Palaeoceanographic differences of early Late Aptian black shale events in the Vocontian Basin (SE France)

A high-resolution study focussing on the distribution of calcareous nannofossils and carbon isotopes was carried out to improve the understanding of mid-Cretaceous black shale formation. The studied interval of the early Late Aptian is characterized by two major black shale couplets, the Niveau Noire 4 (NN4) and Niveau Noire Calcaire 2 (NNC2), of the Serre Chaitieu section in the Vocontian Basin (SE France; Bréhéret, 1997). This interval occurs within a long-term negative carbon isotope excursion of > 1.5‰ following the Early Aptian Oceanic Anoxic Event 1a (OAE1a). In contrast to the local NN4 black shales, the black shales of NNC2 are of regional significance and occur at the end of the major negative carbon isotope excursion of the early Late Aptian. Time equivalent black shales are suggested to be coeval with black shales found in the Western Tethys and Atlantic Ocean (Herrle et al., 2004). Calcareous nannofossil analyses and carbon isotopes indicate higher surface water productivity (mesotrophic), warmer surface water, and higher sea-level during the formation of the NN4 black shales. In contrast, the formation of the NNC2 black shales took place during a cooler phase, lower surface water productivity, and lower sea-level. A sea-level fall may cause a restriction of water mass exchange between the open-marine Western Tethys and the Vocontian Basin. This resulted in a longer residence time of the bottom water, decreased ventilation and less mixing of surface waters and thus enhanced preservation of organic matter at the sea floor. Our results indicate that the black shale formation of NN4 and NNC2 was caused by different processes such as increased surface water productivity and enhanced preservation of organic matter at the sea floor. Thus, we emphasize the role of different forcing factors which control the formation of local and regional black shales. The most important factors are sea-level fluctuations, increasing productivity, and changes in precipitation and evaporation rates.     

High-resolution chemo- and biostratigraphic records of the Early Aptian oceanic anoxic event in Cantabria (N Spain): Palaeoceanographic and palaeoclimatic implications

During the Early Aptian, major palaeoenvironmental changes occurred leading to an oceanic anoxic event (OAE 1a) and a perturbation of the global carbon cycle. New detailed litho-, bio-, and chemostratigraphic (TOC, δ¹³C_carb, δ¹³C_org) records of two superbly exposed and expanded Lower Aptian sections in Cantabria (La Florida and Cuchía) allow to recognize the expression of the OAE 1a in shallow shelf environments of northern Spain. The succession consists of shallow platform limestones that include a marly unit (Patrocinio Formation), the deposition of which occurred mostly at the onset of the OAE 1a (~ 120.5 Ma). This study presents a new integrated biostratigraphy based on ammonites, planktonic foraminifera, calcareous nannofossils and palynomorphs that allows an accurate age resolution of the succession. The marly unit records an abrupt negative δ¹³C excursion in both bulk organic matter (up to 5‰) and carbonate (up to 6‰, mean 3‰), as has been already observed at the onset of the OAE 1a in other Lower Aptian deposits worldwide. In detail, however, the negative excursion presents two minima in the studied sections. This negative spike is confidently attributed to the upper half of the Hayesites irregularis nannofossil Zone, to the upper part of the Blowiella blowi foraminiferal Zone, and to the middle–upper part of the Deshayesites weissi ammonite Zone. A third negative excursion occurs at the base of the Rhagodiscus angustus nannofossil Zone, which may be correlatable with the Dufrenoyia furcata ammonite Zone. This data set refines the age of the OAE 1a and reveals the existence of a stratigraphic gap in the westernmost margin of the Basque Cantabrian Basin that covers at least a portion of the upper part of the Early Aptian. Sedimentary facies and quantitative analysis of palynomorphs and nannofossils document significant environmental changes associated with the OAE 1a: compositional changes of neritic carbonates and calcareous nannofossils data indicate the occurrence of a biocalcification crisis inferred to have been related to CO₂-induced changes in seawater chemistry, and palynomorphs identify a thermal maximum followed by a cooling phase. The latter show a Classopollis maximum during the OAE 1a, which is followed by a decrease in Classopollis and an increase of bisaccate pollen after the event.     

Changes in sedimentary patterns of coastal and deep-sea successions from the North Atlantic (Portugal) linked to Early Cretaceous environmental change

Coastal mixed carbonate-siliciclastic and carbonate deposits of Late Barremian to Early Aptian age from the Lusitanian Basin (Portugal) are compared with a deep-sea succession (ODP 641) off the coast of Portugal. The coastal deposits show an abrupt cessation of rudist-dominated carbonate deposition marked by an emersion horizon and followed by the deposition of orbitolinid-rich marls during the Early Aptian. The subsequent development of the carbonate platform during the Late Aptian is masked by a hiatus. For this time interval shallow-water debris deposits of the deep-sea succession ODP 641 indicate that carbonate production in shallow-water areas resume in the early Late Aptian. Carbon-isotope stratigraphy in combination with available biostratigraphic data is used for intrabasinal correlation and for the correlation of the Portuguese with shallow-water successions from Switzerland, France, Oman and the Pacific. The correlation reveals that during the Early Aptian similar changes in their sedimentary patterns occur at the beginning of a marked negative shift of carbon-isotope values associated with the global deposition of organic rich black shales in the deep sea (OAE 1a). In all compared sections rudist-dominated carbonate deposition is stopped and followed either by orbitolinid-rich deposits, the deposition of microbial carbonates or by the drowning of the carbonate platform. The comparison shows that the deterioration for carbonate platform growth conditions during the Early Aptian occurred essentially simultaneously at the various localities. During this episode the ocean waters were in a preconditioned state of acidification stressing biocalcifying organisms. Sea level change combined with local effects like elevated nutrient levels and higher temperatures, were probably determining the nature of sedimentary change in shallow-water environments during the carbonate crisis. In the sections studied in Portugal the high abundance of orbitolinids overlying rudist limestones indicates that increased nutrient input may have played an important local role among the factors that caused an overall deterioration for carbonate producers during the late Early Aptian.     

The record of mid Cretaceous oceanic anoxic events from the Ionian zone of southern Albania

Along the Sopoti section of Mali Gjere mountain are found two distinct mid-Cretaceous marly-shaly-siliceous intervals that occur near the top of the Vigla Limestone Formation (Aptian-Albian) of the Ionian zone in southern Albania. The lower interval is relatively rich in carbonate content (36% CaCO₃ on average) and radiolaria (10% on average). It contains black shale levels rich in organic matter (up to 8.5% total organic carbon (TOC)) of marine origin, which did not experience any strong thermal maturation as suggested by their very low T_max values. The age of the lower interval is latest Barremian to Early Aptian based on its calcareous nannofossil and radiolarian assemblages. It is therefore regarded as the equivalent of the Fourcade Level of Greece, reflecting the Oceanic Anoxic Event 1a (OAE1a). The upper interval is richer in both carbonate content (60% CaCO₃ on average) and radiolaria (20% on average) but it is practically devoid of any preserved organic matter. Its age straddles the Aptian-Albian boundary based on integrated biochronologic data of dinoflagellates, calcareous nannofossils and radiolaria. It is tentatively considered as the sedimentary expression of OAE1b (sensu Leckie et al., 2002) in the Ionian zone of Albania. The presence of large Assipetra nannoliths in both shaly-siliceous intervals and the relative abundance of radiolaria suggest that their accumulation took place during periods of higher productivity in the Ionian zone of Albania.     

Toarcian carbon isotope shifts and nutrient changes from the Northern margin of Gondwana (High Atlas, Morocco, Jurassic): Palaeoenvironmental implications

The Early Toarcian is marked by a global perturbation of the carbon cycle and major marine biological changes. These coincide with a general decrease in calcium carbonate production and an increase in organic carbon burial, and culminate in the so-called Toarcian Oceanic Anoxic Event. It is believed that the environmental crisis was triggered by the activity of the Karoo-Ferrar large igneous province. In order to further document the Early Toarcian palaeoenvironmental perturbations, carbon isotope, total organic matter, calcareous nannofossils and phosphorus content of the Amellago section in the High Atlas rift basin of Morocco were investigated. This section is extremely expanded compared to the well-studied European sections. Its position along the northern margin of the Gondwana continent is of critical importance because it enables an assessment of changes of river nutrient input into the western Tethyan realm. The carbon isotope curve shows two negative excursions of equal thickness and amplitude, at the Pliensbachian-Toarcian boundary and at the transition from the Polymorphum to the Levisoni Zone. This confirms the supra-regional nature of these shifts and highlights the possible condensation of the first “boundary” shift in European sections. Phosphorus content is used to trace palaeo-nutrient changes and shows that the two negative carbon isotope shifts are associated with increased nutrient levels, confirming that these episodes are related to enhanced continental weathering, probably due to elevated greenhouse gases in the atmosphere. In the High Atlas Basin, the increase in nutrient levels at the Pliensbachian-Toarcian boundary is moreover likely to be the main factor responsible for the coeval demise of the Saharan carbonate platform. A middle Toarcian event, centered on the boundary between the Bifrons and Gradata Zones, characterized by a positive carbon isotope excursion and nutrient level rise, is documented in the Amellago section.     

Carbon-isotope stratigraphy and temperature change during the Early–Middle Jurassic (Toarcian–Aalenian), Raasay,Scotland, UK

The data presented here provide the first high-resolution investigation of carbon and oxygen isotope derived from Boreal Aalenian–Bajocian belemnites from the Isle of Raasay, Scotland, UK. Further isotope data has been gathered from the Toarcian. The carbon-isotope data reveal a positive carbon-isotope excursion in the early Aalenian Scissum Zone (equivalent to the Tethyan Comptum Subzone). The correspondence between this record and Tethyan records lends support that this excursion is global in origin. Although the excursion is coeval with a transgressive interval less obvious is an association with the deposition of organic-rich deposits. An equivocal excursion toward light isotopic close to the Aalenian–Bajocian boundary may be linked to a major pulse of subduction related magmatism. Light oxygen isotopes (and high temperatures) are derived from belemnites from the Toarcian Falciferum Zone. Akin to coeval data from elsewhere surface waters during the Falciferum Zone may have been episodically reduced. Oxygen isotope data from the Aalenian are interpreted to show a cooling trend to a minimum at the base in the Opalinum–Scissum zonal boundary. This major step in cooling is however not entirely coincident with sea-level data and significantly the inferred cooling occurs prior to the positive carbon-isotope excursion. Hence a carbon drawdown and burial scenario that has been interpreted from positive carbon excursions elsewhere cannot drive a precursory cooling episode.     

Resolving Terreneuvian stratigraphy in subtidal–intertidal carbonates: palaeontological and chemostratigraphical evidence from the Turukhansk Uplift,Siberian Platform

Both diverse assemblages of small skeletal fossils and a representative chemostratigraphical record make the Siberian Platform widely regarded as one of the key regions for the reconstruction of global biotic and abiotic events in the late Ediacaran and early Cambrian. However, the wide distribution of intertidal–subtidal facies in the Ediacaran–Cambrian transitional strata of the central and southwestern Siberian Platform (Turukhansk–Irkutsk–Olekma facies region) produces a dramatic depletion of the palaeontological record and considerably limits their age‐calibration and long‐distance correlation. We report new lithological, palaeontological and carbonate carbon‐isotope data for the Ediacaran–Cambrian sections of the Turukhansk Uplift (northwestern Siberian Platform, western facies region). These data provide a robust framework for the chemostratigraphical correlation of the western facies region with sections of the transitional and eastern regions of the Siberian Platform and further confirms a depositional hiatus at the base of the Tommotian Stage in the stratotype section (Aldan River, SE Siberia). The carbon‐isotope curve from the Turukhansk Uplift sections correlates positively with the most chemostratigraphically representative Ediacaran–Cambrian sections (Siberia, Morocco, South China). It records major carbon‐isotope oscillations globally recognized in the lower Cambrian, enabling localization of the Fortunian and Cambrian Stage 2 boundaries in the Platonovskaya Formation. Although there is extreme paucity and poor preservation of the small skeletal fossils in the western facies region, we report individual Barskovia, Blastulospongia and chancelloriid sclerites from the Platonovskaya Formation. A combination of palaeontological and chemostratigraphical data suggests the base of P. antiqua Assemblage Zone is located in the middle Platonovskaya Formation. The earliest spiral gastropods probably occurred at ~541 Ma, as demonstrated by the discovery of a specimen of Barskovia near the base of the large negative excursion in the lower Platonovskaya Formation, correlated with the BACE negative carbon‐isotope peak in the sections of the Yangtze Platform.     

Integrated stratigraphy and palaeoenvironment of the Cenomanian-Lower Turonian (Upper Cretaceous) of northern Westphalia, North Germany

Summary The present study provides an integrated stratigraphy of the Lower Cenomanian-Lower Turonian of the northwestern Münsterland Basin, Westphalia. This is important to establish a standard section allowing an interregional correlation as well as an interpretation of single environmental conditions, their changes through time and their geographical extent. Numerous sections have been investigated in northern Westphalia, in addition to data from other profiles in North Germany. Macrofossils and thin-sections have been sampled, stable isotope and gamma ray data have been obtained from a part of the sections. Investigation of the sedimentary sequence is based on a analysis of events. Many events are diachronous, whereas others are difficult to define and do not show a wide geographic distribution. For ecological or sedimentological reasons, correlation is not possible. The discussion of events leads to a compound picture of the evolution of the depositional sequence, allowing the reconstruction of palaeo-environmental changes. Sea-level changes and their influence on the fauna is discussed. During maximal sea-level rising, macrofossils occur more frequently for ecological reasons, however, some macrofossil accumulations are lag deposits. Some biostratigraphical problems find their origin in a tectonic separation leading to different habitats. The local tectonics was caused by the intial phase of transpression of the Osning Zone, that can be traced down to the Lower Cenomanian. A correlation of the Cenomanian-Turonian Boundary Event (CTBE) in Westphalia (Lengerich), Colorado (USA) and England (Eastbourne), is possible due to very dense sampling of carbon-isotopes (δ¹³C). In Westphalia, definition of the stage boundary is possible by correlation of carbon isotope curves only. A sequence from the upper Middle Cenomanian, up to the lower Upper Cenomanian, is investigated concerning the controlling factors of biogenic sedimentation. The cyclicity of lithology is investigated by Fast Fourier Trans-formation. It can be shown that sedimentation is forced by orbital cycles, mainly by the precession cycle of the Milankovitch band (P1 and P2, 18 500 and 22 300 years, respectively). This confirms the primary origin of the marlstone-limestone couplets that are obvious in the field. Calculation of sedimentation rates is based on these data. There is a high variability of sedimentation rates, maybe due to a strong variation of productivity in this epicontinental environment. Dedicated to the memory of Jost Wiedmann (1931–1993)     

The Cenomanian–Turonian boundary in the northwestern part of the Adriatic Carbonate Platform (Ćićarija Mtn., Istria,Croatia): characteristics and implications

The Cenomanian–Turonian boundary (CTB) in the ?i?arija Mountain region (northern Istria, Croatia) is characterized by calcisphere limestone successions with a firmground and glauconite horizon, bioturbated intervals, tempestites, and slumped structures as well as microbially laminated and organic-rich interbeds deposited in the northwestern part of the intra-Tethyan Adriatic Carbonate Platform (AdCP). Compilation of the results from three studied sections (Vodice–Jelovica, Martinjak and Planik) of litho-, bio-, and microfacies analyses, X-ray diffraction, SEM, EDS, and stable isotope analyses allowed reconstruction of marine paleoenvironmental conditions during this time period. Shallow-marine carbonate deposits of the Milna Formation underlie a drowned-platform succession of the Sveti (Sv.) Duh Formation. The contact between these two formations is sharp and commonly marked by slumped deposits. The Sv. Duh Formation consists of about 100 m of calcisphere wackestone enriched in organic matter. The results of preliminary δ¹³C and δ¹⁸O stable isotope analyses indicate the influence of the global Oceanic Anoxic Event (OAE2) on the deposition of this carbonate succession. Anoxic and hypoxic conditions in the water column lead to major changes in the shallow-marine carbonate system of the AdCP. Numerous benthic foraminifera declined during that time, but planktonic foraminifera and calcareous dinoflagellates diversified and expanded greatly. The results of this research provide new insights into the character of the CTB interval in this part of the Tethyan realm. Local and regional synsedimentary tectonics combined with global upper Cretaceous sea-level dynamics allows the correlation of the investigated deeper-marine lithostratigraphic units with OAE2.     

Facies pattern and sea-level dynamics of the early Late Cretaceous transgression: a case study from the lower Danubian Cretaceous Group (Bavaria,southern Germany)

The facies development and onlap pattern of the lower Danubian Cretaceous Group (Bavaria, southern Germany) have been evaluated based on detailed logging, subdivision, and correlation of four key sections using an integrated stratigraphic approach as well as litho-, bio-, and microfacies analyses. Contrary to statements in the literature, the transgressive onlap of the Regensburg Formation started in the Regensburg–Kelheim area already in the early Early Cenomanian Mantelliceras mantelli ammonite Zone and not in the Late Cenomanian. In the Early Cenomanian, nearshore glauconitic-bioclastic sandstones prevailed (Saal Member), followed by Middle to lower Upper Cenomanian mid-shelf siliceous carbonates intercalated with fine-sandy to silty marls (Bad Abbach Member). Starting in the mid-Late Cenomanian (Metoicoceras geslinianum ammonite Zone), a considerable deepening pulse during the Cenomanian–Turonian Boundary Event (CTBE) initiated the deposition of the deeper shelf silty marls of the Eibrunn Formation, which range into the early Early Turonian. During the CTBE transgression, also the proximal Bodenwöhrer Senke (ca. 40 km NE of Regensburg) was flooded, indicated by the onlap of the Regensburg Formation onto Variscan granites of the Bohemian Massif, overlain by a thin tongue of lowermost Turonian Eibrunn Formation. A detailed record of the positive δ¹³C excursion of the global Oceanic Anoxic Event (OAE) 2 has been retrieved from this shallow-water setting. An integrated approach of bio-, event-, carbon stable isotope and sequence stratigraphy was applied to correlate the sections and to decipher the dynamics of this overall transgressive depositional system. The Cenomanian successions show five prominent unconformities, which correlate with those being known from basins in Europe and elsewhere, indicating their eustatic origin. The rate of sea-level rise during the CTBE suggests glacio-eustasy as a driving mechanism for Late Cenomanian sea-level changes. The Regensburg and Eibrunn formations of the lower Danubian Cretaceous Group are highly diachronous lithostratigraphic units. Their regional distribution and northeast-directed onlap pattern onto the southwestern margin of the Bohemian Massif can readily be explained by the lateral movements of roughly coast-parallel (i.e., NW/SE-trending) facies belts of a graded shelf system transgressing on a northeastward-rising substrate. It took the Cenomanian coastline ca. 6 Ma to transgress from southwest of Regensburg to the topographically elevated granite cliffs southeast of Roding in the Bodenwöhrer Senke (=60 km distance).     

Dissolved and particulate organic matter in contrasting Zostera marina (eelgrass) sediments

The influence of seagrass Zostera marina on sediment characteristics was examined in two contrasting sediments, one organic-rich and one organic-poor. The presence of plants leads to reduced sediment redox potential in both sediment types compared to bare sediment with the largest effects in the organic-poor sediment. Z. marina stimulated the sulfate reduction rates in organic-poor sediment with ∼50% and higher pools of dissolved organic carbon (DOC) were found. In contrast, sulfate reduction rates were lower in vegetated compared to bare sites in the organic-rich sediment. Despite a low contribution of dissolved carbohydrate (DCHO) to the DOC pool (<5%), the seagrass vegetation was responsible for an increase of ∼50% in DCHO pools with a peak in the root zone suggesting that Z. marina supplied DCHO to the pore waters. The Z. marina meadows also enhanced the contribution of particulate carbohydrate (PCHO) to sedimentary particulate organic carbon (POC) pools by 6-14% compared to bare sediment. Although the PCHO pools were higher in organic-rich than organic-poor sediments, the analyses of carbohydrate composition revealed that three groups of neutral sugars including glucose, galactose and mannose+xylose were the major compounds of PCHO and contributed with >60% to sedimentary carbohydrate pools at both sites. Only glucose showed depletion with depth in the vegetated sediments, whereas the percentage of ribose and rhamnose increased indicating a selective degradation of labile carbohydrates in the meadows. Galactose and mannose+xylose appeared to represent a refractory part of carbohydrate that remained after degradation of the more labile components. The sugar content was rather constant with depth at the bare organic-rich sediment indicating that only recalcitrant carbohydrate pools were buried. There was less difference in the PCHO composition profiles between vegetated and bare organic-poor sediments.     

A sequence stratigraphical model for the Late Ludfordian (Silurian) of Gotland,Sweden: implications for timing between changes in sea level,palaeoecology, and the global carbon cycle

This paper investigates a time interval within the Late Ludfordian (Late Silurian), involving changes in faunal composition (the Lau Event), a major positive carbon isotope excursion (CIE), and contemporaneous sea-level changes in remote palaeo-basins. Based on the Silurian strata of Gotland (Sweden), we integrate sequence stratigraphy, carbon isotope stratigraphy, and platform-scale palaeoecological changes associated with this turbulent time period in Earth history. Three depositional sequences (sequences Nos. 1–3), including two separate periods of forced regression (falling stage systems tracts, FSSTs) are identified from outcrop and drillcore studies. The sequence stratigraphical framework is interpreted to reflect glacio-eustatic sea-level changes. The CIE starts at the onset of the initial FSST (sequence No. 1), just below the last appearance datum of the conodont Polygnathoides siluricus. The values increase through the ensuing lowstand and transgressive systems tracts (LST and TST) of sequence No. 2 and peak in the following highstand systems tract (HST). A second forced regression (FSST of sequence No. 2) took place in the lower Ozarkodina snajdri Zone. δ¹³C data are scarce from these siliciclastic strata, but inferably remain high. The δ¹³C values increase within the LST and earliest TST of sequence No. 3, before a decreasing trend starts within the early TST. δ¹³C values return to pre-excursion levels within the ensuing HST. The CIE is closely associated with an increase in stromatolites (mats and oncoids) across a wide range of depths and sedimentary environments, and correlations to other basins indicate a global increase in cyanobacterial activity. A drastic decline in level-bottom benthic faunas during the FSST of sequence No. 2 is, however, interpreted as a local response to the progradation of a delta complex (the Burgsvik Sandstone). Biological carbonate production replenishes rapidly within the TST of sequence No. 3, succeeding a thin LST dominated by reworked siliciclastics and chemically precipitated carbonates (ooids). The detailed relationship between the CIE and sea-level change presented herein is not fully consistent with previous reports on the CIEs associated with the lower Silurian Ireviken and Mulde events, respectively. Based on our facies analysis and sequence stratigraphical interpretation, two main mechanisms are suggested as responsible for the Late Ludfordian CIE: (1) a change in the riverine C-weathering flux towards the ¹³C end member following glacio-eustatically induced subaerial exposure of carbonate platforms throughout the tropics, and, (2) increased photosynthetic activity by benthic cyanobacteria exaggerating the δ¹³C values of precipitated carbonates.     

Microfacies, biostratigraphy, and geochemistry of the hemipelagic Barremian–Aptian in north-central Tunisia: Influence of the OAE 1a on the southern Tethys margin

Upper Barremian–lower Upper Aptian hemipelagic deposits of the Hamada Formation in the Djebel Serdj area, north-central Tunisia were studied in detail with regard to microfacies, biostratigraphy, δ¹³C stratigraphy, and geochemistry. Our data provide insights into the palaeoenvironmental evolution and sea-level fluctuations of the Tunisian shelf. The unusually expanded deposits consist of mud-, wacke-, and packstones which reflect mid- and outer-ramp depositional environments. Planktonic foraminifer and δ¹³C stratigraphy allowed us to establish a detailed time-frame and the recognition of the Lower Aptian Oceanic Anoxic Event 1a (OAE 1a) as well as time-equivalent deposits of shallow-marine carbonate-platform drowning. Based on our microfacies studies, we subdivide the studied sections into four genetic intervals: a pre-OAE 1a interval, an OAE 1a and platform-drowning-equivalent interval, and a post-platform-drowning interval. We present a 3rd-order sea-level curve for the Tunisian shelf, deriving from the results of our microfacies studies.

Deposits of the OAE 1a in the sections investigated are characterised by bioclastic wacke- and packstones with high abundances of poorly preserved radiolarians and moderately to well preserved planktonic foraminifers, suggesting a transgression and an eutrophication of the upper water column. Scarceness of benthic macrofossils, low abundances of small benthic foraminifers and ostracods possibly suggest dysoxic conditions at the seafloor. Mudstones of the platform-drowning time-equivalent deposits, directly overlying the OAE 1a, are partly showing a pronounced drop in carbonate content and are scarce of macrofossils.     

Benthic foraminiferal assemblages in the severely polluted coastal environment of Drapetsona-Keratsini,Saronikos Gulf (Greece)

Surface sediments were collected from the coastal zone of Drapetsona–Keratsini (Saronikos Gulf, Greece) in December 2012 for determining the local benthic foraminiferal community, identifying their spatial distribution patterns, and evaluating the response of foraminiferal species to geochemical composition through the hierarchical cluster analysis, principal component analysis and Spearman's rho correlation. Foraminifera can be classified into three distinct assemblages associated with the granulometry, elemental geochemistry, particulate organic carbon content and degree of sediment contamination. A relatively low-diversity assemblage, dominated by stress-tolerant taxa with Ammonia tepida Bolivina spathulata and Bulimina elongata being the prevailing species, is characteristic of the silty seabed of the main part of Drapetsona coastal zone and the Keratsini Port central basin, where organic carbon content, aliphatic and polycyclic aromatic hydrocarbons concentrations and trace metal loads are greatly elevated. On the sandy bottom of the investigated area, relatively high frequencies of miliolids prevail. An epiphytic rotaliid-dominated assemblage is recorded in the slightly-polluted sedimentary bottom of the inner and western part of the Keratsini Port.     

An Early Cenomanian (Late Cretaceous) maximum flooding bioevent in NW Europe: Correlation,sedimentology and biofacies

The Early Cenomanian (mid-Mantelliceras dixoni Zone) Schloenbachia/Inoceramus virgatus Event of NW Europe is recorded in five calcareous ‘marl’-limestone couplets rich in inoceramid bivalves of the Inoceramus virgatus plexus. The 1.3–6.8m thick beds represent a bundle of precession couplets inferred to have been deposited during a short-term cycle of sea-level change triggered by the 100kyr short eccentricity cycle within the Milankovitch periodicities. A burrowed omission surface terminates the Schloenbachia/Inoceramus virgatus Event in most places. Rock accumulation rates vary in different sections between 13 and 68Bubnoff units (mm/kyr) for the complete 100kyr cycle with extreme values ranging from 10 to 100Bubnoffs at shorter (i.e., 20kyr precession couplet) scales. Taphonomic and microfacies trends suggest deposition below fair-weather wave base with frequent to sporadic reworking of the sea-floor by storm waves for proximal and medial sections. The distal sections were close to, or even slightly below, maximum storm wave base. The biofacies of the Schloenbachia/Inoceramus virgatus Event is strongly dominated by inoceramid bivalves (65–85%). The two representatives of Inoceramus virgatus, I. virgatus virgatus Schlüter and I. virgatus scalprum Böhm, are not geographic subspecies as they co-occur in the same beds. However, proximal morphs tend to be more strongly ribbed and flatter whereas distal ones are more inflated and smoother, representing ecophenotypes of one polymorphic species. Taphonomic observations and shell morphology suggest that the inoceramids lived as epibyssate recliners. The low species richness and fossil content of the event strata (without the inoceramid bivalves) are attributed to low fluxes of organic matter to the sea-bed during the maximum flooding interval of Cenomanian depositional sequence Ce III. The inoceramids of the virgatus plexus are inferred to have been very effective filter feeders flourishing under nutrient-depleted, low-energy conditions. The Schloenbachia/Inoceramus virgatus Event is an example of a maximum flooding bioevent, forming a widespread marker dominated largely by autochthonous biota.     

Preservation of overmature,ancient, sedimentary organic matter in carbonate concretions during outcrop weathering

Concretions are preferentially cemented zones within sediments and sedimentary rocks. Cementation can result from relatively early diagenetic processes that include degradation of sedimentary organic compounds or methane as indicated by significantly ¹³C‐depleted or enriched carbon isotope compositions. As minerals fill pore space, reduced permeability may promote preservation of sediment components from degradation during subsequent diagenesis, burial heating and outcrop weathering. Discrete and macroscopic organic remains, macro and microfossils, magnetic grains, and sedimentary structures can be preferentially preserved within concretions. Here, Cretaceous carbonate concretions of the Holz Shale are shown to contain relatively high carbonate‐free total organic carbon (TOC) contents (up to ~18.5 wt%) compared to the surrounding host rock (with <2.1 wt%). TOC increases with total inorganic carbon (TIC) content, a metric of the degree of cementation. Pyrite contents within concretions generally correlate with organic carbon contents. Concretion carbonate carbon isotope compositions (δ¹³C_carb) range from ?22.5 to ?3.4‰ (VPDB) and do not correlate strongly with TOC. Organic carbon isotope compositions (δ¹³C_org) of concretions and host rock are similar. Thermal maturity data indicate that both host and concretion organic matter are overmature and have evolved beyond the oil window maturity stage. Although the organic matter in general has experienced significant oxidative weathering, concretion interiors exhibit lower oxygen indices relative to the host. These results suggest that carbonate concretions can preferentially preserve overmature, ancient, sedimentary organic matter during outcrop weathering, despite evidence for organic matter degradation genetic mechanisms. As a result, concretions may provide an optimal proxy target for characterization of more primary organic carbon concentrations and chemical compositions. In addition, these findings indicate that concretions can promote delayed oxidative weathering of organic carbon in outcrop and therefore impact local chemical cycling.     

Palynofacies and TOC analysis of marine and non-marine sediments across the Middle–Upper Jurassic boundary in the central-northern Lusitanian Basin (Portugal)

The Middle–Upper Jurassic transition is a geodynamic benchmark in the evolutionary history of several peri-Atlantic basins. Contrary to the vast Tethyan and peri-Tethyan areas, in the Lusitanian Basin (Portugal), this interval corresponds to a major basin-wide disconformity preceded by a complex forced regression that induced sharp facies variations across the depositional systems. The Middle Jurassic units below are fully marine, whereas the Upper Jurassic sediments (Lower?-Middle Oxfordian), correspond to freshwater/brackish lacustrine, grading into punctuated brackish-restricted lagoonal and shallow-marine paleoenvironments. This study presents total organic carbon and palynofacies data of 34 samples (total analyzed thickness about 85 m) collected from three key sections encompassing the Middle–Upper Jurassic transition in the central-northern sector of the Lusitanian Basin. The palynofacies of the analyzed part of Middle Jurassic units (Cabo Mondego Formation) are characteristic of marine environments at their base, evidencing upwards a regressive trend of the depositional systems. Total organic carbon content is generally low, reaching up to 1.94 wt%. The Upper Jurassic Cabaços Formation presents kerogen assemblages mostly of continental origin, although punctuated by minor intervals of marine influence. Total organic carbon content is more variable, reaching up to 30.56 wt%. Intraclasts of re-deposited fragments of microbial mats were found incorporated in the kerogen assemblages, which point to highly dynamic erosional and depositional processes. Diversity of Botryococcus sp. occurrences was confirmed as an indicator of the degree of paleoenvironmental stability. The vertical distribution and comparison of the kerogen assemblages of the different sections indicate major changes of these parameters among relatively close settings and along narrow vertical intervals, attesting to the high sedimentary dynamics observed in the Lusitanian Basin.     

Calcareous nannofossil biostratigraphy of the Thomel Level (OAE2) in the Lambruisse section, Vocontian Basin, southeast France

The Thomel Level of the Lambruisse section in the Vocontian Basin (southeast France), which is marked by intercalations of black shales and organic-rich marls, accumulated during the oceanic anoxic event 2 (OAE2) occurring in the Cenomanian-Turonian (C-T) boundary interval. Calcareous nannofossil biostratigraphic investigation of this interval revealed a total of five nannofossil zones, corresponding to the UC3-UC8 zones (Middle Cenomanian-Middle Turonian) as defined by Burnett. Biostratigraphically important taxa observed in the section include Cretarhabdus striatus, Axopodorhabdus albianus, Lithraphidites acutus, Corollithion kennedyi, Helenea chiastia, Quadrum gartneri, Q. intermedium, Eiffellithus eximius, Eprolithus octopetalus and E. eptapetalus. The two nannofossil events commonly used in the delineation of the C-T boundary, namely the LO of H. chiastia and the FO of Q. gartneri, occur less than 2 m apart in the studied section. These two bioevents define the limits of the UC6 nannofossil Zone and occur within the Whiteinella archaeocretacea foraminifer Zone. Previous litho- and chemostratigraphic analyses indicate that the δ¹³C profile of the section corresponds well with changes in lithofacies and fluctuations in the total organic carbon (TOC) and calcium carbonate content of the section. Initial increase in the δ¹³C values occurs within the UC3-UC4a undifferentiated zone, coinciding with the onset of the deposition of the organic-rich sediments of the Thomel Level and a drastic decline in the CaCO₃ values. The plateau of high δ¹³C values, on the other hand, occurs within the UC5 zone, between the LO of C. kennedyi and the LO of H. chiastia (and FO of Q. gartneri). This interval of high δ¹³C values also corresponds to the interval of high TOC and low CaCO₃ values. The integrated nannofossil, planktonic foraminifer and δ¹³C data provide a precise biostratigraphic and chemostratigraphic framework of the C-T boundary in the Lambruisse section that can be used in future studies in the Vocontian Basin and allow correlations with other well-studied C-T boundary sections.     

Carbonate mud mounds,conglomerates, and sea-level history in the Katian (Upper Ordovician) of central Sweden

The Katian (Upper Ordovician) facies succession of the Siljan district, central Sweden, records some of the most prominent environmental changes in the Ordovician of Baltoscandia. These changes include two separate phases of major sea-level drawdown that were of basinwide and presumably global importance. The first regression and lowstand terminated an entire generation of carbonate mud mounds (the Kullsberg Limestone) and resulted in the formation of polymict carbonate conglomerates (Skålberg Limestone) belonging to the Amorphognathus superbus Zone. New stable isotope data from the Amtjärn quarry shows that this is immediately after the peak of the Guttenberg Carbon Isotope Excursion (GICE), which reaches a δ¹³C peak value at 3.3‰ in the uppermost Amorphognathus tvaerensis Conodont Zone. A second major regression and sea-level lowstand is manifested by palaeokarst morphologies in the Slandrom Limestone, which formed close in time to the comparably minor Waynesville positive carbon excursion in the basal Amorphognathus ordovicicus Conodont Zone. The widespread exposure associated with this latter lowstand terminated carbonate production in much of the basin, and, during the subsequent flooding, organic-rich, graptolitic shale formed across most of Baltoscandia. The two corresponding sequence boundaries are amalgamated at the top of truncated carbonate mud mounds in the Siljan district, resulting in a pronounced Middle Katian hiatus in the immediate mound areas.