High-resolution Chemostratigraphy Research Articles

The application of chemostratigraphy and proximality trends to the Silurian Coralliferous Formation of SW Wales: rhythmical sedimentation during the transgression of a palaeo-shoreline

The Silurian Coralliferous Formation of Pembrokeshire comprises 94–143 m of shelfal sediments deposited on the southern margin of the Welsh Basin, UK. The succession begins with rudites (Renney Slip Member), which are overlain by interbedded silty mudstone and sandstone (Deadman's Bay Member). Biostratigraphical control is weak between the two principal sections (Renney Slip and Marloes Sands), but high-resolution chemostratigraphy permits 11 subdivisions to be distinguished. Some chemostratigraphical units can be traced between the sections, whereas others are locally absent. The correlation of the chemostratigraphical units has been compared with sea-level cycles identified by a study of the frequency, distribution and characteristics of storm-beds using proximality trend analysis. This suggests a synchroneity of traceable chemostratigraphical units within the Deadman's Bay Member, indicating that deposition began in the Renney Slip Section, and that a greater water depth was achieved there. A holistic assessment shows that the Coralliferous Formation onlaps an unconformity surface with a palaeo-relief of 57 m, over a current distance of 3 km. Further considerations of benthic assemblages, subsidence rates and global sea-level fluctuations support the presence of a late Telychian transgression with glacio-eustasy contributing between 40 and 70 m to that sea-level rise.

Temporal and spatial variations of high-resolution strontium, carbon, and oxygen isotopic chemostratigraphy at the end-Permian crisis boundary in South China

Variable degrees of strontium, carbon, and oxygen isotopic perturbations related to the end-Permian mass extinction (EPME) were recorded from the Latest Permian (i.e., the Lopingian stage prior to the extinction event) to the Earliest Triassic recovery (i.e., the Induan stage), while the underlying causes for their temporal and spatial variations and marine paleoenvironmental indications were poorly understood. In this study, integrated high-resolution 87Sr/86Sr, δ13C, and δ18O proxies are presented across the Wuchiapingian/Changhsingian and Permian/Triassic boundaries within the stratigraphic framework calibrated with high-resolution conodont biostratigraphy and high-precision geochronology in three South China sections, i.e., the Liangfengya, the Meishan, and the Shangsi sections. High-resolution strontium isotopes revealed that the water-mass segregation and accelerated precipitation rate of strontium probably resulted in local heterogeneity and provincial features of 87Sr/86Sr values at short-time interval less than 1 Myr. This provincialism of 87Sr/86Sr values can serve high-resolution chemostratigraphy. Integrated δ13C and 87Sr/86Sr isotopes from the studied sections at the Wuchiapingian/Changhsingian boundary (WCB) revealed that spatial heterogeneity with respect to drift magnitudes and peak values of δ13C negative excursion was not associated with oceanic stratification or high terrestrial input, while the releasing of 13C-depleted CO2 by deep-marine volcanism could be a plausible cause. Moreover, integrated δ18O and 87Sr/86Sr isotopes indicated that the “cooling event” occurring at the WCB was a provincial event, which was influenced by water depth and marine volcanism. The integrated high-resolution 87Sr/86Sr, δ13C, and δ18O proxies confirmed that the oceanic stratification was an ultimate cause for shallow-to-deep-water δ13C gradient at the Permian/Triassic boundary (PTB), but having no link to paleoproductivity increase. The positive excursion of 87Sr/86Sr postdated negative shift of δ13C by ∼ 40 Kyr across the PTB from the studied intervals. This temporal chemostratigraphic variation revealed that terrestrial influx was not a major cause for negative excursion of δ13C. The shallow-to-deep-water 87Sr/86Sr gradient was observed across the PTB in the studied sections, which was probably caused by water segregation resulting from oceanic stratification and distances from high- or low-radiogenic strontium sources supplied by terrestrial input or marine volcanism.

Volcanically-Induced Environmental and Floral Changes Across the Triassic-Jurassic (T-J) Transition

The End-Triassic Mass Extinction (ETME) saw the catastrophic loss of ca. 50% of marine genera temporally associated with emplacement of the Central Atlantic Magmatic Province (CAMP). However, the effects of the ETME on land is a controversial topic. Evaluation of the disparate cause(s) and effects of the extinction requires additional, detailed terrestrial records of these events. Here, we present a multidisciplinary record of volcanism and environmental change from an expanded Triassic-Jurassic (T-J) transition preserved in lacustrine sediments from the Jiyuan Basin, North China. High-resolution chemostratigraphy, palynological, kerogen, and sedimentological data reveal that terrestrial conditions responded to and were defined by large-scale volcanism. The record of sedimentary mercury reveals two discrete CAMP eruptive phases during the T-J transition. Each of these can be correlated with large, negative C isotope excursions (CIE-I of −4.7‰; CIE-II of −2.9‰), significantly reduced plant diversity (with ca. 45 and 44% generic losses, respectively), enhanced wildfire (marked by increased fusinite or charcoal content), and major climatic shifts toward drier and hotter conditions (indicated by the occurrence of calcareous nodules, increased Classopollis pollen content, and PCA analysis). Our results show that CAMP eruptions may have followed a bimodal eruptive model and demonstrate the powerful ability of large-scale volcanism to alter the global C cycle and profoundly affect the climate, in turn leading to enhanced wildfires and a collapse in land plant diversity during the T-J transition.

Volcanically driven lacustrine ecosystem changes during the Carnian Pluvial Episode (Late Triassic)

The Late Triassic Carnian Pluvial Episode (CPE) saw a dramatic increase in global humidity and temperature that has been linked to the large-scale volcanism of the Wrangellia large igneous province. The climatic changes coincide with a major biological turnover on land that included the ascent of the dinosaurs and the origin of modern conifers. However, linking the disparate cause and effects of the CPE has yet to be achieved because of the lack of a detailed terrestrial record of these events. Here, we present a multidisciplinary record of volcanism and environmental change from an expanded Carnian lake succession of the Jiyuan Basin, North China. New U-Pb zircon dating, high-resolution chemostratigraphy, and palynological and sedimentological data reveal that terrestrial conditions in the region were in remarkable lockstep with the large-scale volcanism. Using the sedimentary mercury record as a proxy for eruptions reveals four discrete episodes during the CPE interval (ca. 234.0 to 232.4 Ma). Each eruptive phase correlated with large, negative C isotope excursions and major climatic changes to more humid conditions (marked by increased importance of hygrophytic plants), lake expansion, and eutrophication. Our results show that large igneous province eruptions can occur in multiple, discrete pulses, rather than showing a simple acme-and-decline history, and demonstrate their powerful ability to alter the global C cycle, cause climate change, and drive macroevolution, at least in the Triassic.

Milankovitch cyclicity in the latest Cretaceous of the Gulf Coastal Plain, USA

Upper Cretaceous marine sequences in the Gulf Coastal Plain (USA) span the Cretaceous–Paleogene (K–Pg) transition, allowing for detailed studies of one of the most severe extinction events of the Phanerozoic. To improve the temporal resolution of the stratigraphic record that represents environmental change leading up to the K–Pg boundary, we constructed a high-resolution chemostratigraphy and cyclostratigraphy of upper Maastrichtian shallow marine deposits located in the state of Mississippi (USA). Upper Maastrichtian strata in this area consist of alternating decimeter scale chalk and marl rhythmites deposited in a hemipelagic setting. New geochemical proxy records were used to test whether rhythmic sedimentation was driven by Milankovitch cycles. Stable isotopes (δ13Ccarb, δ18Ocarb), carbonate content (wt% CaCO3), and elemental concentrations (Ti, K, Fe) integrated with microfossil and ammonite biostratigraphy reveal astronomical forcing in the studied record. Spectral estimation suggests that rhythmic bedding was associated with climate change driven by precession (~20 kyr). Obliquity signals are also apparent in our analysis, and short eccentricity (~100 kyr) is inferred from amplitude modulation of precession. Studied sections were correlated at the precession scale with the recently tuned K–Pg succession near Morello, Italy which is stratigraphically equivalent to the well-characterized K–Pg sites in Gubbio, Italy (Bottaccione, Contessa Highway). Additionally, carbon isotope records from the study area exhibit large scale trends throughout the latest Maastrichtian, similar to those observed in the Morello and Bottaccione sections. Thus, we show that Milankovitch-scale climatic signals and low-amplitude carbon isotope shifts (<0.5‰) of the late Maastrichtian of the Gulf Coastal Plain are well-preserved and can be correlated globally.

Multiproxy sedimentological and geochemical analyses across the Lower\u2013Middle Pleistocene boundary: chemostratigraphy and paleoenvironment of the Chiba composite section, central Japan

The Chiba composite section (CbCS) in the Kokumoto Formation, Kazusa Group, central Japan is a thick and continuous marine succession that straddles the Lower–Middle Pleistocene boundary and the well-recognized Matuyama–Brunhes paleomagnetic polarity boundary. Although recent studies extensively investigated the CbCS, its chemostratigraphy, particularly around the Lower–Middle Pleistocene boundary, is poorly understood. Therefore, in this study, we performed multiproxy sedimentological and geochemical analyses of the CbCS, including the Chiba section, which is the Global Boundary Stratotype Section and Point for defining the base of the Middle Pleistocene Subseries. The aim of these analyses is to establish the high-resolution chemostratigraphy and to reconstruct the paleoenvironments of its sedimentary basin in detail. We used the K/Ti ratio as a broad proxy for the clastic material grain size of the sediments. Although the K/Ti ratio generally varies throughout the studied interval, the K/Ti ratio especially during Marine Isotope Stage (MIS) 19a shows a variation pattern like those of the foraminiferal oxygen isotope (δ18O) records. The records of the C/N ratio of bulk samples and carbon isotope ratio of the organic carbon (δ13Corg) suggest that the organic matter in the CbCS sediments during MIS 19c mostly originated from marine plankton, whereas the organic matter during MIS 18 and 19a was characterized by a mixture of marine plankton and terrestrial plants. These records are clearly indicative of changes in mixing ratio of marine vs. terrestrial organic matter in association with glacial–interglacial cycles from the late MIS 20 to the early MIS 18. In addition, we calculated the mass accumulation rates (MARs) of organic carbon, biogenic carbonate, and terrigenous material for quantitative interpretations on the paleoenvironmental changes. MAR calculations revealed that the contribution of marine organic carbon relative to terrestrial organic carbon increased during MIS 19c, and that the contribution of the terrigenous material relative to biogenic carbonate decreased during MIS 19c. Furthermore, we observed relatively large variations in the total organic carbon and total nitrogen contents during MIS 19a. These variations were probably caused by the relative decrease in bottom-water oxygen level, which is also supported by our trace-fossil data, although it is not certain whether the increase in organic-carbon flux at ~ 760 ka was due to the synchronous increase in biogenic productivity in surface water. Such a relative decrease in bottom-water oxygen level was partly due to the increased ocean stratification because of the northward displacement of the Kuroshio Extension Front.

Carbon-isotope events during the Pliensbachian (Lower Jurassic) on the African and European margins of the NW Tethyan Realm

Early Jurassic palaeoenvironments and palaeoclimates, based on chemostratigraphy, have been extensively studied for the northern Tethyan margin. For the first time, high-resolution chemostratigraphy for the entire Pliensbachian is proposed for the African Tethyan margin (Morocco), and compared with new sections from the European Tethyan margin (France). Precise ammonite determinations, completed by calcareous nannofossil distribution, provide an accurate biostratigraphic framework from the Upper Sinemurian to the Upper Pliensbachian. Bulk inorganic carbon (δ13Ccarb) and organic carbon (δ13Corg) isotope signals are used to characterise and compare carbon-cycle trends in the Central High Atlas Basin (Amellago-Ikis section, Morocco), the Paris Basin (Montcornet borehole, France), and the Subalpine Basin (Serre-Poncon section, France). Comparison of this new dataset with previously published geochemical data from European basins confirms the existence of five distinct carbon-isotope events, synchronous at the scale of the NW Tethyan Realm: (1) a clear –2‰ negative excursion of δ13Ccarb and δ13Corg, corresponding to the Sinemurian–Pliensbachian Boundary Event (SPBE), probably associated with a massive release of light carbon by volcanism and hydrothermalism during the breakup of Pangea; (2) a positive excursion (+1 to +1.5‰) in the Valdani and Luridum subzones (Ibex Zone), reflecting increased organic matter storage under warm climate conditions; (3) a broadly negative event from the Davoei Zone to the lower part of the Margaritatus Zone, associated with major oceanographic reorganisation in a transgressive context, and possible enhanced hydrothermalism related to the opening of the Hispanic corridor; (4) a positive carbon excursion (+2‰) during the Subnodosus and Gibbosus subzones (Margaritatus Zone), corresponding to the Late Pliensbachian Event (LPE), associated with organic matter storage; (5) an end-Margaritatus–Spinatum negative excursion, associated with sea-level fall, probably driven by glacio-eustasy. This new high-resolution carbon-isotope dataset highlights the synchroneity of carbon-cycle trends throughout the NW Tethyan Realm. Several major isotopic events, prior to those of the Toarcian, must therefore have occurred at a supra-regional to global scale during the Pliensbachian, involving significant internal and external geodynamic mechanisms.

Paleoenvironmental conditions and sedimentation dynamics in Central Europe inferred from geochemical data of the loess-paleosol sequence at Süttő (Hungary)

This study provides a detailed geochemical record of one of the best studied loess-paleosol sequences (LPS) in Hungary covering Marine Isotope Stage (MIS) 6 to MIS 2. Süttő is situated in the northwestern Carpathian Basin at the transition between oceanic and continental climate. We investigated weathering intensity, dust provenance and sedimentation dynamics by high-resolution X-ray fluorescence (XRF) scanning of discrete samples in comparison with quantitative XRF and other published proxy data such as grain-size, magnetic susceptibility and micromorphology. Multivariate statistics unravel pedogenesis in terms of decalcification and grain-size dynamics as processes controlling the elemental composition. Furthermore, high-resolution chemostratigraphy indicates geochemical homogeneity for mature MIS 5 paleosols and the modern soil. Together with the A-CN-K plot providing no hint for K-feldspar weathering, semi-arid continental paleoenvironmental conditions are considered as prevailing. Dust provenance is studied by geochemical reference literature data from potential ultimate dust source areas, Th/metal ratios and comparison with the composition of the upper continental crust (UCC). Results indicate a felsic rock composition and a stable dust source, albeit immediate (adjacent Danube floodplains) and ultimate dust sources (surrounding mountain belts) need to be distinguished. According to the Sr-K-Rb plot, the Austroalpine cover nappes area northwest of Süttő might be the dominant ultimate dust source. The weathering-resistant elements Al, Ti and Zr as well as Zr/metal ratios indicate Zr addition and thus strongest sediment sorting and/or recycling during MIS 6. Since Al, Ti and Zr show minimal correlations with grain-size, we suggest a cascade-alike transport pathway from ultimate source to sink involving multiple sediment recycling and sorting cycles in both fluvial and eolian domains. Therefore, this study highlights how high-resolution qualitative geochemical data contributes to a better understanding of paleoenvironmental and sedimentation dynamics at LPS. Application of such a versatile geochemical screening tool is of broad interest to the scientific community working on (terrestrial) paleoenvironmental archives that are commonly discretely sampled.

High-resolution chemostratigraphy of a Miocene wedge-top carbonate shelf (San Marino Fm., Northern Apennines, Italy): The major role of the Monterey global fertility event

The evolution of the San Marino carbonate succession, developed on a wedge-top basin of the northern Apennines during the Middle Miocene (Torriana outcrop, Marecchia Valley), was studied through a high-resolution stable isotope analysis on different carbonate components. A marked positive carbon isotopic excursion is identified at ca. 16 Ma. The excellent correlation of the San Marino δ13C carbonates with the global δ13C reference curve from Zachos et al. (2008) allows to link the marked positive δ13C present in the San Marino section with the global carbon isotope maximum of the Monterey event. Subordinate long-eccentricity-driven δ13C cycles (~405 kyr) as recorded by Holbourn et al. (2007) were also identified. The correlation with carbon signatures of coeval successions of the Mediterranean region shows that this main carbon isotopic excursion at the Burdigalian–Langhian boundary is widely recorded and predates the crisis of these heterozoan shelves. The demise of the San Marino shelf resulted from a combination of global and regional factors that controlled the nutrient budget, the detrital input and the subsidence of the basin. The high-resolution chemostratigraphy of San Marino succession shows that even though the shelf evolved in the complex setting of a wedge-top basin, which should be largely influenced by local factors (i.e. tectonic subsidence and detrital input), it records the Monterey event and its eccentricity paced rhythms with high precision. This highlights the strong connection of the Monterey event with the development and subsequent demise of shallow water carbonate depositional systems in the Mediterranean, irrespective of variable and changing local conditions.

Chemostratigraphic and depositional characterization of the Niobrara Formation: Techniques for assessing lateral heterogeneity, CEMEX Quarry, Lyons, Colorado, Denver-Julesburg Basin

The Late Cretaceous (89–82 Ma) Niobrara Formation is the primary target for unconventional horizontal well activity in the Denver Basin. With an abundance of horizontal activity, the question of well-length scale lateral heterogeneity becomes paramount addressing steering practices, completion, and ultimately well performance. We have incorporated detailed core description, high-resolution chemostratigraphy, and petrophysical logs from four near-surface cores taken at the CEMEX Lyons Quarry to assess lateral heterogeneity. Cores were originally intended to determine the geochemistry and areal extent of the Fort Hays Limestone Member of the Niobrara Formation, CEMEX’s primary target for cement production. However, due to the cores’ proximity to one another (91.4–609.6 m [300–2000 ft]) and the presence of not only the Fort Hays Limestone but also the petroliferous chalks and marls of the Lower Smoky Hill Member (including the D Chalk, C Marl, and a portion of the C Chalk), the data set can also be used as a proxy to model changes along a pseudohorizontal wellbore. Observations include lateral bentonite discontinuity, trace element and organic carbon variations, and distinct changes in sedimentary structures. Well-length sedimentological and stratigraphic changes may have unique implications for unconventional oil and gas development.

Correlations of some Neoproterozoic carbonate-dominated successions in South America based on high-resolution chemostratigraphy

ABSTRACT: This report reviews and incorporates new elemental and isotope chemostratigraphic data for correlation of Neoproterozoic carbonate-dominated successions in South America (Argentina, Bolivia, Brazil, Paraguay and Uruguay). These thick mixed carbonate/siliciclastic successions were largely deposited in epicontinental basins or accumulated on passive margins on the edges of cratons (e.g. São Francisco, Amazonia, Rio Apa Block, Pampia and Río de la Plata paleocontinents) during extensional events related to the rifting of the Rodinia Supercontinent. From the stratigraphic point of view, these successions occur as three mega-sequences: glaciogenic, marine carbonate platform (above glaciomarine diamictites or rift successions), and dominantly continental to transitional siliciclastics. In the orogenic belts surrounding cratons, carbonate-dominated successions with important volcanoclastic/siliciclastic contribution have been, in most cases, strongly deformed. The precise ages of these successions remain a matter of debate, but recently new paleontological and geochronological data have considerably constrained depositional intervals. Here, we report high-resolution C, O, Sr, and S isotope trends measured in well-preserved sample sets and mainly use Sr and C isotopes in concert with lithostratigraphic/biostratigraphic observations to provide detailed correlations of these successions. The establishing of a high-level and definite chemostratigraphic correlation between Neoproterozoic basins in South America is the main goal of this work.

Refining 2 km of Ordovician chronostratigraphy beneath Anticosti Island utilizing integrated chemostratigraphy

New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the chronostratigraphic resolution of the nearly 2 km thick Ordovician section below Anticosti Island. A total of 1414 horizons, spaced at 1.0–1.5 m intervals, were sampled from the New Associated Consolidated Paper (NACP) drill core and analyzed for major and trace elemental composition by portable X-ray fluorescence analyzer (pXRF). Select micrite and calcareous shale powders were then analyzed for δ13Ccarb (number of samples, N = 364) and 87Sr/86Sr (N = 25). Our results indicate a Floian to early Darriwilian (F3–Dw1) age for the Romaine Formation, a middle Darriwilian to Sandbian (Dw2–Sa1) age for the Mingan Formation, a lower to mid-Katian (Ka1–Ka2) age for the Macasty Formation, an upper Katian (Ka3) age for the lower Vauréal Formation, an uppermost Katian (Ka4) age for the upper Vauréal, and a Hirnantian (H1) age for the Ellis Bay Formation. This integrated chemostratigraphic and biostratigraphic synthesis establishes the position of numerous unconformities, the duration of the intervening depositional sequences, and rates of sedimentation.

A high-resolution chemostratigraphy of post-Marinoan Cap Carbonate using drill core samples in the Three Gorges area, South China

Cap Carbonates overlie the Marinoan Snowball Earth-related glacial diamictite, and possibly record the drastic surface environmental change and biological evolution after the Snowball Earth. We conducted on-land drilling from the Liantuo Formation, through the Nantuo, to the lower Doushantuo Formation in the Three Gorges area of South China to collect fresh, continuous samples in the Three Gorges area. We obtained high-resolution chemostratigraphies of δ13C and δ18O values of carbonates from the topmost part of the Nantuo Formation to the Cap Carbonate, in order to decode the detailed surface environmental change in the shallow marine setting. The δ13C chemostratigraphy possesses some unique characteristics: (1) stable δ13C values as a whole, but ubiquitous low δ13C anomalies through the Cap Carbonate, (2) increase of the δ13C values from −3 to +5‰ across the C2/C3 boundary, (3) no δ13C anomaly between the C1 and C2 boundary, and (4) presence of an anomalous high δ13C value (+2.3‰) and a faint positive correlation between δ13C and δ18O values in the C1 unit.Evidence of quite low δ13C anomalies (with a nadir of −41‰), ubiquitous negative δ13C anomalies through the Cap Carbonate, and a high δ13C anomaly accompanied with a faint positive correlation between δ13C and δ18O values in the C1 unit supports decomposition and formation of methane hydrate during Cap Carbonate formation. The drastic increase of δ13C values from the upper C2 to C3 units indicates enhancement of primary productivity and organic carbon burial, possibly due to high continental fluxes after the Snowball Earth event, evidenced by high Sr isotope values. The increase is restricted to the proximal side of the inner shelf in South China, and the timing of the increase of δ13C values of carbonates is earlier at Three Gorges area than any other area, suggesting that the enhancement of primary productivity started in the proximal environment because of higher continental influxes. The increase in oxygen contents of seawater due to the enhanced primary productivity possibly resulted in the emergence of multicellular animals soon after Cap Carbonate deposition.

High-resolution chemostratigraphy of the late Aptian–early Albian oceanic anoxic event (OAE 1b) from the Poggio le Guaine section (Umbria–Marche Basin, central Italy)

Abstract An integrated study of multiple geochemical proxies (δ13Ccarb, δ13Corg, TOC, HI, CaCO3, trace elements/Al ratios) of the late Aptian–early Albian oceanic anoxic event (OAE) 1b has been performed on the pelagic sedimentary sequence of Poggio le Guaine (Umbria–Marche Basin, central Italy). A comparison of the newly collected stable isotope carbon curve with the records from the Vocontian Basin (SE France), DSDP Site 545 and Hole 1049C provided a reliable and precise identification of the four main prominent black shale levels (113/Jacob, Kilian, Urbino/Paquier and Leenhardt) that characterize the OAE 1b. The studied record shows an increase in the marine organic carbon accumulation rate, in particular in the 113/Jacob and Urbino/Paquier levels. In the other black shales, TOC values are Notably, the Kilian and Urbino/Paquier levels from the PLG section are characterized by the absence of correlative shifts in δ13Ccarb and δ13Corg. The increase in the δ13Corg values in these levels is explained by an increase in the relative contribution of 13C enriched marine planktonic archaeal biomass, while the concomitant negative excursions recorded in the δ13Ccarb could reflect a major contribution of isotopically light terrestrial carbonate ions from increased continental runoff during documented more humid conditions.

Integrated high-resolution chemostratigraphy and facies-based stratigraphic architecture of the Lower Cretaceous (Aptian), Shu’aiba Formation, Saudi Arabia

High-resolution carbon isotope signatures were integrated with core descriptions and gamma-ray logs and used as a correlation tool for better age control to refine the sequence-stratigraphic framework of the Shu’aiba Formation in Saudi Arabia. The carbon isotope variations of the shallow carbonate Shu’aiba Formation correlate well with the Tethyan pelagic record and indicate an original marine signature for the Lower Cretaceous (Aptian) Shu’aiba Formation. Carbon isotope values of the Shu’aiba Formation range from 1.5‰ to 6‰ with minimal or no diagenetic effects. Oxygen isotope values range from −2.7‰ to −6.7‰ but were reset during diagenesis and cannot be applied for chemostratigraphic analysis. The Shu’aiba strontium isotope records range from 0.707356 to 0.707454 and differ slightly from the standard Aptian record because of diagenesis. The Shu’aiba Formation platform is a large-scale composite sequence (∼7 m.y.) composed of seven lower Aptian high-frequency sequences and two additional upper Aptian prograding sequences. Carbon isotope data were calibrated with core descriptions and gamma-ray logs to construct two detailed high-resolution stratigraphic cross sections. Carbon isotope data help refine the internal stratigraphic architecture of the Shu’aiba Formation especially on the slope and open-marine settings across the lower to upper Aptian boundary. The carbon isotope values of the Hawar “dense” unit in the base of the Shu’aiba Formation record major depletion corresponding to the global dissociation of methane hydrates, followed by major positive excursion associated with the deposition of Lithocodium and Bacinella facies coeval with the global oceanic anoxic event 1a. The rudist buildups on the platform have a value of approximately 4.5‰ at their base in most wells with a general uniform carbon isotope trend, followed by a gradual depletion to the top of the Shu’aiba Formation. Although some variations are observed in carbon isotope values associated with the lateral facies change from lagoon, margin, slope, open-marine, and basinal settings, carbon isotope trends are still similar and can be correlated fieldwide. Little evidence exists of meteoric diagenesis associated with the depletion of carbon isotope values. However, oxygen isotope records were possibly affected by meteoric diagenesis associated with subaerial exposure surfaces but did not get affected by the late Aptian hiatus, despite the massive karstification observed in cores. The good correlation between the original carbon isotope fluctuations and the third-order sequence framework of the Shu’aiba Formation fits well with the established carbon isotope curves that have been used as a proxy for global sea level changes during the Early Cretaceous. This study also shows that small-scale parasequences (fifth-order or higher) can be calibrated with carbon isotope curves, but they most likely represent relative sea level changes with local effects instead of global signatures. Application of high-resolution carbon isotope stratigraphy for the Shu’aiba Formation significantly constrain the stratigraphic framework and will lead to better geologic and simulation models for reservoir characterization and development.

High-resolution chemostratigraphy of the Cambrian–Ordovician GSSP: Enhanced global correlation tool

The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) represents the Global Stratotype Section and Point (GSSP) for the Cambrian–Ordovician boundary on Laurentia. The formation consists of the Martin Point (lower) and the Broom Point (upper) members, which constitute a thick (~170m) deep subtidal to slope marine carbonate sequence. Preservation of the micritic carbonates of the Green Point Formation was evaluated by multiple petrographic and geochemical screening tools. The δ13C and δ18O values of near-primary micrites range from −4.7‰ to +1.7‰ (VPDB) and from −8.7‰ to −5.5‰ (VPDB), respectively, with no significant correlation (R2=0.002). Similarly, the δ13C values show no significant correlation with Mn/Sr ratios or total REE contents, which suggest that the investigated carbonates retain their near-primary δ13C signatures that can be utilized to construct a high-resolution carbon-isotope profile for the GSSP. The δ13Ccarb profile about the GSSP trends generally positive into Bed 22 of the Green Point Formation. The top of the C. intermedius Zone marks the onset of a negative δ13Ccarb shift to about 6.0‰ culminating in the upper part of the zone. Subsequently, the δ13Ccarb profile trends positive with a ‘double switch-back’ about the boundary point and then continues on to the highest value in the uppermost part of Bed 23. In addition, the Ce*/Ce values approximately between 0.8 and 1.0, through the GSSP suggest generally dysoxic waters before, during, and immediately after the boundary. The high-resolution chemostratigraphic results, curve, and trends covering the GSSP should facilitate correlation of the Cambrian–Ordovician boundary sections/sequences from other locations.

Trace and rare earth elemental investigation of a Sturtian cap carbonate, Pocatello, Idaho: Evidence for ocean redox conditions before and during carbonate deposition

This study examines the trace element and REE + Y geochemistry of a thin pink dolomite in the Scout Mountain Member of the Pocatello Formation (Idaho). The pink dolomite meets the criteria of a cap carbonate and provides an opportunity to examine ocean redox state and hydrothermal inputs on the margin of Laurentia at the terminal stages of the Sturtian glaciations. Here, high resolution chemostratigraphy of the dolomite shows that the local marine conditions were rapidly changing during cap emplacement. Normalized trace metal concentrations of Zn, Cd, Cu, Cr, Mn, and Ni suggest that transiently reducing conditions prevailed immediately following the initial deposition of the pink dolomite. Enrichments in Eu suggest a hydrothermal vent source for marine waters present during deposition. The pink dolomite lacks evidence for persistent sulfide mineralization and its associated enrichments in Mo suggesting that anoxia was restricted to areas below the sediment–water interface. No notable enrichments are recorded in the glacial diamictite, though the signal might be masked by high rates of sedimentation. Our data suggest that the deposition of the pink dolomite progressed through (1) the overturn of local ocean containing a strongly stratified hydrothermally influence water column, (2) the delivery of micronutrients to the shallow ocean, (3) a biotic bloom in response to micronutrient availability, and (4) related organic matter scavenging of metals. Finally, we argue that this period of anoxia was brief and conditions quickly returned to an environment of oxic deposition.

Permian–Triassic boundary interval in the Middle East (Iran and N. Oman): Progressive environmental change from detailed carbonate carbon isotope marine curve and sedimentary evolution

The end Permian negative δ 13C excursion is well known but its cause and chronology are still in question. In an attempt to decipher the timing and reasons for this strong decrease, we established high-resolution sedimentological, biostratigraphical and carbonate carbon isotopic studies in four sections in Iran (Abadeh, Shareeza, Zal and Djulfa) and two in Oman (Wadi Sahtan). The position of the sections on two different margins of the Neotethys has allowed us to distinguish between local and global signals. This high-resolution chemostratigraphy, tested for isochrony on the basis of an updated high-resolution conodont and ammonoid biostratigraphy, allows the discrimination of nine successive isotopic events ( IE 0– 8) for the Permian–Triassic Boundary Interval (PTBI). The negative excursion of the PTBI has been separated into four distinct parts. A first drop ( IE 0) occurs in the Late Wuchiapingian with an amplitude of around 1.3‰. A second decrease begins ( IE 1– 2) at the base of the Dzhulfites ammonoid beds (Changhsingian) and continues gradually until the extinction event (top jolfensis conodont interval, without important second-order variations. Its amplitude varies between 1.5‰ and 2.8‰ and its duration has been estimated around 2.2 Ma, revealing a non-catastrophic phenomenon. The lower meishanensis– praeparvus conodont Zone records stationary to slightly increasing values ( IE 3). The interval of the curve after the extinction event, between the upper meishanensis– praeparvus Zone and the base of the I. isarcica Zone, records numerous second-order variations ( IE 4– 7). Its amplitude is of 1.2–3.1‰. These second-order variations are correlatable in Iran but with some uncertainty with Oman. Poor correlation of these small peaks with others published sections might be due to a higher sensitivity to local perturbations and a lower buffer capacity of the whole ocean. We assume that the processes leading to the extinction event also caused the isotopic decrease before the extinction level with a maximal amplitude of only 2.8‰. This allows consideration of some possible mechanisms that were rejected as unrealistic to cause a 4–6‰ excursion.

A Whiff of Oxygen Before the Great Oxidation Event?

High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transition metals molybdenum and rhenium in the late Archean Mount McRae Shale in Western Australia. Correlations with organic carbon indicate that these metals were derived from contemporaneous seawater. Rhenium/osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 +/- 8 million years ago (Ma). Molybdenum and rhenium were probably supplied to Archean oceans by oxidative weathering of crustal sulfide minerals. These findings point to the presence of small amounts of O2 in the environment more than 50 million years before the start of the Great Oxidation Event.

Comparative high-resolution chemostratigraphy of the Bonarelli Level from the reference Bottaccione section (Umbria–Marche Apennines) and from an equivalent section in NW Sicily: Consistent and contrasting responses to the OAE2

The Bonarelli Level (BL) from the upper Cenomanian portion of the reference Bottaccione section (central Italy) is characterized by the presence of black shales containing high TOC concentrations (up to 17%) and amounts of CaCO 3 near to zero. In the absence of carbonate and, consequently, of relative carbon- and oxygen-isotopic data, the elemental geochemistry revealed to be a very useful tool to obtain information about the palaeoclimatic and palaeoceanographic evolution of the Tethys Ocean during the OAE2. Based on several geochemical proxies (Rb, V, Ni, Cr, Si, Ba), the BL is interpreted as a high-productivity event driven by increasingly warm and humid climatic conditions promoting an accelerated hydrological cycle. The enrichment factors of peculiar trace metals (Zn, Cd, Pb, Sb, Mo, U) provide further insight about the H 2S activity at the seafloor during the organic-rich sediment deposition and permitted us to evaluate the use of Ba as palaeoproductivity tracer in conditions of high rate of sulphate reduction. By comparing geochemical records from the reference Bottaccione section (central Italy) with those previously obtained for the coeval Calabianca section (northwestern Sicily), different degrees of oceanic anoxia were delineated and ascribed to different abundance and type (degradable or refractory) of organic matter, which are limiting factors in the bacterial sulphate reduction reactions and in subsequent euxinic conditions at seafloor in the Tethys realm. Based on a ciclostratigraphic approach, consistent fluctuations at 100 ky scale in the chemostratigraphic signals from the two sections are inferred to be expression of a strong orbital-climatic forcing driving changes in the oceanic environment during the BL deposition.