Carbon cycle history through the Middle Jurassic (Aalenian – Bathonian) of the Mecsek Mountains, Southern Hungary

AbstractA carbonate carbon isotope curve from the Aalenian–Bathonian interval is presented from the Óbánya valley, of the Mecsek Mountains, Hungary. This interval is certainly less well constrained and studied than other Jurassic time slices. The Óbánya valley lies in the eastern part of the Mecsek Mountains, between Óbánya and Kisújbánya and provides exposures of an Aalenian to Lower Cretaceous sequence. It is not strongly affected by tectonics, as compared to other sections of eastern Mecsek of the same age. In parts, a rich fossil assemblage has been collected, with Bathonian ammonites being especially valuable at this locality. The pelagic Middle Jurassic is represented by the Komló Calcareous Marl Formation and thin-bedded limestones of the Óbánya Limestone Formation. These are overlain by Upper Jurassic siliceous limestones and radiolarites of the Fonyászó Limestone Formation. Our new data indicate a series of carbon isotope anomalies within the late Aalenian and early-middle Bajocian. In particular, analysis of the Komló Calcareous Marl Formation reveals a negative carbon isotope excursion followed by positive values that occurs near the base of the section (across the Aalenian–Bajocian boundary). The origin of this carbon-isotope anomaly is interpreted to lie in significant changes to carbon fluxes potentially stemming from reduced run off, lowering the fertility of surface waters which in turn leads to lessened primary production and a negative δ13C shift. These data are comparable with carbonate carbon isotope records from other Tethyan margin sediments. Our integrated biostratigraphy and carbon isotope stratigraphy enable us to improve stratigraphic correlation and age determination of the examined strata. Therefore, this study of the Komló Calcareous Marl Formation confirms that the existing carbon isotope curves serve as a global standard for Aalenian–Bathonian δ13C variation.</jats:p

(Table 1, page 661) Average composition of fossil nodules from three localities studied in the Transdanubian Central Mountains of Hungary

Fossil manganese nodules and encrustations from seamount' and basin' localities in the Transdanubian Central Mountains of Hungary are lithologically, mineralogically and chemically similar to some modern marine ferromanganese oxide deposits, and show no evidence of postdepositional changes other than cementation. Five groups of deposits were encountered: Fe/Mn nodules, encrusted shells, pavements, stains, and Fe oxide encrusted intraclasts, the first three of which are specific to the 'seamount' environment and the last to the basins'. Optical and electron microprobe investigation of the samples shows them to exhibit many similarities with modern ferromanganese oxide deposits, and that many of the nodules are surrounded by a halo of dispersed ferromanganese oxides, strongly suggesting that they continued to accrete metals through the pore waters of unlithified sediments for a period after burial. By contrast, pavements which appear to have grown on hardgrounds at the sea floor show little or no evidence of derivation of metals from underlying sediments. Geochemical investigations on the deposits show the seamount' varieties to be closer in composition to most modern nodules and crusts than the basin' varieties, and that the latter are essentially manganese and trace-element-poor ferruginous deposits. Nevertheless, all can be more or less compositionally equated with modern ferromanganese oxide deposits forming in marginal Atlantic environments, which would be in accord with the proposed depositional environment of the Transdanubian Central Mountains based on other evidence

Dimorphism and evolution of Albarracinites (Ammonoidea, Lower Bajocian) from the Iberian Range (Spain)

Several tens of specimens of Lower Bajocian Albarracinites (type species A. albarraciniensis Fernandez-Lopez, 1985), including microconchs and macroconchs from the Iberian Range, have been studied. This ammonite genus ranges in the Iberian Range from at least the Ovale Zone to the uppermost Laeviuscula Zone of the Lower Bajocian (Middle Jurassic). The macroconch counterpart is thought to be a group of stephanoceratids previously attributed to Mollistephanus, Riccardiceras and other new forms described in this paper. Two chronologically successive species of Albarracinites have been identified: A. albarraciniensis and A. submediterraneus sp. nov. The evolution of the Albarracinites lineage represents a hypermorphic peramorphocline starting from depressed, small and slender serpenticones of A. westermanni, to larger planorbicones with more cadiconic phragmocones and body chamber of subcircular cross section belonging to A. submediterraneus sp. nov., through A. albarraciniensis Fernandez-Lopez. In contrast, Mollistephanus planulatus (Buckman), M. cockroadensis Chandler & Dietze and M. mollis Buckman represent a peramorphocline by acceleration, producing adults of similar size but more compressed and with increasing ontogenic variation of shell ornament. Albarracinites and Mollistephanus subsequently developed two opposite peramorphoclines or gradational series of morphological changes undergoing greater development and ontogenic variation. These two genera show diverse palaeobiogeographical distributions too. Albarracinites is rarely recorded in the Mediterranean and Submediterranean from the Discites to the Laeviuscula Zone, whereas Mollistephanus is more common in north-western Europe and other biochoremas of the western Tethys from the Discites Zone to the Sauzei Zone. Albarracinites seems to be the earliest stephanoceratid lineage in western Tethys, branching off from the otoitid Riccardiceras by proterogenetic change and resulting in paedomorphosis at the Aalenian/Bajocian boundary