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Hubert Wierzbowski - One of the best experts on this subject based on the ideXlab platform.
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seawater temperatures and carbon isotope variations in central european basins at the middle late jurassic transition late Callovian early kimmeridgian
Palaeogeography Palaeoclimatology Palaeoecology, 2015Co-Authors: Hubert WierzbowskiAbstract:Abstract Oxygen and carbon isotope values and elemental ratios of well-preserved belemnite rostra, brachiopod shells and bulk-carbonates from the Upper Callovian–Lower Kimmeridgian of the Polish Jura Chain, Kujawy (Poland) and Swabian Alb (Germany) are investigated to reconstruct environmental conditions and perturbations in the marine carbon cycle. Belemnite δ18O values show relatively constant temperatures (ca. 12 °C) of bottom waters in the Polish Jura Chain basin during a major part of the Late Callovian–Middle Oxfordian, except for a short-term cooling (to ca. 9 °C) at the Callovian–Oxfordian transition. Belemnite and brachiopod δ18O values show a gradual increase in temperature during the Submediterranean Late Oxfordian; the highest temperatures (ca. 16 °C) are calculated for the Submediterranean Oxfordian–Kimmeridgian transition. Belemnite and brachiopod Mg/Ca and Sr/Ca ratios are disregarded as a paleotemperature proxy because of their weak correlation with δ18O values. The belemnite and brachiopod isotope data confirm that the carbon isotope composition of belemnite rostra is affected by a metabolic effect, which results in a depletion of belemnite calcite in the 13C isotope. Belemnite rostra are considered, nevertheless, as a valuable tracer of temporal variations in the carbon isotope composition of marine carbonates. Belemnite δ13C data show the presence of two positive excursions (in the Upper Callovian and the Middle Oxfordian) in the carbon isotope record of peri-Tethyan carbonates. The excursions are divided by a Lower Oxfordian interval characterized by decreased δ13C values. This is most likely a regional feature caused by upwelling. Lowest belemnite and brachiopod δ13C values are observed in the lower part of the Submediterranean Upper Oxfordian and are linked to a well-mixed state of the seawater in the basins studied. The carbon isotope record of bulk carbonates differs from those of belemnites and brachiopods probably because of strong variations in carbonate production in the Polish Jura Chain basin.
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middle upper jurassic upper Callovian lower kimmeridgian stable isotope and elemental records of the russian platform indices of oceanographic and climatic changes
Global and Planetary Change, 2013Co-Authors: Hubert Wierzbowski, M A Rogov, Bronis£aw A.matyja, D N Kiselev, Alexei P IppolitovAbstract:Abstract New isotope (δ18O, δ13C) and elemental (Mg/Ca, Sr/Ca) data of well-preserved belemnite rostra, ammonite and gastropod shells from the Middle Oxfordian–Lower Kimmerdgian (Densiplicatum–Kitchini zones) of the Russian Platform are presented. This record is supplemented with published data from the Upper Callovian–Lower Kimmeridgian interval (Athleta–Kitchini zones). Significant differences in average temperatures calculated from δ18O values of particular fossil groups (5–15 °C) show the thermal gradient and the presence of cold bottom waters in the Middle Russian Sea during the Late Callovian–Middle Oxfordian. An Upper Oxfordian–lowermost Kimmeridgian decrease in δ18O values and an increase in Sr/Ca ratios of cylindroteuthid belemnite rostra likely reflect a warming of the bottom waters of ca. 3.5 °C. The gradual Late Oxfordian–earliest Kimmeridgian warming is followed by an abrupt temperature rise of 3–6 °C that occurred at the transition of the Early Kimmeridgian Bauhini and Kitchini chrons. The occurrences of cold bottom waters and of (Sub)Mediterranean ammonites and belemnites in the Middle Russian Sea at the Middle–Late Jurassic transition are regarded as a result of the opening of seaways during a sea level highstand. The bottom waters are considered to have been formed in the cool Boreal Sea. The subsequent retreats of the cold bottom waters and of the (Sub)Mediterranean cephalopods from the Middle Russian Sea in the Late Oxfordian are explained by the restriction of water circulation during a sea-level fall. The Early Kimmeridgian rise of bottom temperatures of the sea is linked to a global climate warming. The data presented do not support a major cooling of the Arctic and a consequent glaciation in this region at the Middle–Late Jurassic transition. Since occurrences of cold water masses are diachronous in different European basins, the observed variations in sea water temperatures are interpreted as a result of changes in marine currents and water circulation. δ13C values of belemnite rostra from the Russian Platform are scattered but show the long-term Upper Callovian–Middle Oxfordian positive excursion consistent with the previously published isotope records of the Boreal Realm and terrestrial organic matter.
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strontium isotope variations in middle jurassic late bajocian Callovian seawater implications for earth s tectonic activity and marine environments
Chemical Geology, 2012Co-Authors: Hubert Wierzbowski, Robert Anczkiewicz, J Bazarnik, Jacek PawlakAbstract:Abstract Strontium isotope ratios of well-preserved belemnite rostra and ammonite shells are presented from stratigraphically well-dated Upper Bajocian–Callovian sediments of the Polish Jura Chain in central Poland. Nekto-benthic belemnites show a narrow trend of changes of 87 Sr/ 86 Sr ratio that are interpreted to record secular variations in the strontium isotope composition of seawater. A new fit of the seawater strontium isotope curve for the Late Bajocian–Callovian is presented based on current and literature data. The data show a gradual decrease of seawater 87 Sr/ 86 Sr ratio throughout the Late Bajocian–earliest Middle Callovian, lowest 87 Sr/ 86 Sr ratios (0.706850) and the flat minimum of the curve were reached in the latest Middle and the Late Callovian. The Middle Jurassic decrease in seawater 87 Sr/ 86 Sr ratio is interpreted to have been a result of the increased volcanic activity of the seafloor. Studied nektonic ammonites are considered to have lived in surface or shallow near-coastal waters of the Polish Jura Chain basin and been affected by the freshwater influx. A slight enrichment of ammonite shells in 87 Sr isotope is linked to a change in strontium isotope composition of the surface or near-coastal waters due to the riverine strontium input.
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oxygen and carbon isotope composition of Callovian lower oxfordian middle upper jurassic belemnite rostra from central poland a record of a late Callovian global sea level rise
Palaeogeography Palaeoclimatology Palaeoecology, 2009Co-Authors: Hubert Wierzbowski, Krzysztof Dembicz, Tomasz PraszkierAbstract:Abstract Oxygen and carbon isotope data of well-preserved and stratigraphically well-dated belemnite rostra are presented from a Callovian–Lower Oxfordian succession of the Polish Jura Chain (south-central Poland) that was deposited in a deep shelf environment. Palaeotemperatures calculated from belemnite δ 18 O values (9 to 14 °C) indicate relatively constant climatic conditions during studied Callovian and Early Oxfordian intervals. Belemnite δ 13 C temporal trends, which are interpreted to record primary variation in the isotope composition of dissolved inorganic carbon reservoir, display a prominent positive carbon isotope shift (up to around 1.7‰ VPDB) in the Upper Callovian. This excursion is preceded and followed by a drop in average δ 13 C values of belemnite rostra of around 1‰. The Late Callovian carbonate carbon isotope excursion is likely linked to the enhanced storage of organic carbon in shelf sediments during a global sea-level rise. The presented data are not compatible with the theory of prolonged global glaciation at the Middle–Late Jurassic transition.
Annachiara Bartolini - One of the best experts on this subject based on the ideXlab platform.
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Integrated stratigraphy of the Oxfordian global stratotype section and point (GSSP) candidate in the Subalpine Basin (SE France)
Volumina Jurassica, 2014Co-Authors: Pierre Pellenard, Jacques Thierry, Didier Marchand, Annachiara Bartolini, Slah Boulila, Bruno Galbrun, Pierre-yves Collin, Dominique Fortwengler, Raymond Enay, Silvia GardinAbstract:An integrated biostratigraphic approach, based on ammonites, calcareous nannofossils, dinoflagellates, combined with sedimentology, carbon-isotope and physical stratigraphy, is proposed for the Subalpine Basin (Thuoux and Saint-Pierre d'Argençon sections). Within the expanded marl deposits of the Terres Noires Fm., the Callovian-Oxfordian boundary is particularly well defined by ammonite taxa from different families (i.e. Cardioceratidae, Oppeliidae, Aspidoceratidae and Perisphinctidae), calcareous nannoplankton (first occurrence of large-sized Stephanolithion bigotii) and dinoflagellate cysts (first occurrence of Wanaea fimbriata). This precise biostratigraphy and diversity of ammonites permit long-range correlations with Boreal and Pacific domains. The exceptional outcrop conditions and continuous sedimentation allow high-resolution chemostratigraphy and cyclostratigraphy to be used. Variations of the δ13C record (1.5‰ increase during the Lamberti Zone), consistent with data from other sections of the Subalpine Basin and with the Paris Basin, provide additional markers to characterise the Callovian-Oxfordian boundary. Promising results from cyclostratigraphy should serve to constrain the duration of biostratigraphic units and thus improve the Late Jurassic Geologic Time Scale. Comparisons are made with the two other candidates for an Oxfordian GSSP, Redcliff Point (UK) and Dubki (Russian Platform). Finally, the advantages of the Subalpine Basin sections support the proposal of Thuoux as a suitable GSSP candidate for the base of the Oxfordian Stage.
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Paleoclimatic control of biogeographic and sedimentary events in Tethyan and peri-Tethyan areas during the Oxfordian (Late Jurassic)
Palaeogeography Palaeoclimatology Palaeoecology, 2005Co-Authors: F. Cecca, B Martin Garin, Didier Marchand, Bernard Lathuilière, Annachiara BartoliniAbstract:Abstract The paleobiogeographical distribution of Oxfordian ammonites and coral reefs in northern and Central Europe, the Mediterranean area, North and East Africa, and the Middle East and Central Asia is compared with the distribution in time and space of the most important lithofacies. Interest in the Oxfordian is focused on changes in facies and in biogeographical patterns that can be interpreted as the results of climatic events. Paleotemperature trends inferred from oxygen isotopes and paleoclimatic simulations are tested against fossil and facies data. A Late Callovian–Early Oxfordian crisis in carbonate production is indicated by the widespread absence of Lower Oxfordian reefal formations. There is a gap (hiatus) in deposition on epicontinental platforms, with Middle Oxfordian deposits resting paraconformably on Upper Callovian, while shales accumulated in adjacent intracratonic basins. Simultaneously, in Mediterranean Tethys, radiolarites accumulated in deep troughs while Rosso Ammonitico facies formed on pelagic swells. However, deposition on swells was also discontinuous with numerous gaps (hiatuses) and sequences that are much reduced in thickness. Middle Callovian deposits are generally overlain by Middle Oxfordian limestones. The dearth of carbonates is consistent with a cooling event lasting about 1 My. By the middle Oxfordian a warming, leading to “greenhouse” type conditions, is suggested on the basis of both biogeographical (mostly coral-reef distribution) and geochemical data. Carbonates spread onto an extensive European platform while radiolarites reached a maximum development in the Mediterranean Tethys. Two distinct latitudinal belts, with seemingly different accumulation regimes, are therefore inferred. Similar latitudinal belts were also present in the late Oxfordian, when carbonates were widespread. The distribution of reefal facies in the late Oxfordian–early Kimmeridgian fits relatively well with GCMs simulations that imply low rainfall in the Tethyan Mediterranean area and slightly higher precipitation in central and northern Europe. Local salinity variations, reflecting more arid or humid conditions, may bias the paleotemperature signal inferred from δ18O values. Biogeographical and facies distributions, combined with δ18O values, unravel the ambiguity and support a Late Callovian–Early Oxfordian cooling followed by warming in the later Oxfordian.
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middle and late jurassic radiolarian palaeoecology versus carbon isotope stratigraphy
Palaeogeography Palaeoclimatology Palaeoecology, 1999Co-Authors: Annachiara Bartolini, Jean GuexAbstract:Abstract Jurassic radiolarians are sensitive to palaeoclimatic–palaeoceanographic changes, recorded by the stable carbon-isotope curve. Carbon-isotopic variations have been linked to changes in ocean structure, productivity and concentrations of greenhouse gases in the atmosphere. In the Middle–Upper Jurassic Terminilletto section of the Umbria–Marche–Sabina Apennines (central Italy), well preserved radiolarians are present in a sequence of cherty limestones interbedded with platform-derived carbonates. The δ 13 C curve in the Aalenian–Kimmeridgian interval indicates three main positive δ 13 C `events' in the lower Bajocian, late Bathonian–lower Callovian and middle Oxfordian. The positive δ 13 C shifts are correlated to increases in visible chert in the outcrop and may represent periods of increased biological productivity of surface sea-water. Spumellaria/Nassellaria ratios and the fluctuations of relative abundance of selected genera tend to correlate with δ 13 C and chert abundance values, and seem to be related to episodes of high nutrient mobilisation. From the base of the Bajocian, the radiolarian associations show a slight tendency towards diversification, coincident with increasing δ 13 C values. In the upper part of the middle Bajocian, last appearances prevail over first appearances associated with the peak of the δ 13 C event. This may indicate maximum eutrophic conditions. Coincident with the late Bathonian–early Callovian positive δ 13 C shift, the radiolarian associations record a different trend with respect to the Bajocian event. When the δ 13 C values are increasing to a maximum, radiolarian associations tend to be reduced in species diversity, followed by a small diversification trend when the δ 13 C values are decreasing. We speculate that during the Callovian a more `stressful' palaeoenvironment, due to high level of eutrophication, is indicated by the combination of high productivity of biogenic silica and low carbonate input. Although varying preservational conditions may have influenced the observed trends in the radiolarian associations, the good correlation between changes of sedimentation patterns, radiolarian assemblages and δ 13 C positive anomalies, open interesting avenues for further investigations concerning the palaeoecology of Mesozoic radiolarians.
Alain Trouiller - One of the best experts on this subject based on the ideXlab platform.
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facies and climate environmental changes recorded on a carbonate ramp a sedimentological and geochemical approach on middle jurassic carbonates paris basin france
Sedimentary Geology, 2009Co-Authors: Benjamin Brigaud, Jacques Thierry, Christophe Durlet, Jeanfrancois Deconinck, Benoit Vincent, Emmanuelle Puceat, Alain TrouillerAbstract:A detailed sedimentological, geochemical and mineralogical study is carried out on the Early Bajocian to Early Callovian (Middle Jurassic) limestones of the Paris Basin. Objectives are to document and explain the facies changes in the context of the climate/environmental evolution at the NW European scale. Deposits include 18 lithofacies which are stacked into 10 third-order depositional sequences. At a greater time scale, 4 biosedimentary packages are distinguished by their allochems associations. (1) An intracratonic carbonate environment with coral reefs and crinoid-rich facies is typical of the Early Bajocian. (2) A major facies change occurred at the Early/Late Bajocian transition with a shift from crinoid- and coral-rich facies to ooid-rich facies. During the Late Bajocian, a southward-dipping ooid ramp with successive progradational trends was emplaced. (3) A large lagoon protected by ooid shoals developed during the Bathonian in a muddy ramp setting. (4) At the Bathonian/Callovian boundary, a second major change occurred with the lagoonal facies being superseded by an ooid-bioclastic (crinoids, corals) ramp associated with a waning of carbonate productivity (retrogradational trend). Our geochemical study including a compilation of bibliographic data allows us to refine the existing sea surface palaeotemperature pattern. A cooling from the latest Early Bajocian to the Late Bajocian and a subsequent warming from the earliest Callovian to the Early/Middle Callovian transition are described using a proxy of sea surface temperatures. Together with δ13C and mineralogical data, our new δ18O values suggest a palaeo-climatic/palaeo-environmental control of facies in this shallow carbonate ramp environment.
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vascular plant biomarkers as proxies for palaeoflora and palaeoclimatic changes at the dogger malm transition of the paris basin france
Organic Geochemistry, 2006Co-Authors: Yann Hautevelle, Raymond Michels, Fabrice Malartre, Alain TrouillerAbstract:Vascular plant biomarkers are used here as a chemostratigraphic tool for tracing both palaeoflora and palaeoclimatic changes during the Callovian and Oxfordian in the Paris Basin. The retene/cadalene ratio shows low values in claystones from the Middle Callovian to the end of the Lower Oxfordian, then increases drastically from the end of the Lower Oxfordian (cordatum Zone) synchronously with the progressive development of the Oxfordian carbonate platform. The relative proportion of retene, a conifer biomarker, is strongly correlated with the abundance of some of its diagenetic precursors and with saturated diterpanes. Some of these are very common and abundant in the Pinaceae family while specific markers for non-Pinaceae conifers are not detected. Thus, the increase in the retene/cadalene ratio around the cordatum Zone reflects the rising proportion of Pinaceae on the London-Brabant Massif, which was the nearest land from the locations studied. As Pinaceae show many morphological adaptations to dry climate compared to the other plants reported in the Jurassic taphofloras of the Paris basin, their increasing proportion indicates an increasing aridity. This climatic change is also supported by published geochemical, palaeobiogeographical, palynological, palaeobotanical and mineralogical data. A comparable increase in the relative proportion of retene was also described for the Oxfordian of Western Australia, which could be an argument for a worldwide increase in aridity at the end of the Lower Oxfordian.
D N Kiselev - One of the best experts on this subject based on the ideXlab platform.
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the potential of petromagnetic methods in paleoecological reconstructions based on the example of jurassic deposits Callovian lower oxfordian of the mikhailovtsement section ryazan region
Moscow University Geology Bulletin, 2019Co-Authors: I A Stepanov, M A Rogov, D N Kiselev, Ekaterina Tesakova, Yu A Kazansky, L R Kosareva, E V Shchepetova, Ya A ShurupovaAbstract:Based on a detailed integrated petromagnetic, lithological, and micropaleontological study of the Mikhailovtsement reference section of the Moscow syneclise (Ryazan Region), the fluctuations in the Middle Russian Sea level in the Callovian–Early Oxfordian were reconstructed. According to the variations of petromagnetic parameters through the section, seven petromagnetic intervals corresponding to different stages of the paleobasin evolution were established. These stages are correlated with sea level fluctuations established on the basis of changes in the rock lithology and the ostracod complexes. In general, the character of sea level change in the Callovian–Oxfordian that was revealed during the study of the Mikhailovtsement section is in agreement with the global trends.
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middle upper jurassic upper Callovian lower kimmeridgian stable isotope and elemental records of the russian platform indices of oceanographic and climatic changes
Global and Planetary Change, 2013Co-Authors: Hubert Wierzbowski, M A Rogov, Bronis£aw A.matyja, D N Kiselev, Alexei P IppolitovAbstract:Abstract New isotope (δ18O, δ13C) and elemental (Mg/Ca, Sr/Ca) data of well-preserved belemnite rostra, ammonite and gastropod shells from the Middle Oxfordian–Lower Kimmerdgian (Densiplicatum–Kitchini zones) of the Russian Platform are presented. This record is supplemented with published data from the Upper Callovian–Lower Kimmeridgian interval (Athleta–Kitchini zones). Significant differences in average temperatures calculated from δ18O values of particular fossil groups (5–15 °C) show the thermal gradient and the presence of cold bottom waters in the Middle Russian Sea during the Late Callovian–Middle Oxfordian. An Upper Oxfordian–lowermost Kimmeridgian decrease in δ18O values and an increase in Sr/Ca ratios of cylindroteuthid belemnite rostra likely reflect a warming of the bottom waters of ca. 3.5 °C. The gradual Late Oxfordian–earliest Kimmeridgian warming is followed by an abrupt temperature rise of 3–6 °C that occurred at the transition of the Early Kimmeridgian Bauhini and Kitchini chrons. The occurrences of cold bottom waters and of (Sub)Mediterranean ammonites and belemnites in the Middle Russian Sea at the Middle–Late Jurassic transition are regarded as a result of the opening of seaways during a sea level highstand. The bottom waters are considered to have been formed in the cool Boreal Sea. The subsequent retreats of the cold bottom waters and of the (Sub)Mediterranean cephalopods from the Middle Russian Sea in the Late Oxfordian are explained by the restriction of water circulation during a sea-level fall. The Early Kimmeridgian rise of bottom temperatures of the sea is linked to a global climate warming. The data presented do not support a major cooling of the Arctic and a consequent glaciation in this region at the Middle–Late Jurassic transition. Since occurrences of cold water masses are diachronous in different European basins, the observed variations in sea water temperatures are interpreted as a result of changes in marine currents and water circulation. δ13C values of belemnite rostra from the Russian Platform are scattered but show the long-term Upper Callovian–Middle Oxfordian positive excursion consistent with the previously published isotope records of the Boreal Realm and terrestrial organic matter.
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Stratigraphy of the Bathonian-Callovian boundary deposits in the Prosek section (Middle Volga Region). Article 1. Ammonites and infrazonal biostratigraphy
Stratigraphy and Geological Correlation, 2007Co-Authors: D N Kiselev, Mikhail A. RogovAbstract:In European Russia, the most complete succession of Boreal sediments of the terminal Bathonian and lower Callovian is exposed near the Prosek Settlement. After its revision, the infrazonal division of the upper Bathonian and lower Callovian and position of the Bathonian-Callovian boundary are difined more carefully. The Calyx Zone and bodylevskyi Biohorizon are established in the upper Bathonian. The base of the lower Callovian is defined at the first occurrence level of Macrocephalites jacquoti. Based on four successive ammonite assemblages occurring in lower part of the Elatmae Zone, the breve, frearsi, quenstedti, and elatmae biohorizons are identified. The joint occurrence of Boreal, Subboreal, and Tethyan ammonites in the section facilitate its correlation with the other sections of the Panboreal paleobiogeographic superrealm.
Michal Zaton - One of the best experts on this subject based on the ideXlab platform.
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diverse sclerozoan assemblages encrusting large bivalve shells from the Callovian middle jurassic of southern poland
Palaeogeography Palaeoclimatology Palaeoecology, 2011Co-Authors: Michal Zaton, Mark A Wilson, Elyse ZavarAbstract:A diverse sclerozoan assemblages consisting of both encrusting and boring biota are described from the large limid bivalves Ctenostreon from the Callovian hardground setting of Zalas in southern Poland. At least 27 encrusting and seven bioerosion taxa are reported here, which makes this assemblage not only one of the most diverse in the Middle Jurassic, but the richest in encrusting taxa from the Callovian. The encrusters consist of cryptic biota of which sedentary polychaetes and cyclostome bryozoans dominate with respect to both species number and abundance. The bioerosion traces are dominated by tiny pits referred to the ichnogenus Oichnus, probably made by some soft-bodied biota in the present case, followed by the borings of acrothoracican barnacles (Rogerella). The first colonizers of the bivalve shells probably were borers as they only occur in the host shells. The encrusting pioneers presumably were oysters and oyster-like bivalves, followed by opportunistic serpulid/sabellid polychaetes and cyclostome bryozoans. The last colonizers were calcisponges and thecideide brachiopods. In comparison to the only known Late Callovian shallow and reef-associated, tropical sclerozoans of Israel, the assemblage from the open-marine, deeper setting of Poland is much richer in encrusting taxa. Such a surprising high encruster diversity in the marine northern paleo-latitude may have resulted from the deeper and calmer environment with a both reduced sedimentation rate and algal cover, and without any significant salinity changes, the factors that are thought to have impacted the tropical and shallow-marine sclerozoans from Israel.
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organic matter from the Callovian middle jurassic deposits of lithuania compositions sources and depositional environments
Applied Geochemistry, 2010Co-Authors: Leszek Marynowski, Michal ZatonAbstract:Abstract This study presents the first organic geochemical and petrographical investigation of the Callovian deposits of the eastern part of the Central European Basin. It is shown that in both the terrigenous Papilė Formation (Lower Callovian) and shallow- to deeper-marine facies of the Papartinė and Skinija formations (Middle and Upper Callovian, respectively), terrestrial organic matter predominates. This is reflected by the carbon preference index values higher than 1.2 for all samples and in some cases higher than 2, as well as the occurrence of characteristic higher plant biomarkers like cadalene, dehydroabietane, simonellite and retene. Moreover, in the case of the Papilė Formation, sugiol – a natural product terpenoid produced by distinct conifer families, has been detected in clay sediments. The occurrence of such a biomolecule in the Middle Jurassic clays is reported for the first time. Its occurrence is probably connected with the presence of small wood debris in the clay sediments. In samples of the Papilė Formation, charcoal fragments co-occurring with unsubstituted polycyclic aromatic hydrocarbons were detected, indicating that wildfires took place during the Early Callovian of Lithuania and/or neighbouring areas. In the Middle and Upper marine Callovian sediments of Lithuania there is no evidence of anoxic conditions occurring in the water column. However, periodic anoxic or strongly dysoxic episodes may have occurred, most probably below the photic zone, during the deepest phase of the Late Callovian transgression, as is evidenced from pyrite framboid diameter distribution and general impoverishment of benthic fauna. Huminite reflectance ( R r ) values for the investigated area are in the range of 0.21–0.31%, suggesting the occurrence of immature organic matter. Such values indicate that these investigated deposits were close to the surface during their whole diagenetic history, and the thickness of younger cover did not exceed ca. 500 m. This is also supported by a biomarker analysis in which less thermally stable ββ -hopanes and hopenes significantly dominated.
