Abstract. Bulk-carbonate carbon isotope measurements are a widely applied proxy for investigating the ancient biogeochemical carbon cycle. Temporal carbon isotope trends serve as a prime stratigraphic tool, with the inherent assumption that bulk micritic carbonate rock is a faithful geochemical recorder of the isotopic composition of seawater dissolved inorganic carbon. However, bulk-carbonate rock is also prone to incorporate diagenetic signals. The aim of the present study is to disentangle primary trends from diagenetic signals in carbon isotope records which traverse the Permian–Triassic boundary in marine carbonate-bearing sequences of Iran and South China. By pooling newly produced and published carbon isotope data we confirm that a global first-order trend towards depleted values exists. However, a large amount of scatter is superimposed on this geochemical record. In addition, we observe a temporal trend in the amplitude of this residual δ13C variability, which is reproducible for the two studied regions. We suggest that (sub)seafloor microbial communities and their control on calcite nucleation as well as on ambient porewater dissolved inorganic carbon-δ13C pose a viable mechanism to induce bulk-rock δ13C variability. Numerical model calculations highlight that early diagenetic carbonate rock stabilization, and linked carbon isotope alteration, can be controlled by organic matter supply and subsequent microbial remineralization. Low marine sulfate and a major biotic decline among late Permian bottom-dwelling organisms facilitated a spatial increase in heterogeneous organic carbon accumulation, causing varying degrees of carbon isotope overprint. A simulated time series suggests that a 50 % increase in the spatial scatter of organic carbon relative to the average, in addition to an imposed increase in the likelihood of sampling cements formed by microbial calcite nucleation to one out of 10 samples, is sufficient to induce the observed signal of carbon isotope variability. These findings put constraints on the application of Permian–Triassic carbon isotope chemostratigraphy based on whole-rock samples, which appears less refined than classical biozonation dating schemes. On the other hand, this signal of increased carbon isotope variability concurrent with the largest mass extinction of the Phanerozoic may inform about local carbon cycling mediated by spatially heterogeneous (sub)-seafloor microbial communities under suppressed bioturbation.
Two limestone bone-beds in the early Frasnian of the Chahriseh section, central Iran, yielded numerous chondrichthyan teeth and scales. The fauna includes, most probably, only two taxa: a hitherto unknown aztecodontid omalodontiform, Manberodus fortis gen. et sp. nov., and a multicuspid phoebodontiform, provisionally referred to as Phoebodus cf. latus GINTER & IVANOV, 1995. A new omalodontiformfamily, Aztecodontidae, including Aztecodus LONG& YOUNG, 1995 and Manberodus gen. nov., is proposed.
Հոդվածում քննարկվում են Փոքր Կովկասի վերին դևոն-ստորին քարածխային նստվածքային հաջորդականություններից հայտնի Paraparchitidae և Geisinidae ընտանիքներին պատկանող օստրակոդների տեսակները` հաշվի առնելով ժամանակակից տաքսոնոմիական դասակարգումները և թարմացված շերտագրական կորելացիաները: Այս օստրակոդները հայտնի են Հայաստանի և Նախիջևանի վերին ֆամենից մինչև ստորին վիզե։ Հատկանշական է, որ Geisinidae-ները հայտնի են վերին ֆամենի Paurogastroderhynchus nalivkini բրախիոպոդային զոնայից մինչև վերին տուրնեի Spirifer baiani–Marginatia burlingtonensis բրախիոպոդային զոնա: Paraparchitidae-ները հայտնի են Paurogastro-derhynchus nalivkini բրախիոպոդային զոնայից մինչև ստորին վիզե: Մեր վերանայումն ընդգծում է Փոքր Կովկասի վերին ֆամեն-ստորին վիզեի հաջորդականություններից Չիժովայի կողմից նկարագրված paraparchitid և geisinid օստրակոդների ընտանիքների տեսակների տաքսոնոմիական և շերտագրական առանջնակատկությունները։ Ի վերջո, մենք քննարկում ենք նրանց գլոբալ շերտագրական և հնակենսաշխարհագրական տարածվածությունը՝ այս պահի դրությամբ հայտնի տվյալներով։ В данной статье рассматриваются виды остракод из семейств Paraparchitidae и Geisinidae,известные из верхнедевонских и нижнекарбоновых осадочных толщ Малого Кавказа, с учётом современных таксономических системизаций и обновлённых стратиграфических корреляций. Эти остракоды известны из верхнего фамена донижнего визе Армении и Нахичевани. Примечательно, что Geisinidae известны из верхнефаменской брахиоподовой зоны Paurogastroderhynchus nalivkini до верхнетурнейской зоны Spirifer baiani–Marginatia burlingtonensis. Paraparchitidae известны из зоны брахиоподовой зоны Paurogastroderhynchus nalivkini до нижнего визе. Наш обзор подчеркивает отличительные характеристики и обновленное стратиграфическое распространение видов остракодиз семейств Paraparchitidae и Geisinidae, выявленных Чижовой из верхнефаменских–нижневизейских последовательностях Малого Кавказа. Наконец, мы обсуждаем их глобальное стратиграфическое и палеобиогеографическое распространение, известное на данный момент. A review of the paraparchitid and geisinid ostracod species known from the Upper Devonian–Lower Carboniferous sequences of the Lesser Caucasus is conducted here with respect to modern taxonomic assignments and updated stratigraphic correlations. These ostracods are known from the upper Famennian–lower Visean successions of Armenia and Nachichevan. Most notably, Geisinidae are known from the upper Famennian Paurogastroderhynchus nalivkini Brachiopod Zone to the upper Tournaisian Spirifer baiani–Marginatia burlingtonensis Brachiopod Zone. Paraparchitidae are known from the Paurogastroderhynchus nalivkini Brachiopod Zone to the lower Visean. Our review highlights the distinguishing characters and updated stratigraphic occurrence of the paraparchitid and geisinid ostracod species documented by Tschigova in the upper Famennian–lower Visean successions of the Lesser Caucasus. Finally, we discuss their global stratigraphic and palaeobiogeographic distribution known so far.
Abstract. The Devonian reef limestone complex of Rösenbeck near Brilon (Rhenish Mountains) shows numerous neptunian dykes and other hollows which have been filled with Carboniferous siliciclastic as well as fossil-rich carbonate sediments with ammonoids, conodonts, and chondrichthyan fish. While the shales of the infill can be interpreted as autochthonous sediments, the carbonates represent erratic blocks of sediments which were deposited in elevated areas but subsequently eroded and transported as erratic blocks into the karstic cavities. The biota of the carbonates demonstrates that two stratigraphic intervals are represented: (1) the Tournaisian–Viséan boundary interval, which can be identified by the presence of trilobites, and (2) the late Viséan (Asbian–Brigantian), proven by the presence of ammonoids and conodonts embedded in shales that probably have an early Serpukhovian age. Two carbonate facies can be distinguished in the Late Viséan blocks: (1) low-energy micritic sediments with Goniatites crenistria, which are probably equivalent to the crenistria Limestone and (2) high-energy (tempestite) sediments with numerous ammonoid specimens and in a more subordinate role other to fossil groups.
Abstract The devastating end-Permian mass extinction is widely considered to have been caused by large-scale and rapid greenhouse gas release by Siberian magmatism. Although the proximate extinction mechanisms are disputed, there is widespread agreement that a major extinction pulse occurred immediately below the biostratigraphically defined Permian–Triassic boundary. Our statistical analyses of stratigraphic confidence intervals do not comply with a single end-Permian extinction pulse of ammonoids in Iran. High turnover rates and extinction pulses are observed over the last 700 k.y. of the Permian period in two widely separated sections representative of a larger area. Analyses of body sizes and morphological complexity support a gradual decline over the same interval. Similar pre–mass extinction declines and disturbances of the carbon cycle have sometimes been reported from other regions, suggesting a widespread, but often overlooked, environmental deterioration at a global scale, well before the traditional main extinction pulse.
Abstract Sections at Baghuk Mountain, 45 km NNW of Abadeh (Central Iran), have excellent exposures of fossiliferous marine Late Permian to Early Triassic sedimentary successions. Detailed bed-by-bed sampling enables the analysis of microfacies changes of three successive rock units across the Permian–Triassic boundary. The Late Permian Hambast Formation is mainly the result of biogenic carbonate production. Its carbonate microfacies is dominated by biogen-rich and bioturbated nodular limestones, indicating a well-oxygenated aphotic to dysphotic environment. The biogen-dominated carbonate factory in the Permian ceased simultaneously with the main mass extinction pulse, which is marked by a sharp contact between the Hambast-Formation and the overlaying Baghuk Member (= ‘Boundary Clay’). The clay and silt deposits of the Baghuk Member with some carbonate beds show only a few signs of bioturbation or relics of benthic communities. The Early Triassic Claraia Beds are characterised by a partly microbially induced carbonate production, which is indicated by frequent microbialite structures. The depositional environment does not provide evidence of large amplitude changes of sea level or subaerial exposure during the Permian–Triassic boundary interval. The deposition of the Baghuk Mountain sediments took place in a deep shelf environment, most of the time below the storm wave base.
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