Karoo-Ferrar Large Igneous Province Research Articles

Late Triassic to Early Jurassic carbon isotope chemostratigraphy and organo-facies evolution in a distal to proximal transect of the North German Basin

AbstractThroughout the Latest Triassic and the Early Jurassic, major changes in paleogeography, climate and eustatic sea-level impacted on the development of shelf depositional environments. Secular trends in environmental conditions were punctuated by transient perturbations that occurred in relation to large-scale volcanic events, such as the emplacement of the Central Atlantic Magmatic Province at the Triassic/Jurassic boundary and the Karoo–Ferrar Large Igneous Province in the early Toarcian. We here present bulk organic (HAWK programmed pyrolysis) and organic carbon isotope (δ13Corg) data for three drill cores recovering Latest Triassic and Early Jurassic strata (Rhaetian to Toarcian). Study sites are located in the northeastern part of the Central European Epicontinental Sea and were positioned along a distal–proximal transect of the North German Basin. This allows discussing the differential response of depositional settings and organo-facies toward secular and transient environmental change. Biostratigraphically anchored trends in δ13Corg values allow the precise correlation along the transect, as well as with distant sites. At all North German locations, diagnostic secular trends in δ13Corg are punctuated by transient negative carbon isotope excursions, reflecting perturbations of the global carbon cycle at the Triassic/Jurassic boundary and in the early Toarcian. Stratigraphic gaps occurred during sea-level lowstands and are most pronounced at shallow proximal sites. Programmed pyrolysis data indicate spatiotemporal organo-facies trends that on a temporal scale occurred in response to sea-level and climate trends, while spatial patterns were governed by basin morphology and paleobathymetry. Substantial marine organic matter accumulations occurred at high sea level during the Toarcian only, and were most continuous at distal sites. Graphical Abstract

Partial paleobathymetric restriction from the local North Sea Dome in the Viking Corridor during the Early-Middle Jurassic

The Early-Middle Jurassic (∼201–161.5 Ma) was characterized by major fluctuations in Earth's climate, tectonic activity, and large-scale magmatic events. There has been considerable focus on understanding the mechanisms that triggered the Early Jurassic Toarcian Oceanic Anoxic Event (TOAE; ∼183 Ma)—a warming episode associated with a perturbation of the carbon cycle. This was marked by a negative δ13C isotope excursion (CIE) and CO2 release from the Karoo-Ferrar Large Igneous Province (KF-LIP)—resulting in widespread organic carbon burial under strong euxinic marine conditions. Despite extensive study of the TOAE from the NW European epicontinental shelf, less focus has been placed on the influence of North Sea Dome (NSD) uplift (late Toarcian-early Aalenian) and eruption (Bajocian-Bathonian) on Jurassic paleoceanography. The NSD is a local feature commonly invoked as a driver of regional paleoclimatic conditions—a bathymetric barrier between warm Tethyan Ocean and cooler Boreal Sea waters. This study contributes new, high-resolution geochemical data derived from Viking Corridor (VC) cores and cuttings that fills a critical spatial and temporal gap in records spanning the Early-Middle Jurassic. Redox-sensitive trace elements (RSTEs; Mo/Al, U/Al, V/Al) as well as Cd*Mn and Cd/Mo ratios were used to reconstruct paleoceanographic and redox conditions. We track magmatic events and their implications using the relative enrichment of Hg, and attempt to decouple global versus regional/local volcanic systems with the enrichment factor values of mafic trace metals (MTM-EFs; Co, Cu, Ni, and Cr). Varying concentrations of RSTEs and a particulate shuttle mechanism during the Early-Middle Jurassic suggest that the VC had fluctuating paleoredox conditions, and that the NSD did not fully restrict the VC. We document a negative CIE as well as high Hg influxes during the TOAE—likely associated with KF-LIP eruptions (Early Jurassic). In addition, pulses of elevated Hg and MTM-EF values were observed during the Aalenian and Bajocian (Middle Jurassic) that were likely a result of NSD uplift and eruptions. The subaerial exposure of the NSD created progressively restricted conditions during the Aalenian-Bajocian, but the seaway between the VC and European epicontinental shelf remained open. Together, this work provides evidence that the NSD had a critical role in driving paleobathymetric and paleoceanographic conditions during the Early-Middle Jurassic. Furthermore, this study provides a geochemical insight into the interplay between δ13C isotope excursions, the KF-LIP, the NSD, and Hg-RSTE-MTM systems during an important transition in the Earth's biogeochemical history.

Long eccentricity forcing of the Late Pliensbachian to Early Toarcian (Jurassic) terrestrial wildfire activities in the Tarim Basin, northwestern China

Wildfires are an important disturbance driver for terrestrial ecosystems and even Earth systems, which reflect crucial information about the interactions between the atmosphere and the biosphere. However, the relatively long-term secular evolution controlled by climate change in the Early Jurassic wildfires are poorly understood. Here, we conduct cyclostratigraphic, chemostratigraphic, and molecular compound analyses on the Late Pliensbachian to Early Toarcian terrestrial stratigraphy of the Kuqa River section in the Tarim Basin, northwestern China, in order to obtain a holistic understanding the evolutionary processes of wildfire on long time scales during a past warming event, the Pliensbachian-Toarcian event (Pl-To). Based on the long eccentricity cycle identified in the time series analysis, we establish a ∼ 1.8 Ma high-resolution floating astronomical timescale (FATS) and reconstruct the variable lake level by using the sedimentary noise model. The Pl-To warming event associated with the Karoo-Ferrar Large Igneous Province (K-F LIP) may have led to a shift in climate from warm and wet to dry and hot, with a significant decrease in lake levels in the Tarim Basin. Besides, polycyclic aromatic hydrocarbons (PAHs), C29 sterane and dibenzofuran evidence indicates that the global warming event (Pl-To event) also may regulate the wildfires by affecting the fuel abundance (vegetation). Notably, the astrochronology and PAHs suggest that the periodically enhanced wildfires correspond to the long eccentricity cycles, indicating that astronomical forcing plays a key role in triggering the wildfires. Our results report the first documentation of multiple full long eccentricity cycles forced wildfires during the Late Pliensbachian to Early Toarcian and reveal orbital forcing as a driving mechanism for periodically enhanced wildfires superimposed on a long-time gradual weakening trend associated with Pl-To warming event.

Dinosaur extinctions related to the Jenkyns Event (early Toarcian, Jurassic)

The early Toarcian Jenkyns Event (~183 Ma) was characterized by a perturbation of the global carbon cycle, global warming, which at continental areas led to intensified chemical weathering, enhanced soils erosion, and intensified wildfires. Warming and acid rain affected diversity and composition of land plant assemblages, caused a loss of forests and thereby impacted on trophic webs. The Jenkyns Event, triggered by volcanic activity of the Karoo-Ferrar Large Igneous Province, changed terrestrial ecosystems, and also affected the dinosaurs. Fossil macroplant assemblages and palynological data reveal reductions in the diversity and richness of plant communities. A substantial loss of land plant biomass and a shift to forests dominated by Cheiropelidiaceae conifers occurred as a consequence of seasonally dry and warm conditions. Major changes occurred to hervivore dinosaurs, with extinction of diverse basal families of Sauropodomorpha (‘prosauropods’) as well as some basal sauropods. Ornithischian dinosaurs show patchy records; some heterodontosaurids disappeared and the scelidosaurids (Thyreophora) went extinct during the Jenkyns Event. The dominant carnivorous dinosaurs, the Coelophysoidea (Theropoda), died out during the Jenkyns Event. We interpret the Jenkyns Event as a terrestrial crisis for ecosystems, marked especially by floral changes and the extinction of some dinosaur clades, both hervivores and carnivores.

Life and land engulfed in the late Early Jurassic Karoo lavas of southern Gondwana

AbstractThe rock record from the late Early Jurassic in southern Africa encompasses the history of voluminous continental flood basalt outpourings associated with the magmatic events in the Karoo–Ferrar Large Igneous Province (LIP) in southern and eastern Gondwana. This multiphase magmatism produced one of Earth’s largest continental flood basalt successions volumetrically and is assumed to have been a main driving mechanism in late Early Jurassic global environmental perturbations, including mass extinctions and changes in climate. In southern Africa, these Lower Jurassic flood basalts are interbedded with fossiliferous sedimentary rocks, which in turn host the last signs of ‘Karoo life’ in the form of fossil plants, invertebrates and vertebrates, including the trackways of hopping mammals and the ultimate Karoo dinosaurs. The sedimentology and palaeontology of the interbeds archived depositional and biotic processes in running water as well as in and around shallow, up to ∼10 m deep freshwater lakes and ponds in the late Early Jurassic. This study explains how a complex freshwater palaeo-habitat prevailed – albeit temporarily – in this extremely stressful environment, which was unlike any modern volcanic system. The evidence collectively points to seasonally wet, warm temperate climatic conditions during the early phases of Karoo volcanism. Moreover, the evidence in the rocks also suggests that the dynamic volcanic conditions resulted in shifting habitats that likely facilitated the migration of the ultimate Karoo biota towards the north and west, away from the main Karoo land of fire, just before Gondwana started to disassemble. This refinement of the environmental dynamics in southern Gondwana presented herein lays the groundwork for future high-resolution volcanological, geochronological and chemostratigraphical studies aimed at the nuanced understanding of the global environmental effect of the Karoo–Ferrar LIP.

Reduced plate motion controlled timing of Early Jurassic Karoo-Ferrar large igneous province volcanism.

Past large igneous province (LIP) emplacement is commonly associated with mantle plume upwelling and led to major carbon emissions. One of Earth’s largest past environmental perturbations, the Toarcian oceanic anoxic event (T-OAE; ~183 Ma), has been linked to Karoo-Ferrar LIP emplacement. However, the role of mantle plumes in controlling the onset and timing of LIP magmatism is poorly understood. Using global plate reconstruction models and Lower Toarcian sedimentary mercury (Hg) concentrations, we demonstrate (i) that the T-OAE occurred coevally with Karoo-Ferrar emplacement and (ii) that timing and duration of LIP emplacement was governed by reduced Pangean plate motion, associated with a reversal in plate movement direction. This new model mechanistically links Earth’s interior and surficial processes, and the mechanism is consistent with the timing of several of the largest LIP volcanic events throughout Earth history and, thus, the timing of many of Earth’s past global climate change and mass extinction events.

Testing for ocean acidification during the Early Toarcian using δ44/40Ca and δ88/86Sr

During the Early Toarcian, volcanic gases released by the Karoo-Ferrar large igneous province are widely believed to have caused severe environmental disturbances, including ocean acidification. Here we show records of δ44/40Ca and δ88/86Sr through the Early Toarcian, as recorded in three groups of biogenic calcite: Megateuthididae belemnites, Passaloteuthididae belemnites, and brachiopods of the species Soaresirhynchia bouchardi. We evaluate the data to eliminate the influence on isotopic composition of varying temperature, calcification rate, and salinity, through the section that may mask the environmental signals.Neither δ44/40Ca nor δ88/86Sr show negative isotope excursions across the suggested acidification interval as would be expected had acidification occurred. A profile of δ11B, re-interpreted from a published study, shows no variation through the interval. Taken together, these data provide little support for ocean acidification at this time. In our belemnites, values of δ88/86Sr are independent of temperature or Sr/Ca. For brachiopods, too few data are available to determine whether such dependences exist. Values of δ44/40Ca show a weak temperature control of magnitude +0.020 ± 0.004 ‰/°C (2 s.d.). In belemnites, δ44/40Ca also correlates positively with Mg/Ca and Sr/Ca.

Changes in organic matter composition during the Toarcian Oceanic Anoxic Event (T-OAE) in the Posidonia Shale Formation from Dormettingen (SW-Germany)

During the Early Toarcian, deposition of organic carbon-rich-shales occurred throughout the epicontinental sea across Europe. Climate instability and high extinction rates in the marine realm were associated with profound environmental changes. The Toarcian Oceanic Anoxic Event (T-OAE) has been linked to the injection of greenhouse gases (e.g. oceanic methane) into the atmosphere triggered by the emplacement of the Karoo-Ferrar large igneous province (LIP) volcanism. The data presented are obtained from the Posidonia Shale Formation in Dormettingen (southwestern Germany), ~2 km from the well-known Dotternhausen section. Despite the intense palaeontological and geochemical research, studies on the particulate organic matter (POM) across the T-OAE are scarce. Here, we provide a detailed study of POM of the Dormettingen section as a tool to evaluate changes in the depositional environment. Integrated POM (i.e. amorphous organic matter, marine and terrestrial palynomorphs) and geochemical (i.e. carbon isotope δ13C) analyses reveal different episodes of palaeoecological upheavals during the studied time interval. In this study, we will integrate new palynofacies data and combine it with the existing sedimentological and palaeoecological data of Dotternhausen in order to interpret relative sea-level fluctuations and climatic changes at the local palaeogeographic setting.

The palaeoenvironmental context of Toarcian vertebrate-yielding shales of southern France (Hérault)

AbstractThe Early Jurassic was marked by several episodes of rapid climate change and environmental perturbation. These changes culminated during the Toarcian Oceanic Anoxic Event (T-OAE), an episode of global warming that led to the widespread deposition of organic-rich shales. The Toarcian shales of NW Europe have also yielded exceptionally preserved fossils of marine vertebrates and invertebrates, but the potential links between the occurrences of these exceptionally preserved fossils and the T-OAE remain poorly investigated. Palaeontological excavations realized in Toarcian strata near Lodève (Hérault, southern France) have yielded several specimens of marine vertebrates and abundant invertebrate fauna. We have developed a multiproxy approach (ammonite biostratigraphy, X-ray diffraction-bulk mineralogy, Rock-Eval pyrolysis, stable isotopes, trace element, phosphorus and mercury contents) to place these findings in a well-defined temporal and palaeoenvironmental context, and hence constrain the factors that led to their remarkable preservation. The Jenkyns Event interval, unambiguously identified at the base of the Toarcian organic-rich shales by a 5‰ negative carbon isotope excursion, records higher mercury fluxes, which suggest a causal link with intense volcanic activity of the Karoo–Ferrar large igneous province. This interval is very condensed and unfossiliferous, and might have been deposited under abnormally low-salinity conditions. Our data show that the deposition of the vertebrate-yielding horizons post-dated the T-OAE by several hundreds of ka, and took place during a prolonged period of widespread oxygen-deficiency and elevated carbon burial. Our results indicate that the unusual richness in vertebrates of the studied site can be explained by a combination of regional factors such as warming-induced, prolonged seafloor anoxia, and more local factors, such as extreme condensation owing to reduced dilution by carbonate and detrital input.

Ocean acidification during the early Toarcian extinction event: Evidence from boron isotopes in brachiopods

Abstract The loss of carbonate production during the Toarcian Oceanic Anoxic Event (T-OAE, ca. 183 Ma) is hypothesized to have been at least partly triggered by ocean acidification linked to magmatism from the Karoo-Ferrar large igneous province (southern Africa and Antarctica). However, the dynamics of acidification have never been directly quantified across the T-OAE. Here, we present the first record of temporal evolution of seawater pH spanning the late Pliensbachian and early Toarcian from the Lusitanian Basin (Portugal) reconstructed on the basis of boron isotopic composition (δ11B) of brachiopod shells. δ11B declines by ∼1‰ across the Pliensbachian-Toarcian boundary (Pl-To) and attains the lowest values (∼12.5‰) just prior to and within the T-OAE, followed by fluctuations and a moderately increasing trend afterwards. The decline in δ11B coincides with decreasing bulk CaCO3 content, in parallel with the two-phase decline in carbonate production observed at global scales and with changes in pCO2 derived from stomatal indices. Seawater pH had declined significantly already prior to the T-OAE, probably due to the repeated emissions of volcanogenic CO2. During the earliest phase of the T-OAE, pH increased for a short period, likely due to intensified continental weathering and organic carbon burial, resulting in atmospheric CO2 drawdown. Subsequently, pH dropped again, reaching the minimum in the middle of the T-OAE. The early Toarcian marine extinction and carbonate collapse were thus driven, in part, by ocean acidification, similar to other Phanerozoic events caused by major CO2 emissions and warming.

Water-level and redox fluctuations in a Sichuan Basin lacustrine system coincident with the Toarcian OAE

Increasing cases have indicated that oceanic anoxic events (OAEs) appear to have records in terrestrial sediments, which provide valuably high-resolution materials for the debated and challenging issues of OAEs. However, the current works are preliminary and need more geological analyses to test. Here we present a case study in the Lower Jurassic Da'anzhai Member of the Toarcian (ca. 180 Ma) Sichuan Basin, SW China. The petrology and sedimentary facies of the Da'anzhai Member were used to reconstruct the relative water-level fluctuations in the Sichuan Basin, which are revealed to be consistent with coeval changes in global sea levels. The Da'anzhai Member was deposited during a period of lake shallowing with enhanced salinities corroborated by the Sr/Ba and S/TOC ratios, when sea level was generally high after global transgression. During the lake shallowing, there were four complete secondary lacustrine deepening–shallowing cycles over shorter time-scales. The total organic carbon contents and organic carbon isotopic stratigraphy show there was a basin wide terrestrial response to the T-OAE as recorded by the Da'anzhai Member. The enrichment factors of Mo relative to PAAS (MoEF) and Corg/P values imply a dysoxic redox condition. The terrestrial response of OAEs is likely caused by changes in hydrological conditions, which were related to the global warming triggered by the Karoo–Ferrar Large Igneous Province. The associated organic carbon accumulation mechanism is a combined effect of elevated primary productivity and oxygen-depleted preservation conditions. Our data suggest that OAEs do have terrestrial response and induce organic carbon accumulation, but the mechanisms are still not much clear and need further studies.

Pliensbachian environmental perturbations and their potential link with volcanic activity: Swiss and British geochemical records

The mechanisms leading to environmental and climate change prior to and anticipating the well-studied Toarcian oceanic anoxic event are not completely understood. Specifically, the role of the continuing break-up of Pangea, associated palaeogeographic rearrangements, and the influence of volcanic activity are less well known. Therefore, we studied the Pliensbachian interval in the open marine, hemipelagic section of the Breggia Gorge in southern Switzerland, to elucidate the potential impact of these major changes on the environment and climate. We identified carbon isotope excursions towards lower values in whole-rock carbonates dated from the Sinemurian/Pliensbachian boundary, davoei, margaritatus, and spinatum Zones, and possibly from the Pliensbachian-Toarcian boundary. Using the whole-rock oxygen isotope record and phosphorus content, we associated these events with phases of increased runoff possibly related to climate warming. We also established the first mercury (Hg) record for the entire Pliensbachian, and in order to verify its validity, we analysed an additional section located along the Dorset coast in southern England. The presence of Hg spikes common in both sections may indicate the influence of episodes of increased volcanic activity. The Hg spike during the earliest Pliensbachian may coincide with a late phase of the central Atlantic magmatic province, and/or increased rifting linked with the opening of the Hispanic corridor. Further significant maxima in Hg contents are observed within the davoei and margaritatus Zones, and both phases may correspond to early volcanic activity of the Karoo-Ferrar large igneous province. Further analyses are, however, needed to corroborate this interpretation and evaluate the importance of organic-matter scavenging and detrital input in comparison to volcanic activity.

Rhenium‑osmium geochronology of the Toarcian Posidonia Shale, SW Germany

Black shale samples from sedimentary layers below and between the Unterer Stein, Oberer Stein, Inoceramus Bank, and Nagelkalk horizons from the Dormettingen quarry, SW Germany were analysed for their Re and Os isotope composition and content. The ~12-m-thick sedimentary sequence of the Posidonia Shale Formation, composed of multiple layers of black shale and intercalated limestone beds, was deposited during the Early Jurassic (Toarcian), a time during which black shale sedimentation was ubiquitous in Western Europe during a widespread oceanic anoxic event. Both the marl and shale layers beneath the black shales and adjacent to the Oberer Stein limestone layer show signs of bioturbation. The excellent biostratigraphic constraints and the high Re and Os contents (24–290 ppb and 290–1050 ppt, respectively) of the Dormettingen shales thus make these samples ideal for testing the influence of bioturbation on the Re-Os isotope system, as well as refining age constraints for the Toarcian OAE and potential triggers for this event. The calculated isochron age for the Dormettingen shales is 183.0 ± 2.0 Ma, with a low initial 187Os/188Osi of 0.377 ± 0.065, indicating a Pliensbachian to Toarcian age. This age is in agreement with published U-Pb and Re-Os ages for other Lower Jurassic sites, astronomical constraints, and biostratigraphic correlations, suggesting limited disturbance of the isotope system by minor degrees of biological activity. The low 187Os/188Osi is consistent with significant influx of mantle-derived material into the Toarcian ocean, likely from weathering of Karoo-Ferrar Large Igneous Province basalts.

Terrestrial sources as the primary delivery mechanism of mercury to the oceans across the Toarcian Oceanic Anoxic Event (Early Jurassic)

This study evaluates the utility of sedimentary mercury (Hg) contents as a proxy for fingerprinting ancient massive volcanism, which is often associated with biogeochemical perturbations. Herein we present new Hg geochemical data from anoxic marine basins across the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma) as a test of the complex Hg cycle. The T-OAE was likely initiated by the main eruptive phase of the Karoo–Ferrar large igneous province, which caused a subsequent cascade of environmental perturbations and resulting mass extinction. At present the leading interpretation of sedimentary Hg anomalies has been volcanogenic outgassing as the primary source. Our study and compilation results suggest, however, that Hg/TOC anomalies were restricted to shallow-water, and/or proximal environments, while deep-water, more distal depositional settings document no significant Hg-related anomalies. Furthermore, asynchronous stratigraphic deviations in Hg enrichments favor terrestrially sourced materials and local redox variability, rather than direct volcanogenic emissions, as a primary control mechanism. Additionally, Hg isotope signatures from our only study site documenting an Hg anomaly are also consistent with a terrestrial Hg origin during the T-OAE. Therefore, our results suggest that Hg anomalies in the geological record need to be re-evaluated as a “smoking gun” proxy that only infers volcanogenic inputs.

The Toarcian Oceanic Anoxic Event in southwestern Gondwana: an example from the Andean Basin, northern Chile

Palaeoenvironmental and palaeoclimatic changes during the Toarcian Oceanic Anoxic Event (T-OAE) are reported from two successions deposited in the Andean Basin (Chile), based on a high-resolution biostratigraphic (calcareous nannofossils), sedimentological, mineralogical and geochemical analysis. A large negative carbon-isotope excursion was identified in a lower Toarcian storm-dominated marl–limestone alternation, underlining the global extent of the carbon-cycle perturbation and the occurrence of episodic high-energy conditions during the T-OAE. Low total organic carbon content and absence of redox-sensitive trace-element enrichment indicate that the dynamic depositional conditions did not favour the development of oxygen-depleted conditions and the preservation of organic matter. The decrease in kaolinite reflects a shift to more arid conditions, whereas the increase in the chemical index of alteration at the onset of the T-OAE is probably related to a change in the sediment source. Less hydrolytic activity probably promoted a decrease in nutrient input in the Andean Basin. Mercury anomalies support the link between the onset of the Karoo–Ferrar large igneous province activity, the carbon cycle perturbation and the generated environmental change. This paper presents one of the most complete T-OAE studies from the southern hemisphere, and reveals that regional conditions strongly modulated the expression of this global event in the Andean Basin. Supplementary material: the complete dataset, the coefficient of correlation ( r ) for major and trace elements, and the nannofossil assemblages are available at: https://doi.org/10.6084/m9.figshare.c.4133687

Investigating the distribution of magmatism at the onset of Gondwana breakup with novel strapdown gravity and aeromagnetic data

Massive volumes of mafic magmatism forming the Karoo-Ferrar Large Igneous Province (LIP) in Southern Africa and Antarctica preceded Jurassic breakup of the Gondwana Supercontinent. This widespread LIP magmatism is attributed to a major mantle plume, or plumes, impacting an area thousands of kilometres across. Magmas in lava flows and shallow sills, which flowed laterally hundreds to thousands of kilometres, form most of the exposed LIP. Hence, the distribution of shallow level mafic rocks may not reflect the location of mantle melting. In contrast, large deep-seated mafic intrusions such as gabbros likely more directly overlie areas of mantle melting. Antarctic exposures of such intrusions are limited to the Dufek Intrusion and outcrops >1000 km to the north, hence the true pattern of mantle melting is poorly constrained. Regional aeromagnetic and aerogravity data suggest other Jurassic mafic intrusions are present, but detailed analysis of these bodies is lacking. To define more precisely the distribution of mafic intrusions we use data from the first stand-alone strapdown gravity survey in Antarctica. This innovative technique allows collection of aerogravity data during draped flight, resolving anomalies with a wavelength of ∼6 km and a root-mean-square error of 1.8 mGal. Combining this new gravity data with coincident aeromagnetic data we investigate a ∼50 mGal gravity and associated >1000 nT magnetic high in Coats Land, East Antarctica. Our interpretation is that the so called ‘Halley High’ reflects a large gabbroic body ∼80 km long, 30 km wide and ∼6 km thick, equivalent to the inferred total size to the better known Stillwater layered mafic intrusion in the US. Our interpretation of a large mafic intrusion supports the suggestion from reconnaissance aeromagnetic data that this and other similar anomalies are Jurassic mafic intrusions. These large mafic intrusions, and hence underlying mantle melting, appear restricted to a linear band parallel to the continental margin. This structured pattern of mantle melting is consistent with shallow mantle convection and lithospheric extension playing a significant role in the later stages of the Karoo-Ferrar magmatism. An apparent 650 km gap in mafic intrusions adjacent to the continental Weddell Sea Rift System suggests different mantle processes were occurring here. Future joint magnetic and strapdown gravity surveying presents a key opportunity to fully constrain the location and extent of mantle melting during Gondwana breakup.

The Lower Toarcian Serrone Marls (Northern Apennines, Italy): A 3.5 Myr record of marl deposition in the aftermath of the T-OAE

The early Toarcian sedimentary record is frequently characterized by the occurrence of marls, shales, and organic rich facies comprising the Toarcian oceanic anoxic event (T-OAE), which are broadly synchronous to the emplacement of the Karoo-Ferrar large igneous province (LIP) with a main pulse at ~181.7 Ma. Here we describe the chronology and rock magnetic properties of the early Toarcian Marne del Serrone Formation (Serrone Marls) from the Monte Serrone section in the Northern Apennines of Italy, and interpret them as reflecting the local environmental response to the transient CO2 rise and accelerated hydrological cycle most likely induced by the emplacement of the Karoo-Ferrar LIP. The studied 62 m-thick section is characterized by a succession of micritic limestones and marls, interrupted by a few slumps, and contains a record of the T-OAE represented by posidonia beds and a ~40 cm-thick black shale interval. The age of the section has been constrained using magnetostratigraphy and ammonite biostratigraphy to the Spinatum to Variabilis zones. Rock-magnetic properties have been used to characterize fluctuations in the magnetite and hematite input throughout the section. Spectral analysis of continuous IRM2.5T and IRM0.1T/IRM2.5T records indicates a duration of the studied marly interval of ca. 3.5 Myr, regarded as a first-order estimate of the duration of the response of the environmental system (hydrological cycle) to the transient climate perturbation triggered by the emplacement of the Karoo-Ferrar LIP, and a duration of the Bifrons zone of ca. 3.4 Myr.

Magnetostratigraphy of the Toarcian Stage (Lower Jurassic) of the Llanbedr (Mochras Farm) Borehole, Wales: basis for a global standard and implications for volcanic forcing of palaeoenvironmental change

The Lower Jurassic Toarcian Stage ( c. 183–174 Ma) is marked by one of the largest global exogenic carbon-cycle perturbations of the Phanerozoic, which is associated with the early Toarcian Oceanic Anoxic Event (T-OAE; c. 183 Ma). Climatic and environmental change at the T-OAE is reasonably well constrained in the marine realm, with marine anoxic or euxinic conditions developing locally across both hemispheres, at the same time as the T-OAE negative carbon-isotope excursion. However, high-resolution stratigraphic comparison between different palaeo-ocean basins and with the continental realm can be complicated. Palaeomagnetic reversals can provide a precise and accurate stratigraphic correlation tool between marine and continental sedimentary archives, and even between sedimentary and igneous successions. Here, we present a high-resolution magnetostratigraphic record for the Toarcian Stage in the biostratigraphically complete and expanded Llanbedr (Mochras Farm) Borehole, Cardigan Bay Basin, Wales. This study provides the first geomagnetic polarity reversal scale that is integrated with high-resolution biostratigraphy and carbon-isotope stratigraphy for the entire Toarcian Stage. This stratigraphic framework also provides a new, precise correlation with the basalt lava sequence of the Karoo–Ferrar Large Igneous Province, linking the Pliensbachian–Toarcian boundary and T-OAE climatic and environmental perturbations directly to this episode of major volcanic activity. Supplementary material: Details of the palaeomagnetic data and dip direction are available at https://doi.org/10.6084/m9.figshare.c.4052720

Jurassic high heat production granites associated with the Weddell Sea rift system, Antarctica

The distribution of heat flow in Antarctic continental crust is critical to understanding continental tectonics, ice sheet growth and subglacial hydrology. We identify a group of High Heat Production granites, intruded into upper crustal Palaeozoic metasedimentary sequences, which may contribute to locally high heat flow beneath the West Antarctic Ice Sheet. Four of the granite plutons are exposed above ice sheet level at Pagano Nunatak, Pirrit Hills, Nash Hills and Whitmore Mountains. A new UPb zircon age from Pirrit Hills of 178.0±3.5Ma confirms earlier RbSr and UPb dating and that the granites were emplaced approximately coincident with the first stage of Gondwana break-up and the developing Weddell rift, and ~5m.y. after eruption of the Karoo-Ferrar large igneous province. Aerogeophysical data indicate that the plutons are distributed unevenly over 40,000km2 with one intruded into the transtensional Pagano Shear Zone, while the others were emplaced within the more stable Ellsworth-Whitmore mountains continental block. The granites are weakly peraluminous A-types and have Th and U abundances up to 60.7 and 28.6ppm respectively. Measured heat production of the granite samples is 2.96–9.06μW/m3 (mean 5.35W/m3), significantly higher than average upper continental crust and contemporaneous silicic rocks in the Antarctic Peninsula. Heat flow associated with the granite intrusions is predicted to be in the range 70–95mW/m2 depending on the thickness of the high heat production granite layer and the regional heat flow value. Analysis of detrital zircon compositions and ages indicates that the high Th and U abundances are related to enrichment of the lower-mid crust that dates back to 200–299Ma at the time of the formation of the Gondwanide fold belt and its post-orogenic collapse and extension.

Timing and genesis of the Karoo-Ferrar large igneous province: New high precision U-Pb data for Tasmania confirm short duration of the major magmatic pulse

The Karoo-Ferrar igneous province is one of the largest igneous provinces on Earth. It extends from South Africa, along the Trans-Antarctic Mountains to Tasmania and South Australia. Reconstruction of the continents back to the Gondwana configuration in the Early Jurassic reveals a total length of the Karoo-Ferrar province of >5000km. New isotope dilution thermal ionization mass spectrometry (ID-TIMS) single grain U-Pb ages for zircon and baddeleyite from Tasmanian dolerites combined with ID-TIMS literature single grain U-Pb ages from the Ferrar and Karoo suites are consistent with the major pulse of synchronous magmatism throughout the province lasting about 1Ma or less for the major pulse of magmatism at the time of the Toarcian mass extinction event. We argue that the mechanism of synchronization of magmatism over such a short period of time along such a long distance is the major question which has to be answered in search of the correct model for the origin of the Karoo-Ferrar large igneous province. It cannot be reconciled with the lower mantle plume head model with the plume impingement beneath the Karoo. Plume material could not spread beneath the lithosphere at a rate of ~5–10m/yr (5000km per 0.5–1Myr), at least based on the current knowledge of the mantle physical properties. It seems unlikely that the entire Karoo-Ferrar large igneous province formed due to long distance magma migration through dykes from the same mantle plume irrespective on the proposed plume centre location. In such case, magma would have had to cross the boundaries (and thus weakness zones) between three future continents. In the framework of the dyke propagation model we would expect dykes to follow these weakness zones, not cross them. In addition to this, the Karoo and Ferrar contain geochemically different igneous rocks, which were not formed from the same magma source, preventing interpretations based on one single plume. Both the Karoo and Ferrar contain low-Ti tholeiites, which are similar by their trace element patterns to modern arc analogues – the Central Andes and Kamchatka, respectively. Thus, our preferred model for the origin of the Karoo-Ferrar large igneous province is associated with subduction of the Phoenix plate beneath the southern Gondwana. Probably, deep slab dehydration at the depth of the mantle transition zone modulated surface volcanism or the Toarcian tectonic event triggered voluminous but short-term melting of mantle, which was metasomatized by subduction-derived fluids.