Reply to comment on “Mantle plume: the invisible serial killer — Application to the Permian–Triassic boundary mass extinction” by E. Heydari, N. Arzani and J. Hassanzadeh [Palaeogeography, Palaeoclimatology, Palaeoecology 264 (2008) 147–162]

In a recent article, Heydari et al. (2008) suggested that the perturbation at the Permian–Triassic boundary (PTB) was initiated by processes associated with an end-Permian mantle plume including igneous intrusions and uplift. These events resulted in the massive release of CH₄ primarily from the dissociation of marine gas hydrates, and secondarily from maturation of organic-rich sediments and fracturing of petroleum reservoirs. Injection of CH₄ into the ocean changed seawater composition (the acid-bath ocean) leading to marine mass extinction. Transfer of CO₂ and CH₄ from the ocean to the atmosphere created a hot climate (the end-Permian inferno) which caused the terrestrial mass extinction. We suggested that the Siberian trap volcanism and marine anoxia played little role in this catastrophe.Wignall and Racki (2009-this issue) have raised three criticisms to our article. The first is that our interpretation has been previously advocated by others. Our re-evaluation indicates that our interpretation was in fact opposite of those considered by Wignall and Racki (2009-this issue) to have presented scenarios similar to ours.The second, Wignall and Racki (2009-this issue) also suggest that our proposed change in carbonate mineralogy across the PTB did not occur because such a change “should produce a large positive excursion rather than the observed negative excursion”. Wignall and Racki (2009-this issue) have made a basic mathematical error in evaluating the effect of carbonate mineralogy on δ¹³C values. Therefore, they have reached two wrong conclusions: one about the validity of a change in carbonate mineralogy and the other regarding its effect on the shift in δ¹³C values at the PTB. A change in carbonate mineralogy produced a larger negative excursion rather than a positive shift.The third, Wignall and Racki (2009-this issue) indicate that the PTB ocean was anoxic to the rim. This criticism is not supported by the rock record because highly bioturbated strata were deposited in environments ranging from shallow shelves to deep waters under oxygenated water column at the time of the PTB mass extinction. If the ocean were totally stratified for 20 Ma, and if anoxia extended all the way to the shoreline, and if the ocean were anoxic to the rim and H₂S were oozing out of it, then we should see at least 100 m of organic-rich, varved-laminated strata in areas ranging from the abyssal plain to the shoreline environments. Such strata have not yet been found.     

Isotopic evidence of environmental changes during the Devonian–Carboniferous transition in South China and its implications for the biotic crisis

The Devonian–Carboniferous (D–C) transition coincides with the Hangenberg Crisis, carbon isotope anomalies, and the enhanced preservation of organic matter associated with marine redox fluctuations. The proposed driving factors for the biotic extinction include variations in the eustatic sea level, paleoclimate fluctuation, climatic conditions, redox conditions, and the configuration of ocean basins. To investigate this phenomenon and obtain information on the paleo-ocean environment of different depositional facies, we studied a shallow-water carbonate section developed in the periplatform slope facies on the southern margin of South China, which includes a well-preserved succession spanning the D–C boundary. The integrated chemostratigraphic trends reveal distinct excursions in the isotopic compositions of bulk nitrogen, carbonate carbon, organic carbon, and total sulfur. A distinct negative δ¹⁵N excursion (~−3.1‰) is recorded throughout the Middle Si. praesulcata Zone and the Upper Si. praesulcata Zone, when the Hangenberg mass extinction occurred. We attribute the nitrogen cycle anomaly to enhanced microbial nitrogen fixation, which was likely a consequence of intensified seawater anoxia associated with increased denitrification, as well as upwelling of anoxic ammonium-bearing waters. Negative excursions in the δ¹³C_carb and δ¹³C_org values were identified in the Middle Si. praesulcata Zone and likely resulted from intense deep ocean upwelling that amplified nutrient fluxes and delivered ¹³C-depleted anoxic water masses. Decreased δ³⁴S values during the Middle Si. praesulcata Zone suggests an increasing contribution of water-column sulfate reduction under euxinic conditions. Contributions of organic matter produced by anaerobic metabolisms to the deposition of shallow carbonate in the Upper Si. praesulcata Zone is recorded by the nadir of δ¹³C_org values associated with maximal △¹³C. The integrated δ¹⁵N-δ¹³C-δ³⁴S data suggest that significant ocean-redox variation was recorded in South China during the D–C transition; and that this prominent fluctuation was likely associated with intense upwelling of deep anoxic waters. The temporal synchrony between the development of euxinia/anoxia and the Hangenberg Event indicates that the redox oscillation was a key factor triggering manifestations of the biodiversity crisis.     

Evolution of C isotopes in the Cambrian of China: implications for Cambrian subdivision and trilobite mass extinctions

Carbon and oxygen isotopes were studied in fossiliferous Cambrian carbonates in northwestern Hunan Province (South China) and in northern Anhui and southern Shandong provinces (North China). Two major C isotope excursions related to biological events occur in the Wangcun section (Yongshun County, northwestern Hunan), which consists of a slope carbonate sequence (510 m thick) containing abundant trilobites. The first C isotope excursion (δ¹³C value shifts from -2.3‰ to 2‰) occurs near the boundary between the Qingxudong and Aoxi formations, close to the traditional Lower-Middle Cambrian boundary. The second excursion (δ¹³C value shifts from 0‰ to 3‰) occurs in the interval between the Linguagnostus reconditus Zone and the Glyptagnostus reticulatus Zone. The base of the G. reticulatus Zone define the base of the Paibi Stage and Furongian Series. Similar C isotope excursions also occur in shallow - water carbonate sections in North China. In Jiagou section near Huainan (Anhui Province), recently considered an important interval for defining the lower-middle Cambrian boundary because of dramatic changes in the trilobite fauna (extinction of redlichiids and appearances of ptychopariids), a negative C isotope excursion (δ¹³C value shifts from +1.21‰ to -1.93‰) occurs at the top of the lower member of the Mantou Formation. In the Gushan section (Changqing County, Shandong Province), a C isotope excursion (δ¹³C value shifts from -0.04‰ to 2.23‰) occurs at the base of the Changshan Formation and is coincident with the base of the Chuangia Zone. This excursion can be correlated with the excursion in the lower part of Glyptagnostus reticulatus Zone in the Wangcun section. The above two distinct C isotope excursions, which occur both in slope carbonates in South China and in shallow - water carbonates in North China, have also been recognized in Cambrian sections on other continents, and they coincide with global mass extinctions of trilobites. The two excursions evidently reflect global changes of Cambrian sea level, and they have utility for Cambrian subdivisions and for both regional and global stratigraphic correlation. In addition, a negative carbon excursion below the base of the Ptychagnostus atavus Zone in the Wangcun section supports previous suggestions that the FAD of P. atavus can be considered as a global correlatable horizon within the middle Cambrian.     

A negative carbon isotope excursion defines the boundary from Cambrian Series 2 to Cambrian Series 3 on the Yangtze Platform,South China

A globally recorded negative carbon isotope excursion characterizes the transition from Cambrian Series 2 to Cambrian Series 3. This transition is also well exposed in sedimentary successions on the Yangtze Platform, and the Wuliu–Zengjiayan section, Guizhou Province, South China has been proposed as a potential Global Stratotype Section and Point (GSSP) for this boundary. Here, we report δ¹³C_carb values for the Jianshan and the Wuliu–Zengjiayan sections. Both sections display a progressive decrease in δ¹³C from values around + 3‰ upwards in stratigraphy to a pronounced δ¹³C minimum with values as low as ? 6.9‰ at the proposed boundary level, and a return to δ¹³C values between 0 and + 1‰ in the upper part of the sections. The δ¹³C minimum is thought to be caused by a transgressive event, flooding the shelf area with ¹³C depleted basinal anoxic bottom water. Our δ¹³C data are in good agreement with carbon isotope profiles recorded elsewhere. These define the so called ROECE event (Redlichiid–Oleneliid Extinction Carbon Isotope Excursion, cf. Zhu et al., 2006, 2007) and may reflect the perturbation of the global carbon cycle during the Cambrian Series 2 to Cambrian Series 3 transition.     

Uncovering the spatial heterogeneity of Ediacaran carbon cycling

Records of the Ediacaran carbon cycle (635–541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to ?12‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ¹³C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ¹³C_carb and δ¹³C_org) records from the Ediacaran Doushantuo Formation along a proximal‐to‐distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ¹³C_org are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ³⁴S of carbonate‐associated sulfate (CAS) and increased ⁸⁷Sr/⁸⁶Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ¹³C_org records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O₂ and sulfate) limited to marginal near‐surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ¹³C presented in this study.     

Depositional conditions during the Lower Kellwasser Event (Late Frasnian) in the deep‐shelf Łysogóry Basin of the Holy Cross Mountains Poland

Integrated biostratigraphical, microfacial and geochemical studies of the Lower Kellwasser Event in the P?ucki succession (southern Poland) provide details about redox conditions during the deposition of this horizon in the deep‐shelf ?ysogóry basin of the Holy Cross Mountains. The environment is characterized by calm sedimentation and soft, muddy carbonate substrate. However, microfacies changing from wackestones to grainstones, the presence of crushed or current‐oriented nautiloid shells and the occurrence of redeposited material from shallow‐water Dyminy Reef environments (such as calcispheroids, algae and girvanellid cyanobacteria) suggest episodes of a higher‐energy regime. Uranium/thorium ratios indicate that bottom‐water redox conditions changed periodically from being mainly anoxic in the middle part of the Lower Kellwasser Horizon to dysoxic in the lower and upper parts. During a short‐term episode of bottom‐water ventilation, the seafloor was rapidly colonized by a dense assemblage of opportunistic buchiolid bivalves, which suffered mass mortality upon the return to anoxic conditions. A very rich concentration of cephalopods and homoctenids may be regarded as reflecting a bloom of high‐density populations during high‐productivity events. Similarly, they suffered mass mortality when episodically increasing anoxia/euxinia reached the upper part of the water column. The Late Frasnian inorganic carbon isotope records in the P?ucki section show a positive shift with a maximum amplitude of 3‰. This enrichment in δ¹³C can be correlated with the deposition of the Lower Kellwasser Horizon and reflects the expansion of anoxic and probably high‐productivity regimes.     

Permian and early Triassic isotopic records of carbon and strontium in Australia and a scenario of events about the Permian‐Triassic boundary

The negative shift in δ¹³C values of carbonate carbon at the Permian/Triassic boundary is one of the better documented geochemical signatures of a mass extinction event. The similar negative shift in δ¹³C values in organic carbon from Permian/Triassic boundary marine sediments in Austria and Canada is shown to occur also in marine and non‐marine sediments from Australian sedimentary basins. This negative shift in δ¹³C values is used to calibrate Australian sections lacking diagnostic faunal elements identifying the Permian/Triassic boundary. The minimum in the carbonate ⁸⁷Sr/⁸⁶Sr seawater curve from carbonates across the Guadalupian/Ochoan Stage boundary, mainly from North America, is shown to occur also in brachiopod calcite mainly from the Bowen Basin of eastern Australia, hence providing a second calibration point in the Australian sedimentary record. These two geochemical events support a model of a runaway greenhouse developing about the Permian/Triassic boundary; this is inferred to have contributed to the end‐Permian mass extinction.     

Revisiting marine redox conditions during the Ediacaran Shuram carbon isotope excursion

The Neoproterozoic carbonate record contains multiple carbon isotope anomalies, which are the subject of intense debate. The largest of these anomalies, the Shuram excursion (SE), occurred in the mid-Ediacaran (~574–567 Ma). Accurately reconstructing marine redox landscape is a clear path toward making sense of the mechanism that drives this δ¹³C anomaly. Here, we report new uranium isotopic data from the shallow-marine carbonates of the Wonoka Formation, Flinders Ranges, South Australia, where the SE is well preserved. Our data indicate that the δ²³⁸U trend during the SE is highly reproducible across globally disparate sections from different depositional settings. Previously, it was proposed that the positive shift of δ²³⁸U values during the SE suggests an extensive, near-modern level of marine oxygenation. However, recent publications suggest that the fractionation of uranium isotopes in ferruginous and anoxic conditions is comparable, opening up the possibility of non-unique interpretations of the carbonate uranium isotopic record. Here, we build on this idea by investigating the SE in conjunction with additional geochemical proxies. Using a revised uranium isotope mass balance model and an inverse stochastic carbon cycle model, we reevaluate models for δ¹³C and δ²³⁸U trends during the SE. We suggest that global seawater δ²³⁸U values during the SE could be explained by an expansion of ferruginous conditions and do not require a near-modern level of oxygenation during the mid-Ediacaran.     

A diagenetic control on the Early Triassic Smithian–Spathian carbon isotopic excursions recorded in the marine settings of the Thaynes Group (Utah,USA)

In the aftermath of the end‐Permian mass extinction, Early Triassic sediments record some of the largest Phanerozoic carbon isotopic excursions. Among them, a global Smithian‐negative carbonate carbon isotope excursion has been identified, followed by an abrupt increase across the Smithian–Spathian boundary (SSB; ~250.8 Myr ago). This chemostratigraphic evolution is associated with palaeontological evidence that indicate a major collapse of terrestrial and marine ecosystems during the Late Smithian. It is commonly assumed that Smithian and Spathian isotopic variations are intimately linked to major perturbations in the exogenic carbon reservoir. We present paired carbon isotopes measurements from the Thaynes Group (Utah, USA) to evaluate the extent to which the Early Triassic isotopic perturbations reflect changes in the exogenic carbon cycle. The δ¹³C_carb variations obtained here reproduce the known Smithian δ¹³C_carb‐negative excursion. However, the δ¹³C signal of the bulk organic matter is invariant across the SSB and variations in the δ³⁴S signal of sedimentary sulphides are interpreted here to reflect the intensity of sediment remobilization. We argue that Middle to Late Smithian δ¹³C_carb signal in the shallow marine environments of the Thaynes Group does not reflect secular evolution of the exogenic carbon cycle but rather physicochemical conditions at the sediment–water interface leading to authigenic carbonate formation during early diagenetic processes.     

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.     

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

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

Stepwise and large-magnitude negative shift in δ13Ccarb preceded the main marine mass extinction of the Permian–Triassic crisis interval

Large perturbations to the global carbon cycle occurred during the Permian–Triassic boundary mass extinction, the largest extinction event of the Phanerozoic Eon (542 Ma to present). Controversy concerning the pattern and mechanism of variations in the marine carbonate carbon isotope record of the Permian–Triassic crisis interval (PTCI) and their relationship to the marine mass extinction has not been resolved to date. Herein, high-resolution carbonate carbon isotope profiles (δ¹³C_carb), accompanied by lithofacies, were generated for four sections with microbialite (Taiping, Zuodeng, Cili, and Chongyang) in South China to better constrain patterns and controls on δ¹³C_carb variation in the PTCI and to test hypotheses about the temporal relationship between perturbations to the global carbon cycle and the marine mass extinction event. All four study sections exhibit a stepwise negative shift in δ¹³C_carb during the Late Permian–Early Triassic, with the shift preceding the end-Permian crisis being larger (> 3‰) than that following it (1–2‰). The pre-crisis shifts in δ¹³C_carb are widely correlatable and, hence, represent perturbations to the global carbon cycle. The comparatively smaller shifts following the crisis demonstrate that the marine mass extinction event itself had at most limited influence on the global carbon cycle, and that both Late Permian δ¹³C_carb shifts and the mass extinction must be attributed to some other cause. Their origin cannot be uniquely determined from C-isotopic data alone but appears to be most compatible with a mechanism based on episodic volcanism in combination with collapse of terrestrial ecosystems and soil erosion.     

High-resolution chemo- and biostratigraphic records of the Early Aptian oceanic anoxic event in Cantabria (N Spain): Palaeoceanographic and palaeoclimatic implications

During the Early Aptian, major palaeoenvironmental changes occurred leading to an oceanic anoxic event (OAE 1a) and a perturbation of the global carbon cycle. New detailed litho-, bio-, and chemostratigraphic (TOC, δ¹³C_carb, δ¹³C_org) records of two superbly exposed and expanded Lower Aptian sections in Cantabria (La Florida and Cuchía) allow to recognize the expression of the OAE 1a in shallow shelf environments of northern Spain. The succession consists of shallow platform limestones that include a marly unit (Patrocinio Formation), the deposition of which occurred mostly at the onset of the OAE 1a (~ 120.5 Ma). This study presents a new integrated biostratigraphy based on ammonites, planktonic foraminifera, calcareous nannofossils and palynomorphs that allows an accurate age resolution of the succession. The marly unit records an abrupt negative δ¹³C excursion in both bulk organic matter (up to 5‰) and carbonate (up to 6‰, mean 3‰), as has been already observed at the onset of the OAE 1a in other Lower Aptian deposits worldwide. In detail, however, the negative excursion presents two minima in the studied sections. This negative spike is confidently attributed to the upper half of the Hayesites irregularis nannofossil Zone, to the upper part of the Blowiella blowi foraminiferal Zone, and to the middle–upper part of the Deshayesites weissi ammonite Zone. A third negative excursion occurs at the base of the Rhagodiscus angustus nannofossil Zone, which may be correlatable with the Dufrenoyia furcata ammonite Zone. This data set refines the age of the OAE 1a and reveals the existence of a stratigraphic gap in the westernmost margin of the Basque Cantabrian Basin that covers at least a portion of the upper part of the Early Aptian. Sedimentary facies and quantitative analysis of palynomorphs and nannofossils document significant environmental changes associated with the OAE 1a: compositional changes of neritic carbonates and calcareous nannofossils data indicate the occurrence of a biocalcification crisis inferred to have been related to CO₂-induced changes in seawater chemistry, and palynomorphs identify a thermal maximum followed by a cooling phase. The latter show a Classopollis maximum during the OAE 1a, which is followed by a decrease in Classopollis and an increase of bisaccate pollen after the event.     

Widespread seafloor anoxia during generation of the Ediacaran Shuram carbon isotope excursion

Reconstructing the oxygenation history of Earth's oceans during the Ediacaran period (635 to 539 million years ago) has been challenging, and this has led to a polarizing debate about the environmental conditions that played host to the rise of animals. One focal point of this debate is the largest negative inorganic C-isotope excursion recognized in the geologic record, the Shuram excursion, and whether this relic tracks the global-scale oxygenation of Earth's deep oceans. To help inform this debate, we conducted a detailed geochemical investigation of two siliciclastic-dominated successions from Oman deposited through the Shuram Formation. Iron speciation data from both successions indicate formation beneath an intermittently anoxic local water column. Authigenic thallium (Tl) isotopic compositions leached from both successions are indistinguishable from bulk upper continental crust (ε²⁰⁵Tl_A ≈ −2) and, by analogy with modern equivalents, likely representative of the ancient seawater ε²⁰⁵Tl value. A crustal seawater ε²⁰⁵Tl value requires limited manganese (Mn) oxide burial on the ancient seafloor, and by extension widely distributed anoxic sediment porewaters. This inference is supported by muted redox-sensitive element enrichments (V, Mo, and U) and consistent with some combination of widespread (a) bottom water anoxia and (b) high sedimentary organic matter loading. Contrary to a classical hypothesis, our interpretations place the Shuram excursion, and any coeval animal evolutionary events, in a predominantly anoxic global ocean.     

Local paleoenvironmental controls on the carbon‐isotope record defining the Bitter Springs Anomaly

Large magnitude (>10‰) carbon‐isotope (δ¹³C) excursions recorded in carbonate‐bearing sediments are increasingly used to monitor environmental change and constrain the chronology of the critical interval in the Neoproterozoic stratigraphic record that is timed with the first appearance and radiation of metazoan life. The ~10‰ Bitter Springs Anomaly preserved in Tonian‐aged (1000–720 Ma) carbonate rocks in the Amadeus Basin of central Australia has been offered as one of the best preserved examples of a primary marine δ¹³C excursion because it is regionally reproducible and δ¹³C values covary in organic and carbonate carbon arguing against diagenetic exchange. However, here we show that δ¹³C values defining the excursion coincide with abrupt lithofacies changes between regularly cyclic grainstone and microbial carbonates, and desiccated red bed mudstones with interbedded evaporite and dolomite deposits, recording local environmental shifts from restricted marine conditions to alkaline lacustrine and playa settings that preserve negative (?4‰) and positive (+6‰) δ¹³C values, respectively. The stratigraphic δ¹³C pattern in both organic and carbonate carbon recurs within the basin in a similar way to associated sedimentary facies, reflecting the linkage of local paleoenvironmental conditions and δ¹³C values. These local excursions may be time transgressive or record a relative sea‐level influence manifest through exposure of sub‐basins isolated by sea‐level fall below shallow sills, but are independent of secular seawater variation. As the shallow intracratonic setting of the Bitter Springs Formation is typical of other Neoproterozoic carbonate successions used to construct the present δ¹³C seawater record, it identifies the potential for local influences on δ¹³C excursions that are neither diagenetic nor representative of the global exogenic cycle.     

End-Permian stratigraphic timeline applied to the timing of marine and non-marine extinctions

A latest Permian timeline (251.9 Ma) can be constructed from the perspectives of: a global nickel spike attributed to emissions from the coeval Siberian flood-basalt eruptions, the correlative end-Permian marine mass extinction (EPME), a transition from reversed to normal paleomagnetism, and a negative anomaly in δ¹³C_carb and δ¹³C_org. In a number of marine and non-marine localities, this timeline is also correlated (to within ≤30 ky) with palynological evidence for the latest Permian destruction of terrestrial vegetation and the accompanying short-lived global fungal (Reduviasporonites) event. This correlation suggests that devastation in marine and non-marine environments was essentially coeval at a time marked by hyperthermal conditions and anoxic oceans.We utilized this proposed timeline to estimate the relative timing of the extinction of latest Permian vertebrates in the Karoo Basin of South Africa. In several sections in the Karoo, the LAD of the therapsid Dicynodon, is correlated with the proposed timeline. In the Carlton Heights section in the Karoo we estimate that the palynological changes and the fungal event occurred within ≤30 ky of the LAD of Dicynodon. Further sampling in the Karoo and other Permian–Triassic non-marine basins would help to clarify the relative timing of the global marine extinctions, plant devastation and the disappearance of non-marine vertebrates.     

Stable isotope ecology of Miocene bovids from northern Greece and the ape/monkey turnover in the Balkans

Eurasia was home to a great radiation of hominoid primates during the Miocene. All were extinct by the end of the Miocene in Western Eurasia. Here, we investigate the hypothesis of climate and vegetation changes at a local scale when the cercopithecoid Mesopithecus replaced the hominoid Ouranopithecus along the Axios River, Greece. Because they are herbivorous and were much more abundant than primates, bovids are preferred to primates to study climate change in the Balkans as a cause of hominoid extinction. By measuring carbon stable isotope ratios of bovid enamel, we conclude that Ouranopithecus and Mesopithecus both evolved in pure C₃ environments. However, the large range of δ¹³C values of apatite carbonate from bovids combined with their molar microwear and mesowear patterns preclude the presence of dense forested landscapes in northern Greece. Instead, these bovids evolved in rather open landscapes with abundant grasses in the herbaceous layer. Coldest monthly estimated temperatures were below 10 °C and warmest monthly temperatures rose close to or above 20 °C for the two time intervals. Oxygen isotope compositions of phosphate from bulk samples did not show significant differences between sites but did show between-species variation within each site. Different factors influence oxygen isotope composition in this context, including water provenience, feeding ecology, body mass, and rate of amelogenesis. We discuss this latter factor in regard to the high intra-tooth variations in δ¹⁸O_p reflecting important amplitudes of seasonal variations in temperature. These estimations fit with paleobotanical data and differ slightly from estimations based on climate models. This study found no significant change in climate before and after the extinction of Ouranopithecus along the Axios River. However, strong seasonal variations with relatively cold winters were indicated, conditions quite usual for extant monkeys but unusual for great apes distributed today in inter-tropical regions.     

Evaluation of the Salinic I tectonic, Cancañiri glacial and Ireviken biotic events: Biochemostratigraphy of the Lower Siluriansuccession in the Niagara Gorge area, Canada and U.S.A.

Well-preserved brachiopods from the Niagara Gorge area, Anticosti Island, Britain, Gotland and Estonia were utilised to delineate a complex isotopic evolution for Llandovery-Wenlock seawater. The Sr-isotope record reflects the Salinic I tectophase of the Late Llandovery in the continuous increase in ⁸⁷Sr/⁸⁶Sr values from 0.708070 to 0.708346. The Salinic II tectophase is marked by relative constancy of Sr isotope values until the Late Wenlock when it rises from 0.708345 to 0.708430. The second tectonic phase was therefore likely only of a regional nature. The carbon isotopes during the Llandovery fall within a band of about − 1‰ to + 3‰, a range comparable to modern low-latitude brachiopods. A large positive δ¹³C excursion of about 3‰, identifies the Ireviken event/excursion, characterizes the Early Wenlock. The biotic crisis and the isotope excursion itself may be ultimately related to the onset and duration of the Cancañiri glaciation, although a direct causative scenario is as yet unknown. The oxygen isotopic trends of well-preserved brachiopods clearly reflect a warm climate interval during the latest Llandovery associated with the Silurian sea level highstand. Subsequently, in the Early Wenlock, the sea level fell with the onset of the Cancañiri glaciation in the southern hemisphere. This is reflected in a significant positive δ¹⁸O excursion, particularly in brachiopods from the Niagara Gorge area. Brachiopods from lower latitudes were awash in warm tropical currents and therefore exhibit somewhat more negative δ¹⁸O values, indicating a lesser cooling gradient.     

Methanogenesis in Eocene Arctic soils inferred from δC of tree fossil carbonates

We report on the carbon and oxygen stable isotope composition of fossil tree material collected at the White Mountain locality of the Buchanan Lake Formation on Axel Heiberg Island in the High Arctic of Canada. The fossils are Middle Eocene in age and have been permineralized with carbonate. Microscopic examination of fossils revealed them to be the remains of Metasequoia stems, composed of secondary carbonate (calcite) and original wood intermingled at the cellular level. Because the specimens show little compression, crushing, or tissue degradation, we believe that carbonate permineralization occurred soon after burial, and therefore provides insight into Eocene carbon cycling at the locality. The carbon isotope signature of the carbonate suggests that methanogenesis resulted in a ¹³C-enriched CO₂ pool that equilibrated with soil water and gave rise to unusually ¹³C-enriched CaCO₃. Tree fossil carbonate exhibited strikingly high δ¹³C values (+4.0 to +7.4‰) compared to published Phanerozoic pedogenic carbonate δ¹³C values. These δ¹³C values, in conjunction with fractionation factors (α) previously determined for carbonate precipitation and methanogenic pathways, indicate an Eocene soil CO₂ pool containing 80-95% CO₂ produced as a by-product of acetate-fermentation methanogenesis. Because methane in the atmosphere is a powerful greenhouse gas, we suggest that methane emissions from Axel Heiberg soils contributed to the relatively warm Arctic climate during the Middle Eocene.     

The influence of seawater carbonate ion concentration [CO32−] on the stable carbon isotope composition of the planktic foraminifera species Globorotalia inflata

We sampled the upper water column for living planktic foraminifera along the SW-African continental margin. The species Globorotalia inflata strongly dominates the foraminiferal assemblages with an overall relative abundance of 70–90%. The shell δ¹⁸O and δ¹³C values of G. inflata were measured and compared to the predicted oxygen isotope equilibrium values (δ¹⁸O_eq) and to the carbon isotope composition of the total dissolved inorganic carbon (δ¹³C_DIC) of seawater. The δ¹⁸O of G. inflata reflects the general gradient observed in the predicted δ¹⁸O_eq profile, while the δ¹³C of G. inflata shows almost no variation with depth and the reflection of the δ¹³C_DIC in the foraminiferal shell seems to be covered by other effects. We found that offsets between δ¹⁸O_shell and predicted δ¹⁸O_eq in the surface mixed layer do not correlate to changes in seawater [CO₃²⁻].To calculate an isotopic mass balance of depth integrated growth, we used the oxygen isotope composition of G. inflata to estimate the fraction of the total shell mass that is grown within each plankton tow depth interval of the upper 500 m of the water column. This approach allows us to calculate the Δδ¹³C_{interval added-DIC}; i.e. the isotopic composition of calcite that was grown within a given depth interval. Our results consistently show that the Δδ¹³C_IA-DIC correlates negatively with in situ measured [CO₃²⁻] of the ambient water. Using this approach, we found Δδ¹³C_IA-DIC/[CO₃²⁻] slopes for G. inflata in the large size fraction (250–355 μm) of − 0.013‰ to 0.015‰ (μmol kg^− 1)^− 1 and of − 0.013‰ to 0.017‰ (μmol kg^− 1)^− 1 for the smaller specimens (150–250 μm). These slopes are in the range of those found for other non-symbiotic species, such as Globigerina bulloides, from laboratory culture experiments. Since the Δδ¹³C_IA-DIC/[CO₃²⁻] slopes from our field data are nearly identical to the slopes established from laboratory culture experiments we assume that the influence of other effects, such as temperature, are negligibly small. If we correct the δ¹³C values of G. inflata for a carbonate ion effect, the δ¹³C_shell and δ¹³C_DIC are correlated with an average offset of 2.11.