Carbon Cycle Disturbance Research Articles

Mercury isotope evidence for Middle Ordovician photic-zone euxinia: Implications for termination of the Great Ordovician biodiversification event

The Darriwilian (Middle Ordovician) is marked by a striking peak of the Great Ordovician Biodiversification Event (GOBE) followed by an abrupt ∼50 % decline in invertebrate species. Understanding the ultimate driving mechanism behind this biotic turnover and its correlation with those proximate climatic-oceanic changes that directly result in elevated biotic mortality have garnered considerable attention. One hypothesis posits that oceanic redox evolution actively influenced biodiversification, and progressive expansion of marine anoxia caused the termination of this biotic event. In this study, we present sedimentological and geochemical profiles for three outcrops of different lithofacies across the Middle-Upper Ordovician transition, ranging from carbonate platform to deep-water slope-basinal facies of the Tarim and South China cratons. Mercury isotopes reveal that recurrent and spatially dynamic photic-zone euxinia (PZE) occurred in the slope-basinal facies while being absent in platform facies. Mass balance model results suggest that ∼23 % of local atmospheric Hg was sequestered in marine sediments due to elevated dissolved H2S in surface waters. This finding suggests that destabilized oceanic redox conditions developed during climatic cooling, particularly shoaling and upwelling of deep-marine euxinic waters into the photic zone, coupled with global carbon-cycle disturbances, resulted in biodiversity decline following the peak of the GOBE.

Carbon cycle perturbations and environmental change of the middle permian and Late Triassic Paleo-Antarctic circle

During the middle Permian through the Triassic, Tasmania moved from paleo-latitudes of 78° to 69°S, wedged between Antarctica and Australia, within the paleo-South polar circle. During this time, significant global carbon cycle disturbances triggered major environmental and climatic changes and mass extinction events globally. The Bicheno-5 core from Eastern Tasmania, Australia, provides the opportunity to examine middle Permian and Upper Triassic sediments from the paleo-Antarctic, using high-resolution organic carbon isotope (δ13CTOC) chemostratigraphy, pXRF, and sedimentology, combined with new palynological data integrated with the existing radiometric age model. While there is a significant unconformity in the Upper Permian to the middle Triassic associated with eustatic sea-level fall as a result of regional uplift in eastern Australia, three distinct carbon isotope excursions (CIEs), characterized by negative shifts of up to − 6 ‰ were identified; the middle Permian Guadalupian Carbon Isotope Excursions (G-CIE), the Carnian Pluvial Episode (CPE), and the mid-Norian Event (MNE). These three events highlight a significant climate shift through glacial and interglacial cycles to warmer non-glacial intervals in the Late Triassic, with evidence of the polar record of the Carnian Pluvial Episode and the mid-Norian Event, which are poorly studied in the Southern Hemisphere, specifically within the Paleo-Antarctic circle.

Palaeoenvironmental changes in the Late Triassic lacustrine facies of the Ordos Basin of Northwest China were driven by multistage volcanic activity: Implications for the understanding the Carnian Pluvial Event

The Carnian Pluvial Event (CPE) induced an intense carbon-cycle disturbance and biological crisis at a global scale. The eruption of the Wrangellia Large Igneous Province (W-LIP) is considered a possible triggering mechanism; however, this cannot fully explain the significant differences in environmental perturbations across different regions. In this paper, we present detailed records of petrography, mercury geochemistry, carbon and sulfur isotopes, trace elements, and provenance data for the Triassic lacustrine facies of the Ordos Basin in the North China Plate, which provide insights into the important effects of volcanic activities, other than the W-LIP, on palaeoenvironment during the CPE interval. Massive emissions of SO2, CO2, and pyroclastic deposits resulting from these volcanic activities triggered four humid episodes, each characterized by lake-level rise, negative δ13Corg excursion, a transition from mudstones to organic-rich shales facies, as well as algal bloom within a time span of 180 kyrs. The establishment of an age framework in the Ordos Basin enables comparative analyses of environmental evolution during the Carnian age across the North China Plate, the western Tethys domain, and the South China Plate. During the period of global climate perturbation driven by the W-LIP eruption, we conclude that short-term localized volcanic activities may have further induced high frequentcy environmental perturbations, which influenced biological shifts and increase organic carbon burial. These short-term volcanic activities with different occurrence times and frequencies complicate local-scale palaeoenvironmental evolutions, which may be an important reason contributing to significant differences in environmental perturbations during the CPE interval across different regions of the globe.

Frequent high-temperature volcanic combustion events delayed biotic recovery after the end-Permian mass extinction

The Early Triassic after the end-Permian mass extinction was a time of repeated environmental degradation, which delayed biotic recovery. Although repeated volcanic activity during the Early Triassic has been cited as the cause, the evidence for this hypothesis needs to be confirmed with convincing proxies. This study presents sedimentary molecular evidence for the continuity of volcanic combustion events into the Early Triassic. Geochemical analysis of combustion-derived organic molecules from the uppermost Permian to the Lower Triassic exposed in the Chaohu and Qingyan sections, South China, reveals multiple spikes of a polyaromatic hydrocarbon, coronene, formed by high-temperature organic matter combustion (>1200 K), in the transitions of the Permian–Triassic, Dienerian, Smithian–Spathian, and Olenekian–Anisian boundaries. The frequent high-temperature volcanic combustions ceased 2.7 Myr after the end-Permian mass extinction, and substantial biotic recovery began. The Earth's surface system has mechanisms to buffer high CO2 levels and global warming in the long term, as evidenced in the Cretaceous greenhouse period. Nevertheless, if volcanic activity is overly frequent, the Earth's surface buffering mechanisms cannot keep pace. A simple box model based on excess carbon input revealed by frequent combustion events reproduces the observed carbon isotopic record of carbonate. This model implies that combustion events impacted carbon cycle disturbances and the global climate (e.g., global warming and related environmental destruction). Frequent high-temperature volcanic combustion was a crucial mechanism of delayed biotic recovery during the Early Triassic.

Intensive ocean anoxia and large δ13Ccarb perturbations during the Carnian Humid Episode (Late Triassic) in Southwest China

The Carnian Humid Episode (CHE) represents a dramatic dry to wet climate transition in the Late Triassic. Manifestations of this climate shift and its associated biological and environmental responses are not fully understood. Here, we carried out carbonate carbon isotope, trace metal, and pyrite framboid analyses at Wolonggang in southwest China to trace palaeoenvironmental changes during this critical interval. The CHE at Wolonggang is marked by the development of fine laminated carbonaceous siltstones and black shales overlying the intensely bioturbated Zhuganpo limestone deposited in the latest Julian 1. This lithological change is accompanied by two negative δ 13 C carb excursions, a strong enrichment of redox-sensitive trace metals U and Mo, and the development of abundant small pyrite framboids with an average diameter < 5 μm. The sedimentary and geochemical evidence collectively suggest a sharp decrease in carbonate production and a major carbon cycle disturbance, coinciding with the onset of intense ocean anoxia and the mass mortality of burrowing faunas. Unlike other areas in the western Tethys, the aftermath of the CHE at Wolonggang is characterized by long-lasting ocean anoxia, with weak carbonate production persisting at least to the late Tuvalian. This is evidenced by the deposition of finely laminated silty carbonates, marls, and carbonaceous siltstones with rare benthic fossils in the Wayao Formation. Our study suggests that anoxic-euxinic environments widely developed along the southwest margins of the Yangtze Platform and in the Nanpanjiang Basin, probably contributing to the loss of marine faunas during the CHE. • Sedimentary and bio-chemostratigraphic investigations were carried out in a key Carnian section in SW China. • Carbon cycle instability was seen during the Carnian Humid Episode. • Anoxia was basin-wide and long-lasting during the CHE and its aftermath. • Warming and anoxia played key roles in the loss of marine faunas during Carnian.

Stable ocean redox during the main phase of the Great Ordovician Biodiversification Event

The Great Ordovician Biodiversification Event (GOBE) represents the greatest increase in marine animal biodiversity ever recorded. What caused this transformation is heavily debated. One hypothesis states that rising atmospheric oxygen levels drove the biodiversification based on the premise that animals require oxygen for their metabolism. Here, we present uranium isotope data from a Middle Ordovician marine carbonate succession that shows the steepest rise in generic richness occurred with global marine redox stability. Ocean oxygenation ensued later and could not have driven the biodiversification. Stable marine anoxic zones prevailed during the maximum increase in biodiversity (Dapingian–early Darriwilian) when the life expectancy of evolving genera greatly increased. Subsequently, unstable ocean redox conditions occurred together with a marine carbon cycle disturbance and a decrease in relative diversification rates. Therefore, we propose that oceanic redox stability was a factor in facilitating the establishment of more resilient ecosystems allowing marine animal life to radiate.

Benthic foraminiferal turnover across the Dan-C2 event in the eastern South Atlantic Ocean (ODP Site 1262)

The Paleogene was punctuated by perturbations of the global carbon cycle, many associated with transient global warming events (hyperthermals). The Dan-C2 event (~160 kyr after Cretaceous/Paleogene boundary; K/Pg) was the oldest of these eccentricity-linked carbon cycle disturbances (ELCD). In contrast to other hyperthermals, the Dan-C2 event was not characterised by bottom water warming, and surface water warming probably was not global. Benthic foraminiferal assemblages across Dan-C2 at SE Atlantic Ocean Drilling Program (ODP) Site 1262 are diverse and strongly dominated by calcareous species. Epifaunal and infaunal morphogroups are equally abundant, suggesting meso-oligotrophic seafloor conditions. Assemblages decreased in diversity gradually before Dan-C2, and Nuttallides truempyi decreased in relative abundance while Stensioeina beccariiformis and the agglutinant Spiroplectammina spectabilis increased, suggesting enhanced food supply to the seafloor. Benthic foraminifera were not highly affected by the Dan-C2 event. An increase in relative abundance of the opportunistic species Bulimina kugleri and Seabrookia cretacea after Dan-C2 points to a change in the type of organic matter arriving at the seafloor. These changes may have been caused by ongoing environmental and/or evolutionary instability following K/Pg mass extinction of oceanic plankton. Variability in composition of pelagic ecosystems, thus the type and/or amount of food arriving at the seafloor, may have been caused by the gradual recovery of pelagic ecosystems after that extinction, possibly affected by warming and pH changes due to Deccan volcanism.

Expression of the early Aptian Oceanic Anoxic Event (OAE) 1a in lacustrine depositional systems of East China

The early Aptian (~120 Ma) Oceanic Anoxic Event 1a (OAE 1a) was associated with marked changes in palaeoclimate linked to a perturbation of the global carbon cycle. While it is well-studied in the marine realm, OAE 1a is poorly known from continental records. Here, we fill that knowledge-gap by describing OAE 1a from lacustrine facies in the East Asia continental margin (Jiaolai Basin and Lingshan Island in eastern China). Specifically, we integrate data from bulk organic carbon-isotopes (δ 13 C org ), total organic carbon (TOC), and redox-and productivity-sensitive trace elements, as well as existing records on depositional environments, clay mineral compositions, elemental geochemistry and the latest detrital zircon U Pb ages. We propose a conceptual model of the response of the lake system to OAE 1a through analysis of the palaeoenvironmental and palaeoclimatic context. During the initial stage, an extreme greenhouse climatic event resulted in an abrupt increase in palaeosalinity in the lake system. This led to anoxic conditions, resulting in a high preservation rate of TOC and the development of organic-rich black shale. Organic carbon isotopes documented in the organic-rich black shale show negative and then positive excursions, consistent with the carbon cycle disturbance found in marine OAE 1a records. In addition, the data show that this event may have been associated with a transition from an extreme greenhouse climate to a cooling phase. The framework presented here facilitates further research on the expression of OAE 1a in continental ecosystems and its fingerprint in lacustrine depositional systems. • Evidence of OAE1a was identified in lacustrine depositional systems. • Lacustrine black shale in East China preserves information on early Aptian C-cycle perturbation during OAE 1a. • Palaeosalinity and RSTE data indicate environmental instability of inland lakes in East China during OAE 1a. • Terrestrial records suggest an extreme greenhouse climate led to OAE 1a. • The OAE 1a itself may represent a period of transition from an extreme greenhouse climate to a cooling phase.

Carbon and oxygen isotope stratigraphy of Jurassic platform carbonates from Saudi Arabia: Implications for diagenesis, correlations and global paleoenvironmental changes

Major carbon cycle disturbances reflected by carbon isotope excursions (CIE) have been reported in the Jurassic successions of Euro-Boreal and Mediterranean basins. However, sedimentary and geochemical data from other domains are still required to unravel the global or regional nature of paleoenvironmental disturbances and identify the potential triggers. Here, we present biostratigraphically well-constrained (middle Toarcian to lower Kimmeridgian) bulk-rock δ13C and δ18O data from central Saudi Arabia. Five major negative δ18O excursions are identified in back-reef facies below sequence boundaries and disconformities (Aalenian-Bajocian boundary, upper Bajocian, middle Bathonian, upper Callovian and lower Kimmeridgian), suggesting that these carbonates underwent intense meteoric diagenesis during partial or major subaerial exposures of the Arabian Platform. The δ13C record is less altered and shows correlations with the regional relative sea-level changes influencing the productivity levels and inflows of fluvial (12C-enriched) or oceanic (12C-depleted) waters onto the platform. Overall, most of the δ13C variations recorded at the scale of the Arabian Platform within the lower and middle Jurassic are in agreement with NW Tethyan data. Among these variations, the increase in δ13C values recorded in the middle Toarcian, lower Bajocian, and lower Bathonian zigzag Zone indicate a considerable increase of productivity and 12Corg burial concomitant with global sea-level rises. Conversely, the occurrence of major but diachronous negative CIEs at the Aalenian-Bajocian boundary and around the lower-upper Bajocian transition are interpreted as accelerations of terrestrial 12Corg transfer to the ocean and atmosphere linked to regional but widespread emergences of landmasses. Whether global (eustatic) or regional (tectonic), all these sea-level changes and carbon transfers could be distal consequences of successive episodes of the Pangean breakup.

Evolution of the carbon isotope composition of atmospheric CO2 throughout the Cretaceous

Although atmospheric CO2 has been extensively described as a primary driver of Phanerozoic climate and carbon cycle disturbance, little is known about its carbon isotope composition (δ13CCO2) during pre-Cenozoic times. We reconstruct for the first time the evolution of δ13CCO2 during the whole Cretaceous period based on reference curves of δ13C values of Tethyan marine bulk carbonates (δ13Ccarb) and δ18OPO4 values of fish tooth enamel. We test this method against that based on the oxygen and carbon isotope ratios of high-latitude benthic foraminifera (δ13Cforam) recently implemented for the Cenozoic and closely matching δ13CCO2 measurements from ice cores. Highly consistent δ13CCO2 estimates and trends are obtained from the δ13Ccarb method (−5.90‰±0.28) and the δ13Cforam method (−6.18‰±0.64) during the phases of available foraminiferal record encompassing the Albian-Maastrichtian interval. However, the δ13Cforam-based δ13CCO2 signal is strongly biased during the main OAEs, during which heat transport to higher latitudes associated with Haline Euxinic Acidic Thermal Transgression (HEATT) episodes may have affected temperature inference from δ18Oforam values. Our results demonstrate the suitability of using δ13Ccarb records to reconstruct past δ13CCO2 values, which opens the opportunity for pre-Cenozoic, worldwide δ13CCO2 reconstructions even during sharp climate change intervals. The consistency between both methods together with our δ13CCO2 estimates and those previously obtained for the Cenozoic provides about 145Myr of carbon isotope evolution of atmospheric CO2.

Constraining the cause of the end-Guadalupian extinction with coupled records of carbon and calcium isotopes

A negative δC13 excursion in carbonate sediments near the Guadalupian/Lopingian (Middle/Late Permian) boundary has been interpreted to have resulted from a large carbon cycle disturbance during the end-Guadalupian extinction event (ca. 260 Ma). However, the carbon isotope data alone are insufficient to uniquely determine the type and magnitude of perturbation to the global carbon cycle. Calcium isotopes can be used to further constrain the cause of a carbon isotope excursion because the carbon and calcium cycles are coupled via CaCO3 burial. In this study, we present coupled carbon and calcium isotope records from three Guadalupian–Lopingian (G/L) sections in China (Penglaitan and Chaotian) and Turkey (Köserelik Tepe). The δC13 and δCa44/40 records differ among our studied sections and do not co-vary in the same manner. No section shows δC13 and δCa44/40 changes consistent with massive, rapid volcanic CO2 emissions or methane clathrate destabilization. Additionally, many sections with large (>3‰) changes in δC13 exhibit δO18 evidence for diagenetic alteration. Only one section exhibits a large excursion in the δCa44/40 of limestone but the absence of a similar excursion in the δCa44/40 of conodont apatite suggests that the limestone excursion reflects a mineralogical control rather than a perturbation to the global calcium cycle. Hence, we interpret the large isotopic changes observed in some sections to have resulted from local burial conditions or diagenetic effects, rather than from a large carbon and calcium cycle disturbance. Perturbations to the global carbon and calcium cycles across the G/L transition were much less intense than the disturbances that occurred across the subsequent Permian–Triassic boundary. This finding is consistent with the much smaller magnitude of the end-Guadalupian extinction relative to the end-Permian.

Isotopic chemostratigraphy across the Early–Middle Frasnian transition (Late Devonian) on the South Polish carbonate shelf: A reference for the global punctata Event

The coupled high-resolution carbon isotope data from whole-rock limestone and organic matter samples of the transitans (Early Frasnian), punctata and the earliest hassi Zones (Middle Frasnian) from the South Polish carbonate shelf successions reveal the presence of a large, multi-part δ13C excursion, one of the largest known carbon cycle disturbance of Devonian period. This Early–Middle Frasnian (E–MF) δ13C perturbation consists of two positive excursions (a minor event I and a major event III) and two negative shifts (events II and IV). The major positive excursion, up to 5‰, begins near the E–MF boundary and the onset of Middlesex transgressive–anoxic event. The latter broader-scale positive δ13C perturbation correlates with the worldwide punctata Event documented across eastern and western Laurussia as well as northern Gondwana. The large magnitude of the punctata Isotopic Event, paired with negligible biotic effects, is similar to the Silurian biogeochemical perturbations but contrasts markedly with the younger Kellwasser Events. The distinctive protracted (about 1Ma) E–MF δ13C variations may be only partly explained by escalated sea water exchange between epeiric seas and the anoxic open ocean during successive transgressive pulses. Very high δ13C plateau values of the punctata Event correspond to a positive 87Sr/86Sr shift, as well as elevated clastic input proxies and magnetic susceptibility, suggesting altogether that delivery of tectonically-promoted land-derived nutrients to marine basins was a key factor stimulating the biogeochemical perturbation. Extreme increases in primary productivity and enhanced organic matter burial in restricted deeper basinal settings would promote a gradual drawdown in surface-water as well as atmospheric pCO2, and consequently a climatic cooling by 5°C, as indicated by the high-resolution δ18Ophosphate record.

Sedimentological, palynological and geochemical studies of the terrestrial Triassic–Jurassic boundary in northwestern Poland

AbstractThe Kamień Pomorski IG-1 borehole (Pomerania, NW Poland) yields a profile through the Triassic–Jurassic (T–J) transition in continental deposits. An integrated study of the sedimentology, sequence stratigraphy, palynology, biostratigraphy and geochemistry of these deposits has been carried out on the boundary interval, which represents a time of major environmental change. Two lithological units within the transitional section are distinguished: the Lower–Middle Rhaetian Wielichowo Beds of alluvial plain facies, which shows evidence of a semi-arid climate, and the Upper Rhaetian to Lower Hettangian Zagaje Formation, lying above a marked erosional sequence boundary, composed of mudstone-claystone and sandstone deposited in a fluvial-lacustrine environment. Carbon isotope values obtained from palynomaceral separates, and thus reflecting isotopic changes in atmospheric CO2, show significant fluctuations through the Rhaetian; the most conspicuous negative δ13Corgexcursion is correlated with the Rhaetian ‘initial’ excursion and shows two sub-peaks, pointing to short-term carbon-cycle disturbances of lesser magnitude. Above the ‘initial’ negative excursion, there is a positive excursion followed again by more negative values, representing subordinate fluctuation within a positive excursion and is correlated with the T–J boundary. Seventy-two miospore taxa have been determined from the studied T–J transitional section. Two major palynological assemblages have been distinguished: the lower one, typically Rhaetian, named theCingulizonates rhaeticus–Limbosporites lundblandiiassociation, which corresponds to theRhaetipollis–Ricciisporites(=Rhaetipollis–Limbosporites) Zone; and the upper one, typically Hettangian, named theConbaculatisporites mesozoicus– Dictyophyllidites mortoni–Cerebropollenites thiergartiiassociation (with the age-diagnostic pollenC.thiergartii), which corresponds to thePinuspollenites–Trachysporites(= Trachysporites–Heliosporites) Zone. The T–J palynofloral turnover occurred in a humid period and is more conspicuous then palynofloral changes observed in Greenland, the Tethyan domain or other parts of NE Europe. The osmium isotope system is studied herein for the first time from T–J continental deposits and shows marked disturbances similar to those measured in marine deposits and attributed to volcanic fallout. Carbon and osmium isotope correlation and coeval increase in polycyclic aromatic hydrocarbon (PAH) content and darkening of miospores confirm that eruptions of the Central Atlantic Magmatic Province (CAMP) contributed to the perturbances in climate and crisis in the terrestrial biosphere. A series of periodical atmospheric loading by CO2, CH4or alternatively by SO2, sulphate aerosols and toxic compounds is inferred to have caused a series of rapid climatic reversals, directly influencing the ecosystem and causing the Triassic floral crisis. A floral turnover period commenced at the ‘initial’ δ13C excursion, with the onset of CAMP volcanism. Obtained values of initial187Os/186Os between 2.905 and 4.873 and very low iridium content (about 5 ppt) lend no support to a role for an extraterrestrial impact at the T–J boundary event. The position of the ‘initial’ negative carbon isotope excursion about 12 m below the T–J boundary, position of sequence boundaries (emergence surfaces) and other isotope excursions allow reliable correlation with marine profiles, including St Audrie's Bay (UK), Csövár (Hungary) and the GSSP profile at Kuhjoch (Austria).

Multiple phases of carbon cycle disturbance from large igneous province formation at the Triassic-Jurassic transition

The end-Triassic mass extinction (ca. 201.4 Ma) coincided with a major carbon cycle perturbation, based on an ∼5‰−6‰ negative excursion in δ 13 C TOC (total organic carbon) records. Both events coincided directly with the onset of massive flood basalt volcanism in the Central Atlantic Magmatic Province (CAMP). Organic carbon isotope data from the western Tethys Ocean (Austria) and the Germanic basin (UK and Germany), however, demonstrate earlier disruption of the global carbon cycle, preceding CAMP eruptions. A 2‰−3‰ late Rhaetian precursor negative excursion in marine and continental δ 13 C TOC records is matched by a negative perturbation in δ 13 C leaf data, suggesting multiple events of Rhaetian atmospheric 13 C depletion. Intruding dike and sill systems, preceding CAMP eruptive volcanic activity, may have released ∼3000–7000 Gt of isotopically light carbon as thermogenic methane from subsurface organic-rich strata. This possibly caused an end-Triassic atmospheric p CO 2 increase and reduced ecosystem stability before the actual onset of eruptive volcanic activity in the CAMP region. We present a model that identifies three phases of disturbances in global biogeochemical cycles related to the formation of this large igneous province.

Evidence for recurrent Early Triassic massive volcanism from quantitative interpretation of carbon isotope fluctuations

Carbon cycle disturbance associated with mass extinction at the end of the Permian Period continued through the Early Triassic, an interval of approximately 5 million years. Coincidence of carbon cycle stabilization with accelerated Middle Triassic biotic recovery suggests a link between carbon cycling and biodiversity, but the cause of Early Triassic carbon isotope excursions remains poorly understood. Previous modeling studies have focused exclusively on the initial negative excursion in δ 13C across the Permian–Triassic boundary and have not addressed the cycles of positive and negative excursions observed through the Early Triassic. This study uses a simple carbon cycle box model to investigate potential causes underlying the series of Early Triassic carbon isotope excursions and to assess possible relationships between isotope excursions and coeval patterns of carbonate deposition. According to the model, introduction of carbon with the isotope composition of volcanic CO 2 produces small negative carbon isotope excursions followed by larger and more protracted positive excursions. Positive excursions result because increased pCO 2 causes warming, enhancing marine anoxia and associated regeneration of phosphate and thus allowing greater productivity. In addition, carbonate weathering is more sensitive than organic carbon weathering to changes in atmospheric pCO 2 and climate, causing an increase in the overall δ 13C composition of weathered carbon. Therefore, the full Early Triassic record of negative and positive carbon isotope excursions can only be accounted for within the model by several pulses of carbon release characterized by varying mixtures of organic and mantle isotope compositions. Thermal metamorphism of coal and carbonate rocks in the crust of the Siberian craton during eruption of the Siberian Traps flood basalts provides the most plausible mechanism for such a carbon release scenario. If multiple episodes of CO 2 release account for Early Triassic carbon cycle instability (regardless of their precise trigger), then cessation of CO 2 release is likely to explain acceleration of biotic recovery early in the Middle Triassic.

Late Early Triassic climate change: Insights from carbonate carbon isotopes, sedimentary evolution and ammonoid paleobiogeography

The late Early Triassic sedimentary–facies evolution and carbonate carbon-isotope marine record (δ 13C carb) of ammonoid-rich, outer platform settings show striking similarities between the South China Block (SCB) and the widely distant Northern Indian Margin (NIM). The studied sections are located within the Triassic Tethys Himalayan belt (Losar section, Himachal Pradesh, India) and the Nanpanjiang Basin in the South China Block (Jinya section, Guangxi Province), respectively. Carbon isotopes from the studied sections confirm the previously observed carbon cycle perturbations at a time of major paleoceanographic changes in the wake of the end-Permian biotic crisis. This study documents the coincidence between a sharp increase in the carbon isotope composition and the worldwide ammonoid evolutionary turnover (extinction followed by a radiation) occurring around the Smithian–Spathian boundary. Based on recent modeling studies on ammonoid paleobiogeography and taxonomic diversity, we demonstrate that the late Early Triassic (Smithian and Spathian) was a time of a major climate change. More precisely, the end Smithian climate can be characterized by a warm and equable climate underlined by a flat, pole-to-equator, sea surface temperature (SST) gradient, while the steep Spathian SST gradient suggests latitudinally differentiated climatic conditions. Moreover, sedimentary evidence suggests a transition from a humid and hot climate during the Smithian to a dryer climate from the Spathian onwards. By analogy with comparable carbon isotope perturbations in the Late Devonian, Jurassic and Cretaceous we propose that high atmospheric CO 2 levels could have been responsible for the observed carbon cycle disturbance at the Smithian–Spathian boundary. We suggest that the end Smithian ammonoid extinction has been essentially caused by a warm and equable climate related to an increased CO 2 flux possibly originating from a short eruptive event of the Siberian igneous province. This increase in atmospheric CO 2 concentrations could have additionally reduced the marine calcium carbonate oversaturation and weakened the calcification potential of marine organisms, including ammonoids, in late Smithian oceans.