Positive Carbon Isotope Excursion Research Articles

Upper Ordovician (Sandbian-Katian) sub-surface stratigraphy of the Cincinnati Region (Ohio, USA): transition into the Sebree Trough

Facies and carbon isotope analysis of a series of four drill cores from Cincinnati, Ohio provide an opportunity to assess Late Ordovician paleoenvironmental variability along the margin of the Sebree Trough, an intracratonic basin associated with Taconian far-field tectonics. The cores provide stratigraphic data for an approximately 50km long NE-SW transect that roughly parallels the margin of the trough. The cores span the upper High Bridge Group through much of the equivalents of the Lexington Formation, and in one case into the overlying Cincinnatian strata (Kope). The cores were logged at centimeter-scale for litho-, bio-, and tapho-facies.Anumber of marker beds that are present in the cores, including K-bentonites, fossil epiboles (e.g., Prasopora bryozoans), and deformed beds, have been previously documented in the central Kentucky outcrop belt.We also sampled one core for carbon isotopes as a means of providing additional constraints on correlation into the central Kentucky outcrop belt. The facies succession in the lower half of the cores matches that of the lower members of the Lexington Formation in Kentucky (Curdsville, Logana, and lower Grier members). The upper half of the cores contain a more shale-rich facies succession, recording offshore environments that were substantially deeper than those of the Lexington Platform of the central Kentucky outcrop belt. These results corroborate previous studies, which indicated that the Sebree Trough expanded during the earliest Katian. Two positive carbon isotope excursions identified from outcrops of the Logana Member and the Macedonia bed on the Lexington Platform, are clearly recognizable in the sampled Cincinnati region drill core.We believe that one or both of these excursions are equivalent to the Guttenberg carbon isotope excursion (GICE) of the upper Mississippi Valley.

The carbon isotope and sequence stratigraphic record of the Sheinwoodian and lower Homerian stages (Silurian) of the Midland Platform, UK

New high resolution δ13Ccarb isotope data are presented through the Sheinwoodian and lower Homerian stages of the Wenlock Series (Silurian) from sections in Shropshire, Herefordshire and the West Midlands (UK). This data identifies the well-known Early Sheinwoodian positive Carbon Isotope Excursion (ESCIE), the minor mid-ESCIE negative shift, and an Early Homerian positive Carbon Isotope Excursion (EHCIE) in the region of 1.4‰, as developed upon the Midland Platform and in close proximity to the Sheinwoodian and Homerian GSSPs. These carbon isotope excursions have been correlated against graptolite, conodont and chitinozoan frameworks from across the Midland Platform, and alongside details of sedimentology and faunas, have allowed for the establishment of a sequence stratigraphic framework. These integrated data sets reveal that the shallow-water Barr–Buildwas–Woolhope limestones reflect approximately synchronous deposition across the Midland Platform; in that they all contain the ESCIE and five widely traceable depositional cycles. Moreover, biostratigraphic and sequence stratigraphic frameworks from across the Midland Platform (Avalonia) confirm that both the ESCIE and the five cycles are synchronous with records from other palaeocontinents (Baltica and Laurentia) and indicate that the early Sheinwoodian graptolite zones as established in the type area are in need of review.

Silurian conodont biostratigraphy and carbon (δ13Ccarb) isotope stratigraphy of the Victor Mine (V-03-270-AH) core in the Moose River Basin

The Moose River Basin in Ontario, Canada, contains nearly 1 km of Silurian marine strata, and although it has been studied for more than a century, its precise correlation globally has not been constrained. Herein, a core from the Victor Mine in the Moose River Basin was examined for conodont biostratigraphy and carbonate carbon (δ13Ccarb) isotope chemostratigraphy to provide a detailed chronostratigraphic framework for the Silurian strata (Severn River, Ekwan River, and Attawapiskat formations) in the Moose River Basin. The recovery of Aspelundia expansa, Aspelundia fluegeli fluegeli, Distomodus staurognathoides, Ozarkodina polinclinata estonica, Pterospathodus eopennatus, and Aulacognathus bullatus, as well as the lower Aeronian, upper Aeronian, lower Telychian (Valgu), and ascending limb of the Sheinwoodian (Ireviken) positive carbonate carbon (δ13Ccarb) isotope excursions provide significantly improved chronostratigraphic correlation of Llandovery strata in the Moose River Basin.

The global Hangenberg Crisis (Devonian–Carboniferous transition): review of a first-order mass extinction

Abstract The global Hangenberg Crisis near the Devonian–Carboniferous boundary (DCB) represents a mass extinction that is of the same scale as the so-called ‘Big Five’ first-order Phanerozoic events. It played an important role in the evolution of many faunal groups and destroyed complete ecosystems but affected marine and terrestrial environments at slightly different times within a short time span of c. 100–300 kyr. The lower crisis interval in the uppermost Famennian started as a prelude with a minor eustatic sea-level fall, followed rather abruptly by pantropically widespread black shale deposition (Hangenberg Black Shale and equivalents). This transgressive and hypoxic/anoxic phase coincided with a global carbonate crisis and perturbation of the global carbon cycle as evidenced by a distinctive positive carbon isotope excursion, probably as a consequence of climate/salinity-driven oceanic overturns and outer-shelf eutrophication. It is the main extinction level for marine biota, especially for ammonoids, trilobites, conodonts, stromatoporoids, corals, some sharks, and deeper-water ostracodes, but probably also for placoderms, chitinozoans and early tetrapods. Extinction rates were lower for brachiopods, neritic ostracodes, bryozoans and echinoderms. Extinction patterns were similar in widely separate basins of the western and eastern Prototethys, while a contemporaneous marine macrofauna record from high latitudes is missing altogether. The middle crisis interval is characterized by a gradual but major eustatic sea-level fall, probably in the scale of more than 100 m, that caused the progradation of shallow-water siliciclastics (Hangenberg Sandstone and equivalents) and produced widespread unconformities due to reworking and non-deposition. The glacio-eustatic origin of this global regression is proven by miospore correlation with widespread diamictites of South America and South and North Africa, and by the evidence for significant tropical mountain glaciers in eastern North America. This isolated and short-lived plunge from global greenhouse into icehouse conditions may follow the significant drawdown of atmospheric CO 2 levels due to the prior massive burial of organic carbon during the global deposition of black shales. Increased carbon recycling by intensified terrestrial erosion in combination with the arrested burial of carbonates may have led to a gradual rise of CO 2 levels, re-warming, and a parallel increase in the influx of land-derived nutrients. The upper crisis interval in the uppermost Famennian is characterized by initial post-glacial transgression and a second global carbon isotope spike, as well as by opportunistic faunal blooms and the early re-radiation of several fossil groups. Minor reworking events and unconformities give evidence for continuing smaller-scale oscillations of sea-level and palaeoclimate. These may explain the terrestrial floral change near the Famennian–Tournaisian boundary and contemporaneous, evolutionarily highly significant extinctions of survivors of the main crisis. Still poorly understood small-scale events wiped out the last clymeniid ammonoids, phacopid trilobites, placoderms and some widespread brachiopod and foraminiferan groups. The post-crisis interval in the lower Tournaisian is marked by continuing eustatic rise (e.g. flooding of the Old Red Continent), and significant radiations in a renewed greenhouse time. But the recovery had not yet reached the pre-crisis level when it was suddenly interrupted by the global, second-order Lower Alum Shale Event at the base of the middle Tournaisian.

The onset of the ‘Ordovician Plankton Revolution’ in the late Cambrian

The ‘Great Ordovician Biodiversification Event’ comprises the rapid diversification of marine organisms during the Ordovician Period. It is now clear that this adaptive radiation started for some organisms already in the Cambrian and continued for others beyond the end of the Ordovician, making the ‘Great Ordovician Biodiversification Event’ part of a long-term late Proterozoic and Early Palaeozoic radiation, that in part is expressed by the fossil record as the ‘Cambrian Explosion.’ A significant diversification of different groups of the plankton is observed in the late Cambrian–Early Ordovician interval, leading to the subsequent ‘Ordovician Plankton Revolution.’ The possible causes of this ‘plankton revolution’ are currently debated. They include changes in palaeoclimate, palaeogeography or tectonic and volcanic activity, as well as a modified nutrient supply. In this context, the Steptoean Positive Carbon Isotope Excursion δ13Ccarb (SPICE) event in the late Cambrian (Paibian Stage, Furongian Series) has been related to a major increase in atmospheric O2 (from 10–18% to some 20 – 29%) and to increased oceanic nutrient availability. Here we analyze the diversification of the planktonic groups during the late Cambrian and Early Ordovician, in particular in relation to the SPICE event. Our analyses include the changing diversities of the phytoplankton (acritarchs), diverse groups of zooplankton (e.g., radiolarians, graptolites, chitinozoans) and the switch to a planktonic mode of life of fossil groups (e.g., arthropods, molluscs) that were part of the Cambrian benthos and that later occupied pelagic niches. In addition, we focus also on data indicating evidence for a late Cambrian to Ordovician origin of planktotrophy in invertebrate larvae. It can be concluded that none of the diversifications of the different planktonic organisms can be related directly to the SPICE event. However, a long term (10–20millionyears) oxygenation pulse related to the SPICE event might have fuelled the explosion of phytoplankton diversity observed in the latest Cambrian–Early Ordovician that led to completely modified trophic structures permitting an increase in diversity and abundance of plankton-feeding groups during the Ordovician.

The Palaeoproterozoic global carbon cycle: insights from the Loch Maree Group, NW Scotland

Two Palaeoproterozoic events have particularly interested Earth scientists. These are the global Lomagundi–Jatuli Event, the greatest magnitude positive carbonate carbon isotope excursion in Earth history, and the Shunga Event, the world’s largest organic carbon burial event. Analysis of newly acquired high-resolution C–O isotope data and U–Pb zircon geochronology refine understanding of carbon isotope characteristics and timing of deposition of the Palaeoproterozoic Loch Maree Group of NW Scotland. Petrographic examination reveals a basal unconformity between the Loch Maree Group and Archaean basement, permitting a stratigraphy and younging direction to be assigned. Detrital zircon ages from immediately above the unconformity are dated at c . 2.3 Ga. δ 13 C carbonate data on two temporally discrete carbonate packages range from c . +15 to 2‰ in the older unit and c . 2 to −5‰ in the younger carbonate unit. Current age constraints indicate that the Loch Maree Group is too young to be fully coeval with the Lomagundi–Jatuli Event but is within the age range of the Shunga Event. This revives consideration of a straightforward mass-balance process involving burial of organic carbon as an explanation for at least some of the C-cycle perturbations of Palaeoproterozoic time. Supplementary Material: Sample descriptions from the Loch Maree Group, geochemical data, sample preparation and geochronology data are available at http://www.geolsoc.org.uk/SUP18865 .

Euxinic conditions recorded in the ca. 1.93 Ga Bravo Lake Formation, Nunavut (Canada): Implications for oceanic redox evolution

The composition of seawater changed dramatically during the initial rise of atmospheric oxygen in the earliest Paleoproterozoic, but the emerging view is that atmosphere–ocean system did not experience an irreversible transition to a well-oxygenated state. Instead, it has been suggested that the oxygen content of the atmosphere–ocean system decreased considerably after ca. 2.06billionyearsago (Ga), which resulted in a crash in marine sulfate concentrations. The end of the deposition of major granular iron formations at ca. 1.85Ga has been linked either to the development of extensive euxinic conditions along continental shelves or a decrease in hydrothermal flux. The record of oceanic redox state is not well constrained for the period between ca. 2.06Ga, the end of the Lomagundi positive carbon isotope excursion, and ca. 1.88Ga when major granular iron formations appeared. We address this gap by presenting new iron-speciation, major and trace element data, as well as sulfur, organic carbon, and molybdenum isotopic data for greenschist facies organic matter-rich mudrocks (ORMs) of the ca. 1.93Ga Bravo Lake Formation, Piling Group, Baffin Island. The iron speciation data suggest deposition of the Bravo Lake Formation under a euxinic (anoxic and sulfidic) water column. Trace metal enrichments and Mo isotope data suggest extensive marine euxinia ca. 90millionyears before the disappearance of large-scale, economic granular iron formations. The addition of new Mo data in this time interval is important, as it contributes to filling in the sparse Proterozoic record. Lastly, this work provides further support for the idea that there was widespread anoxia shortly after the end of the Lomagundi Event.

Large igneous provinces and organic carbon burial: Controls on global temperature and continental weathering during the Early Cretaceous

There is an abundance of evidence for short intervals of cold climatic conditions during the Early Cretaceous. However, the lack of a high-resolution, long-term Early Cretaceous paleotemperature record hampers a full-scale synthesis of these putative “cold snap” episodes, as well as a more holistic approach to Early Cretaceous climate changes. We present an extended compilation of belemnite-based oxygen, carbon and strontium isotope records covering the Berriasian–middle Albian from the Vocontian Basin (SE France). This dataset clearly demonstrates three intervals of cold climatic conditions during the Early Cretaceous (late Valanginian–earliest Hauterivian, late early Aptian, latest Aptian–earliest Albian). Each of these intervals is associated with rapid and high amplitude sea-level fluctuations, supporting the hypothesis of transient growth of polar ice caps during the Early Cretaceous. As evidenced by positive carbon isotope excursions, each cold episode is associated with enhanced burial of organic matter on a global scale. Moreover, there is a relatively good match between the timing and size of large igneous province eruptions and the amplitude of Early Cretaceous warming episodes. Altogether, these observations confirm the instrumental role of atmospheric CO2 variations in driving Early Cretaceous climate change. From a long-term perspective, the coupling of global paleotemperature and seawater strontium isotopic ratio during the Early Cretaceous is best explained by temperature-controlled changes of continental crust weathering rates.

Carbon cycling across the southern margin of Laurentia during the Late Ordovician

The Guttenberg Isotope Carbon Excursion (GICE), a positive carbon isotope excursion that occurs near the base of the Katian Stage, is thought to be a global event possibly related to Late Ordovician cooling. Documenting how much regional and global variability exists in carbon isotopic trends prior to and during the GICE is a critical aspect in understanding the implications of the excursion for interpreting changes in the global carbon cycle, paleoclimate, and chemostratigraphic correlation during the Ordovician. To investigate carbon isotopic trends along the southern margin of the Laurentian carbonate platform during the Late Ordovician, we measured bulk carbonate δ13Ccarb and δ18Ocarb as well as organic carbon δ13Corg values from four locations in Alabama. These sections are excellent study sites because they are well exposed, contain the regionally well-correlated Deicke and Millbrig K-bentonites, and are in a region where δ13C trends have not been studied.Carbonate δ13Ccarb results from one section (i.e. Fort Payne) in northeastern Alabama record a 1.5‰ positive excursion above the Millbrig K-bentonite. We interpret the 1.5‰ positive excursion as a primary feature and identify it as the GICE. The characteristic GICE excursion is absent in the other study sections and we attribute this absence to the presence of unconformities (either non-depositional or erosional) in these sections. We find neither evidence for sea level control on δ13C trends nor conclusive evidence for δ13CDIC gradients along the southeast margin of Laurentia's epeiric sea. Combined with the proximity to the Iapetus Ocean, these results suggest that carbon isotopic trends in Alabama are a reliable proxy for open ocean conditions. Therefore, despite the absence of GICE excursion in all but one of our study sections, increasing δ13C values in Alabama prior to the GICE provides evidence for a steady 13C enrichment of the global surficial carbon reservoir consistent with a drawdown of atmospheric CO2 during the early Late Ordovician

Faunal and facies changes through the mid Homerian (late Wenlock, Silurian) positive carbon isotope excursion in Podolia, western Ukraine

The late Wenlock is characterized by two global regressive-transgressive eustatic cycles in association with a double-peaked positive carbon isotope excursion. The onset of the excursion coincides with an extinction event affecting graptolites (the lundgreni event) and proposed to affect conodonts (the Mulde Event) and proliferation of non-skeletal carbonates. In order to test the hypothesized relationships between eustatic and ecological changes, the tropical carbonate Homerian succession in Podolia has been examined with respect to conodont, sequence and δ13C stratigraphy. Four depositional sequences (DS) have been identified. The onset of the δ13C excursion occurs at the boundary between DS1 and DS2, corresponding to a forced regression of proposed glacioeustatic origin. The following rapid eustatic transgression associated with the highest δ13C values of 5.2‰ includes a higher-order shallowing episode recorded in Podolia as normal regression and a boundary between DS2 and DS3. This interval is distinguished by the presence of oncoids and thrombolitic buildups. The latest Wenlock eustatic fall and the corresponding second peak of the δ13C excursion corresponds in Podolia to a stratigraphic gap. The first δ13C peak (top of DS1 and DS2) corresponds to the O. bohemica longa conodont Zone, the interval between the two peaks (DS3) – to K. ortus absidata and C. murchisoni zones, and DS4 is tentatively placed in the lowermost Ludlow Series. The record of relative sea-level changes in Podolia is consistent with reconstructions based on successions in England and Sweden. The moderate drop in conodont taxonomic richness may reflect the primary depositional control over their proposed extinction.

Integrated stratigraphy of Lower Cretaceous sediments (Ryazanian–Hauterivian) from North-East Greenland

Abstract The reconstruction of past climates and oceanography requires a solid stratigraphic framework ideally applicable on a global scale. The earliest Cretaceous, however, was a time of strong faunal provincialism, making supra-regional correlation of biostratigraphical zonations difficult. The step-by-step correlations between neighbouring provinces/subprovinces that are commonly utilized bear the risk of loosing accuracy in every step. Here we present 87Sr/86Sr- and stable isotope-data (δ13C, δ18O) from belemnite rostra of the Rodryggen section in North-East Greenland. The integrated stratigraphy based on Sr-isotope ratios, ammonite and calcareous nannofossil biostratigraphy offers the opportunity for a direct comparison of the different stratigraphic zonations. These are complemented by δ13Cbel-data recording the positive carbon isotope excursion of the Valanginian Weissert Event, which is a reliable stratigraphic event. The geochemical data furthermore allow a reliable correlation of Tethyan and Boreal strata. The stratigraphic range of the Rodryggen section resulting from Sr-isotope stratigraphy (Ryazanian–Barremian) is in agreement with the biostratigraphic findings. Mismatches regarding stage/substage boundaries demand a reconsideration of the nannofossil biostratigraphy of the Boreal Lower Cretaceous. Our findings suggest stratigraphic ranges for two nannofossil index species (Sollasites arcuatus, Micrantholithus speetonensis) different from published ranges. The new observations imply changes in the Boreal Ryazanian–Valanginian nannofossil zonation scheme. Specifically the base of calcareous nannofossil zone BC3, originally defined as uppermost Ryazanian, is shifted to the lower Valanginian. Based on these new stratigraphic interpretations a decrease in the abundance of nannoconids observed in the Rodryggen section can now be identified as the Valanginian nannoconid crises. This nannoconid decline has been observed in Tethyan sections along with the Weissert Event. A positive trend in the δ18Obel-data agrees with a late Valanginian cooling that has been postulated based on independent proxies from the Boreal Realm and the Tethys.

Questioning a widespread euxinia for the Furongian (Late Cambrian) SPICE event: indications from δ13C, δ18O, δ34S and biostratigraphic constraints

AbstractResults from a high-resolution study of δ13Ccarb, δ18Ocarb, δ34SCAS, δ34SCRSand elemental concentrations (Ca, Fe, Mg, Mn and Sr) in the Furongian Kyrshabakty section, southern Kazakhstan, are reported here. The investigated interval covers the Drumian to Jiangshanian stages of the Cambrian Period, respectively the regionalPtychagnostus atavustoIvshinagnostus ivshini-Irvingella majortrilobite zones. δ13Ccarbdata include the Steptoean positive carbon isotope excursion (SPICE) with a local peak value of +5‰. The onset of SPICE corresponds to theKormagnostus simplex–Glyptagnostus stolidotuszones and pre-dates the base of the Paibian Stage/Furongian Series. δ34SCASdata already increase during theLejopyge armatabiozone culminating in three positive excursions prior and after the SPICE maximum. Differences in onset, peak values and shape of the δ13Ccarband/or δ34SCASexcursions at Kyrshabakty, but also in almost all sections characterized by the SPICE, are pointing towards Furongian seawater that was low in sulphate concentration and heterogeneous in its carbonate carbon and sulphate sulphur isotopic composition. The occurrence of benthic faunal elements in almost all SPICE-related sections strongly supports oxygenated conditions at the seafloor, therefore excluding widespread anoxia or euxinia. Regional anoxic conditions are most probable. A positive δ18Ocarbexcursion parallel to the SPICE could probably be explained by a decline in seawater pH associated with a sea-level rise. Again, no euxinic conditions would be mandatory for explaining the SPICE event.

Late Wenlock carbon isotope excursions and associated conodont fauna in the Podlasie Depression, eastern Poland: a not‐so‐big crisis?

The middle Silurian ‘big crisis’ refers to a graptolite extinction event and faunal turnover at the onset of a double‐peaked positive carbon isotope excursion. The crisis has been proposed to affect conodonts, giving rise to a sophisticated palaeoecological model of their stepwise extinction, known as the Mulde Event. However, the impact of the event on conodont faunas outside Gotland and the Silurian Baltic Basin remains unknown. Here, it is examined in the Widowo IG‐1 core (E Poland). The middle Silurian succession in this core represents foreshoal, shoal, and lagoonal settings on a tropical carbonate ramp on the shelf of the Eastern European Craton. Three positive δ13Ccarb excursions have been identified; the two upper excursions correlate with the Mulde isotope anomaly and with two global eustatic regressions. Conodont species proposed to be affected by the extinction event either were not observed or ranged through the extinction interval. Changes in their frequencies are best explained by the sequence stratigraphic architecture of the late Wenlock strata in the Widowo IG‐1 core. The little impact of the ‘big crisis’ may reflect facies homogeneity across the studied interval, supporting the hypothesis that the late Wenlock conodont turnover in epicontinental settings was primarily driven by eustatically controlled facies shifts. Copyright © 2015 John Wiley & Sons, Ltd.

Sulfur isotope evidence for low and fluctuating sulfate levels in the Late Devonian ocean and the potential link with the mass extinction event

High amplitude positive carbon isotope excursions in the Late Devonian, the punctata and Kellwasser events, reflect major perturbations in the global carbon cycle that have been attributed to increased continental weathering and subsequent ocean eutrophication. Despite the comparable carbon isotope anomalies, however, a major extinction has been reported only for the Kellwasser Events, while the punctata Event is marked by low extinction intensity. This study presents multiple sulfur isotope records of carbonate-associated sulfate (CAS) and pyrite from Late Devonian sections in the Great Basin, USA, in order to document changes in the coupled (or decoupled) geochemical cycles of carbon and sulfur during the punctata and Upper Kellwasser events. A positive sulfur isotope shift in both CAS and pyrite accompanies the onset of the punctata Event, but to a larger extent in the latter. As a result, the sulfur isotope offset between CAS and pyrite (Δ34SCAS-py) dropped to less than 10‰. In the middle of the punctata Event, a sharp negative δ34SCAS excursion and negative Δ34SCAS-py values coincide with the Alamo impact. Unlike the rapid δ34Spy and δ34SCAS oscillations associated with the punctata Event, the Upper Kellwasser was a period of relative stability, except for a brief δ34SCAS drop before the event. Paired sulfur isotope data, aided by a simple box model, suggest that the geochemical cycle of sulfur may have been partly responsible for the contrasting biological responses that define these events. High stratigraphic δ34Spy and δ34SCAS variability, coupled with strong reservoir effect, demonstrates a relatively small oceanic sulfate pool existed during the punctata Event. Further, the Alamo impact likely triggered the rapid oxidation of microbially-produced sulfide within this event. The expansion of sulfidic bottom water thus may have been impeded during the punctata Event. In contrast, the lack of a positive shift in δ34SCAS and sizable Δ34SCAS-py values (>15‰) throughout the Upper Kellwasser Event imply higher relative sulfate levels. A larger seawater sulfate reservoir may have promoted the development of sulfidic bottom waters in the eutrophic epicontinental seas, increasing biological stress and potentially contributing to the mass extinction.

Palaeoclimate and palaeoenvironmental changes through the onset of the Valanginian carbon–isotope excursion: Evidence from the Polish Basin

Abstract The Valanginian Weissert episode of environmental change is associated with a positive carbon–isotope excursion (CIE) in δ13Ccarb and δ13Corg records and a crisis in pelagic and neritic carbonate production. The CIE was interpreted to represent the first oceanic anoxic event (OAE) of the Cretaceous, linked with the formation of the Parana–Etendeka large igneous province (LIP). Recent studies suggest though that the extent of oceanic anoxia was limited to high-latitude epicontinental seas and the Pacific, and that the Parana–Etendeka LIP postdates the Valanginian CIE. With these new interpretations, the palaeoenvironmental changes behind the observed perturbation of the carbon cycle remain to be elucidated. Here we present sedimentological and geochemical results from a drill core near Wąwal, central Poland. The excavated succession is of particular interest due to its near-shore depositional setting within the former Polish Basin and its preservation (up to 17 wt.% of aragonite). The core consists of lower to upper Valanginian silty to sandy clays deposited under fully marine conditions on top of an Upper Jurassic karstified limestone. A change in weathering mode from very humid and highly hydrolysing towards less humid and seasonally contrasted conditions is indicated by the abrupt change from a kaolinite- to a smectite-dominated clay-mineral association near the boundary between the early and late Valanginian. Moreover, two phosphate-rich horizons were identified, which correlate in time to northern Tethyan occurrences in the Helvetic Alps. The lower level is associated with the early Valanginian transgression and corresponds to peak humidity recorded in the Wąwal core. The upper layer corresponds to the paroxysmal phase of the Valanginian crisis in carbonate production. In the Wąwal core, evidence for anoxic conditions during the Valanginian CIE is lacking. The climatic conditions and changes identified in the Polish Basin are different from those of the northern Tethyan area, where an increase in humidity is observed near the early–late Valanginian boundary. The contrasting climate conditions are probably an expression of latitudinal shifts in the position of the northern mid-latitude humid belt.

Latitudinal temperature trends in the northern hemisphere during the Early Cretaceous (Valanginian–Hauterivian)

We present the first detailed stable isotope record (δ13C, δ18O) for the Valanginian (Lower Cretaceous) of the southern Boreal Realm. The observations are based on the geochemical analysis of 286 belemnite rostra from stratigraphically well-dated sections in northwest Germany. A major positive carbon isotope excursion (CIE) of ~ 2‰ in the early late Valanginian documents a perturbation of the carbon cycle. This positive CIE, which is well known as the ‘Weissert Event’ in the Tethys, is also displayed in composite carbon isotope records based on published and own data from three different palaeolatitudes (35°, 39°, 65° North). These findings suggest that the environmental changes indicated by the CIE in the early late Valanginian are synchronous, thereby pointing to a global control. The combined oxygen isotope data sets show long-term fluctuations of up to 7 °C with a cooling in the late Valanginian for both the southern and the arctic part of the Boreal Realm, whilst Tethyan data document only a ca. 1 °C cooling. Based on a reliable bio- and chemostratigraphic correlation between the different realms this study highlights the amplitude of the environmental perturbations in the Valanginian and supports the idea that a combination of regional and global factors caused such shifts.

A multi-peak organic carbon isotope excursion in the late Aeronian (Llandovery, Silurian): Evidence from Arisaig, Nova Scotia, Canada

The late Aeronian is now recognized as one of several time intervals of the Silurian Period that is characterized by a positive carbon isotope excursion believed to be of global significance. There is also some recent evidence that this interval may include several distinct δ13C peaks within a time interval of less than 300kyr. The late Aeronian to earliest Telychian succession in the Ross Brook Formation near Arisaig, Nova Scotia, Canada, preserves a thick succession of marine, clastic strata that spans the interval of this excursion, and it shows evidence of three or possibly four positive organic carbon isotope (δ13Corg) excursions ranging in magnitude from at least +0.7‰ to approximately +3.5‰ above local baseline values. These excursions occur within an interval beginning near the base of the Stimulograptus sedgwickii Zone and ending near the base of the Telychian, as defined by the last occurrence of the brachiopod Eocelia intermedia. Comparison with a recently published δ13Corg record from Bohemia suggests that the three or four peaks may be correlated between these two regions indicating that these multiple excursions may represent a regional or global environmental signal of cyclical palaeoenvironmental change. In the Arisaig region, these δ13Corg peaks all occur within an overall shallowing-upward cycle but otherwise our study provides no new insights into the nature of the specific palaeoenvironmental processes that may have driven these short-term fluctuations in δ13Corg values. If the level of last occurrence of Eocelia intermedia can be reliably regarded as indicating correlation with the GSSP for the Base of Telychian, and if our proposed correlation between the Arisaig and Bohemian successions is correct, then the biostratigraphic level of the GSSP correlates with a level near the base of the Bohemian Lituigraptus rastrum Zone, which correlates closely with the base of the Stimulograptus halli Zone in eastern Avalonia and Baltica.

The North China Steptoean Positive Carbon Isotope Event: New insights towards understanding a global phenomenon

Precise biostratigraphic correlation of the base of the Paibian Stage, Cambrian, within and beyond the North China craton has been difficult, mostly because of the strong endemism of the trilobite fauna on the shallow platform facies. The use of the Steptoean Positive Carbon Isotope Excursion (SPICE) in defining the Guzhangian-Paibian stage boundary has become a standard practice. Thus, a chemostratigraphic study of the Huangyangshan section from North China has been carried out. This section in Shandong Province clearly records the SPICE, with the most detailed fossil and carbonate carbon isotope data to date. Compared to other sections in the world, all the published SPICE in North China display both reduced peak and reduced difference (Δ13C) values. These may imply unique conditions on the North China craton during the upper Guzhangian to Paibian stages. This study shows that the SPICE is recorded in relatively condensed sections in North China; thus, ≤ 0.5 m sampling density is suggested here in order to evaluate the presence of the SPICE in North China and condensed sections.

Chronology of the Early Toarcian environmental crisis in the Lorraine Sub-Basin (NE Paris Basin)

Early Toarcian (Jurassic; ∼183 Ma) sediments recorded profound environmental changes, including mass extinction, global warming, marine transgression as well as widespread bottom water anoxia and organic matter accumulation on the Western Tethyan shelf. Enhanced organic matter accumulation was accompanied by a positive carbon isotope excursion (CIE) in pelagic carbonate, which marks the Toarcian Oceanic Anoxic Event. These environmental changes were accompanied by a major perturbation of the global carbon cycle, expressed by negative CIE, interrupting the positive trend. The duration of the carbon cycle perturbation is still debated, with estimates for the negative CIE range from ∼200 to ∼600 kyr. Here we present ultra high-resolution (<1 kyr) measurements of magnetic susceptibility and sediment color from a marine section located in the Lorraine Sub-Basin (NE Paris Basin) documenting Milankovitch-controlled fluctuations in depositional conditions that occurred superimposed onto the overall sea level evolution. Differences in the wavelength of the sedimentary cycles indicate variable sediment accumulation rates that mainly resulted from rapid sea level fluctuations. The most pronounced sea level rise that took place within the uppermost tenuicostatum zone resulted in a strong condensation of the basal Schistes Carton formation. Strong condensation can explain the discrepancy between durations previously calculated for the CIE placed at this stratigraphic interval. Our data support durations of ∼900 kyr and ∼600 kyr for the positive and negative CIE, respectively. The cyclostratigraphy-based timescale further proposes a duration of >555 kyr for the tenuicostatum zone and 1310 kyr for the serpentinum zone. The durations of the elegantulum and falciferum subzones can be estimated to ∼790 kyr and ∼520 kyr, respectively. A change in the orbital response from eccentricity- to obliquity-forcing, evident from other locations, is well-expressed in the Lorraine Sub-Basin and occurred within the CIE interval. The strong impact of the obliquity component in post-event deposits hints to processes most effective at high latitudes, such as the waxing and waning of polar ice. Paleogeographic features of the Western Tethyan shelf supported the tele-connection of higher to lower latitude processes via water exchange through the Viking Corridor.

Enhanced primary productivity and magnetotactic bacterial production in response to middle Eocene warming in the Neo-Tethys Ocean

Earth's climate experienced a warming event known as the Middle Eocene Climatic Optimum (MECO) at ~40Ma, which was an abrupt reversal of a long-term Eocene cooling trend. This event is characterized in the deep Southern, Atlantic, Pacific and Indian Oceans by a distinct negative δ18O excursion over 500kyr. We report results of high-resolution paleontological, geochemical, and rock magnetic investigations of the Neo-Tethyan Monte Cagnero (MCA) section (northeastern Apennines, Italy), which can be correlated on the basis of magneto- and biostratigraphic results to the MECO event recorded in deep-sea sections. In the MCA section, an interval with a relative increase in eutrophic nannofossil taxa (and decreased abundances of oligotrophic taxa) spans the culmination of the MECO warming and its aftermath and coincides with a positive carbon isotope excursion, and a peak in magnetite and hematite/goethite concentration. The magnetite peak reflects the appearance of putative magnetofossils, while the hematite/goethite apex is attributed to an enhanced detrital mineral contribution, likely as aeolian dust transported from the continent adjacent to the Neo-Tethys Ocean during a drier, more seasonal climate during the peak MECO warming. Based on our new geochemical, paleontological and magnetic records, the MECO warming peak and its immediate aftermath are interpreted as a period of high primary productivity. Sea-surface iron fertilization is inferred to have stimulated high phytoplankton productivity, increasing organic carbon export to the seafloor and promoting enhanced biomineralization of magnetotactic bacteria, which are preserved as putative magnetofossils during the warmest periods of the MECO event in the MCA section. Together with previous studies, our work reinforces the connection between hyperthermal climatic events and the occurrence (or increased abundance) of putative magnetofossils in the sedimentary record.