Carbon Isotope Curve Research Articles

The oxic Coniacian-Santonian interval in the Kopet-Dagh Basin (NE Iran): Carbon isotope and benthic-planktic foraminiferal assemblages at the time of the last Cretaceous OAE

A middle Coniacian-middle Santonian interval has been studied in the Kopet-Dagh Basin (NE Iran) to reconstruct the paleoenvironmental conditions in the eastern part of Tethyan realm at the time of the last Cretaceous Oceanic Anoxic Event (OAE3). This succession is composed of a chalky limestone unit, sandwiched by light gray calcareous mudstone and marlstone beds. The δ 13 C curve indicates several minor positive shifts up to 1‰ in the studied section that is correlative to other carbon isotope curves from the Tethys and Boreal realms. A prominent upward increase of P/B ratio in the studied section suggests a relative sea-level rise toward the Santonian. Lithology (low carbonate contents) and faunal contents (high %benthic and higher infaunal benthic foraminifera) reveal that the lower part of the studied section was deposited in a neritic environment influenced by continental-derived nutrients and mesotrophic to eutrophic conditions. Enhanced carbonate contents, higher diversity of benthic foraminifera and dominance of epifaunal taxa suggest more oligotrophic conditions and/or higher oxygen concentrations in the interval equivalent with a δ 13 C positive shift, caused by a transgression phase at this time. The overlying deposits are characterized by the maximum flooding surface and the highstand phase. The stratification of water column and more oxygen consumption in this interval resulted in higher abundance of infaunal benthic foraminifera and decline of carbonate production in a bathyal environment. • Coniacian/Santonian boundary interval was deposited under oxic conditions in the E Tethys. • No black shales or anoxia mark this inferred OAE3 interval. • Sea-level fluctuations control foraminiferal assemblages in this study. • The δ 13 C curve reveals the diagnostic features of global Co/Sa boundary record.

Carbon and Oxygen Isotopic Characteristics of Longmendong Formation in Muxu River Section of Qishan, Shanxi Province and Its Significance

The lower and upper Longmen cave formation of Late Ordovician in the Muxu river section of Qishan in the southwest margin of Ordos are Sandaogou formation of Middle Ordovician and beiguoshan formation of Upper Ordovician respectively, with continuous and complete strata. The average values of carbon and oxygen isotopes of carbonate rocks are 0.57‰ and -10.73‰ respectively, with poor correlation, which basically preserves the characteristics of original isotopic composition. The paleosalinity Z is between 120 and 130, which reflects that the late Ordovician was a marine sedimentary environment, which is consistent with the geological research results. According to the base value characteristics and drift changes of carbon isotope curve of Longmenshan formation, it can be divided into XH1-XH4 cycle segments. Generally speaking, the early carbon isotope values are mainly positive drift, with large scale and long time; The late stage is dominated by negative drift, with small scale and short time. This reflects that the sea level fluctuated greatly in the early Late Ordovician, with strong activity, and the sea level fluctuated slightly in the late Ordovician, showing a downward trend as a whole. Carbon and oxygen isotope variation curve of longmendong formation and global Ordovician δ13C variation curves shows that longmendong formation is deposited in the late Ordovician, which is equivalent to Sandbian and Katian periods. There is a positive drift event of GICE carbon isotope, which is regionally comparable.

Baghuk Mountain (Central Iran): high-resolution stratigraphy of a continuous Central Tethyan Permian–Triassic boundary section

Abstract. Permian–Triassic boundary sections at Baghuk Mountain (Central Iran) are investigated with respect to their lithological succession, biostratigraphy (particularly conodonts, nautiloids and ammonoids) as well as chemostratigraphy (carbon isotopes). The rock successions consist of the Late Permian Hambast Formation, the youngest Permian Baghuk Member (new name for the “Boundary Clay”) and the Early Triassic Claraia beds. Correlation of the data allows the establishment of a high-resolution stratigraphy based on conodonts with seven Changhsingian zones. Abundant ammonoids enable the separation of ammonoid assemblages with the successive Wuchiapingian genera Prototoceras, Pseudotoceras and Vedioceras, as well as the Changhsingian genera Shevyrevites, Paratirolites, Alibashites, Abichites and Arasella. Griesbachian and Dienerian ammonoids are usually poorly preserved. Nautiloids occur predominantly in the Wuchiapingian part of the section with two successive assemblages dominated by the Liroceratidae and Tainoceratidae, respectively. Numerous Early Triassic strata contain microbialites of various outer morphology and microstructure. The carbon isotope curve (δ13Ccarb) shows a continuous late Changhsingian negative excursion continuing across the Baghuk Member with the lightest values at the base of the Triassic.

Magnetostratigraphy and stable isotope stratigraphy of the middle-Eocene succession of the Ainsa basin (Spain): New age constraints and implications for sediment delivery to the deep waters

Stratigraphic cycles preserved in sedimentary successions are controlled by the interaction of tectonics, climate, sediment supply and sea-level variations. Understanding the influence of these drivers on sedimentary systems dynamics is crucial to understand and extract information from sedimentary archives. In the deep marine deposits of the Ainsa foreland basin (lower to middle Eocene, southern central Pyrenees, Spain), the origin of well-preserved cyclicity between channelized sandy submarine fans and hemipelagic deposits remains subject to debate because of the absence of chronostratigraphic constraints on high resolution geochemical proxies. Here, we contrast a combination of newly acquired and legacy geochemical datasets (carbon and oxygen stable isotopes, organic matter content, major and trace elements and the mineralogical composition of bulk hemipelagic sediments) tuned by a new age model to global “target” curves to discuss the possible drivers of the stratigraphic cycles found in the deep marine sediment gravity flow deposits of the Hecho Group. Our new age model is based on magnetostratigraphic and biostratigraphic datasets acquired on a composite section going from the Banaston system to the distal deposits of the Sobrarbe deltaic complex. Four major polarity zones are identified in the studied succession covering the time span from C21n (lower members of the Banaston system) to C19r (Sobrarbe deltaic complex). Our age model is corroborated by the identification of the nannoplankton zone NP16 in the Guaso system (C20n).Comparison between global carbon isotope curve and local isotope dataset shows some differences, suggesting the preservation of a local isotope signal modulated by eustatism, changes in the environment of deposition and/or restriction in water circulation in the Ainsa basin. Yet, comparison of the stratigraphic record with global sea-level curves highlights that sands are mainly delivered to the basin during lowstands, pointing to the important role of eustasy even in an active foreland tectonic context. The exception to the rule is the Banaston member II, whose deposition during a sea-level highstand seems to be controlled by the growth of tectonic structures.

The Cenomanian–Turonian boundary event in Egypt: Foraminiferal turnover and carbon isotopic change

This study deals with the integration between geochemical data (carbon isotope and, total organic carbon) and foraminiferal turnover to track the Oceanic Anoxic Event 2 (OAE2) from the OCT-A8st1 well, which located in October field, central part of Gulf of Suez, Egypt. The studied well is subdivided into two formations, namely the Galala (Early–Late Cenomanian) and Abu Qada (Late Cenomanian–Early Turonian). Concerning the foraminiferal assemblage, agglutinated foraminifera dominate the Galala Formation, while planktonic taxa only occur within a few levels in the Abu Qada Formation. Due to the absence of keeled planktonic forms, the biostratigraphic framework is established on the integration between benthonic and planktonic foraminifera. Three biozones are recognized, including Thomasinella aegyptia–Merlingina cretacea Interval Zone (Early–Middle Cenomanian), Thomasinella fragmentaria–Praebulimina aspera Interval Zone (Middle–Late Cenomanian), and Whiteinella archaeocretacea Taxon Range Zone (Late Cenomanian–Early Turonian). Therefore, the Cenomanian–Turonian boundary interval (CTBI) is placed within the lower part of the Abu Qada Formation, which is characterized by the presence of black shales and low richness of both foraminifera and ostracods. A remarkable increase in TOC values is observed close to the CTBI. Also, the carbon isotope curve shows a distinctive positive δ13C anomaly at the same level of the maximum TOC. The comparison between the recorded δ13C data and those obtained from other sections worldwide, indicates that the base of the Turonian is placed directly above the δ13C excursion. Furthermore, the recorded foraminiferal community shows evidence for the deepening of the environment and extension of the oxygen minimum zone during the CTBI. This conclusion is based on a sequence of bioevents starting with the disappearance of some agglutinated benthic foraminifera, followed by the dominance of opportunistic groups including buliminids, whiteinellids and heterohelicids. Consequently, the studied well may be considered as a local reference section for OAE2.

Chemostratigraphy of the lower Cretaceous Mural Limestone, Rancho Bufalo section, Sonora, Mexico: Implications for OAE 1b

A multidisciplinary approach of the Aptian-Albian Mural Limestone including petrography, geochemistry and stable isotope data is undertaken for the 660-m thick Rancho Bufalo section of northeastern Sonora to characterize source of rare earth elements and to infer paleo-redox conditions. The lithofacies of limestones range from mudstone to boundstone. The unaltered signatures of the δ13C values are confirmed by using Mn and Sr concentrations and their ratios along with the absence of correlation between δ13C and δ18O values. The limestones have large variations in ΣREE and Al2O3 contents (8.0 − 95.8 ppm; 0.45% − 7.10 ppm; respectively). Most limestones show non-seawater-like REE + Y patterns with positive Eu anomalies and negative Ce anomalies. The limestones have positive Ba anomalies in six intervals and most of them were resulted due to weathering and depositional processes and not due to sea level variations. The redox proxies of U, V/Cr and V/(V + Ni) indicate that the bottom water conditions fluctuated from oxic to anoxic. The high-resolution carbon isotope curve reveals four OAE 1 b sub-events: Jacob, Kilian, Paquier, and Leenhardt. The carbon isotope curve of the Rancho Bufalo section is comparable to that of the Cerro Pimas and Peregrina Canyon sections of Mexico.

Carbon, oxygen and strontium isotopes of the Mesoproterozoic Jixian System （1.6-1.4 Ga）in the southern margin of the North China Craton and the geological implications

ABSTRACT The Mesoproterozoic Jixian System (1.6–1.4 Ga) in the Luonan area, southern margin of the North China Craton (NCC), is characterized by complete development, continuous sedimentation and a low degree of structural deformation, resulting in well-preserved palaeontological and paleoenvironmental records. Carbon, oxygen and strontium isotopes of 382 dolostone samples from the Jixian System in Xiong’er Rift, southern margin of the NCC are analysed for the restoration of paleoenvironment and stratigraphic correlation. The results show that the δ13C values of these samples mainly concentrate between −1.8‰ and 2.0‰, and the values can be divided into three segments. The δ18O values are mainly between −6.5‰ and −3.5‰, with frequent fluctuations and weak regularity. The 87Sr/86Sr ratio ranges from 0.7051 to 0.7139, with an average value of 0.7085. Its fluctuations have certain regularity and are negatively correlated with the carbon isotopes, indicating that the sea-level changes during the Jixian period in the southern margin of the NCC. The carbon isotope curve of the samples from the study section is very similar to those of the Xiong’er Rift and the Yanliao Rift of the NCC. The −3.2‰ δ13C negative excursion in the middle and upper parts of the Xunjiansi Formation corresponds to the δ13C negative excursion from the top of the Wumishan Formation to the bottom of the Tieling Formation in the Yanliao Rift. Combined with previous research data, the Jixian System that deposited in the Xiong’er Rift and the Yanliao Rift can be approximately correlated with each other. In addition, the carbon isotope data obtained from this study is comparable to global carbon isotope values during the same period. The rise and oscillation of carbon and strontium isotope values during this period may be a response to the break-up of the Colombian supercontinent.

Carbon isotope chemostratigraphy and conodont biostratigraphy around the Smithian–Spathian boundary in the Panthalassan carbonate succession (SW Japan)

Abstract The Tahogawa Member of the Triassic Taho Formation in Southwest Japan consists predominantly of light, dark, and brownish gray bedded limestones and contains abundant Lower to Middle Triassic conodonts. The late Smithian to early Spathian sequence of the Tahogawa Member is divided into five conodont zones, in ascending order: Novispathodus ex gr. waageni, Nv. pingdingshanensis, Nv. brevissimus, Triassospathodus symmetricus, and Tr. homeri. The Scythogondolella milleri Subzone of the Nv. ex gr. waageni Zone contains a late Smithian Anasibirites bearing ammonoid bed. The carbon isotope (δ13Ccarb) curve for the late Smithian to early Spathian shows a rapid excursion with values increasing from ~0.11‰ to ~5.88‰ (VPDB). The rise in δ13Ccarb starts in the upper part of the Nv. ex gr. waageni Zone and continues into the overlying Nv. pingdingshanensis Zone. The δ13Ccarb values peak in the Nv. pingdingshanensis Zone at 5.88‰, and subsequently decrease to 1.13‰ in the Tr. symmetricus Zone. The carbon cycle perturbation recorded in the Tahogawa Member corresponds to the late Smithian to early Spathian global carbon isotope positive and following negative excursions reported from the Tethyan, Panthalassan, and Boreal regions. The Smithian–Spathian boundary is probably located within the Nv. pingdingshanensis Zone of the Tahogawa Member, coincident with the decreasing limb of the carbon isotope perturbation. The combination of δ13Ccarb and conodont stratigraphic data enable us to correlate between the eastern Tethys and Panthalassan regions.

New Findings and Stratigraphic Distribution of Foraminifera from Permian–Triassic Boundary Deposits in the Southern Verkhoyansk Region

The first study results on foraminifera from the Permian–Triassic boundary deposits in the southern Verkhoyansk region (the lower part of the Nekuchan Formation, Suol section located in the basin of Setorym River, a tributary of the East Khandyga River) are presented. The studied foraminifera constitute a novel group for this section and have not been used in paleontological-stratigraphic studies. The foraminiferal assemblage is represented exclusively by ammodiscids (Ammodiscus, Glomospira, and Glomospirella genera), among which Ammodiscus septentrionalis Gerke dominates. The distribution of foraminifera in the Suol section is compared to the previously constructed carbon isotope curve, which reflects global environmental changes. Three intervals are identified in the stratigraphic distribution of foraminifera. In the lower interval, foraminifera are relatively numerous and diverse. In the middle interval, foraminifera are not detected; the maximum negative values of δ13Corg isotope is also recorded here. This interval obviously corresponds to the main extinction episode in the Tethyan basins. In the upper interval, a gradual restoration of the abundance and structure of the foraminiferal complex occurs. A comparative analysis of the distribution and dynamics of taxonomic reorganization of the foraminiferal assemblages from the Permian–Triassic deposits within the Suol section and the assemblages from the Tethyan and Boreal sections has been carried out; as a result of this analysis, the common features and regularities are established. Brief descriptions of four foraminiferal species and a plate with their images are given.

Trophism, climate and paleoweathering conditions across the Eocene-Oligocene transition in the Massignano section (northern Apennines, Italy)

The Eocene-Oligocene transition represents the latest greenhouse-icehouse shift faced by Earth, a major global climate change associated with carbon cycle perturbation. In this paper, we investigate the Massignano stratigraphic section (northern Apennines, Italy), GSSP of the Eocene-Oligocene boundary, by X-ray diffraction analysis of clay minerals and carbon and oxygen stable isotopes to explore the link among climate, paleoweathering and runoff, and carbon cycle in the Neotethys across this major climatic transition. We examine the interplay between global climate forcing and orogen evolution controlling the Massignano hemipelagic sedimentation. The late Eocene clay mineral assemblages indicate that the Neotethys was sensitive to global eustatic changes as well as changes in composition and rates of weathered sediments accumulating into the basin. The carbon isotope record matches with the global signal, showing productivity pulses linked to cooling phases and sea-level drops before 34.8 Ma. Subsequently, a constant negative trend is recorded, which is consistent with the global carbon isotope curve, and attests for a decrease of primary productivity linked to decreasing atmospheric CO2. This negative trend ends at the Eocene-Oligocene boundary, when the onset of the global carbon isotope positive excursion is recorded also within the Massignano hemipelagic succession. On the contrary, the clay mineral assemblage is quite constant across the Eocene-Oligocene boundary reflecting the complex interplay among fluvial discharge, sea level changes and orogen dynamics which clouded the global climate shift. In this context, the enhanced fluvial discharge likely contributed to sustain high trophic conditions in the Adriatic waters and, in turn, the Oligocene positive carbon isotope shift. These results highlight how clay minerals proved to be a useful proxy to identify the interplay between global and regional controlling factors on hemipelagic sedimentation and, their integration with the carbon isotope record, provides insights into carbon cycle dynamics.

Geochemical characteristics of Middle-Upper Ordovician source rocks in the Kalpin outcrop profiles and marine oil-source correlation, Tarim Basin, NW China

The Middle-Upper Ordovician source rocks developed in six outcrop profiles in the Kalpin area, Tarim Basin, NW China were analyzed using an organic geochemical screening method. Most TOC values of source rocks of Salgan Formation of Middle-Upper Ordovician developed in the Dawangou, Kamatikan, and Tonggusibulong outcrop profiles are greater than 1.0%, whereas the TOC value of the source rocks of Yingan Formation of Upper Ordovician only developed in the Dawangou outcrop ranged from 0.14% to 1.29%, with an average value lower than that of the Salgan Formation source rocks. Combined with the analysis results of source rocks of Yuertusi Formation of Lower Cambrian developed in the Xiaoerbulake, Sugaite and West Yuti outcrop profiles, the source rocks of Yuertusi, Salgan and Yingan Formations showed similar molecular geochemical characteristics, with low Pr/Ph value, C23 greater than C21 tricyclic terpane, high gamacerane and C28 sterane content. The similarity in molecular compounds of Cambrian to Ordovician source rocks in the Kalpin area may be a result of the high maturity. In the relative distribution of C27–C29 regular steranes, the marine oils with C28 sterane lower than 25% showed great difference with that of the source rocks of Lower Cambrian Yuertusi Formation. In the distribution of carbon isotopic curves, the source rocks of Lower Cambrian Yuertusi Formation have lighter carbon isotopes, compared with that of the Middle-Upper source rocks, especially in kerogen carbon isotopes, Lower Cambrian Yuertusi Formation lighter 5‰ than that of Middle-Upper Ordovician source rocks. Overall, the carbon isotopes fractionation of saturate, bitumen, aromatic fraction, resin, asphaltene, and kerogen is small in source rocks from Middle-Upper Ordovician Salgan and Yingan Formations in the Kalpin area. It is suggested that the carbon isotope of kerogen of source rocks can be used as a parameter for oil-source correlation excluding the TSR of the oil reservoirs.

Carbon-isotope, petrological and floral record in coals: Implication for Bajocian (Middle Jurassic) climate change

Much of our existing knowledge of Middle Jurassic paleoclimate is based on well-dated marine isotopic records that show fluctuations between warm (greenhouse effected) and cool climates. In contrast, much less is known from contemporaneous terrestrial deposits that are often difficult to correlate stratigraphically with marine successions, and are typically considered as showing regional rather than global signals. In this paper we show that the carbon-isotopic, petrological and floral record through 20 m thick coals in the Northern Qaidam Basin (NW, China) represents a relatively comprehensive northern hemisphere Bajocian (Middle Jurassic) record for terrestrial climatic change. δ13Ccoal values for 88 samples from the Yuqia area in Northern Qaidam Basin range from–25.3‰ to –22.5‰, indicating that C3 plants were the main coal-forming vegetation in the region during the Middle Jurassic. This isotopic variation follows fluctuations in the composition of ferns and gymnosperms, where higher ratios correspond to more negative δ13C values. The similar carbon isotope values of gymnosperms and coal from the Yuqia area indicate that the coal comprises a high proportion of organic material derived from gymnosperm taxa, which is consistent with the very high abundance of diterpenoids in coal and especially pimarane and abietane that are produced primarily by gymnosperms. Results from the coal matrix provide an opportunity to record paleoclimate changes, showing several striking and regular coal-forming cycles with distinct long- and short-term variations. Trends of δ13Ccoal values changes coupled with the evolution of coal-forming plants may record a gradual increase in paleo-CO2 (pCO2) concentration, water-table level changes and the decrease in abundance of Classopollis conifer pollen through Bajocian. This result is in accordance with the published marine carbonate records for this time, with correlation enabled through matching carbon isotope curves from the terrestrial succession at Yuqia and marine records. The similarity of the terrestrial and marine geochemical and floral record is an indication that the observed paleoclimate signal is a global phenomenon. Furthermore, the high-frequency fluctuation of δ13Ccoal values, along with the coal petrologic variations may record short-term changes of environmental factors (e.g. temperature or humidity/precipitation), especially during intervals when palaeobotanical composition has not fluctuated intensely.

НОВЫЕ НАХОДКИ И СТРАТИГРАФИЧЕСКОЕ РАСПРЕДЕЛЕНИЕ ФОРАМИНИФЕР В ПОГРАНИЧНЫХ ПЕРМСКО-ТРИАСОВЫХ ОТЛОЖЕНИЯХ ЮЖНОГО ВЕРХОЯНЬЯ

The first results made on the study of foraminifera from Permian-Triassic boundary deposits in the South Verkhoyan Region (the lower part of the Nekuchan Formation, the Suol section, the Setorym River basin, tributary of the Vostochnaya Khandyga River) are presented. The foraminifera are a new group for this section and were not used in paleontological stratigraphic studies before. The foraminiferal assemblage is represented exclusively by ammodiscids (genera Ammodiscus, Glomospira, and Glomospirella), among which Ammodiscus septentrionalis Gerke dominates. The distribution of foraminifera in the Suol section is compared with the previously constructed carbon isotope curve, which reflects global environmental changes. Three intervals are identified in the stratigraphic distribution of foraminifera. In the lower interval, foraminifera are relatively numerous and diverse. In the middle interval foraminifera were not detected; here the maximum negative values of the δ13Сorg isotope are also recorded. This interval, obviously, corresponds to the time of the main fauna extinction in the Tethyan basins. In the upper interval, a gradual recovery of the abundance and structure of the foraminiferal complex occurs. A comparative analysis of the distribution and dynamics of taxonomic rearrangements of foraminiferal associations in the Permian-Triassic deposits in the Suol section with Tethyan and Boreal sections was carried out, some general features and patterns are established. A brief description of four of foraminiferal species and a photo table with their image are given.

High-resolution correlation of the Homerian carbon isotope excursion (Silurian) across the interior of the Midland Platform (Avalonia), UK

AbstractNew δ13Ccarb and microfacies data from Hereford–Worcestershire and the West Midlands allow for a detailed examination of variations in the Homerian carbon isotope excursion (Silurian) and depositional environment within the Much Wenlock Limestone Formation of the Midland Platform (Avalonia), UK. These comparisons have been aided by a detailed sequence-stratigraphic and bentonite correlation framework. Microfacies analysis has identified regional differences in relative sea-level change and indicates an overall shallowing of the carbonate platform interior from Hereford–Worcestershire to the West Midlands. Based upon the maximum δ13Ccarb values for the lower and upper peaks of the Homerian carbon isotope excursion (CIE), the shallower depositional setting of the West Midlands is associated with values that are 0.7 ‰ and 0.8 ‰ higher than in Hereford–Worcestershire. At the scale of parasequences the effect of depositional environment upon δ13Ccarb values can also be observed, with a conspicuous offset in the position of the trough in δ13Ccarb values between the peaks of the Homerian CIE. This offset can be accounted for by differences in relative sea-level change and carbonate production rates. While such differences complicate the use of CIEs as a means of high-resolution correlation, and caution against correlations based purely upon the isotopic signature, it is clear that a careful analysis of the depositional environment can account for such differences and thereby improve the use of carbon isotopic curves as a means of correlation.

The lead-up to the Sturtian Snowball Earth: Neoproterozoic chemostratigraphy time-calibrated by the Tambien Group of Ethiopia

AbstractThe Tonian-Cryogenian Tambien Group of northern Ethiopia is a mixed carbonate-siliciclastic sequence that culminates in glacial deposits associated with the first of the Cryogenian glaciations—the Sturtian “Snowball Earth.” Tambien Group deposition occurred atop arc volcanics and volcaniclastics of the Tsaliet Group. New U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates demonstrate that the transition between the Tsaliet and Tambien Groups occurred at ca. 820 Ma in western exposures and ca. 795 Ma in eastern exposures, which is consistent with west to east arc migration and deposition in an evolving back-arc basin. The presence of intercalated tuffs suitable for high-precision geochronology within the Tambien Group enable temporal constraints on stratigraphic data sets of the interval preceding, and leading into, the Sturtian glaciation. Recently discovered exposures of Sturtian glacial deposits and underlying Tambien Group strata in the Samre Fold-Thrust Belt present the opportunity to further utilize this unique association of tuffs and carbonate lithofacies. U-Pb ID-TIMS ages from zircons indicate that Tambien Group carbonates were deposited from ca. 820 Ma until 0–2 m.y. before the onset of the Sturtian glaciation, making the group host to a relatively complete carbonate stratigraphy leading into this glaciation. New δ13C and 87Sr/86Sr data and U-Pb ID-TIMS ages from the Tambien Group are used in conjunction with previously published isotopic and geochronologic data to construct newly time-calibrated composite Tonian carbon and strontium isotope curves. Tambien Group δ13C data and U-Pb ID-TIMS ages reveal that a pre-Sturtian sharp negative δ13C excursion (referred to as the Islay anomaly in the literature) precedes the Sturtian glaciation by ∼18 m.y., is synchronous in at least two separate basins, and is followed by a prolonged interval of positive δ13C values. The composite Tonian 87Sr/86Sr curve shows that, following an extended interval of low and relatively invariant values, inferred seawater 87Sr/86Sr rose ca. 880–770 Ma, then subsequently decreased leading up to the ca. 717 Ma initiation of the Sturtian glaciation. These data, when combined with a simple global weathering model and analyses of the timing and paleolatitude of large igneous province eruptions and arc accretion events, suggest that the 87Sr/86Sr increase was influenced by increased subaerial weathering of radiogenic lithologies as Rodinia rifted apart at low latitudes. The following 87Sr/86Sr decrease is consistent with enhanced subaerial weathering of arc lithologies accreting in the tropics over tens of millions of years, lowering pCO2 and contributing to the initiation of the Sturtian glaciation.

Late Devonian carbon isotope chemostratigraphy: A new record from the offshore facies of South China

We present a new biostratigraphically constrained carbon isotope record from the Lali section (South China) to document perturbations in the Late Devonian carbon cycle. Positive carbon isotope excursions (CIEs) are most important components of the Late Devonian long-term carbon isotope cycle: Frasnes (+2‰), Middlesex (+1‰), Lower Kellwasser (+3‰), Upper Kellwasser (+3‰) and Upper annulata (+1‰) events, as well as a small positive CIE (+1‰) in the Famennian expansa-aculeatus zones were recorded. It is noteworthy that no significant carbon isotope shifts are found for the Rhinestreet, Nehden, Condroz and Enkeberg events. The Late Devonian long-term carbon isotope curve from South China is in agreement with contemporaneous world-wide trends in Euramerica (Europe and North America) and Gondwana. Global data suggests a higher Famennian δ13Ccarb baseline (about +2.5 ‰) than that of the Frasnian (about +1‰). Previous studies have related the positive CIE to ocean anoxia, but its ultimate driver remains unclear. Other interpretations include the spread of land plants or increased tectonic activity, but these explanations are questionable due to the relative short durations of the CIEs compared to long-term processes of plant evolution and plant cover extension as well as tectonic movements.

Did early land plants produce a stepwise change in atmospheric oxygen during the Late Ordovician (Sandbian ~458 Ma)?

A stepwise change in atmospheric oxygen (O2) levels during the Ordovician has been attributed to the emergence of land plants. This phenomenon is tied to a major baseline shift in the stable carbon isotope (δ13C) curve and inferred increase in nutrient delivery and enhanced primary productivity in nearshore settings, which led to high organic carbon burial. The timing and magnitude of this baseline shift, however, is still elusive in part because of the lack of high-resolution δ13C data that span this period. Much of the existing Ordovician δ13C literature is focused on isotopic excursions with less emphasis on identifying long-term shifts in baseline (pre- and post-excursion) values.This study presents new high resolution δ13C data from stratigraphic sections at Germany Valley (West Virginia) and Union Furnace (Pennsylvania) in the Central Appalachian Basin. These sections span the entire Sandbian Stage and continue into the lower Katian Stage. The δ13C data from both sections are characterized by relative stability carbon isotope values (mean = −0.61‰) in the lower Sandbian, followed by a + 1.2‰ shift in the upper Sandbian (mean = +0.62‰). Herein, we propose that the positive shift represents a long-term global shift in baseline δ13C values of dissolved inorganic carbon. The timing of this positive shift coinciding with the diversification of early land plants (i.e., bryophytes) supports earlier models that suggested enhanced organic carbon burial rates served as a mechanism for the stepwise oxygenation of the atmosphere during the Late Ordovician.

Enhanced weathering as a trigger for the rise of atmospheric O2 level from the late Ediacaran to the early Cambrian

A shift toward a higher oxygen level in both ocean and atmosphere systems during the late Ediacaran to the early Cambrian has been suggested from multiple indirect proxies. However, the mechanism and magnitude of this oxidation remain unclear. To solve this issue, we measured carbon isotopes in both carbonate and organic matter as well as their trace element compositions for an Ediacaran-Cambrian sequence in the Lower Yangtze basin, South China. The δ13Corg and δ13Ccarb excursions of this sequence are coupled and can be compared with contemporaneous global carbon isotope curves. A 2‰ rise in Δ13Ccarb-org occurred from the late Ediacaran to the early Cambrian, suggesting a substantial increase in atmospheric oxygen level from 16% to 30% of the present atmospheric level (PAL). Furthermore, the distribution pattern of rare earth elements and the concentrations of water-insoluble elements in the carbonates indicate a sudden enhancement in chemical weathering of the continental crust during the early Cambrian, which may be a trigger for the rise of atmospheric O2 level. Both the supply of a large amount of nutrients due to the enhanced continental weathering and the contemporary increase of atmospheric oxygen concentrations may have promoted the appearance of large metazoans in the early Cambrian.

Chemostratigraphy of Late Sinemurian – Early Pliensbachian shallow-to deep-water deposits of the Central High Atlas Basin: Paleoenvironmental implications

Shallow water carbonate rocks are especially prone to diagenetic alteration. As such they are sometimes problematic archives of past carbon cycle perturbations, casting doubt on the reliability of shallow-water carbonates carbon isotopes analyses for chemostratigraphic purposes. In this paper, bulk organic carbon isotopes (δ13Corg) is used as a robust replacement of the more sensitive carbonate carbon isotope systems for the establishment of a refined chronostatigraphic framework for the evolution of Sinemurian-Pliensbachian neritic carbonate systems in the Central High Atlas Basin of Morocco. These data show very similar patterns and characteristics in the organic carbon isotope curves of basinal and platform sections and thus illustrate the reliability of these archives. Simultaneously, we explore the expression of the global carbon isotope excursion at the Sinemurian-Pliensbachian boundary (SP event) in the study area in Morocco. This event has been previously described as a negative carbon isotope excursion of ∼2‰ associated with a large transgression in numerous basins. In Morocco and other neritic carbonate records, the SP event is better described as a return to previous values after a positive carbon isotope excursion coinciding with a major latest Sinemurian regression recognised on a global scale. Similar patterns are also found in other sections, although sometimes poorly expressed. However, it shows, that the SP event has ambiguous characteristics and questions the conventional hypothesis of it being linked to the massive injection of 13C-depleted carbon into the atmosphere-ocean system.

Comparative analysis of characteristics and genesis of crude oils of the Zs block in the Tazhong area and the Ha 6 block in the Tabei area, Tarim basin

Ha6 block is located in the north of the Tarim Basin. In this study, the characteristics of the Ordovician crude oil in the Ha6 block were analyzed by the gas chromatography-mass spectrometry (GC-MS) method on the basis of exploration and development data. The results show that C7 compounds are produced by type-I kerogen. According to the comparison, the medium-molecular-weight hydrocarbon fingerprint of the Cambrian crude oil in the Zs block is consistent with that of the Ordovician crude oil in the Ha6 block, which means that their sources are the same. In the two blocks, biological marker compounds of crude oil are also very similar: C27R, C28R, and C29R steranes are in a V-shaped pattern; the Ts/Tm ratio and the content of tricyclic terpanes are both high; and the presence of a large amount of aromatized isoprenoid indicates the crude oil was generated in a strong-reducing environment. In addition, the crude oil single molecule carbon isotope curve is similar with that of Wells Zs1 and Zs5. Therefore, it is comprehensively concluded that the crude oil in the Ordovician reservoirs in the Repu area is mainly supplied by the Cambrian source rock.