δ13C Curve Research Articles

Carbon isotope (δ13Ccarb) stratigraphy of the Lower–Middle Ordovician (Tremadocian–Darriwilian) in the Great Basin, western United States: Implications for global correlation

New stable carbon isotope data (δ13Ccarb) from Lower–Middle Ordovician (Tremadocian to Darriwilian) carbonate mudstone and wackestone rocks of the Pogonip Group are presented from two sections in the Great Basin region (USA) — Shingle Pass (east-central Nevada) and the Ibex area (western Utah). The Pogonip Group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the Late Cambrian (Furongian) to Middle Ordovician (Darriwilian). The Shingle Pass and Ibex area sections have been previously studied for their conodont biostratigraphy and contain a North American Midcontinent conodont fauna that range from the Cordylodus intermedius Zone (uppermost Cambrian) to the Phragmodus polonicus Zone (Darriwilian). The δ13C trend has four distinct characteristics recognized in both Great Basin sections: 1) a drop in δ13C from +1‰ at the base of the Ordovician (Tremadocian) to −0.7‰, 2) a 1 to 2‰ positive δ13C shift in the uppermost Rossodus manitouensis Zone during the late Tremadocian, 3) a gradual δ13C increase from −2‰ to ca. 0‰ during the end of the Early Ordovician (Floian), and 4) a steady δ13C decrease from 0‰ to −4 to −5‰ during Middle Ordovician (Dapingian–Darriwilian).In the Lower Ordovician, δ13C trends reported here from the Great Basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies. Instead, these Lower Ordovician isotope data most likely reflect primary seawater chemistry and changes in δ13C on a global scale. This interpretation is supported by the excellent correlation of δ13C in the Lower Ordovician to other δ13C trends reported from the sections in the Argentine Precordillera (La Silla and San Juan formations) and in western Newfoundland (St. George and Table Head groups). These correlations using δ13C are consistent with published biostratigraphic data and provide an integrated and high-resolution chemo-biostratigraphic framework for the Lower Ordovician sedimentary record of the Laurentian margin. The Middle Ordovician portion of the δ13C curves in the Great Basin represented by the Kanosh and Lehman formations shows significant isotopic depletion relative to the section in Argentina. Thus, although there is some indication that minima and maxima in the Middle Ordovician curves can be correlated, the Great Basin sections show clear evidence of overprinting by local variables related to both diagenesis (dolomitization) and platform restriction.

Carbon and oxygen isotopic composition and foraminifers of condensed basal Zechstein (Upper Permian) strata in western Poland: environmental and stratigraphic implications

The basinal facies of the Lopingian Zechstein Limestone in SW Poland consists of thin (often less than 1 m thick) limestones and/or dolomites, often containing the Kupferschiefer (few tens of centimetres thick) at their base, and local thick (up to 90 m) reefal carbonates. The δ13C curve of these basal Zechstein deposits strongly suggests that even when the Kupferschiefer is lacking, the thin (condensed) sequences record the entire interval of the Zechstein prior to the onset of evaporite deposition, in contrast to the thick reef sequences which lack the characteristic δ13C curve for the lowermost part of the Zechstein. The calcite samples show considerable ranges of δ18O values. If the maximum δ18O values are considered to be the closest to the pristine original ones and if δ18Owater value = 0 is assumed, then the calculated range of palaeotemperatures for the Kupferschiefer and Zechstein Limestone calcite ranges from 19 to 34 °C. The faunal restriction, common dwarf foraminifers and the predominance of lagenids in the foraminiferal assemblage indicate continual dysaerobic conditions and possibly elevated salinity of seawater during deposition of thin basinal Zechstein Limestone deposits. The mixing of shallow and deeper waters in the stratified Zechstein Basin caused by upwelling could result in prolific carbonate precipitation in reefs located at the slope of the marginal carbonate platform of the Zechstein Limestone and in isolated reefs related to palaeohighs within the basin; however, there is no isotopic record of eventual upwelling. Copyright © 2014 John Wiley & Sons, Ltd.

Geochemical, δ13C and Eustatic curves as means for reconstructing and characterising deep marine carbonate stratigraphic sequences (Upper Cretaceous, Tunisia)

A geochemical study of a thick chalk Tunisian onshore section of the Campanian–early Maastrichtian Abiod Formation was undertaken in order to assess the previously proposed linkages between chemostratigraphy, sequence stratigraphy and eustatic variations. An up-to-date correlation of the Abiod Formation δ13C curve at Le Kef to other Tethyan, Boreal and NW Pacific Ocean sections adds important data to improve and revise previous correlations. Fourteen correlation sequences including earlier published tie points are proposed, two of which are considered to be major isotopic events. The CaCO3, Al2O3, MnO, SiO2, TiO2 and Zr contents of Le Kef section were determined in order to assess the vertical elemental variations of such elements in pelagic sediments in response to eustatic fluctuations. For that, a correlation of δ13C, CaCO3, Mn, Ti/Al, Zr/Al and Si/Al of the Campanian–early Maastrichtian sediments with published regional and global sea-level curves is proposed and overall infers that: (1) δ13C can be used for estimating eustatic sea-level cycles and have subsequently permitted to allocate the Abiod Formation, at Le Kef, to five third order cycles (UZA) corresponding to five stratigraphic sequences; (2) sequence boundaries are mainly characterised by minima in Mn–CaCO3 and a low Ti/Al ratio (or Si/Al, Zr/Al); (3) transgressive system tracts are characterised by increasing δ13C, Mn, CaCO3 and Ti/Al ratio; (4) maximum flooding surfaces correspond to a rise in carbonate, manganese and Ti/Al or (Si/Al, Zr/Al); (5) and highstand system tracts show a Ti/Al ratio (or Si/Al, Zr/Al) decrease, a carbonate increase, a manganese rise and a δ13C reaching high levels. These conclusions fairly agree with the idealised relationship between chemostratigraphy, depositional sequences and sea-level curves proposed by Jarvis et al. (J Geol Soc Lond 158:685–696, 2001). Furthermore, the Abiod Formation is a proven reservoir in many oil fields in Tunisia, and we believe that the present sequence stratigraphic characterization, via geochemical signatures, can be of great future usefulness to better identify and correlate similar pelagic and hemi-pelagic horizons.

Kolosjoki Sedimentary Formation: A record in the aftermath of the Paleoproterozoic global positive δ13C excursion in sedimentary carbonates

The Kolosjoki Sedimentary Formation of the Pechenga Greenstone Belt, north eastern Fennoscandia, records the aftermath of the Paleoproterozoic global positive δ13C excursion in sedimentary carbonates (the Lomagundi-Jatuli isotope excursion). The goals of this research were to provide detailed new data on the δ13C trend in the Kolosjoki Sedimentary Formation and generate a secular δ13C curve for the time period immediately succeeding the Lomagundi-Jatuli isotope excursion. For these purposes, sedimentary carbonate samples from two new drillcores representing the Kolosjoki Sedimentary Formation were collected. In total 88 whole-rock samples and 67 micro-drilled subsamples (ca. 1–3mg) were analyzed for the isotope composition of C and O, the whole-rock elemental abundances, and for the composition of the acid-soluble fraction. The Kolosjoki carbonate rocks are dolostone or calcitic dolostone (Mg/Ca=0.50–0.67) with minor limestone (Mg/Ca=0.04–0.06). Only weak correlation was observed between the Mn/Sr ratios and the δ13C and δ18O values, and the carbonate samples have likely retained their near-primary δ13C values. The sedimentary carbonate rocks show first low δ13C values at −2 to −1‰ and then a subtle, upward increasing δ13C trend from ca. 1 to 3‰. The increasing δ13C trend is interpreted to record a recovery from a δ13C minimum following the Lomagundi-Jatuli isotope excursion.

Carbon cycle and sea-water palaeotemperature evolution at the Middle–Late Jurassic transition, eastern Paris Basin (France)

A very high-resolution carbon and oxygen stable isotope analysis (bulk-carbonate) of a biostratigraphically well-constrained Callovian–Oxfordian series is provided here for the first time. The homogeneity of the clayey series and the weak diagenetic alteration allow the isotopic signal variations to be considered as primary in origin. A prominent and brief negative excursion in the δ13C curve (−2‰), occurring at the start of the Middle Callovian (Jason Zone – Obductum Subzone) and correlated regionally, suggests a possible methane release. The increasing δ13C values thereafter up to the Early Oxfordian, concomitant with a warming episode, highlight the burial of carbon in organic-rich layers which, in return, may have triggered a decrease in atmospheric pCO2. At higher frequencies, observed fluctuations of the δ13C and δ18O values are orbitally driven (405-kyr and 100-kyr eccentricity cycles) and may correspond to the salinity and temperature variability recorded in sea water. The δ18O isotopic measurements from well-preserved diagenetically screened belemnites and bivalves along the series, compared to available data from Tethyan domains, agree with the scenario of a global cooling at the Middle-Late Jurassic transition. The well-dated δ18O isotopic curve suggests that the onset of this cooling event occurred at the end of the Coronatum Zone (Middle Callovian).

Graptoloid evolutionary rates track Ordovician–Silurian global climate change

AbstractGraptoloid evolutionary dynamics show a marked contrast from the Ordovician to the Silurian. Subdued extinction and origination rates during the Ordovician give way, during the late Katian, to rates that were highly volatile and of higher mean value through the Silurian, reflecting the significantly shorter lifespan of Silurian species. These patterns are revealed in high-resolution rate curves derived from the CONOP (constrained optimization) scaled and calibrated global composite sequence of 2094 graptoloid species. The end-Ordovician mass depletion was driven primarily by an elevated extinction rate which lasted forc. 1.2 Ma with two main spikes during the Hirnantian. The early Silurian recovery, although initiated by a peak in origination rate, was maintained by a complex interplay of origination and extinction rates, with both rates rising and falling sharply. The global δ13C curve echoes the graptoloid evolutionary rates pattern; the prominent and well-known positive isotope excursions during the Late Ordovician and Silurian lie on or close to times of sharp decline in graptoloid species richness, commonly associated with extinction rate spikes. The graptoloid and isotope data point to a relatively steady marine environment in the Ordovician with mainly background extinction rates, changing during the Katian to a more volatile climatic regime that prevailed through the Silurian, with several sharp extinction episodes triggered by environmental crises. The correlation of graptoloid species diversity with isotopic ratios was positive in the Ordovician and negative in the Silurian, suggesting different causal linkages. Throughout the history of the graptoloid clade all major depletions in species richness except for one were caused by elevated extinction rate rather than decreased origination rate.

The Cenomanian–Turonian boundary on the Saharan Platform (Tunisia and Algeria)

Several transects made of correlated stratigraphic sections and well logs have been constructed spanning southern Tunisia and the Algerian Sahara (Tinrhert) for comparison with earlier results obtained in the Saharan Atlas. The study is based on facies analysis, sedimentology, biostratigraphy focused on ammonites and foraminifers) as well as whole rock geochemistry (δ13C). These suggest that the entire northern Sahara Platform underwent marine flooding that commenced just prior to the onset of the global positive δ13C shift documented for the Cenomanian–Turonian boundary. This flooding occurred in two phases. The first phase is expressed by the deposition of deeper-water, light-coloured bioturbated mudstones overlying the shallow-water deposits comprising the local Cenomanian successions. But in some places in the Central Sahara (Hassi Messaoud area, Tihemboka Arch) as well as in the Saharan Atlas, shallow-water carbonates kept up locally with the relative sea-level rise to build up isolated carbonate platforms. The topographic lows or saddles between these areas could have been formed through differential accumulation rates. During the second phase, flooding resumed and black shales were deposited over the mudstones in the saddles. The occurrence of black shales in these saddles is limited to the northern edge of the platform (Saharan Atlas of Algeria, Gafsa Trough in southern Tunisia). On the platform, this phase is represented by the same kind of mudstones deposited during the first phase of the flooding (southern Tunisia), or by ammonite-rich chalks in the intra-cratonic basin of the Tinrhert (southern Algeria). Black-shale deposition ceased in the early Turonian. Based on the δ13C curve, the latest Cenomanian flooding of the Sahara Platform is roughly coeval with that documented for the US Western Interior.During the first phase of the transgression, that is before the occurrence of the large Whiteinella of the W. archeocretacea Zone in the black shale unit, planktic foraminifers are dominated by small globulose forms of the Hedbergella delrioensis type, associated with Heterohelicidae. Keeled forms (rotaliporids, dicarinellids) are scarce and always very small when present. Perhaps these dwarfed forms were adapted to the restricted environments of the extensive intracratonic seaways crossing the Saharan Platform to the Benoué Trough in Nigeria.

Tracking lateral δ13Ccarb variation in the Paleoproterozoic Pechenga Greenstone Belt, the north eastern Fennoscandian Shield

The Kuetsjärvi Sedimentary Formation (KSF) records the global Paleoproterozoic (ca. 2200–2100Ma) positive δ13C excursion of sedimentary carbonates. This event is here called as the Lomagundi-Jatuli isotopic excursion. In this study, lateral δ13Ccarb variation in the KSF and local basinal factors amplifying global δ13Ccarb value were investigated and a secular δ13C curve for the KSF was constructed. Sedimentary carbonate samples from a new drillcore (Drillcore 5A) from the KSF were analysed for δ13C, δ18O and selected major and trace elements. The δ13C values of the samples vary from 5 to 8‰ (VPDB) and the δ18O values from −18 to −10‰ (VPDB). Most of the samples have retained their primary C isotopic composition, as they do not show correlation between the Mn/Sr ratios and the δ13C and δ18O values. The new results were compared to those previously obtained from another drillcore (Drillcore X) from the KSF (ca. 25km along strike from Drillcore 5A). Both cores show a similar kind of generally upward decreasing trend in δ13C, excluding some negative and positive spikes in Drillcore X and samples close to a contact with a Fe-picrite dyke in Drillcore 5A. A secular δ13C curve of the KSF was constructed based on the least altered δ13C data from the cores 5A and X. The secular δ13C curve of the KSF may represent the latest part of the Lomagundi-Jatuli isotopic excursion.

Carbon and oxygen-isotope stratigraphy of the Early Cretaceous carbonate platform of Pădurea Craiului (Apuseni Mountains, Romania): A chemostratigraphic correlation and paleoenvironmental tool

Stable C and O isotope records were obtained from carbonate rocks spanning the Hauterivian to Cenomanian interval collected in several sections from the carbonate platform of Pădurea Craiului (Apuseni Mountains, Romania). In the absence of some key biostratigraphic marker species, stable isotopes were applied as a tool for stratigraphic correlation and dating. The composite δ13C and δ18O curves for the Early Cretaceous shows variable conditions with large positive and negative excursions and provide information on past environmental changes. The Hauterivian and the Barremian limestones (Blid Formation) display lower δ13C values (−2.8‰ to +2.9‰) relative to the Aptian–Albian deposits (−2.6‰ to +5.4‰) (Ecleja, Valea Măgurii and Vârciorog Formations). The red detrital formation (Albian–Cenomanian) is characterized by a highly variable distribution of the δ13C values (−3.5‰ to +3.9‰). Based on the similarities between the C-isotope curve established in Pădurea Craiului and from other sections in the Tethyan and the Pacific regions, two major oceanic anoxic events characterized by δ13C positive excursions were clearly recognized. The first is the OAE1a event (Early Aptian) in the upper part of the Ecleja Formation and the Valea Măgurii Formation. The second is the OAE1b event (Late Aptian–Albian) in the upper part of the Vârciorog Formation and in the Subpiatră Member. The position of the Aptian/Albian boundary is estimated to be at the upper part of the Vârciorog Formation, immediately after the beginning of the δ13C positive excursion. The δ13C data show major negative excursions during the Barremian (Blid Formation), Early Aptian (Ecleja Formation), and Late Aptian (Vârciorog Formation). The O isotope variation pattern (−10.2‰ to −2.1‰) is consistent with progressively warming temperatures during the Early Barremian followed by a cooling period. A subsequent warming period culminated in the Early Aptian. A significant cooling phase corresponds to the Late Aptian and Early Albian and the climate cooled again during the Late Albian and into the Early Cenomanian stage. The data provide a better understanding of the Early Cretaceous sedimentation cycles in Pădurea Craiului and create a more reliable framework for regional correlations.

"OAE 3" – regional Atlantic organic carbon burial during the Coniacian–Santonian

Abstract. The Coniacian–Santonian time interval is the inferred time of oceanic anoxic event 3 (OAE 3), the last of the Cretaceous OAEs. A detailed look on the temporal and spatial distribution of organic-rich deposits attributed to OAE 3 suggests that black shale occurrences are restricted to the equatorial to mid-latitudinal Atlantic and adjacent basins, shelves and epicontinental seas like parts of the Caribbean, the Maracaibo Basin and the Western Interior Basin, and are largely absent in the Tethys, the North Atlantic, the southern South Atlantic, and the Pacific. Here, oxic bottom waters prevailed as indicated by the widespread occurrence of red deep-marine CORBs (Cretaceous Oceanic Red Beds). Widespread CORB sedimentation started during the Turonian after Oceanic Anoxic Event 2 (OAE 2) except in the Atlantic realm where organic-rich strata continue up to the Santonian. The temporal distribution of black shales attributed to OAE 3 indicates that organic-rich strata do not define a single and distinct short-time event, but are distributed over a longer time span and occur in different basins during different times. This suggests intermittent and regional anoxic conditions from the Coniacian to the Santonian. A comparison of time-correlated high-resolution δ13C curves for this interval indicates several minor positive excursions of up to 0.5‰, probably as a result of massive organic carbon burial cycles in the Atlantic. Regional wind-induced upwelling and restricted deep basins may have contributed to the development of anoxia during a time interval of widespread oxic conditions, thus highlighting the regional character of inferred OAE 3 as regional Atlantic event(s).

A new upper Middle Ordovician–Lower Silurian drillcore standard succession from Borenshult in Östergötland, southern Sweden: 2. Significance of δ13C chemostratigraphy

A total of 239 isotope samples are used for establishing the δ13C chemostratigraphy in the upper Middle Ordovician to Lower Silurian succession in the approximately 70 m long Borenshult drillcore. The study interval starts in the upper Darriwilian Furudal Limestone and ends in the Rhuddanian Motala Formation. Four named δ13C excursions are recognized in 3–4 formations, namely the Guttenberg isotope carbon excursion (GICE) in the lower-middle Freberga Formation, the Kope (Rakvere) excursion in the uppermost Freberga Formation and possibly the Slandrom Formation, the Whitewater (Moe) excursion in the Lower Member of the Jonstorp Formation and the Hirnantian isotope carbon excursion (HICE) in the Loka Formation. The Middle Darriwilian isotope carbon excursion (MDICE) is missing in the drillcore and it is suggested that the study succession starts just above the interval of this excursion, which is in agreement with the range of MDICE in other Baltoscandic successions. The widespread Waynesville (Saunja) excursion may be cut out by the prominent unconformity below the Fjäcka Shale. The two most conspicuous excursions are the GICE (peak value ∼+1.9‰) and the HICE (peak value ∼+3.7‰), whereas the other excursions are represented by relatively minor perturbations in the δ13C curve. The Borenshult drillcore chemostratigraphy, which has been closely tied to conodont biostratigraphy, is quite similar to that of Estonian drillcores and is useful for not only trans-Baltic but also trans-Atlantic correlations. It is the first drillcore-based δ13C chemostratigraphy from the Swedish Ordovician.

Orbital control on carbon cycle and oceanography in the mid‐Cretaceous greenhouse

We established a new high‐resolution carbonate carbon isotope record of the Albian interval of the Marne a Fucoidi Formation (Central Apennines, Italy), which was deposited on the southern margin of the western Tethys Ocean. Bulk carbonate sampled with 10–15 cm spacing was used for the construction of a continuous carbon isotope curve through the Albian stage. Spectral analyses reveal prominent 400 kyr cyclicity in the δ13C curve, which correlates with Milankovitch long eccentricity changes. Cycles occurring in our record resemble those observed in several Cenozoic δ13C records, suggesting that a link between orbital forcing and carbon cycling existed also under mid‐Cretaceous greenhouse conditions. Based on comparisons with Cenozoic eccentricity‐carbon cycle links we hypothesize that 400 kyr cycles in the mid‐Cretaceous were related to a fluctuating monsoonal regime, coupled with an unstable oceanic structure, which made the oceanic carbon reservoir sensitive to orbital variations. In the Tethys these oceanographic conditions lasted until the Late Albian, and then were replaced by a more stable circulation mode, less sensitive to orbital forcing.

Revision of the position of the Ordovician–Silurian boundary in southern Ontario: regional chronostratigraphic implications of δ13C chemostratigraphy of the Manitoulin Formation and associated strata

δ13C values of 142 samples from the Manitoulin Formation and subjacent strata collected from 14 exposures and two drill-cores on Manitoulin Island, Bruce Peninsula, and the region south of Georgian Bay suggest that the Manitoulin Formation is latest Ordovician (Hirnantian) rather than earliest Silurian in age. A δ13C excursion identified as the Hirnantian isotope carbon excursion (HICE), which has a magnitude of nearly 2.5‰ above baseline values, is present in an interval from the upper Queenston Formation to the lower to middle part of the Manitoulin Formation in most of Bruce Peninsula and in the area south of Georgian Bay, whereas on Manitoulin Island the HICE appears to be absent. This indicates that a significant part of the Manitoulin Formation is older on the Bruce Peninsula and in its adjacent region than on Manitoulin Island. The chemostratigraphically based conclusions are consistent with biostratigraphic data from conodonts and brachiopods. The Hirnantian δ13C curve from Anticosti Island, Quebec is closely similar to those of southern Ontario. Traditionally, the Ordovician–Silurian boundary has been placed at the base of the Manitoulin Formation, but the new results suggest that it is more likely to be at, or near, the base of the overlying Cabot Head Formation. These new results have major implications for the interpretation of the geologic history and marine depositional patterns of the latest Ordovician of a large part of the North American Midcontinent.

THE TRIASSIC-JURASSIC BOUNDARY CARBON-ISOTOPE EXCURSIONS EXPRESSED IN TAXONOMICALLY IDENTIFIED LEAF CUTICLES

Abstract A negative stable carbon-isotope excursion (CIE) has been identified at sites across the globe in strata that span the Triassic–Jurassic boundary. Different studies have suggested that this negative CIE could be the result of either a change in vegetation or a massive perturbation in the global carbon cycle at this time. To determine which, 84 hand-picked leaf cuticle fragments from plant macrofossils previously identified to genus level were analyzed for stable carbon-isotope values. The samples were taken from known heights in nine plant beds spanning the Rhaetian–Hettangian (Upper Triassic–Lower Jurassic) at Astartekloft, East Greenland. We have constructed taxon-specific stable carbon-isotope curves for Ginkgoales and Bennettitales and compared these to an existing δ13C curve based on fossil wood from the same section. This study reveals that taxon-specific carbon-isotope curves based on the leaf data from these two seed-plant groups both record the same negative CIE as the fossil wood, despi...

Simulating the biogeochemical effects of volcanic CO2 degassing on the oxygen-state of the deep ocean during the Cenomanian/Turonian Anoxic Event (OAE2)

Cretaceous anoxic events may have been triggered by massive volcanic CO2 degassing as large igneous provinces (LIPs) were emplaced on the seafloor. Here, we present a comprehensive modeling study to decipher the marine biogeochemical consequences of enhanced volcanic CO2 emissions. A biogeochemical box model has been developed for transient model runs with time-dependent volcanic CO2 forcing. The box model considers continental weathering processes, marine export production, degradation processes in the water column, the rain of particles to the seafloor, benthic fluxes of dissolved species across the seabed, and burial of particulates in marine sediments. The ocean is represented by twenty-seven boxes. To estimate horizontal and vertical fluxes between boxes, a coupled ocean–atmosphere general circulation model (AOGCM) is run to derive the circulation patterns of the global ocean under Late Cretaceous boundary conditions. The AOGCM modeling predicts a strong thermohaline circulation and intense ventilation in the Late Cretaceous oceans under high pCO2 values. With an appropriate choice of parameter values such as the continental input of phosphorus, the model produces ocean anoxia at low to mid latitudes and changes in marine δ13C that are consistent with geological data such as the well established δ13C curve. The spread of anoxia is supported by an increase in riverine phosphorus fluxes under high pCO2 and a decrease in phosphorus burial efficiency in marine sediments under low oxygen conditions in ambient bottom waters. Here, we suggest that an additional mechanism might contribute to anoxia, an increase in the C:P ratio of marine plankton which is induced by high pCO2 values. According to our AOGCM model results, an intensively ventilated Cretaceous ocean turns anoxic only if the C:P ratio of marine organic particles exported into the deep ocean is allowed to increase under high pCO2 conditions. Being aware of the uncertainties such as diagenesis, this modeling study implies that potential changes in Redfield ratios might be a strong feedback mechanism to attain ocean anoxia via enhanced CO2 emissions. The formation of C-enriched marine organic matter may also explain the frequent occurrence of global anoxia during other geological periods characterized by high pCO2 values.

DYNAMICS OF A PALEOECOSYSTEM REEF ASSOCIATED WITH OCEANIC CHANGE IN CARBONATE SEDIMENTARY REGIME AND CARBON CYCLING (OXFORDIAN, SWISS JURA)

Abstract Herein we report an analysis of an Oxfordian (Upper Jurassic) paleoreef located in the Swiss Jura Mountains. The paleoreef is located in a Middle Oxfordian transitional interval in which sedimentation switched from marl-dominated to carbonate-dominated deposits. The paleoecosystem is composed of four successive fossil communities characterized by microsolenid corals and organisms that specialized in suspension feeding. Carbon isotopes measured from echinoid spine carbonates exhibit a positive trend from ∼1.0‰ to 2.5‰ in δ13C values from the base to the top of the paleoreef. Comparison of δ13C curves with organic matter and belemnites shows different patterns not compatible with a global variation of the carbon cycle. Similar fossil assemblages and stratigraphic sequences identical in age are found along the continental margin of the Tethys–Atlantic Ocean. This biolithostratigraphic succession corresponds to increasing δ13C values of marine and biogenic carbonates, to the transition from marl-domi...

Facies and diagenetic evaluation of the Permian–Triassic boundary interval and basal Triassic carbonates: shallow and deep ramp sections, Hungary

The Permian–Triassic boundary and basal Triassic shallow-marine successions were studied and correlated in sections of two structural units in Hungary (Transdanubian Range and Bukk units). Core sections in the Transdanubian Range unit recovered inner ramp deposits whereas outcrops in the Bukk unit expose deposits of the deeper ramp area of the western Tethys. The inner ramp section (studied ca. 10 m in thickness) is characterized by a succession of dolomites overlain by bioclastic limestones, peloidal grainstones (which recorded the biotic decline) and oolites with finely crystalline limestone interlayers. The deeper ramp section (studied ca. 15 m in thickness) is characterized by a succession of bioclastic limestones and marlstones, mudstone beds (recording the first biotic decline), the ‘boundary shales’ (recording the second biotic decline and the stable carbon isotope marker), mudstones with wackestone laminae, and stromatolite boundstones. Accordingly, oolite formation and microbial micrite precipitation represent carbonate sedimentary responses of end-Permian mass extinction on the carbonate shelf. In both successions, mudstones predominate the upsection, suggesting a relative sea-level rise. The succession of the deep ramp area exhibits a continuous sediment accumulation and the diagenesis here was influenced by marine and marine-derived pore water. The δ13C curve shows a continuous change towards more negative values, starting in bioclastic limestones, followed by a sharp symmetric negative peak at the second biotic decline that is a chemostratigraphic marker of the boundary event. Facies and microfacies trend of the inner ramp carbonates in the Transdanubian Range unit exhibits close similarities to that found in many South Alpine sections. Relict peloidal deposits, formed cemented submarine hardground substrate, indicate the extinction level. Sedimentary and diagenetic features of the overlying oolite bedset revealed slightly different depositional environments in the two studied Transdanubian Range unit sections. Petrography of the oolites highlighted shallow burial diagenetic alterations which includes marine cementation, marine-burial replacement and dolomitization. A lack of the specific negative peak in the δ13C values is most likely due to the multiple redeposition events of the sedimentary grains. This led to the conclusion that the deeper ramp deposits (e.g., in Bukk unit) have greater potential for recognizing trends in processes, affecting the marine environments and related to the end-Permian mass extinction, at the western Tethys.

The new chronostratigraphic classification of the Ordovician System and its relations to major regional series and stages and to δ 13 C chemostratigraphy

The extensive work carried out during more than a decade by the International Subcommission on Ordovician Stratigraphy has resulted in a new global classification of the Ordovician System into three series and seven stages. Formal Global Boundary Stratotype Section and Points (GSSPs) for all stages have been selected and these and the new stage names have been ratified by the International Commission on Stratigraphy. Based on a variety of biostratigraphic data, these new units are correlated with chronostratigraphic series and stages in the standard regional classifications used in the UK, North America, Baltoscandia, Australia, China, Siberia and the Mediterranean-North Gondwana region. Furthermore, based mainly on graptolite and conodont zones, the Ordovician is subdivided into 20 stage slices (SS) that have potential for precise correlations in both carbonate and shale facies. The new chronostratigraphic scheme is also tied to a new composite δ13C curve through the entire Ordovician.

The Cenomanian Turonian of the Wunstorf section (North Germany): global stratigraphic reference section and new orbital time scale for Oceanic Anoxic Event 2

The Cenomanian–Turonian Boundary Event (CTBE) is reflected by one of the most extreme carbon cycle perturbations in Earth's history and is characterized by the widespread occurrence of sediments indicating oxygen deficiency in oceanic waters (Oceanic Anoxic Event 2 = OAE 2). At Wunstorf (northern Germany) the CTBE is represented by a 26.5 m thick sedimentary succession consisting of rhythmically bedded laminated black shales, dark organic-rich marls and marly limestones yielding abundant micro- and macrofossils, making the locality particularly well suited to serve as an international standard reference section for the CTBE. In 2006 a newly drilled continuous core recovered 76 m of middle Cenomanian to middle Turonian sediments. A high-resolution carbonate δ13C curve derived from core samples resolves all known features of the positive δ13C anomaly of OAE 2 with high accuracy. Throughout the middle Cenomanian – middle Turonian succession, the δ13C curve shows numerous small-scaled positive excursions, which appear to be cyclic. High-resolution borehole geophysics and XRF core scanning were performed to generate two time series of gamma-ray data and Ti concentrations for the CTBE black shale succession. Hierarchical bundling of sedimentary cycles as well as spectral analysis and Gaussian filtering of dominant frequencies reveal cycle frequency ratios characteristic for short eccentricity modulated precession (100 kyr, 21 kyr). This new orbital time scale provides a time estimate of 430–445 kyr for the duration of OAE 2 and refines the existing orbital age models developed at localities in the English Chalk, the Western Interior Basin and the Tarfaya Basin. Based on the new age model and high-resolution carbon isotope correlation, our data allow for the first time a precise basin-wide reconstruction of the palaeoceanographic modifications within the European shelf sea during OAE 2.

Correlation of carbon isotope events in the Danish Upper Cretaceous chalk

A high resolution carbon isotope (δ13C) profile through the upper Campanian to Maastrichtian chalk was recently completed based on material from the Stevns-1 core from the Stevns peninsula, eastern Denmark. The δ13C variation of marine carbonates essentially reflects global perturbations in the carbon cycle, i.e. the burial fluxes of carbonate carbon versus organic carbon. It is widely observed that the δ13C variation broadly tracks the eustatic sea-level curve, and that δ13C curves can be used for stratigraphic correlation (e.g. Jarvis et al. 2002). In the Stevns-1 core, a total of 29 notable isotope changes have been identified in the upper Cam panian to Maastrichtian succession. In order to evaluate the stratigraphic significance of the isotope changes, the variation in δ13C values of the mid-Maastrichtian chalk from cores in eastern Denmark and the Danish North Sea, and from outcrops at Rørdal, northern Jylland has been examined (Fig. 1). The selected interval is characterised by distinct chalk and marl cycles in the Stevns-1 and Karlslunde-1 cores and in the Rørdal quarry (Fig. 2), whereas a non-cyclic clean chalk is found in the M-10X well from the North Sea. In the Rørdal quarry, the chalk–marl unit spans the upper–lower Maastrichtian boundary in the Boreal brachiopod and belemnite stratigraphies (Surlyk 1984; unpublished data, B. Lauridsen & F. Surlyk). In Stevns-1 and Karlslunde-1 the chalk–marl unit was deposited during the younger part of nannofossil subzone UC20b (Sheldon 2006, in press). This paper presents preliminary results of a high-resolution study of carbon isotopes, carried out by the Geological Survey of Denmark and Greenland (GEUS) in co-operation with partners from the Department of Geography and Geology at the University of Copenhagen. This paper is a product of the Cretaceous Research Centre (CRC) at Geo center Denmark.