenomanian-Turonian), Gongila/Pindiga/Dukul Formation (Turonian-Coniacian) and Gombe Formation (Campanian-Maastrichtian). To evaluate the maturity and source rocks potential, vitrinite reflectance, Rock-Eval pyrolysis and infrared spectroscopy were carried out on 52 shale samples collected from boreholes, mine quarries and outcrop sections. In the Gongola Basin, mean random vitrinite reflectance (Rom) values range from 0.45% in the Gombe Formation to 0.69% in the Pindiga Formation and to 0.82% in the Bima Formation. Reflectance values in the Yola Basin also increase with stratigraphic age ranging from 0.73% in the Dukul Formation to 0.94% in the Yolde Formation and up to 1.37% in the Bima Formation. Total organic carbon (TOC) values in the Pindiga and Gongila Shales are between 0.4 to 2.4% averaging 0.75%. TOC contents from 0.10 to 12.9 averaging 1.2% are contained in the Yolde Formation of the Yola Basin. Tmax values from the pyrolysis of shales in the Gongola Basin range from 419 to 435°C whereas for shales in the Yola Basin they range from 431 to 442°C. Plots of HI vs Tmax for kerogen classification indicate the prevalence of Type III kerogens in the Gongila and Pindiga Shales although there are some indications of Type II–III kerogens in the Yolde Shales of the Yola Basin. Our preliminary data suggest that Cretaceous successions in the Gongola Basin are thermally immature to marginally mature whereas source rocks in the Yola Basin are thermally mature with respect to hydrocarbon generation. The predominance of Type III kerogens in the Gongola Basin suggest their potential to generate gas in the deeply buried sections. The Dukul and Yolde formations with Type II–III kerogens may have generated some quantities of oil and gas in the deeper non-emergent sections.

Origin of variations in organic matter abundance and composition in a lithologically homogeneous maar-type oil shale deposit (Gerce, Pliocene, Hungary)

Despite having an homogeneous lithology, the largest Hungarian maar-type deposit (Gerce oil shale, Pliocene) has previously been shown to exhibit substantial variations in organic matter quantity and quality with depth. This het- erogeneity is also reflected, in the present study, by large variations in bitumen abundance and composition, for 23 samples from Gerce well-6 core. Based on the above bitumen data, four samples were selected that were representative of the whole set which exhibit contrasting features. Scanning and transmission electron microscopy showed the occurrence of extensively altered Botryococcus colonies in this deposit. GC/MS and GC-C-ir-MS of the saturated hydrocarbon fractions of the bitumen of these samples reveal a predominant algal contribution along with a variable bacterial input. The relative abundance of these two contributions in the four selected samples is also reflected by dif- ferences in FTIR and solid-state 13 C NMR spectra of the isolated kerogens. Curie point pyrolysis/GC/MS of these kerogens revealed a relatively high terrestrial contribution in one sample and confirmed the variable input of algae and bacteria. The above diAerences in relative contributions account for the variations in organic matter quantity and quality observed along the core. # 2000 Elsevier Science Ltd. All rights reserved.

Simulated thermal maturation of type I and III kerogens in the presence, and absence, of calcite and montmorillonite

Abstract The effects of a mineral matrix on hydrocarbon generation from different types of kerogen were examined. Simulated thermal maturation of type I ∗ , III/a and III/b kerogens, alone and mixed with minerals, was investigated using anhydrous pyrolysis.Both calcite and montmorillonite modified the products of simulated thermal maturation of the kerogens. Montmorillonite slightly reduced, and calcite slightly enhanced the formation of oil-like products from the coals (type III/a and III/b kerogens). At higher levels of maturation, however, the catalytic effect of calcite resulted in cracking of the pyrolysates. For retention of the pyrolysis products of type I kerogen, montmorillonite proved to be a very active matrix whereas the products were less affected by calcite. The amount of soluble bitumen extracted from heated type I kerogen was higher in the presence of minerals than in their absence. On the contrary, the amount of soluble bitumen obtained from heated type III/a kerogen was smaller in the presence of minerals. In the case of type III/b kerogens this effect of minerals changed as a function of the temperature of simulated thermal maturation.

Primary productivity and early diagenesis in the Toarcian Tethys on the example of the Mn-rich black shales of the Sachrang Formation, Northern Calcareous Alps

Organic, inorganic and isotope geochemical investigations have been performed on 25 samples - 20 laminated marl samples and 5 re-sediment samples - taken from a 27m thick black shale section of early Toarcian age, cropping out in the Northern Calcareous Alps. On the basis of MnO, Fe, organic carbon (TOC) contents and Hydrogen Index (HI) values, the laminated marls of the section can be divided into two parts. TOC contents and HI values are below 2% and 500 mg HC/g TOC in the lower, Mn-rich (and Fe-rich) part (MnO between 2 and 16%, Fe between 3.9 and 6.4%), while they range between 5 and 9% and 600-700 mg HC/g TOC, respectively in the upper, Mn-poor (and Fe-poor) part (MnO < 1%, Fe between 1.5 and 3.8%). Both original amount of organic carbon (TOC or ), calculated from TOC, MnO and sulphur contents, and CaO concentration show a strong upward increase in the lower, Mn-Fe-rich part of the section. TOC or and CaO are believed to represent planktonic particles hence their simultaneous upward increases are interpreted as the result of an at least 2.5 fold increase of productivity during the first part of the black shale event. Following the same logic, in the second part of the black shale event productivity is believed to have slightly decreased from the previously reached high level. Stratigraphic variation of the sulphur isotopic ratio support this scenario. Comparison of organic geochemical and δ 34 S data of the re-sediments with those of the neighbouring marls suggests that increase and decrease (?) of productivity was paralleled by expansion and withdrawal of oxygen-depleted waters. During the black shale event the extent of the oxygen-depleted bottom water was governed by changes in intensity of productivity. Changes in rates of deposition of Mn and Fe were not related to those of the productivity but they deeply influenced nature and intensity of bacterial degradation of organic matter and especially the incorporation of sulphur into kerogen. The high contribution of Mn- and Fe-minerals in the case of the lower Mn-rich part prevented by both dilution and degradation of OM due to Mn-reduction an intense incorporation of sulphur into OM. In the upper, Mn- (and iron) poor part the high initial C org /Fe ratio led to an important incorporation of sulphur into OM and an early termination of sulphate reduction.

Organic geochemistry of an anoxic cycle: A case history from the Oligocene section, Hungary

Total organic carbon, extractable organic matter, sulfur content, kerogen type and saturated hydrocarbons have been determined in 25 core samples from an approximately 100 m thick section of the Oligocene Tard Clay, near Budapest. The Tard Clay records a five million year long anoxic cycle initiated by isolation of the sea, development of a positive water balance and resulting density stratification, and terminated by the re-opening of access to the sea. The lower, non- and moderately laminated shales record the build-up of anoxia, they contain 0.5–5.0% of organic carbon and Type III kerogen. Their saturated hydrocarbon fractions show high abundance of phyllocladanes, biomarkers characterising Coniferae-derived organic matter. The overlying moderately and strongly laminated sediments containing 1.5–5.0% organic carbon, a predominance of hopanes over the n-alkanes and Type II kerogen show the imprint of increased planktonic productivity. The nutrients needed for this high productivity were liberated by alteration processes of contemporaneous volcanic glass. The upper part of the strongly laminated shales containing a maximum of 2% organic carbon and Type III kerogen, deposited after the termination of volcanic ash falls, records a drop of planktonic productivity. At the top of the Tard Clay the weak lamination and the low content (<1%) of organic carbon signifies the end of the bottom water oxygen depletion due to the destruction of density stratification of the water column.