Ocean acidification and photic‐zone anoxia at the Toarcian Oceanic Anoxic Event: Insights from the Adriatic Carbonate Platform

Severe global climate change led to the deterioration of environmental conditions in the oceans during the Toarcian Stage of the Jurassic. Carbonate platforms of the Western Tethys Ocean exposed in Alpine Tethyan mountain ranges today offer insight into this period of environmental upheaval. In addition to informing understanding of climate change in deep time, the effect of ancient carbon cycle perturbations on carbonate platforms has important implications for anthropogenic climate change; the patterns of early Toarcian environmental deterioration are similar to those occurring in modern oceans. This study focuses on the record of the early Toarcian Oceanic Anoxic Event (ca 183.1 Ma) in outcrops of the north‐west Adriatic Carbonate Platform in Slovenia. Amidst environmental deterioration, the north‐west Adriatic Platform abruptly transitioned from a healthy, shallow‐water environment with diverse metazoan ecosystems to a partially drowned setting with low diversity biota and diminished sedimentation. An organic carbon‐isotope excursion of ?2.2‰ reflects the massive injection of CO₂ into the ocean‐atmosphere system and marks the stratigraphic position of the Toarcian Oceanic Anoxic Event. A prominent dissolution horizon and suppressed carbonate deposition on the platform are interpreted to reflect transient shoaling of the carbonate compensation depth to unprecedentedly shallow levels as the dramatic influx of CO₂ overwhelmed the ocean’s buffering capacity, causing ocean acidification. Trace metal geochemistry and palaeoecology highlight water column deoxygenation, including the development of photic‐zone anoxia, preceding and during the Toarcian Oceanic Anoxic Event. Ocean acidification and reduced oxygen levels likely had a profoundly negative effect on carbonate‐producing biota and growth of the Adriatic Platform. These effects are consistent with the approximate doubling of the concentration of CO₂ in the ocean‐atmosphere system from pre‐event levels, which has previously been linked to a volcanic triggering mechanism. Mercury enrichments discovered in this study support a temporal and genetic link between volcanism, the Toarcian Oceanic Anoxic Event and the carbonate crisis.     

Pacing of the Toarcian Oceanic Anoxic Event (Early Jurassic) from astronomical correlation of marine sections

The Early Toarcian Oceanic Anoxic Event (OAE) in the Early Jurassic Period is associated with a major negative carbon isotope excursion (CIE), mass extinction, marine transgression and global warming. The Toarcian OAE is thought to have been caused by flood basalt magmatism, and may have been a trigger for mass extinction. However, these proposed causes of the Toarcian OAE and associated biotic crisis are not adequately resolved by a precise chronology. The duration of the Toarcian OAE has been estimated to be anywhere from ~ 0.12 to ~ 0.9 Myr, most recently 0.74 to 3.26 Myr from U–Pb dating. The CIE associated with the Toarcian OAE has a similar pattern at numerous localities, and there is evidence that the marine carbon isotope variations recorded astronomical forcing signals. Here we estimate a duration of ~ 620 kyr for the main negative CIE, ~ 860 kyr for the polymorphum zone and > 1.58 Myr for the levisoni zone based on 405-kyr astronomical eccentricity tuning of the marine section at Peniche (Portugal). This 405-kyr tuned series provides a ~ 2.5 Myr continuous high-resolution chronology through the Early Toarcian. There are 6, or possibly 7 short eccentricity cycles in the main CIE interval at Peniche. To confirm this astronomically based estimate, we analyzed three other sections at Yorkshire (UK), Dotternhausen (Germany), and Valdorbia (Italy) from marine carbon isotopic series. These four stratigraphic sections from Early Jurassic western Tethys record the Toarcian OAE with ~ 6 prominent carbon isotope cycles in the CIE that span a 600 ± 100 kyr duration. The Peniche 405-kyr tuned series indicates that the pre- and post-CIE intervals experienced strong precession–eccentricity-forced climate change, whereas the CIE interval is marked by dominant obliquity forcing. These dramatic and abrupt changes in astronomical response in the carbon isotopes point to fundamental shifting in the Early Toarcian paleoclimate system that was directly linked to the global carbon cycle.     

Organic Carbon Isotope Geochemistry of the Neoproterozoic Doushantuo Formation, South China

The Neoproterozoic Doushantuo Formation on the Yangtze Platform, South China, documents a sedimentary succession with different sedimentary facies from carbonate platform to slope and to deep sea basin, and hosts one of the world-class phosphorite deposits. In these strata, exquisitely preserved fossils have been discovered： the Weng＇an biota. This study presents carbon isotope geochemistry which is associated paired carbonate and organic matter from the Weng＇an section of a carbonate platform （shelf of the Yangtze Platform, Guizhou Province） from the Songtao section and Nanming section of a transition belt （slope of the Yangtze Platform, Guizhou Province） and from the Yanwutan section （basin area of the Yangtze Platform, Hunan Province）. Environmental variations and bio-events on the Yangtze Platform during the Late Neoproterozoic and their causal relationship are discussed. Negative carbon isotope values for carbonate and organic carbon （mean δ^13Corg = -35.0‰） from the uppermost Nantuo Formation are followed by an overall increase in δ^13C up-section. Carbon isotope values vary between -9.9‰ and 3.6‰ for carbonate and between -35.6‰ and -21.5‰ for organic carbon, respectively. Heavier δ^13Ccarb values suggest an increase in organic carbon burial, possibly related to increasing productivity （such as the Weng＇an biota）. The δ^13C values of the sediments from the Doushantuo Formation decreased from the platform via the slope to basin, reflecting a reduced environment with minor dissolved inorganic carbon possibly due to a lower primary productivity. It is deduced that the classical upwelling process, the stratification structure and the hydrothermal eruption are principally important mechanisms to interpret the carbon isotopic compositions of the sediments from the Doushantuo Formation.     

Evaluation of bulk carbonate δ13C data from Triassic hemipelagites and the initial composition of carbonate mud

Bulk carbonate samples of hemipelagic limestone–marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane‐bedded limestone–marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between δ¹³C and δ¹⁸O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane‐bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane‐bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 μm in diameter), whereas samples of the plane‐bedded facies are composed of calcite crystals ca 10 μm in size showing pitted, polished surfaces. These observations suggest that nodular and plane‐bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high‐Mg calcite in the nodular facies and aragonite in the case of the plane‐bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated δ¹³C and δ¹⁸O values and exhibit small, less than 5 μm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate δ¹³C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low‐Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing δ¹³C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies.     

Neptunian dykes along a drowned carbonate platform margin: an indication for recurrent extensional tectonic activity?

Three successive Mesozoic neptunian dyke generations and related unconformities suggest recurrent extensional fracturing and periods of relative sea-level rise along the NW Trento Plateau margin in the Southern Alps, Italy. The first neptunian dyke generation was induced by NNW–SSE directed extension of Early Jurassic skeletal oolitic periplatform deposits generating micritic early Middle Liassic neptunian dykes with orthogonal orientation. The second generation of neptunian dykes was possibly caused by marginal extension at the drowned platform edge penetrating Late Jurassic, red pelagic limestones with a pelagic matrix of Albian/Cenomanian age and nearly orthogonal fracture orientation. The third generation of neptunian dykes occurred after a prolonged period of submarine exposure and erosion (Aptian/Albian to Late Maastrichtian) during the rapid burial of the submarine Trento Plateau margin relief. The Late Maastrichtian neptunian dykes were caused by extension of Early to Middle Jurassic oolitic periplatform limestones along steep (inclination > 10°) submarine slopes. Generally successive neptunian dyke generations along drowned carbonate platform margins could be caused by repeated extensional brittle fracturing of lithified periplatform deposits and the filling of micritic matrix derived from overlying pelagic sediment sequences under substantial hydrostatic pressure. This would suggest that recurrent extensional fracturing is continuously recorded by neptunian dyke formation which could be used to indicate extensional tectonic activity at a foundering deep-marine carbonate platform edge.     

Sr chemostratigrphy, δ<Superscript>13</Superscript>C,and δ<Superscript>18</Superscript>O of rocks in the Crimean carbonate platform (Late Jurassic,northern Peri-Tethys)

The paper presents the results of study of the Sr, C, and O isotope compositions in Upper Jurassic carbonate rocks of the Baidar Valley and Demerdzhi Plateau in the Crimean Mountains represented by different facies of the carbonate platform at the northern active margin of the Tethys. The ⁸⁷Sr/⁸⁶Sr value in them varies from 0.70699 to 0.70728. Based on the Sr chemostratigraphic correlation, the age of massive and layered limestones in the western part of the Ai-Petri and Baidar yailas (pastures) is estimated as late Kimmeridgian–early Tithonian, whereas the age of flyschoids of the Baidar Valley are estimated as late Tithonian–early Berriasian. The nearly synchronous formation of carbonate breccias of the Baidar Valley and Demerdzhi Plateau in late Tithonian–early Berriasian is substantiated. A summary section of Upper Jurassic rocks is compiled based on the Sr chemostratigraphic data. It has been established that δ¹⁸O values in the studied carbonate sediments vary from–2.9 to 1.3‰ (V-PDB). At the same time, shallow-water sediments in the internal part and the edge of the Crimean carbonate platform are depleted in ¹⁸O (–2.9 to +0.1‰) relative to sediments on the slope and foothill (–0.5 to +1.3‰). It is demonstrated that δ¹³C values do not depend on the facies properties and decrease in younger carbonate sediments from 3–3.5‰ to 1–1.5‰ in line with the Late Jurassic general trend. The δ¹³C values obtained for the Crimean carbonate platform turned out to be 0.5–1‰ higher than the values typical of the deep-water marine setting at the western margin of the Tethys. These discrepancies are likely related to peculiarities of water circulation and high bioproductivity in marine waters of the northern Peri-Tethys.     

Sedimentation in shallow to deep water carbonate environments across a sequence boundary: effects of a fall in sea-level on the evolution of a carbonate system (Ladinian-Carnian, eastern Lombardy, Italy)

In the Concarena‐Pizzo Camino Massif (Lombardy Basin, Southern Alps, Italy) the lateral transition from Ladinian‐Carnian carbonate platforms to coeval intraplatform basins is preserved. The succession records the sedimentological evidence of a sea‐level fall on a flat‐topped platform with a narrow marginal reef rim and its effects in the adjacent deeper‐water basin. Repeated high‐frequency exposures of the platform top are recorded by a peritidal–supratidal succession that overlies subtidal inner platform facies of the former highstand system tract (HST). On the slope and in the basin, the sea‐level fall is recorded by a few metre thick succession of bioclastic packstones. These facies directly lie on coarse clinostratified breccia bodies (slope facies of the former HST) or on resedimented, well‐bedded, dark laminated limestones (basinal facies of the HST). This facies distribution indicates that during the sea‐level fall carbonate production on the platform top decreased rapidly and that sedimentation in the basin was mainly represented by condensed facies. Microfacies record an enrichment, during low stand, in pelagic biota (packstones with radiolarians and spiculae), whereas the occurrence of platform‐derived, shallow‐water materials is limited to thin lenses of reworked and micritized Fe‐rich oolites and bioclasts (mainly pelecypods and echinoderms). The facies association in the Concarena‐Pizzo Camino Massif demonstrates that a highly‐productive carbonate factory was almost completely turned off during the emergence of the platform top at a sequence boundary, leading to low‐stand starvation in the basin. The reconstruction of the stratigraphic evolution of the Concarena‐Pizzo Camino carbonate platform therefore represents a significant case history for the study of the behaviour of ancient carbonate systems during a fall in sea‐level, independent of its origin (eustatic or tectonic).     

Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia

Thick Aptian deposits in north central Tunisia comprise hemipelagic lower Aptian, reflecting the sea-level rise of OAE 1a, and an upper Aptian shallow marine environment characterized by the establishment of a carbonate platform facies. Carbon stable isotope data permit recognition of the OAE 1a event in the Djebel Serdj section. Cephalopods are rare throughout these successions, but occurrences are sufficient to date the facies changes and the position of the OAE1a event. Ammonite genera include lower Aptian Deshayesites, Dufrenoyia, Pseudohaploceras, Toxoceratoides and ?Ancyloceras; and upper Aptian Zuercherella, Riedelites and Parahoplites. Correlation of carbon isotope data with those of other Tethyan sections is undertaken together with the integration of planktonic foraminiferal data.     

Spatial variability of watermass conditions within the European Epicontinental Seaway during the Early Jurassic (Pliensbachian–Toarcian)

In order to constrain spatial variability in watermass conditions within the European Epicontinental Seaway prior to, during and after the Toarcian Oceanic Anoxic Event, carbon (δ¹³C_bel, δ¹³C_carb) and oxygen (δ¹⁸O_bel, δ¹⁸O_carb) isotope records were obtained from three sections in the Grands Causses Basin (southern France). These data were then compared with similar records along a north–south transect across the European Epicontinental Seaway. As the conclusions reached here strongly depend on the reliability of belemnite calcites as archives of palaeoceanographic changes, an attempt was made to improve the understanding of isotope signals recorded in belemnite calcite. Intra‐rostral carbon and oxygen‐isotope data from six belemnite specimens belonging to the genus Passaloteuthis were collected. Intra‐rostral carbon‐isotopes are influenced by vital effects, whereas oxygen‐isotopes reflect relative changes in temperature and salinity. Palaeotemperatures calculated from δ¹⁸O_bel‐isotope records from the Grands Causses Basin confirm relatively low temperatures throughout the Late Pliensbachian. Similar cool water conditions have previously been shown in Germany, England, Spain and Portugal. A temperature increase of up to 6 °C is observed across the Pliensbachian–Toarcian boundary. A pronounced negative shift of at least ?3‰ (Vienna‐Pee Dee Belemnite) is recorded in bulk carbonate carbon during the lower Harpoceras serpentinum zone, typical of the Toarcian Oceanic Anoxic Event. Before and after the Toarcian Oceanic Anoxic Event, a good correlation between δ¹³C_carb and δ¹³C_bel exists, indicating well‐ventilated bottom‐waters and normal marine conditions. Instead, data for the Toarcian Oceanic Anoxic Event indicate the development of a strong north–south gradient in salinity stratification and surface‐water productivity for the Western Tethyan realm. This study thus lends further support to a pronounced regional overprint on carbon and oxygen‐isotope records in epicontinental seaways.     

Early Jurassic climatic trends in the south-Tethyan margin

The Early Jurassic period was characterized by extreme environmental changes, as reflected by major global carbon isotope anomalies and abrupt changes in oxygen isotope and elemental records of marine organisms. Available data suggest an overall warm Early Jurassic climate interrupted by periods of severe cooling, with a climatic optimum during the early Toarcian. Available geochemical studies, however, have mainly focused on the northern margin of the Tethys Ocean, so that the palaeogeographic extent of these environmental perturbations, latitudinal palaeotemperature gradients and climate belt boundaries remain poorly constrained. Here we report the first stable isotope records of brachiopod shells (δ¹³C and δ¹⁸O values) from the Upper Sinemurian-Middle Toarcian interval in the southern margin of the Tethys Ocean (northwest Algeria). These data were used to better constrain the palaeoenvironmental evolution of the North Gondwana margin during the Early Jurassic, which likely played an important role on supra-regional climate. The diagenetic history of the analysed brachiopod shells was monitored using scanning electron microscopy, and elemental (manganese and strontium) compositions. The brachiopod δ¹³C and δ¹⁸O data show very similar trends as those reported for various Tethyan regions, and record negative carbon and oxygen isotope excursions near the Sinemurian–Pliensbachian and Pliensbachian–Toarcian transitions and during the Toarcian oceanic anoxic event (T-OAE). Despite these similarities, the carbon and oxygen isotope records are systematically offset towards more positive δ¹³C values (average +0.5‰) and more negative δ¹⁸O values (−1.0‰) compared to those obtained from sites of higher palaeolatitudes in the northern Tethyan margin. These offsets suggest a spatial heterogeneity in the stable isotope composition of dissolved inorganic carbon in the Early Jurassic Ocean and a marked latitudinal temperature gradient between the southern and northern margins of the Tethys.     

Latitudinally different responses of Tethyan shoal‐water carbonate systems to the Early Aptian oceanic anoxic event (OAE 1a)

Ocean‐wide anoxic events represent intensively investigated anomalies in the global carbon cycle. Most previous research has focussed on hemipelagic and pelagic settings and on the relationship between black‐shale deposition and carbon‐isotope excursions. The study of ocean‐wide anoxic events and coeval shallow‐water settings is now increasingly seen as an interesting complementary approach, but one that is not without problems. Whereas platform drowning characterizes the Early Aptian of the northern Tethyan margin, Lithocodium–Bacinella‐rich facies and ongoing shoal‐water sedimentation at the southern Tethyan margin (Oman) bears important information on potential causes of carbon‐cycle perturbations. The present paper seeks to test the supra‐regional relevance of the Oman data by investigating coeval central Tethyan limestones. Three Lower Aptian shoal‐water sections in Istria (Croatia), deposited on the isolated Adriatic Carbonate Platform, are investigated applying chemostratigraphy (carbon and strontium) and detailed sedimentological analysis. The focus is on peritidal to lagoonal facies characterized by mass occurrences of Lithocodium–Bacinella, an enigmatic microencruster community. Lithocodium–Bacinella facies occurs predominantly in layers ranging from one to several centimetres in stratigraphic thickness, with several layers merging to metre‐thick packages. Growth fabrics within the layers include oncoidal morphotypes, lumps, interconnected patches and columns, layers and rare nodular to massive bindstone facies. These growth patterns show a remarkable regional extent and consistency over study sites distributed several kilometres apart. This widespread distribution suggests that specific Lithocodium–Bacinella morphotypes might serve as regional stratigraphic markers. The high‐resolution carbon‐isotope chemostratigraphy presented here is based on pristine rudist shells and matrix micrite samples and calibrated against strontium‐isotope data obtained from screened rudist low‐Mg calcite. The chemostratigraphic data are consistent with existing biostratigraphic data and place the studied strata at the onset of Early Aptian oceanic anoxic event 1a. Moreover, results indicate the near‐coeval nature of Lithocodium–Bacinella bloom facies in Istria and Oman. The outcomes of this study point to latitudinally different responses of Tethyan shoal‐water carbonate systems (platform drowning versus Lithocodium–Bacinella blooms) to the ocean‐wide anoxic event 1a.     

Primary and diagenetic isotopic signals in fossils and hemipelagic carbonates: the Lower Jurassic of northern Spain

Stable isotope and trace element analyses of 230 Jurassic (Pliensbachian–Toarcian) samples from northern Spain have been performed to test the use of geochemical variations in fossils (belemnites and brachiopods) and whole‐rock hemipelagic carbonates as palaeoceanographic indicators. Although the succession analysed (Reinosa area, westernmost Basque–Cantabrian Basin) has been subject to severe thermal alteration during burial diagenesis, the samples appear to be well preserved. The degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic, chemical and cathodoluminescence analyses. It is demonstrated that brachiopods and whole‐rock carbonates, although widely used for palaeoceanic studies, do not retain their primary marine geochemical composition after burial diagenesis. In contrast, there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock. Well‐preserved belemnite shells (non‐luminescent to slightly luminescent) typically show stable isotope values of +4·3‰ to –0·7‰δ¹³C, +0·7‰ to –3·2‰δ¹⁸O, and trace element contents of <32 μg g^–1 Mn, <250 μg g^–1 Fe, >950 μg g^–1 Sr and Sr/Mn ratios >80. This study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low‐Mg calcite fossil shells and hemipelagic bulk carbonates, behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated. Multiple non‐luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders, yet their δ¹³C and δ¹⁸O values exhibited moderate scatter. Such variability is likely to be related to the palaeoecological behaviour of belemnites and/or high‐frequency secular variations in sea‐water chemistry superimposed on the long‐term isotopic trend. A pronounced positive carbon‐isotope excursion (up to +4·3‰) is documented in the early Toarcian serpentinus biozone, which correlates with the Toarcian δ¹³C maximum reported in other European and Tethyan regions.     

Expression of the Toarcian Oceanic Anoxic Event: New insights from a Swiss transect

A sedimentological, biostratigraphical and geochemical (stable isotopes and Rock‐Eval parameters) analysis was performed on four Swiss successions, in order to examine the expression of the Toarcian Oceanic Anoxic Event along a north–south transect, from the Jura through the Alpine Tethys (Sub‐Briançonnais and Lombardian basins). The locations were selected to represent a range of palaeoceanographic positions from an epicontinental sea to a more open marine setting. The Toarcian Oceanic Anoxic Event was recognized by the presence of the characteristic negative carbon‐isotope excursion in carbonate (ca 2 to 4‰) and organic matter (ca 4 to 5‰) at the base of the falciferum ammonite Zone (NJT6 nannofossil Zone). The sedimentary expression of the Toarcian Oceanic Anoxic Event varies along the transect from laminated mudstone rich in total organic carbon (≤11 wt.%) in the Jura, to thin‐bedded marl (≤5 wt.% total organic carbon) in the Sub‐Briançonnais Basin and to hemipelagic reddish marly limestone (total organic carbon <0·05 wt.%) in equivalent levels from the Lombardian Basin. The carbon‐isotope excursion is thus independent of facies and palaeoceanographic position. The low nannofossil abundance and the peak in Calyculaceae in the Jura and the Sub‐Briançonnais Basin indicate low salinity surface waters and stratified water masses in general. Sedimentological observations (for example, obliquely‐bedded laminae and homogeneous mud layers containing rip‐up clasts) indicate the presence of dynamic conditions, suggesting that water mass stratification was episodically disrupted during the Toarcian Oceanic Anoxic Event. The proposed correlation highlights a stratigraphic gap and/or condensed interval between the Pliensbachian–Toarcian boundary and the Toarcian Oceanic Anoxic Event interval (most of the tenuicostatum ammonite Zone is missing), which is also observed in coeval European sections and points to the influence of sea‐level change and current dynamics. This transect shows that the sedimentary expression of the Toarcian Oceanic Anoxic Event is not uniform across the Alpine Tethys, supporting the importance of local conditions in determining how this event is recorded across different palaeoceanographic settings.     

Climatic and tectonic controls on carbonate deposition in syn‐rift siliciclastic fluvial systems: A case of microbialites and associated facies in the Late Jurassic

This work provides new insights to assess the factors controlling carbonate deposition in the siliciclastic fluvial systems of rift basins. Sedimentological and stable‐isotope data of microbialites and associated carbonate facies, along with regional geological information, are shown to reveal the influence of climate and tectonics on the occurrence and attributes of carbonate deposits in these settings. The Vega Formation – a 150 m thick Lower Kimmeridgian siliciclastic fluvial sequence in Asturias Province (northern Spain) – constitutes a candidate for this approach. This unit includes varied facies (stromatolites; rudstones, packstones and wackestones containing oncoids, intraclasts, charophytes and shell bioclasts; marlstones and polygenic calcareous conglomerates) that formed in a low‐gradient fluvial–lacustrine system consisting of shallow, low‐sinuosity oncoid‐bearing channels and pools within marshy areas, with sporadic coarse alluvial deposition. The sedimentological attributes indicate common erosion by channel overflow and rapid lateral changes of subenvironments caused by water‐discharge variations. The carbonate fluvial–lacustrine system developed near uplifted marine Jurassic rocks. The occurrence of the system was conditioned by normal faults (active during the deposition of the unit) that favoured: (i) springs of HCO₃–Ca‐rich water from a Rhaetian–Sinemurian carbonate rock aquifer; and (ii) carbonate deposition in areas partially isolated from the adjacent siliciclastic fluvial system. The microbialite δ¹³C and δ¹⁸O values support deposition in a hydrologically open system, fed by ambient‐temperature meteoric water, with riparian vegetation. Three types of lamination in the stromatolites and oncoids reflect distinct morphological types of cyanobacterial communities. The textural pattern of lamination parallels δ¹³C and δ¹⁸O changes, suggesting short‐term cycles of precipitation and temperature. A moderately to strongly contrasted seasonal and/or pluriannual precipitation regime is inferred from the cyclic δ¹³C pattern of the lamination and from the discontinuous and asymmetrical growth of oncoids. Thus, the isotopic and sedimentological attributes of the carbonate deposits were linked to short‐term climate changes associated with semi‐arid conditions, consistent with the studied climatic zone.     

Integrating outcrop data and forward computer modelling to unravel the development of a Messinian carbonate platform in SE Spain (Sorbas Basin)

Geological mapping, definition of facies distributions and reconstruction of platform‐interior growth geometries of the Messinian Cariatiz carbonate platform (Sorbas basin, South Spain), were performed to evaluate the controlling factors in platform growth and to test a 3‐D computer simulation program. For the simulation with the program REPRO, five platform‐related facies were modelled: (1) the reef crest facies by the numerical solution of a Fisher equation; (2) the lagoonal facies by a function of water depth‐dependent carbonate production; (3) the proximal and middle slope facies (breccia and block facies, calcarenite facies) by a subroutine simulating gravity‐driven particle export from the reef crest; (4) a distal slope; and (5) a basinal facies by a pelagic rain function. Development of a fan delta conglomeratic system is simulated by using a siliciclastic point source and gravity‐driven particle redistribution. A best fit between the observed platform growth geometries and modelling results is achieved by assuming that high‐frequency sea‐level changes superimposed onto a longer term sea‐level fall controlled platform growth. For the modelling, a relative sea‐level curve was reconstructed, which is based on a deep‐sea benthic foraminiferal stable oxygen isotope record at ODP Site 926 with a 45 m eustatic sea‐level fall, and a tectonic uplift component of 20 m. The consistency of 3‐D simulation results is corroborated by the coral growth rates provided by the Fisher‐equation subroutine. These rates of 2–8 mm year⁻¹ compare well to the coral growth rates in Recent fringing reefs. We propose that during the early stage of platform evolution the high‐frequency fluctuations were obliquity‐modulated precessional cycles, whereas precessional cycles control later stages of platform growth. REPRO provides a separate visualization of the different facies bodies as a function of time and space, showing the intrinsic pattern of facies distribution in the platform. This is the result of a combination of platform growth and syndepositional subaerial erosion. For example, only the youngest stages of reef framework facies in the development of the Cariatiz carbonate platform are preserved.     

Ocean current intensification during the Cretaceous oceanic anoxic event 2 – evidence from the northern Tethys

Mesozoic Oceanic Anoxic Events (OAEs) are expressions of major physical oceanographic changes at times of perturbation of the global carbon cycle. A northern Tethyan record of OAE2 is preserved in expanded Cenomanian–Turonian pelagic limestone sections (Seewen Formation) in Eastern Switzerland. The new carbonate carbon‐isotope stratigraphy extracted from these limestones demonstrates that the OAE2 is condensed in all the studied successions and only the onset of the δ¹³C excursion (5.0‰) is present. The condensed interval is characterized by dissolution features, which are filled by a glauconite quartz sandstone. This bed is overlain by a well‐sorted sandstone with intercalated limestone pebbles (Götzis Member), which can be compared with palimpsest sands forming today along current‐swept shelves. The wide distribution of this thin sandstone layer within OAE2 indicates that an intense, erosive, east‐west trending shelf current was active during the highest sea level and most extreme carbon‐cycle perturbation of the OAE2.     

The Late Jurassic Tithonian, a greenhouse phase in the Middle Jurassic–Early Cretaceous 'cool' mode: evidence from the cyclic Adriatic Platform, Croatia

Well‐exposed Mesozoic sections of the Bahama‐like Adriatic Platform along the Dalmatian coast (southern Croatia) reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding Late Jurassic (Tithonian) shallow platform‐interior (over 750 m thick, ca 5–6 Myr duration). Facies within parasequences include dasyclad‐oncoid mudstone‐wackestone‐floatstone and skeletal‐peloid wackestone‐packstone (shallow lagoon), intraclast‐peloid packstone and grainstone (shoal), radial‐ooid grainstone (hypersaline shallow subtidal/intertidal shoals and ponds), lime mudstone (restricted lagoon), fenestral carbonates and microbial laminites (tidal flat). Parasequences in the overall transgressive Lower Tithonian sections are 1–4·5 m thick, and dominated by subtidal facies, some of which are capped by very shallow‐water grainstone‐packstone or restricted lime mudstone; laminated tidal caps become common only towards the interior of the platform. Parasequences in the regressive Upper Tithonian are dominated by peritidal facies with distinctive basal oolite units and well‐developed laminate caps. Maximum water depths of facies within parasequences (estimated from stratigraphic distance of the facies to the base of the tidal flat units capping parasequences) were generally <4 m, and facies show strongly overlapping depth ranges suggesting facies mosaics. Parasequences were formed by precessional (20 kyr) orbital forcing and form parasequence sets of 100 and 400 kyr eccentricity bundles. Parasequences are arranged in third‐order sequences that lack significant bounding disconformities, and are evident on accommodation (Fischer) plots of cumulative departure from average cycle thickness plotted against cycle number or stratigraphic position. Modelling suggests that precessional sea‐level changes were small (several metres) as were eccentricity sea‐level changes (or precessional sea‐level changes modulated by eccentricity), supporting a global, hot greenhouse climate for the Late Jurassic (Tithonian) within the overall ‘cool’ mode of the Middle Jurassic to Early Cretaceous.     

Primary dolomite in the Late Triassic Travenanzes Formation,Dolomites, Northern Italy: Facies control and possible bacterial influence

In the late Carnian (Late Triassic), a carbonate‐clastic depositional system including a distal alluvial plain, flood basin and sabkha, tidal flat and shallow carbonate lagoon was established in the Dolomites (Northern Italy). The flood basin was a muddy supratidal environment where marine carbonates and continental siliciclastics interfingered. A dolomite phase made of sub‐micrometre euhedral crystals with a mosaic microstructure of nanometre‐scale domains was identified in stromatolitic laminae of the flood basin embedded in clay. This dolomite is interpreted here as primary and has a nearly stoichiometric composition, as opposed to younger early diagenetic (not primary) dolomite phases, which are commonly calcian. This primary dolomite was shielded from later diagenetic transformation by the clay. The stable isotopic composition of dolomite was analyzed along a depositional transect. The δ¹³C values range between ca ?6‰ and +4‰, with the most ¹³C‐depleted values in dolomites of the distal alluvial plain and flood basin, and the most ¹³C‐enriched in dolomites of the tidal flat and lagoon. Uniform δ¹⁸O values ranging between 0‰ and +3‰ were found in all sedimentary facies. It is hypothesized that the primary dolomite with mosaic microstructure nucleated on extracellular polymeric substances secreted by sulphate reducing bacteria. A multi‐step process involving sabkha and reflux dolomitization led to partial replacement and overgrowth of the primary dolomite, but replacement and overgrowth were facies‐dependent. Dolomites of the landward, clay‐rich portion of the sedimentary system were only moderately overgrown during late dolomitization steps, and partly retain an isotopic signature consistent with bacterial sulphate reduction with δ¹³C as low as ?6‰. In contrast, dolomites of the marine, clay‐free part of the system were probably transformed through sabkha and reflux diagenetic processes into calcian varieties, and exhibit δ¹³C values of ca +3‰. Major shifts of δ¹³C values strictly follow the lateral migration of facies and thus mark transgressions and regressions.     

皖南地区晚震旦世陡山沱期岩相古地理

皖南地区在晚震旦世早期早时发生海侵,沉积了含碳、含硅的陆源碎屑岩,为浅海盆地相沉积;早期晚时,沉积了碳酸盐岩和碳酸盐岩夹粘土岩沉积,为台坡和台盆相沉积,其沉积相变化和古地理分异与当时基底性质、同沉积构造和碳酸盐建隆作用密切相关。     

Spatial analysis of thickness variability applied to an Early Jurassic carbonate platform in the central Southern Alps (Italy): a tool to unravel syn‐sedimentary faulting

Early Jurassic syn‐sedimentary extensional tectonics in the central Southern Alps controlled patterns of deposition within the Calcari Grigi carbonate platform. We used variogram maps to gather model‐independent information on the spatial distribution of thicknesses of selected platform units and investigated whether major syn‐sedimentary faults outlined subsiding domains during platform growth. Thicknesses display a spatial organization that suggests that large fault belts, often coincident with exposed Jurassic extensional structures, transected large parts of the platform. The network of four fault systems (trending NNW–SSE and NE–SW) displays orthorhombic symmetry, suggesting non‐Andersonian faulting and a true triaxial strain field with N100°E maximum extension or transfer shear zones connecting major NNW–SSE‐trending extensional faults. In both cases, inherited structures of Permian to Triassic age may have played a primary role in Jurassic faulting. If confirmed throughout the South‐Alpine domain, this arrangement could shed new light on Early Jurassic rifting mechanisms in the Southern Alps.