Positive Carbon Isotope Excursion Research Articles

Carbon sequestration activated by a volcanic CO2 pulse during Ocean Anoxic Event 2

Ninety-four million years ago, during Ocean Anoxic Event 2, there was a marked increase in the burial of organic carbon in marine sediments. Measurements of stomata in fossil leaves show that the two main pulses of carbon burial were associated with a decline in atmospheric CO2 levels of up to 26%. The Cretaceous Ocean Anoxic Event 2 (about 94 million years ago) is thought to be linked to extensive volcanism, which triggered a biogeochemical chain of events that eventually led to widespread marine anoxia and a remarkable increase in carbon burial in marine sediments1,2,3. It has been suggested that the event was accompanied by a substantial decrease in atmospheric CO2 concentrations4, but the quantification of the drawdown remains controversial5,6. Here we reconstruct atmospheric CO2 concentrations throughout the ocean anoxic event from counts of the stomata in fossil leaves, and use terrestrial carbon isotopes to link the reconstruction to marine records of the event7,8. We find that before the onset of ocean anoxia, atmospheric CO2 concentrations increased by 20% over background levels of 370+100/−70 ppm. This was part of a long-term rise in atmospheric CO2 levels, presumably caused by volcanism, which reached a peak of 500+400/−180 ppm. However, two pulses of extensive carbon burial during the ocean anoxic event, as indicated by positive carbon isotope excursions, are associated with decreased atmospheric CO2 concentrations. We conclude that the sequestration of marine organic carbon led to a decrease in atmospheric CO2 concentration of up to 26% during Ocean Anoxic Event 2.

High-resolution carbon isotope changes, litho- and magnetostratigraphy across Permian-Triassic Boundary sections in the Dolomites, N-Italy. New constraints for global correlation

The Siusi/Seis section has been measured for the δ 13C signature across the Permian–Triassic Boundary (PTB) in high resolution on magnetostratigraphic samples from Scholger et al. (2000), enabling an exact correlation with magnetostratigraphic data. We show that the base of the short normal polarity magnetozone that has been reported from the evento–stratigraphic boundary correlates with a distinctive short positive carbon isotope excursion marking the beginning of the decrease in δ 13C, which has been documented in almost all marine carbonate PTB sections. These findings correlate well with the data from the Abadeh section (Iran) and a composite magnetostratigraphy (Hounslow et al., 2008), establishing a global pattern that should facilitate the accurate correlation of sections and the correct placement of the PTB.

Calcareous nannofossils of the Cenomanian/Turonian boundary interval from the Boreal Realm (Wunstorf, northwest Germany)

The Cenomanian/Turonian boundary interval (CTBI) is marked by an intense climatic warming presumably caused by large magmatic eruptions. This warming was characterised by one of the most prominent Mesozoic perturbations of the carbon cycle, the Oceanic Anoxic Event 2 (OAE2), which is marked by a well pronounced positive carbon isotope excursion (CIE). Sediments of the OAE2-interval often consist of organic rich black shales suggesting widespread bottom water anoxia during the CTBI. This study focuses on calcareous nannofossils from the CTBI of a European section (Wunstorf Core; northwest Germany). A total of 105 samples were examined for calcareous nannofossils using the settling technique. Eight bioevents (last occurrences: Corollithion kennedyi, Lithraphidites acutus; first occurrences: Rotelapillus biarcus, Corollithion exiguum, Eprolithus octopetalus, Eprolithus eptapetalus, Quadrum gartneri, Eiffellithus eximius) have been recognised throughout the middle Cenomanian to middle Turonian interval. With the exception of eleven samples preservation is moderate to good. Calcareous nannofossils are abundant (mean 2.0 billion specimens/g sediment) and highly diverse (mean 58 species/sample). Assemblages are dominated by Watznaueria spp. (32.3%), Prediscosphaera spp. (13.4%), Zeugrhabdotus spp. (11.2%) and Biscutum spp. (10.5%). Pre-OAE2 and also post-OAE2 nannofossil assemblages show high abundances of Biscutum spp. (∼ 20%) indicative for stable mesotrophic conditions. The assemblages of the OAE2 itself are marked by high values for Watznaueria spp. and low frequencies of Biscutum spp. making oligotrophic conditions during the OAE2 likely. High absolute abundances of organic walled dinoflagellates and the occurrence of frequent stress tolerant nannofossil genera like Retecapsa spp. in the organic rich intervals suggest, however, a deposition of black shales enhanced by high primary productivity. Thus dinoflagellates and calcareous nannofossils are interpreted to reflect different seasonal signals during the time of black shale deposition. Short-term high fertile seasons allowed the blooming of the organic walled dinoflagellates whereas calcareous nannofossils dominated the longer oligotrophic seasons. The black shale deposition was supported by the formation of large amounts of organic matter during fertile seasons as well as by surface water stagnation during oligotrophic seasons.

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.

A multilayered water column in the Ediacaran Yangtze platform? Insights from carbonate and organic matter paired δ13C

Large carbon isotope fluctuations have been systematically reported for Ediacaran carbonate rocks, the meaning of which remains controversial. In order to better understand the mechanisms behind such variations, we present new paired δ 13C data on carbonates ( δ 13C carb) and their associated organic matter ( δ 13C org) from a shelf-margin section (Yangjiaping) of the Doushantuo Formation. In this section, the Doushantuo Formation starts with cap dolostones presenting δ 13C carb values around − 5‰ and δ 13C org values between − 30.3 and − 27.6‰. Up-section, phased variations in δ 13C carb and δ 13C org describe positive and negative excursions, while their difference (Δ 13C dol–org = δ 13C carb − δ 13C org) remains around + 29.2‰. These new data allow the first reconstruction of lateral variations of δ 13C carb, δ 13C org and Δ 13C carb–org for a shelf-to-basin cross-section of the Yangtze platform (South China) after integration with results reported previously for two other sections. Across the Yangtze platform, the isotope signals reveal strong lateral heterogeneities, with complex variations of δ 13C carb and Δ 13C dol–org in the inner-shelf section, phased variations in the shelf-margin section with positive δ 13C carb and Δ 13C dol–org close to 29‰, and dominantly negative δ 13C carb with δ 13C org as low as − 35‰ in the basin. These variations are incompatible with the idea that the δ 13C carb can systematically be used as a proxy of ocean surface waters. Assuming that δ 13C carb are acquired in bottom waters and/or upper sediments, we show that the heterogeneous δ 13C carb and Δ 13C dol–org are compatible with a stratified water column composed of up to three layers: (i) an oxic surface layer, where dissolved inorganic carbon (DIC) is probably in isotope equilibrium with the atmosphere; (ii) an intermediate euxinic layer with a lower δ 13C DIC due to organic matter oxidation by ongoing sulphate reduction; (iii) a bottom euxinic layer that seems to be restricted to the inner-shelf lagoonal facies, lacking sulphate, containing methane and with a higher δ 13C DIC due to DIC production by methanogenesis. If our model holds true, it suggests that not only negative but also positive Ediacaran carbon isotope excursions may reflect ocean stratification, the positive excursion possibly recording a sulphate-free methanogenic layer at the bottom of restricted basins.

Cenomanian sequence stratigraphy and sea-level fluctuations in the Tarfaya Basin (SW Morocco)

We investigated the sequence architecture of two expanded Cenomanian successions along a depth transect in the Tarfaya Basin (SW Morocco) and correlated these successions to published records from northwest Europe and India. Changes in terrigenous material, carbonate and organic carbon content, carbonate microfacies and foraminiferal biofacies, as well as nondepositional and erosional surfaces were used to define depositional sequences and systems tracts. We identified two main transgressive cycles in the lower and middle-upper Cenomanian separated by a major regression at the early-middle Cenomanian transition (sequence boundary Ce 3). This regressive interval is characterized by lagoonal low-stand deposits indicating an overall sealevel fall of more than 30 m. Superimposed on the two main transgressive cycles, there are 11 third-order depositional sequences that correlate to globally recognized sealevel fluctuations and appear to be paced by long eccentricity variations (400 Ka period). Positive carbon isotope excursions in the middle Cenomanian (96.0 Ma) and latest Cenomanian (94.0 Ma) following sealevel lowstands together with planktonic foraminiferal and ammonite datums provide a robust framework for stratigraphic correlation. We suggest that the onset of these excursions was triggered by eccentricity minima during periods of low variability in obliquity (nodes), which probably coincided with glacioeustatic lowstands.

Vertebrate extinctions and reorganizations during the Late Silurian Lau Event

During the Silurian Period, as shown here by data from Gotland, Sweden, and other regions of the Baltoscandian paleocontinent, fish faunas were struck hard by extinctions caused by the late Ludlow Lau Event, also known for being associated with the largest positive stable carbon isotope excursion of the Phanerozoic. This event had a profound impact on the early evolutionary history of vertebrates, wiping out two-thirds of the fish taxa and causing significant ecological reorganizations over an estimated time span of similar to 200 ka. Our data show that immediately prior to the event, jawed acanthodians dominated the fish faunas, whereas the event led to a diverse fauna and a brief but marked dominance of the jawless thelodonts. The stepwise changes observed mimic those of conodonts, suggesting a similar mode of life and response to atmospheric oceanic perturbations for these clades. (Less)

Late Cretaceous carbon- and oxygen isotope stratigraphy, nannofossil events and paleoclimate fluctuations in the Haţeg area (SW Romania)

This study presents carbon and oxygen isotope fluctuations identified in marine red marlstones and claystones of the Fizeşti Formation from the SE Haţeg area (Southern Carpathians, Romania). Biostratigraphy based on calcareous nannoplankton allow the identification of nannofossil standard zones and subzones CC16–CC17 up to CC20, and respectively UC11c up to UC15b, indicating that the investigated deposits are latest Coniacian–late Early Campanian in age. Throughout the studied succession, δ 13C values fluctuate between 1.4‰ and 2.65‰, while δ 18O values are between − 3‰ and − 4‰. In the middle Santonian (within the UC12 calcareous nannofossil zone), we found a slight positive carbon isotope excursion, with values increasing by 0.3‰, up to a value of 2.44‰, which we assume to represent the regional expression of the Horseshoe Bay Event, originally recognized in the English Chalk. In the uppermost Santonian, just below the first occurrence of the crinoid Marsupites testudinarius (within the UC13 calcareous nannofossil zone), we recorded a δ 13C increase, from 2.38‰ up to 2.63‰, correlative with the Hawks Brow Event, firstly described in UK. The most significant δ 13C increase identified by us, up to 2.63‰, is coincident with the last occurrence of the crinoid Marsupites testudinarius (= the Santonian–Campanian boundary) and represents the regional expression of the worldwide distributed Santonian/Campanian Boundary Event. The Santonian–Campanian boundary is associated with a slight positive excursion for δ 18O. In the lower part of the Campanian stage, δ 13C values progressively decrease to around 1.5‰. A decrease of the water surface temperature in the region is suggested for the lower part of the Santonian, an assumption based on the significant shift to less negative values of δ 18O and on the mixed (Tethyan and Boreal) character of the identified nannofloras. From the Santonian–Campanian boundary interval upwards, the temperatures were rising, leading to an arid and warm climate in the region. This climate mode continued probably into the Maastrichtian, when the Haţeg area became part of an island situated in the Northern Tethys Realm.

Isotopic signals from Callovian–Kimmeridgian (Middle–Upper Jurassic) belemnites and bulk organic carbon, Staffin Bay, Isle of Skye, Scotland

Abstract: The stable isotope data presented here significantly extend and expand upon previous isotopic investigations of the Middle to Late Jurassic interval. The belemnite samples collected from the Staffin Bay and Staffin Shale formations from the Isle of Skye, Scotland, yielded oxygen isotope values consistent with Callovian–Kimmeridgian palaeotemperatures of 6.7–20.6 °C. The carbon isotope data comprise one of the first moderately high-resolution investigations of the relationship between terrestrial δ 13 C org (predominantly fossil wood debris) and marine δ 13 C carb (belemnites) as derived from a geologically coeval record. The Staffin Bay data reveal a broad Early to Mid-Oxfordian positive carbon isotope excursion. The excursion maximum occurs in the cordatum Zone (British Boreal ammonite zonation), although high values persist into the tenuiserratum Zone. The correspondence between the marine and terrestrial records indicates a strong coupling of the ocean–atmosphere system and suggests that the total exchangeable carbon reservoir would have been affected at this time. The Mid-Oxfordian negative carbon isotope excursions identified in published Tethyan records and commonly attributed to methane release are not recorded in the Staffin Bay data, which may suggest that the Tethyan excursions do not represent fluctuations in the global carbon reservoir and that the fidelity of the methane hypothesis should be re-evaluated. Supplementary material: Isotopic and geochemical data are available at http://www.geolsoc.org.uk/SUP18339 .

Palaeoenvironmental significance of carbon- and oxygen-isotope stratigraphy of marine Triassic–Jurassic boundary sections in SW Britain

Abstract: Carbon-isotope stratigraphy is a useful tool for stratigraphic correlation, especially for strata deposited during major perturbations of the carbon cycle that affected the marine, terrestrial and atmospheric reservoirs. For the Triassic–Jurassic boundary, effectively defined by a first-order mass extinction, major fluctuations in carbon-isotope values have been well documented, but these datasets have generally been derived from bulk-rock samples. Hence, the extent to which features of the isotopic curve reflect diagenetic alteration or changing proportions of constituent materials is unconstrained. Here, carbon- and oxygen-isotope data are presented from well-preserved oyster shells ( Liostrea ) comprising low-magnesium calcite, a mineral species relatively resistant to diagenetic alteration. Samples were obtained from Lavernock Point, Glamorgan, Wales, a coastal section close to a candidate stratotype for the base of the Jurassic at St Audrie's Bay, Somerset, England. The carbon-isotope signature from St Audrie's Bay, previously defined on the basis of analysis of bulk organic matter, is confirmed by our new data. Major features are (1) the upper part of an ’initial' negative isotope excursion in the lowest part of the section, followed by (2) a pronounced positive excursion, and (3) an extended ’main' negative isotope excursion in the highest part of the section. The data confirm that the carbon-isotope stratigraphy previously documented from bulk organic matter in SW England records the chemical composition of the contemporaneous seawater. Bulk carbonates sampled over the same interval near Lyme Regis, England, show similar trends to those from oyster calcite in the lower part of the study section, but there are more 13 C-depleted values up-section. These lower values probably result from an admixture of primary and diagenetic carbonate. Palaeotemperatures calculated from oxygen-isotope values from Lavernock Point oyster shells are relatively cool at the beginning of the positive carbon-isotope excursion, and increased by up to 10 °C during the main negative carbon-isotope excursion. The new results are compatible with the view that positive carbon-isotope excursions correspond to times of low atmospheric carbon dioxide content, whereas negative carbon-isotope excursions correspond to times of high atmospheric carbon dioxide content, as is also found to be the case during the Early Jurassic (Toarcian) Oceanic Anoxic Event. The Mg/Ca and Sr/Ca ratios and δ 18 O of investigated Liostrea hisingeri show no correlation, supporting data from modern bivalves that indicate that incorporation of Mg and Sr is controlled mainly by factors other than temperature.

Turonian to Santonian carbon isotope data from the Tethys Himalaya, southern Tibet

New stable carbon and oxygen isotope data from an Upper Cretaceous section in Tibet are presented, and compared to carbon isotope records from England, Italy, and Germany. Together with a stratigraphic re-interpretation of published carbon isotope data from a nearby section in Tibet, our data can surprisingly well be correlated with the European sections. This indicates that, similar to the distinct positive carbon isotope excursion at the Cenomanian-Turonian boundary, also the broad positive carbon isotope shift in the middle-late Coniacian and early Santonian reflects a major perturbation of the carbon cycle on a global scale, even though organic-rich sediments related to the OAE3 appear to be mainly restricted to the Atlantic Ocean and adjacent basins. The data further show that, apart from the broad Coniacian-Santonian carbon isotope excursion, also isotopic shifts on a smaller scale in the Turonian and Coniacian, such as the Round Down, Pewsey, and Hitchwood Events, can be correlated over both hemispheres. This demonstrates that the development of global oceanic anoxic conditions and associated burial of large amounts of organic carbon do not constitute a prerequisite for globally reflected carbon isotopic shifts. The data from Tibet support the concept of a relation between main carbon isotope excursions and major sea-level variations. Cyclic fluctuations of geochemical and lithological parameters are likely to be orbitally driven. These cycles appear to be preferably reflected in the sediments during periods of lower or variable sea-level, whereas the ocean-atmosphere system seems to have operated in a different mode during long phases of high, stable sea-level, as during the Coniacian-Santonian OAE3.

Paleoenvironmental changes across the Mid Cenomanian Event in the tropical Atlantic Ocean (Demerara Rise, ODP Leg 207) inferred from benthic foraminiferal assemblages

This study is based on Cenomanian sediments of Ocean Drilling Program (ODP) Sites 1258 and 1260 from Demerara Rise (Leg 207, western tropical Atlantic, off Suriname, ~ 1000 and ~ 500 m paleo-water depth, respectively). Studied sediments consist of laminated black shales with TOC values between 3 and 18% and include the Mid Cenomanian Event (MCE), a positive carbon isotope excursion predating the well-known Oceanic Anoxic Event 2 (OAE 2). Benthic foraminiferal assemblages of the continuously eutrophic environment at Demerara Rise are characterized by low diversities (≤ 9 species per sample) and large fluctuations in abundances, indicating oxygen depletion and varying organic matter fluxes. Dominant species at both sites are Bolivina anambra, Gabonita levis, Gavelinella dakotensis, Neobulimina albertensis, Praebulimina prolixa, and Tappanina cf. laciniosa. Benthic foraminiferal assemblages across the MCE show a threefold pattern: (1) stable ecological conditions below the MCE interval indicated by relatively high oxygenation and fluctuating organic matter flux, (2) decreasing oxygenation and/or higher organic matter flux during the MCE with decreasing benthic foraminiferal numbers and diversities (Site 1258) and a dominance of opportunistic species (Site 1260), and (3) anoxic to slightly dysoxic bottom-water conditions above the MCE as indicated by very low diversities and abundances or even the absence of benthic foraminifera. Slightly dysoxic conditions prevailed until OAE 2 at Demerara Rise. A comparison with other Atlantic Ocean and Tethyan sections indicates that the MCE reflects a paleoceanographic turning point towards lower bottom-water oxygenation, at least in the proto-North Atlantic Ocean and in the Tethyan and Boreal Realms. This general trend towards lower oxygenation of bottom waters across the MCE is accompanied by ongoing climate warming in combination with rising sea-level and the development of vast shallow epicontinental seas during the Middle and Late Cenomanian. These changes are proposed to have favoured the formation of warm and saline waters that may have contributed to intermediate- and deep-water masses at least in the restricted proto-North Atlantic and Tethyan Ocean basins, poor oxygenation of the Late Cenomanian sediments, and the changes in benthic foraminiferal assemblages across the MCE.

An isotopic appraisal of the Late Jurassic greenhouse phase in the Russian Platform

Oxygen- and carbon-isotope ratios have been determined from Late Jurassic (Callovian–Volgian) belemnites from three locations on the Russian Platform (Gorodischi, Khanskaya Gora and Marievka). All samples were examined by means of trace element geochemistry and petrography in order screen for diagenetic alteration. Oxygen and carbon isotopes from well-preserved belemnites range from − 2.24 to − 0.09‰ and − 0.57 to 1.77‰ respectively. Oxygen isotopes, if interpreted in terms of temperature, reveal a rise of temperatures during the Oxfordian–Early Kimmeridgian and indicate a prolonged episode of warmth during the Kimmeridgian–Volgian. The isotope data only equivocally reflect a number of significant changes in Boreal–Tethyan ammonite assemblages. A positive carbon isotope excursion is observed within the Volgian, but not seen within composite carbon-isotope stratigraphies of the western Tethys. Hence the Jurassic may have been characterised by regional δ 13C excursions related to non-simultaneous organic matter deposition resulting from localised ponding, semi restricted ocean circulation and a lack of tidal mixing.

Cambrian high‐resolution biostratigraphy and carbon isotope chemostratigraphy in Scania, Sweden: first record of the SPICE and DICE excursions in Scandinavia

A core drilling (Andrarum-3), from the classical locality at Andrarum, Scania, southernmost Sweden, penetrated a 28.90-m-thick Cambrian succession. The core comprises dark grey to black, finely laminated mudstones and shales with early concretionary carbonate lenses (stinkstones or orsten) and a few primary carbonate beds. The middle Cambrian (provisional Series 3) part of the core comprises 17.35 m, whereas the Furongian Series (upper Cambrian) part covers the remaining 11.55 m. Nineteen trilobite and two phosphatocopine genera are present in the middle Cambrian, whereas the less diverse Furongian interval yielded four trilobite and three phosphatocopine genera. Other, less frequent, faunal elements include conodonts (s. l.), brachiopods, sponge spicules, bradoriids, and coprolites. Trilobites and phosphatocopines were used to subdivide the core into seven biozones ranging from the Ptychagnostus atavus Zone to the Parabolina spinulosa Zone (P. spinulosa Subzone). Carbon isotopic analyses (δ13Corg) through the core show two important excursions, the negative DrumIan Carbon isotope Excursion (DICE) in the Pt. atavus Zone, and the Steptoean Positive Carbon Isotope Excursion (SPICE) beginning near the first appearance of Glyptagnostus reticulatus and extending upward into the Olenus and Agnostus (Homagnostus) obesus Zone. The DICE displays a peak value, in the samples at hand, of –30.45‰δ13Corg in the lower part of the P. atavus Zone. The δ13Corg values increase through the overlying L. laevigata and A. pisiformis zones and display peak values of c. –28.00‰δ13Corg in the lowermost Furongian Olenus wahlenbergi and O. attenuatus subzones. Thereafter the values decrease significantly through the O. scanicus Subzone. Both isotopic excursions have been documented from several palaeocontinents, but never before from Baltica. Moreover, for the first time these excursions are recorded from organic matter in an alum shale setting. The recorded shift of +1.50–2.00‰δ13Corg is approximately half the magnitude of the SPICE documented from other regions. This discrepancy may be related to temporal variations in the type, origin, or diagenesis of the organic fraction analysed.

Carbon chemostratigraphy of the Cambrian-Ordovician transition in a midlatitude mixed platform, Montagne Noire, France

Research Article| July 01, 2008 Carbon chemostratigraphy of the Cambrian-Ordovician transition in a midlatitude mixed platform, Montagne Noire, France J. Javier Álvaro; J. Javier Álvaro † 1Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009-Zaragoza, Spain, and LP3, UMR 8014 CNRS, University of Lille I, 59655-Villeneuve d'Ascq, France †E-mail: Jose-Javier.Alvaro@univ-lille1.fr. Search for other works by this author on: GSW Google Scholar Blanca Bauluz; Blanca Bauluz 2Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009-Zaragoza, Spain Search for other works by this author on: GSW Google Scholar Ignacio Subías; Ignacio Subías 2Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009-Zaragoza, Spain Search for other works by this author on: GSW Google Scholar Catherine Pierre; Catherine Pierre 3Laboratoire d'Océanographie Dynamique et de Climatologie, Université Pierre et Marie Curie, 75252-Paris cedex 05, France Search for other works by this author on: GSW Google Scholar Daniel Vizcaïno Daniel Vizcaïno 47 rue Jean-Baptiste Chardin, Maquens, 11090-Carcassonne, France Search for other works by this author on: GSW Google Scholar GSA Bulletin (2008) 120 (7-8): 962–975. https://doi.org/10.1130/B26243.1 Article history received: 13 Apr 2007 rev-recd: 26 Nov 2007 accepted: 08 Dec 2007 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. Javier Álvaro, Blanca Bauluz, Ignacio Subías, Catherine Pierre, Daniel Vizcaïno; Carbon chemostratigraphy of the Cambrian-Ordovician transition in a midlatitude mixed platform, Montagne Noire, France. GSA Bulletin 2008;; 120 (7-8): 962–975. doi: https://doi.org/10.1130/B26243.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Cambrian Drumian Carbon Isotope Excursion (DICE) and the Steptoean Positive Carbon Isotope Excursion (SPICE) have been so far reported in subtropical platforms: the DICE event is a large negative excursion in δ13Ccarb record that nearly coincides with the beginning of the Drumian (Cambrian Series 3), whereas the SPICE event is a large positive shift in δ13Ccarb identified close to the base of the Paibian (Furongian Series). The chemostratigraphic excursions have been recognized in carbonate-dominated platforms, and their application to mid- and high-latitude, siliciclastic-dominated platforms has been problematic. This paper offers, for the first time, a high-resolution stratigraphic analysis of this time span in a temperate-water platform (Montagne Noire, France) that recorded episodes of carbonate productivity in nearshore environments. Two sections were analyzed in detail: a complete Cambrian Series 3–Tremadocian succession, devoid of carbonate interbeds and paleogeographically located in the distal part of the platform (Pardailhan nappe), and a Furongian-Tremadocian succession, which has limestone interbeds and represents the proximal part of the platform (Minervois nappe).The lower part of the La Gardie Formation in the Pardailhan nappe displays background δ13Corg values of −22‰ punctuated by a large negative, middle Languedocian (regional substage) shift, peaking at −24.5‰. This negative excursion is similar to the DICE event reported close to the base of the Drumian, although the latter is exclusively based on δ13Ccarb values. Stable isotopes of δ13Ccarb and δ13Corg from both sections also indicate a common chemostratigraphic shift at the onset of the Furongian. In nearshore deposits of the Minervois nappe, the δ13Ccarb background values for the interbedded limestone strata of the Val d'Homs Formation gradually increase from −3‰ to −1‰ and are punctuated by a sharp increase in δ13Ccarb values to >3.0‰. By contrast, in basinal shales of the La Gardie Formation in the Pardailhan nappe, the δ13Corg background values decrease from −22‰ to −27.1‰. This trend is directly controlled by total organic carbon (TOC) contents and is interpreted to have been initiated by changes in the degree of biodegradation of organic matter, sulfate reduction, and methanogenesis before or soon after burial. Despite this overprinting and the very low degree of metamorphism reached by the shales, the δ13Corg background trend is also punctuated by a sharp positive shift in δ13Corg values to >4.0‰, similar to the SPICE event. In addition, the asymmetric shape of the SPICE excursion in the Montagne Noire shows evidence for the HERB event, a late Furongian negative carbon isotope excursion not yet accepted as a worldwide chemostratigraphic anomaly.The recognition of both chemostratigraphic shifts in mixed and clayey strata opens new possibilities of chemostratigraphic correlation in mid- and high-latitude platforms, where carbonate factories did not widely develop, and strata only contain endemic fauna. This implication has a major consequence because the Global Stratotype Section and Point (GSSP) of the Drumian and Paibian stages occurs near the base of both excursions. 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The SPICE carbon isotope excursion in Siberia: a combined study of the upper Middle Cambrian–lowermost Ordovician Kulyumbe River section, northwestern Siberian Platform

AbstractAn integrated, high-resolution chemostratigraphic (C, O and Sr isotopes) and magnetostratigraphic study through the upper Middle Cambrian–lowermost Ordovician shallow-marine carbonates of the northwestern margin of the Siberian Platform is reported. The interval was analysed at the Kulyumbe section, which is exposed along the Kulyumbe River, an eastern tributary of the Enisej River. It comprises the upper Ust'-Brus, Labaz, Orakta, Kulyumbe, Ujgur and lower Iltyk formations and includes the Steptoean positive carbon isotopic excursion (SPICE) studied here in detail from upper Cambrian carbonates of the Siberian Platform for the first time. The peak of the excursion, showing δ13C positive values as high as +4.6‰ and least-altered87Sr/86Sr ratios of 0.70909, is reported herein from the Yurakhian Horizon of the Kulyumbe Formation. The stratigraphic position of the SPICE excursion does not support traditional correlation of the boundary between the Orakta and Labaz formations at the Kulyumbe River with its supposedly equivalent level in Australia, Laurentia, South China and Kazakhstan, where theGlyptagnostus stolidotusandG. reticulatusbiozones are known to immediately precede the SPICE excursion and span the Middle–Upper Cambrian boundary. The Cambrian–Ordovician boundary is probably situated in the middle Nyajan Horizon of the Iltyk Formation, in which carbon isotope values show a local maximum below a decrease in the upper part of the Nyajan Horizon, attributed herein to the Tremadocian Stage. A refined magnetic polarity sequence confirms that the geomagnetic reversal frequency was very high during Middle Cambrian times at 7–10 reversals per Ma, assuming a total duration of about 10 Ma and up to 100 magnetic intervals in the Middle Cambrian. By contrast, the sequence attributed herein to the Upper Cambrian on chemostratigraphic grounds contains only 10–11 magnetic intervals.

Fractionation between inorganic and organic carbon during the Lomagundi (2.22–2.1 Ga) carbon isotope excursion

The Lomagundi (2.22–2.1 Ga) positive carbon isotope excursion in shallow-marine sedimentary carbonates has been associated with the rise in atmospheric oxygen, but subsequent studies have demonstrated that the carbon isotope excursion was preceded by the rise in atmospheric oxygen. The amount of oxygen released to the exosphere during the Lomagundi excursion is constrained by the average global fractionation between inorganic and organic carbon, which is poorly characterized. Because dissolved inorganic and organic carbon reservoirs were arguably larger in the Paleoproterozoic ocean, at a time of lower solar luminosity and lower ocean redox state, decoupling between these two variables might be expected. We determined carbon isotope values of carbonate and organic matter in carbonates and shales of the Silverton Formation, South Africa and in the correlative Sengoma Argillite Formation, near the border in Botswana. These units were deposited between 2.22 and 2.06 Ga along the margin of the Kaapvaal Craton in an open-marine deltaic setting and experienced lower greenschist facies metamorphism. The prodelta to offshore marine shales are overlain by a subtidal carbonate sequence. Carbonates exhibit elevated 13C values ranging from 8.3 to 11.2‰ vs. VPDB consistent with deposition during the Lomagundi positive excursion. The total organic carbon (TOC) contents range from 0.01 to 0.6% and δ 13C values range from − 24.8 to − 13.9‰. Thus, the isotopic fractionation between organic and carbonate carbon was on average 30.3 ± 2.8‰ ( n = 32) in the shallow-marine environment. The underlying Sengoma shales have highly variable TOC contents (0.14 to 21.94%) and δ 13C values (− 33.7 to − 20.8‰) with an average of − 27.0 ± 3.0‰ ( n = 50). Considering that the shales were also deposited during the Lomagundi excursion, and taking δ 13C values of the overlying carbonates as representative of the δ 13C value of dissolved inorganic carbon during shale deposition, a carbon isotope fractionation as large as ~ 37‰ appears to characterize the production of bulk organic matter in the deeper part of the Pretoria Basin at that time. This enhanced fractionation relative to that observed in shallow-water environments likely reflects heterotrophic (secondary) and chemotrophic productivity at and below a pronounced redoxcline, consistent with the euxinic conditions inferred from independent evidence for the deeper part of the Pretoria Basin. Greater variability in organic carbon vs. carbonate carbon isotopic values on the shallow-marine carbonate platform suggests that the carbon cycling was dominated by a large dissolved inorganic carbon reservoir during the Lomagundi excursion. Our study suggests that in contrast to the Late Neoproterozoic and Phanerozoic, when carbon isotope fractionation between carbonate and organic carbon in the open ocean was mostly controlled by primary producers, in the Paleoproterozoic redox-stratified ocean heterotrophic and chemotrophic productivity overprinted a signal of primary productivity below the redoxcline. This strong imprint of heterotrophic and chemotrophic productivity on organic carbon isotope records complicates the reconstruction of spatial patterns and secular trends in the δ 13C values of dissolved inorganic carbon in the Paleoproterozoic seawater.

Environmental change during the Late Famennian and Early Tournaisian (Late Devonian–Early Carboniferous): implications from stable isotopes and conodont biofacies in southern Europe

AbstractA biostratigraphic correlation of the Devonian/Carboniferous (D/C) boundary sections from the Carnic Alps, the Graz Palaeozoic, the Montagne Noire and the Pyrenees resulted in a high‐resolution record of the carbon isotopic composition of micrites (δ13Ccarb), of sedimentary organic matter (δ13Corg) and of oxygen isotope ratios of conodont apatite (δ18Ophosph). The studies focused on the interval between the Upper postera Zone (Late Famennian) and the sandbergi Zone (Lower Tournaisian). For the first time, weak but significant positive carbon isotope excursions in micrites and in the sedimentary organic matter is reported from the Middle and Upper expansa zones of the Carnic Alps. They coincide with a decrease in the oxygen isotope values of conodont apatite. The excursions indicate changes in the global carbon cycle during an episode of high seawater temperatures, and correlate with sedimentary change and a stepwise eustatic rise in the Rhenish Massif. High carbon isotope values were also measured in limestones from the Graz Palaeozoic in the Upper praesulcata Zone, which were previously reported from the Rhenish Massif, Carnic Alps, Montagne Noire and the North America continent. The change from a palmatolepid–polygnathid conodont biofacies to a palmatolepid–bispathodid–branmehlid biofacies in the expansa Zone in the Carnic Alps is obviously influenced by anoxic conditions and repeated transgressive phases. The protognathodids and the polygnathids start to radiate in the Upper praesulcata Zone, after the main end‐Famennian extinction episode. This is connected with the disappearance of the palmatolepids, and environmental stress created by worldwide anoxic conditions, climate change and sea‐level changes, stimulated the radiation of both protognathodids and the polygnathids. The regional correlation of the geochemical records as well as interpretations of these records on a global scale indicate that changes in conodont biofacies of Late Famennian–Early Tournaisian limestones were caused by a complex pattern of environmental changes. Copyright © 2008 John Wiley & Sons, Ltd.

Carbon cycle perturbation and stabilization in the wake of the Triassic‐Jurassic boundary mass‐extinction event

The Triassic‐Jurassic boundary mass‐extinction event (T‐J; 199.6 Ma) is associated with major perturbations in the carbon cycle recorded in stable carbon isotopes. Two rapid negative isotope excursions in bulk organic carbon (δ13Corg) occur within the immediate boundary interval at multiple locations and have been linked to the outgassing of 12C‐enriched CO2 from the Central Atlantic Magmatic Province. In British Columbia, a positive δ13Corg excursion of +5‰ (Vienna Peedee belemnite (V‐PDB)) spans part or all of the subsequent Hettangian stage. Here, we examine the significance of these carbon isotope excursions as records of global carbon cycle dynamics across the T‐J boundary and test the link between carbon cycle perturbation‐stabilization and biotic extinction‐recovery patterns. A combination of δ13Corg and palynological analyses from the Late Triassic to Early Jurassic in the Mingolsheim core (Germany) suggests that organic carbon isotope variations are best explained as the result of both compositional changes in terrestrial versus marine input and disturbance and recovery patterns of major terrestrial plant groups across the T‐J boundary. A new high‐resolution δ13Ccarb record from the Val Adrara section in the Southern Alps (Italy) spanning from the uppermost Rhaetian through Lower Sinemurian does not exhibit a negative excursion at the T‐J boundary but does record a large positive δ13Ccarb excursion of +4‰ (V‐PDB) in bulk carbonate that begins at the T‐J boundary and reaches a local maximum at the Early Late Hettangian boundary. Values then gradually decrease reaching +0.5‰ at the Hettangian‐Sinemurian boundary and remain relatively constant into the Early Sinemurian. Complementary δ13Ccarb data from 4 more sections that span the Hettangian‐Sinemurian boundary support carbon cycle stabilization within the Upper Hettangian. Our analyses suggest that isotope changes in organic carbon reservoirs do not necessarily require a shift in the global exogenic carbon reservoir and that the positive excursion in the carbonate carbon isotope record is best explained as the combined result of an increase in atmospheric pCO2 leading to accelerated carbon cycling, decreased skeletal carbonate production, and increased organic carbon burial lasting several hundred thousand years. The termination of the positive inorganic carbon isotope excursion coincides with the recovery of marine skeletal carbonate producers and coeval changes in terrestrial vegetation and reflects the gradual reduction in pCO2 and the stabilization of the global carbon cycle during the Sinemurian.

Rise in seawater sulphate concentration associated with the Paleoproterozoic positive carbon isotope excursion: evidence from sulphate evaporites in the ∼2.2–2.1 Gyr shallow‐marine Lucknow Formation, South Africa

Abstract Past oceanic sulphate concentration is important for understanding how the oceans’ redox state responded to atmospheric oxygen levels. The absence of extensive marine sulphate evaporites before ∼1.2 Gyr probably reflects low seawater sulphate and/or higher carbonate concentrations. Sulphate evaporites formed locally during the 2.22–2.06 Gyr Lomagundi positive δ13C excursion. However, the ∼2.2–2.1 Gyr Lucknow Formation, South Africa, provides the first direct evidence for seawater sulphate precipitation on a carbonate platform with open ocean access and limited terrestrial input. These marginal marine deposits contain evidence for evaporite molds, pseudomorphs after selenite gypsum, and solid inclusions of Ca‐sulphate in quartz. Carbon and sulphur isotope data match the global record and indicate a marine source of the evaporitic brines. The apparent precipitation of gypsum before halite requires ≥2.5 mm L−1 sulphate concentration, higher than current estimates for the Paleoproterozoic. During the Lomagundi event, which postdates the 2.32 Gyr initial rise in atmospheric oxygen, seawater sulphate concentration rose from Archean values of ≤200 μm L−1, but dropped subsequently because of higher pyrite burial rates and a lower oceanic redox state.