Carbon Excursion Research Articles

Northward range extension of a diminutive-sized mammal (Ectocion parvus) and the implication of body size change during the Paleocene–Eocene Thermal Maximum

An abrupt global warming event marks the Paleocene–Eocene boundary, known as the Paleocene–Eocene Thermal Maximum (PETM). The event is distinguished in the strata globally by a significant negative excursion of δ13C ratio values. The response of the terrestrial biota to the abrupt climatic change has been well studied in northern Wyoming in the Bighorn Basin, where it has been observed that the mammalian fauna during the global warming event is represented by smaller, but morphologically similar species to those found later in the Eocene. Various hypotheses have been proposed to explain the observation smaller body sizes during the global warming event. In this article, evidence is presented to support the hypothesis that the observed body size decrease during the PETM was influenced by the appearance of smaller southern species who extended their geographic range northward during the abnormal global warming event. Using disperse organic carbon isotopic ratios of bulk sediment, the negative excursion of δ13C was located in the Piceance Creek Basin of western Colorado, 400km to the south of the Bighorn Basin. Below the stratigraphic level marking the negative carbon excursion in the Piceance Creek Basin are five specimens of the phenacodontid mammal (Ectocion parvus), a diminutive species of the genus Ectocion restricted to the basal Eocene (Wa-0 Biozone) in northern Wyoming. The five specimens of E. parvus are associated with a late Paleocene (Clarkforkian) mammalian fauna in Colorado, implying that the diminutive species extended its geographic range northward during the global warming event. This evidence supports biogeographic models that assume poleward biogeographic shifts during global warming events, and will have modern day implications for the conservation of species as global temperatures rise in the near future.

Late Ordovician–Early Silurian δ13C chemostratigraphy in the Upper Mississippi Valley: implications for chronostratigraphy and depositional interpretations

ABSTRACTA pioneer study of the previously unknown δ13C chemostratigraphy in the Ordovician/Silurian boundary interval in eastern Iowa and northeastern Illinois resulted in the discovery of the Hirnantian Isotope Carbon Excursion (HICE). The presence of this major isotope excursion in the Mosalem Formation in Iowa and the Wilhelmi Formation in Illinois, which indicates that the excursion interval in these units is of Hirnantian (latest Ordovician) rather than Early Silurian age, necessitates a revised chronostratigraphic classification of these units. Although the precise level of the Ordovician/Silurian boundary remains somewhat uncertain in the absence of the diagnostic graptolites, it is herein placed in the upper part, but well below the top, of the Mosalem Formation and at the top of the Wilhelmi Formation. During a major regression following the deposition of the Maquoketa Shale, the upper part of the latter clastic unit was in some places deeply eroded, resulting in a topographically dissected landscape with upland areas separated by wide incised valleys. During a subsequent late Hirnantian transgression, these palaeovalleys were gradually filled with marine sediments, but the upland areas were not transgressed until earliest Silurian times. The new chemostratigraphical evidence is in good agreement with the available biostratigraphical data, especially from corals, conodonts, and brachiopods. A preliminary chemostratigraphical study of the presumably coeval Edgewood Group successions in Pike County, northeastern Missouri failed to document any heavy δ13C values characteristic of the HICE and some, or all, of the Hirnantian values obtained there may be diagenetically overprinted.

Productivity response of calcareous nannoplankton to Eocene Thermal Maximum 2 (ETM2)

Abstract. The Early Eocene Thermal Maximum 2 (ETM2) at ~53.7 Ma is one of multiple hyperthermal events that followed the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma). The negative carbon excursion and deep ocean carbonate dissolution which occurred during the event imply that a substantial amount (103 Gt) of carbon (C) was added to the ocean-atmosphere system, consequently increasing atmospheric CO2(pCO2). This makes the event relevant to the current scenario of anthropogenic CO2 additions and global change. Resulting changes in ocean stratification and pH, as well as changes in exogenic cycles which supply nutrients to the ocean, may have affected the productivity of marine phytoplankton, especially calcifying phytoplankton. Changes in productivity, in turn, may affect the rate of sequestration of excess CO2 in the deep ocean and sediments. In order to reconstruct the productivity response by calcareous nannoplankton to ETM2 in the South Atlantic (Site 1265) and North Pacific (Site 1209), we employ the coccolith Sr/Ca productivity proxy with analysis of well-preserved picked monogeneric populations by ion probe supplemented by analysis of various size fractions of nannofossil sediments by ICP-AES. The former technique of measuring Sr/Ca in selected nannofossil populations using the ion probe circumvents possible contamination with secondary calcite. Avoiding such contamination is important for an accurate interpretation of the nannoplankton productivity record, since diagenetic processes can bias the productivity signal, as we demonstrate for Sr/Ca measurements in the fine (<20 μm) and other size fractions obtained from bulk sediments from Site 1265. At this site, the paleoproductivity signal as reconstructed from the Sr/Ca appears to be governed by cyclic changes, possibly orbital forcing, resulting in a 20–30% variability in Sr/Ca in dominant genera as obtained by ion probe. The ~13 to 21% increase in Sr/Ca above the cyclic background conditions as measured by ion probe in dominating genera may result from a slightly elevated productivity during ETM2. This high productivity phase is probably the result of enhanced nutrient supply either from land or from upwelling. The ion probe results show that calcareous nannoplankton productivity was not reduced by environmental conditions accompanying ETM2 at Site 1265, but imply an overall sustained productivity and potentially a small productivity increase during the extreme climatic conditions of ETM2 in this portion of the South Atlantic. However, in the open oceanic setting of Site 1209, a significant decrease in dominant genera Sr/Ca is observed, indicating reduced productivity.

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.

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.

Chromium isotopes in carbonates — A tracer for climate change and for reconstructing the redox state of ancient seawater

Strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic cross correlations of time-equivalent sedimentary sequences and for calibration of the compositional evolution of seawater throughout Earth's history, mainly for the purpose of reconstructing ancient climatic changes. We here present results of a new isotopic tracer system – stable chromium isotopes – applied to a late Ediacaran marine carbonate sequence exposed in the Calera de Recalde syncline, Arroyo del Soldado Group, Uruguay. The aim was to compare Cr isotope signatures directly to δ 13C, 87Sr/ 86Sr and 143Nd/ 144Nd fluctuations in a well defined stratigraphic profile comprising sediments that were deposited during cold–warm periods accompanied by sea-level changes in response to glaciation–deglaciation at higher latitudes. The studied section is characterized by a pronounced negative (down to − 3.3‰) δ 13C excursion in carbonates paralleled by a decrease of 87Sr/ 86Sr values. Chromium isotope signatures over this section also show a correlated decrease in δ 53Cr (δ 53Cr = [(( 53Cr/ 52Cr) sample / ( 53Cr/ 52Cr) SRM979) − 1] × 1000) values from ~+0.29 to − 0.17‰ which mirrors a decrease in positively fractioned seawater signatures to slightly negative values characteristic of high-temperature magmatic sources. Linear correlations between δ 53Cr and εNd(570 Ma), 87Sr/ 86Sr and Cr concentrations can be explained by mixing between two major input sources of Cr, Nd and Sr into the shallow seawater: 1) a source characterized by negative δ 53Cr values of ~−0.2‰ , low 87Sr/ 86Sr values of ~ 0.707, and elevated 147Sm/ 144Nd values of ~ 0.13, recognized as a subaqueous hydrothermal dominated input source, and 2) a source characterized by positively fractionated δ 53Cr values of ~+0.2‰, higher 87Sr/ 86Sr values of ~ 0.708, and lower 147Sm/ 144Nd values of ~ 0.11, a source which is strongly affected by continentally derived input. Chromium isotopes provide a powerful tool for reconstructing the redox state of ancient seawater since positive values indicate that, at least locally, Neoproterozoic shallow ocean waters were sufficiently oxidized to fractionate chromium and/or that oxygen levels of the atmosphere were sufficient to transform Cr(III) into the more mobile hexavalent Cr(VI) formed during weathering processes on land. The fact that 87Sr/ 86Sr values, despite δ 13C fluctuations, remain low (indicative of a strong hydrothermal input into the basin at his time) implies that CO 2 limitation was the cause of negative δ 13C and δ 53Cr excursions in otherwise nutrient rich late Neoproterozoic basins, and that glaciation is only one more consequence of a tectonically driven, biologically mediated system. In such a scenario, glaciation acts as an amplifier of δ 53Cr signals. These signals in marine carbonates are a sensitive tracer for redox processes in the ocean and/or on land and have the potential to contribute significantly, in combination with the other commonly used isotopic tracers, to the reconstruction of climatic changes, particularly those that are associated with major glaciation periods in Earth's history.

A time like our own? Radioisotopic calibration of the Ordovician greenhouse to icehouse transition

Tiered interpolation, a new timescale methodology, was used to construct the first radioisotopically-calibrated composite δ 13C curve for the Ordovician period using sanidine 40Ar/ 39Ar age determinations and existing U–Pb geochronology and biostratigraphic zonation. Tiered interpolation intercalates and temporally scales the numerical age of lithostratigraphic horizons by conducting a series of nested projections between hierarchical temporal control points. For primary control points, new 40Ar/ 39Ar ages and legacy U–Pb geochronology were screened to avoid analyses affected by inheritance and daughter loss and calibrated to reflect modern decay constants and standard values. Ages for secondary, tertiary, etc.… control points are obtained via linear interpolation of between higher order control points. In scaling the Ordovician δ 13C composite, the following control point order was applied: (1) radioisotopic ages (2) graptolite Zones, (3) index taxa-based on speciation events (North Atlantic conodont Zones), (4) North American Mid-continent conodont zones, and (5) stratal thicknesses at δ 13C sampled sections. The resulting timescale utilizes the highest resolution of each component, is internally consistent, and is re-scalable as more precise radioisotopic ages become available. It provides a robust framework for independently assessing the accuracy of biostratigraphic composite timescales because it does not rely an assumption of quasi-continuous sediment accumulation and/or speciation. To better calibrate the Late Ordovician and resolve a discrepancy between U–Pb and 40Ar/ 39Ar ages, three new 40Ar/ 39Ar ages were determined via the laser fusion of multiple single sanidine phenocrysts from three bentonitic ash beds from the Late Ordovician marine strata of the upper Mississippi valley where the record of Taconic volcanism is most complete. Fusions of 275 individual sanidine crystals from the Millbrig, Dygerts, and Rifle Hill bentonites yield largely Gaussian apparent age distributions with a small number of readily identified outliers and stratigraphically-consistent weighted mean ages of 454.1 ± 1.4 Ma (51 of 57), 450.7 ± 1.4 Ma (39 of 74), and 450.3 ± 1.9 Ma (96 of 144) for the Millbrig, Dygerts, and Rifle Hill bentonites, respectively (2σ analytical uncertainties relative to 28.201 Ma for FCs). The Millbrig age is consistent with the existing U–Pb ages for both the underlying Deicke bentonite and the Kinnekulle bentonite of Sweden. The new age model permits the assembly of the first complete radioisotopically-calibrated composite δ 13C curve for the Ordovician, the first icehouse to occur subsequent to the Cambrian explosion. The resulting δ 13C composite integrates all available graptolite and conodont biostratigraphic with radioisotopic ages and indicates that previous biostratigraphic composites incorporate 2σ errors up to ~ 5 Ma. When viewed without temporal distortions, isotopic carbon excursions (ICEs) in the Ordovician appear to have occurred at a similar tempo as ICEs in the better resolved Cenozoic greenhouse to icehouse transition. Although boundary conditions for oceanography, biogeography, and continental configuration are strikingly different, the tempo of isotopic changes, growth of south-polar ice sheets, and concurrent oceanic and geomorphic responses bears both similarities and differences with the better understood Cenozoic era.

Acritarchs from the Ordovician–Silurian boundary beds of the Valga-10 drill core, southern Estonia (Baltica) and their stratigraphical and palaeobiogeographical implications

Fourteen samples of the Valga-10 drill core, south Estonia, from the lower Jelgava Formation (middle Pirgu Regional Stage, Upper Katian) to the lowermost Õhne Formation (lowermost Juuru Regional Stage, Lower Rhuddanian) were investigated for acritarchs. The section is biostratigraphically and chemostratigraphically well constrained; it comprises the rugata, taugourdeaui and scabra chitinozoan zones, the ordovicicus and giradeauensis conodont zones and the Hirnantian Isotopic Carbon Excursion (HICE). The good preservation allowed the identification of three prasinophyte phycomata and 52 acritarch species including the four new species Evittia porkuniensis, Helosphaeridium tongiorgii, Nexosarium leherissei and ? Veryhachium bulliferum. One new combination is proposed: Poikilofusa obliquipunctata (Uutela & Tynni 1991) comb. nov. Comparison with contemporaneaous palynofloras shows that eastern Laurentia and Baltica share a high number of species during the latest Katian–Hirnantian. Some of these species show a potential for future improvement of biostratigraphical correlation between the uppermost Katian–Hirnantian strata of low to mid-latitude carbonate platforms in eastern Laurentia and Baltica. Conversely, significant taxonomic differences exist between the assemblage studied and typical Gondwanan palynofloras. These results suggest that the Laurentian/Baltic and Gonwanan phytoplanktonic palaeoprovinces existed during latest Ordovician times. Published data reveal similar distribution pattern for chitinozoans and graptolites during the Hirnantian. A bathymetric ridge rise associated with the opening of the Rheic Ocean, coupled with the Hirnantian glacially-driven sea-level fall might have prevented water mass exchange and mixing/migration of phytoplankton between Gondwana and Laurentia/Baltica, thus being at the origin of the observed acritarch bioprovincialism. Additionally, distribution and diversity patterns of acritarchs are compared to those of other microfossils in the Valga-10 section. Near the base of the Hirnantian (Porkuni Regional Stage), benthic organisms (ostracods and scolecodonts) and phytoplankton (acritarchs) show increasing diversity with appearances of new taxa and new morphologies. Planktonic (chitinozoans) and nektonic (conodonts) organisms show a different pattern, with a decline in diversity during the earliest Hirnantian and a marked increase in the later part of the stage. Two alternative hyptotheses are proposed to explain these differences in diversification patterns: (1) the development of a shallower, proximal environment in the locality studied during the Hirnantian glaciation may have been more favourable to the diversification of benthonic organisms; (2) the planktonic and nektonic organisms suffered the effects of glaciation more than benthonic ones.

Lower Katian (Upper Ordovician) δ13C chemostratigraphy, global correlation and sea-level changes in Baltoscandia

A long-standing problem in the Ordovician stratigraphy of south-eastern Norway has been to the relations between the Mjøsa Formation in the Lake Mjøsa region and coeval strata in the Oslo region. The recent discovery of the globally distributed Guttenberg δ13C excursion (Guttenberg Isotopic Carbon Excursion) in the Mjøsa region provided the impetus to search for this excellent chemostratigraphic marker in the classical Oslo region succession, where it was found in the Frognerkilen Formation. Another positive δ13C excursion, which we identify as the Kope excursion, was discovered in the Solvang Formation. The new data show that the lower Katian δ13C chemostratigraphy in the Oslo region is closely similar to that from south-eastern and southern Estonia. This permits detailed correlations across Baltoscandia, which are useful for recognising the Baltic stage boundaries in the Oslo region succession. Both the Lake Mjøsa and Oslo regions study successions can be chemostratigraphically correlated with those in North America and eastern Asia. The newly established stratigraphic relations in the Oslo region are also used for a re-assessment of lower Katian local and eustatic sea-level changes.

Geochemical properties of Neoproterozoic “cap dolomites” in the Patom paleobasin and problem of their genesis

The characteristic feature of many Upper Neoproterozoic glacial sequences is their “cap carbonates” (CC) resting without visible unconformity upon glaciogenic diamictites. Such an unusual association, peculiar structures and textures, and negative δ13C values (approximately −4 ± 2‰) that are atypical of marine carbonates provoked long debates about the nature of these carbonates, which play an important role in the Snowball Earth hypothesis. According to this hypothesis, the Earth was entirely covered by ice during large-scale glaciations, and CC accumulation was related to the global change in geochemical processes. In this work, we discuss data on the chemical and isotopic (C, O, Sr) compositions of CCs, which overlie glacial sediments of the Nichatka and Bol’shoi Patom formations accumulated in different parts of the Neoproterozoic Patom paleobasin (Central Siberia). High concentrations of Fe (up to 6400 ppm), Mn (2320 ppm), and radiogenic Sr (87Sr/86Sr0 up to 0.7172) established in CCs indicates a strong influence of the continental flow. Extraordinary Snowball Earth conditions are not necessary for the accumulation of these rocks, geochemical and sedimentological properties of which may be explained by the discharge of thawing waters into partly or completely isolated near-glacier basin, their intermittent freezing, and/or washout of “frozen” carbonates from the surface of thawing glaciers. The peculiar thin-laminated texture of CC may be related to seasonal processes of climatic cycles. They were accumulated in the course of general (relatively long-term) depletion of the atmosphere and hydrosphere in 13C, which has nothing to do with the CC formation as a specific type of carbonate sediments. Amplitude and duration of the negative δ13C excursion in carbonates associated with the Lower Vendian glacial sediments (665–635 Ma) are appreciably lower than the negative anomaly in rocks of the Zhuya Group that likely correspond to the Shuram-Vonoka Event (∼560−580 Ma ago), which probably marks the crucial point in the Precambrian deglaciation: mass destabilization of methane hydrates and degradation of the Early Vendian psychrosphere in oceans.

Chitinozoan biostratigraphy of a new Upper Ordovician stratigraphic framework for Anticosti Island, Canada

Thickness and facies variations in the Upper Ordovician succession of Anticosti Island have made correlations between the western and eastern ends of the island difficult. This chitinozoan study reveals that most strata of the Ellis Bay Formation at the eastern end of the island correlate with the Vaureal Formation at the western end. In the eastern end, chitinozoans typical of the Ellis Bay Formation occur only in the upper part of the Lousy Cove Member, whereas chitinozoans diagnostic of the mid-upper Vaureal occur in the rest of the formation. Moreover, the eastern Schmitt Creek and Mill Bay members of the Vaureal Formation are older than the uppermost Vaureal eponym members in the western end. New east-west correlations, supported by geochemical, sedimentological, and sequence stratigraphic data, are proposed. Comparison with coeval Upper Ordovician successions suggests a Hirnantian age for the gamachiana Zone that occurs in the lower members of the Ellis Bay Formation. It also indicates that the Hirnantian Isotopic Carbon Excursion (HICE) on Anticosti begins in the D. anceps Zone, just below the base of the Hirnantian Stage, reaches a first peak in the lower N. extraordinarius Zone, a second modest peak high in the N. extraordinarius Zone, and a third, more important one, in the lower part of the N. persculptus Zone. These results are in agreement with the biostratigraphic and chemostratigraphic data at the Global Boundary Stratotype Section and Points (GSSPs) for the base of the Hirnantian and Silurian in south-central China and at Dob9s Linn in Scotland.

Astronomical tuning of the La Vedova High Cliff section (Ancona, Italy)—Implications of the Middle Miocene Climate Transition for Mediterranean sapropel formation

Continuous marine successions covering the Middle Miocene Climate Transition (MMCT; ∼ 15–13.7 Ma) are scarce and the lack of a high-resolution magnetobiostratigraphic framework hampers the construction of astronomically tuned age models for this time interval. The La Vedova High Cliff section, exposed along the coast of the Cònero Riviera near Ancona (Italy), is one of the few Mediterranean sections covering the critical time interval of the MMCT. Starting from an initial magnetobiostratigraphic age model, a robust astronomical tuning was constructed for the interval between 14.2 and 13.5 Ma, using geochemical element data and time series analysis. A shift in δ 18O of bulk sediment towards heavier values occurs between ∼ 13.92 and 13.78 Ma and could be related to the Mi3b oxygen isotope event, which reflects the rapid expansion of the East Antarctic Ice Sheet in the middle Miocene. The onset of the CM6 carbon excursion is reflected in the bulk record by a rapid increase in δ 13C at 13.86 Ma. Our results confirm the proposition that these events coincide with a 405-kyr minimum in eccentricity and a node in obliquity related to the ∼ 1.2 Myr cycle. From 13.8 Ma onwards, distinct quadruplet cycles containing sapropelitic sediments were deposited. This may suggest a causal connection between the main middle Miocene cooling step and the onset of sapropel formation in the Mediterranean.

Global nature of the Paleoproterozoic Lomagundi carbon isotope excursion: A review of occurrences in Brazil, India, and Uruguay

Positive carbon isotope excursion is reported from Paleoproterozoic carbonates of the Aravalli Supergroup (northwestern India), the Minas Supergroup (Brazil), and new sections of the Paso Severino Formation (Uruguay). The 2.42 Ga Gandarela Formation, Minas Gerais, Brazil, contains red carbonate-facies BIF grading into dolostones and limestones and yielding δ 13C values ranging from −1.6 to +0.4‰ V-PDB. The positive C-isotope excursion (up to + 11‰ V-PDB) in marine shallow-water carbonates in India and Brazil (Jhamarkotra Formation in northwestern India, and Cercadinho and Fecho do Funil formations in Minas Gerais State, Brazil) is comparable to that observed in 2.22–2.1 Ga carbonate successions worldwide that were deposited during the Lomagundi excursion. In Uruguay, δ 13C values up to +11.6‰ V-PDB in the deep-water Paso Severino Formation of the Piedra Alta Terrane are compatible with deposition at ca. 2.15 Ga, as indicated by the 2146 ± 7 Ma U–Pb age of dacites occurring at the top of the unit. Negative δ 13C values are also present in carbonates of the Paso Severino Formation, but an origin related to organic-matter remineralization cannot be ruled out. Thin carbonate beds in the Rio Itapicuru greenstone belt, Bahia State, Brazil, are associated, as in the Paso Severino Formation, with deep-water black shales and have carbon isotope values up to +9‰ V-PDB. High metamorphic grade carbonates of the Jacurici terrane in the Medrado-Ipueira area, Bahia, Brazil, have carbon isotope values up to +6.9‰ V-PDB, consistent with their minimum age of 2085 ± 5 Ma inferred from the intrusive contact with and the age of the Medrado norite. No evidence was found in India, Brazil, or Uruguay for Paleoproterozoic glacial events recognized in the 2.45–2.22 Ga sedimentary successions worldwide. Unconformities between the Gandarela and Cercadinho formations in Brazil and the banded gneissic Complex and the Lower Aravalli Supergroup in India might explain the absence of glacial record. Compositional and isotopic data presented here for studied Paleoproterozoic carbonate successions allow their integration into the global record of the Paleoproterozoic evolution as well as correlation with other successions of similar age. The study highlights the global nature of the Lomagundi excursion. Furthermore, it indicates that the Lomagundi excursion is recorded in both shallow-water (Aravalli and Minas supergroups) and deep-water carbonates (Paso Severino Formation and Rio Itapicuru greenstone belt) negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow-water, open-marine and isolated basins.

Strontium and carbon isotope stratigraphy of the Llandovery (Early Silurian): Implications for tectonics and weathering

A high-resolution 87Sr/ 86Sr curve and paired δ 13C carbonate-organic data set is generated for the Llandovery Series from the Ikla drill core in Estonia. A δ 13C carbonate curve is also presented from the Pancake Range in Nevada. Observed 87Sr/ 86Sr values in the Ikla drill core are at a minimum in the early Llandovery Rhuddanian Stage (∼ 0.7079 to 0.7080), and then trend to more radiogenic ratios in the basal part of the Telychian Stage. An 87Sr/ 86Sr high near ∼ 0.7084 is observed in the Telychian at the top of the studied section. The range of values is in general agreement with the data from previous sample sets of brachiopods and conodonts recovered from localities in North America and Europe that record a rising trend in the 87Sr/ 86Sr ratio throughout the Llandovery from approximately 0.7080 to 0.7084. The major increase in the 87Sr/ 86Sr ratio during the late Llandovery may be due to weathering of radiogenic source rocks that were uplifted during early Silurian continent–continent collisions. The Sr rise potentially coincides with the occurrence of an unusually thick sequence of K-bentonite beds representing large-magnitude ash falls in the early Telychian. A previously documented negative δ 13C excursion in marine carbonates in the lower Telychian interval of the Ikla core is quasi-synchronous with the increase in 87Sr/ 86Sr. Our new organic matter δ 13C data from the Ikla core confirm that this negative δ 13C carbonate excursion is not a result of diagenesis. Furthermore, a negative δ 13C excursion in carbonates from the early Telychian portion of the Pancake Range section in Nevada seems to confirm the global scope of this carbon cycle perturbation.

Global Upper Ordovician correlation by means of δ13C chemostratigraphy: implications of the discovery of the Guttenberg δ13C excursion (GICE) in Malaysia

AbstractApart from a single study of the early Katian δ13C chemostratigraphy in two regions in China, no investigations of the Sandbian and Katian chemostratigraphy have been published from anywhere in Asia. A recent study of the conodont biostratigraphy of the classical Ordovician succession on Langkawi Islands, peninsular Malaysia, showed the presence there of strata coeval with those having the Guttenberg Carbon Excursion (GICE) on the Yangtze Platform. In an effort to establish for the first time the presence of this widespread δ13C excursion in southern Asia, a series of samples from the upper part of the Kaki Bukit Formation was isotopically analysed. This resulted in the discovery of a conspicuous δ13C excursion with peak values of ~ 2 ‰ above the baseline values. The excursion is located just above theBaltoniodus alobatusSubzone and near the level of the first appearance ofHamarodus europaeus, hence the same stratigraphic position as the GICE on the Yangtze Platform. Using the GICE, the Malaysian study interval is closely correlated with the GICE intervals at three localities representing an approximately 23 000 km long transect from Malaysia across Baltoscandia to central North America. This shows the usefulness of δ13C chemostratigraphy to clarify previously obscure stratigraphic relationships between geographically very widely separated localities.

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.

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 (SkAyenlberg Limestone) belonging to the Amorphognathus superbus Zone. New stable isotope data from the Amtjarn quarry shows that this is immediately after the peak of the Guttenberg Carbon Isotope Excursion (GICE), which reaches a delta C-13 peak value at 3.3aEuro degrees 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.

Paleogene paleosols and changes in pedogenesis during the initial Eocene thermal maximum: Big Bend National Park, Texas, USA

Research Article| November 01, 2008 Paleogene paleosols and changes in pedogenesis during the initial Eocene thermal maximum: Big Bend National Park, Texas, USA Paul D. White; Paul D. White † 1Physics Department, Community College of Rhode Island, 400 East Avenue, Warwick, Rhode Island, 02886, USA †E-mail: pdwhite1@ccri.edu Search for other works by this author on: GSW Google Scholar Judith Schiebout Judith Schiebout 2Museum of Natural Science and the Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2008) 120 (11-12): 1347–1361. https://doi.org/10.1130/B25987.1 Article history received: 10 Feb 2006 rev-recd: 14 Dec 2006 accepted: 25 Jul 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 Paul D. White, Judith Schiebout; Paleogene paleosols and changes in pedogenesis during the initial Eocene thermal maximum: Big Bend National Park, Texas, USA. GSA Bulletin 2008;; 120 (11-12): 1347–1361. doi: https://doi.org/10.1130/B25987.1 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 This paper tests the hypothesis that continental chemical weathering increased during the initial Eocene thermal maximum (IETM) by comparing paleosols that formed before and during the event in western Texas. The chemical index of alteration (CIA) was used to investigate the weathering of silicate minerals. Paleosols generated before the IETM have CIA values ranging from 62 to 72, and CIA values during the IETM range from 67 to 82. The CIA values for pre–initial Eocene thermal maximum paleosols indicate moderate weathering conditions, and CIA values during the event indicate moderate to extreme weathering conditions. The clay mineralogy of the paleosols is dominated by smectite, and it is only within paleosols that formed during the IETM that there is a change. There is a notable increase in the amount of kaolinite in one paleosol horizon that is associated with the carbon excursion. In addition, there is an increase in the translocation of clays and iron, and an increase in the leaching of calcite and plagioclase in initial Eocene thermal maximum paleosols. The differences between soils that formed before and during the initial Eocene thermal maximum indicate that chemical weathering did increase during this ancient global warming event. The mechanism responsible for increased weathering is interpreted to be an increase in hydrolysis reactions caused by an increase in humidity and an increase of carbonic acid in the soil due to elevated CO2 levels during the initial Eocene thermal maximum. Documentation of an increase in chemical weathering during the initial Eocene thermal maximum is significant because it may have served as a negative feedback to reduce atmospheric CO2. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Facies and geochemistry across the Early–Middle Frasnian transition (Late Devonian) on South China carbonate shelf: Comparison with the Polish reference succession

Dongcun and Longmen are two important reference sections for the study of the Frasnian carbonate platform of Guangxi (southern South China). At the Dongcun section, conodonts recovered in the present study provide a robust biostratigraphic framework for the Lower–Middle Frasnian interval. The studied interval (uppermost Givetian to Middle Frasnian basal Palmatolepis hassi Zone) is characterized by repeated shallower and deeper water deposits, but the cyclic depositional trend was mainly deepening upward. Carbon isotopic investigation of bulk samples recognizes a major, four-step perturbation across the Lower–Middle Frasnian boundary, which is also evident in the Holy Cross Mountains (Poland). The pattern is reinforced by correlative organic carbon isotopic secular trends from the reference Polish sections: a prominent long-lasting increase in δ 13C carb up to 4.5‰ (event III) is evident in the Palmatolepis punctata Zone, but is preceded and followed by rapid negative shifts (events II and IV, respectively) to ca. 0‰; a weak incipient positive excursion (event I) is recognized in carbonate carbon only in the upper part of the Palmatolepis transitans Zone. In Chinese sections, different background isotopic values indicate decoupled, isotopically dissimilar, hydrothermally affected and mostly oxygenated water masses over the carbonate shelf, as shown by trace and rare earth elements. The shallower water, upslope, diagenetically unaltered Dongcun succession reveals lighter carbon isotopic ratios, on the order of 2‰, while the less densely sampled and less certainly dated, mostly muddy-siliceous Longmen downslope succession shows δ 13C carb levels similar to those recorded worldwide. The onset of the bipartite event III is marked by 13C enrichment of 4.3‰ at Dongcun. Event IV, a light carbon excursion, is especially well recorded in the shallow-water facies by a 2.5‰ negative shift. The event I excursion in the transitans Zone is obscured, as in some Polish successions, but is probably recorded in organic carbon trends. Slight differences in timing and magnitude of the δ 13C carb oscillations may originate from geochemical decoupling between the open ocean and the tectonically active, morphologically differentiated epeiric basins. Nevertheless, a global extent for this major punctata Event is clear from the similar biogeochemical signals recorded on the distantly separated south Laurussian and South China shelves.

Ostracode biostratigraphy and carbon and oxygen isotopic studies across the Paleocene/Eocene boundary in the subsurface of Jaisalmer Basin, Rajasthan, India

The ostracode fauna across the Paleocene/Eocene boundary has been studied at 5 m intervals between 690–810 m in the Ghotaru Fort Well (GTF Well-A), Jaisalmer Basin, Rajasthan. Ostracodes and foraminifera have been utilized to provide the biostratigraphic framework. Four ostracode zones designated as POS-I to POS-IV in the Late Paleocene and one zone EOS-I in the earliest Early Eocene age sediments have been recognized. Quantitative ostracode and qualitative foraminiferal data were used to interpret the paleoenvironment across the Paleocene/Eocene boundary. Occurrence of the benthic foraminiferaDiscocyclina seunesi andMiscellanea miscella at 735–740 m depth in this well suggests a latest Late Paleocene age. The frequency and diversity of ostracodes at this depth is low. The ostracodes recorded near the top of the Paleocene are:Anommatocytkere indica, Echinocythereis contexta, Phyrocythere alfuraihi andSchizocythere prolata. This ostracode assemblage along with benthic foraminifera suggests that sediments were deposited in an inner neritic environment. This is followed by an earliest Early Eocene age transgression marked by the extinction of preceding Late Paleocene ostracodes and advent of a new ostracode suite represented byAlocopocythere abstracta in the 725–730 m interval. In the 720–725 m interval, however, there is a turnover to an entirely new suite of ostracodes represented byParagrenocythere reticulospinosa, Phalcocythere sentosa, Hornibrookella avadheshi, H. jaisalmerensis, Alocopocythere abstracta, Gyrocytbere parvicarinata, Schizocythere appendiculata, Bairdoppilata kalakotensis, Uroleberis khoslai, Loxoconchella ? guhai, Cytherella jaisalmerensis andC. tawaica. This ostracode assemblage is indicative of Early Eocene age and deposition in an inner neritic environment. The Paleocene/Eocene boundary in well GTF-A is recognised on the basis of ostracodes and foraminifera at 735–740 m, which in turn coincides with an overall sea level fall attributed to global warming, and this is further corroborated by a negative carbon excursion (δ13C=−13.63) across the Paleocene/Eocene boundary.