The early Toarcian extinction event and associated Pliensbachian-Toarcian palaeoenvironmental perturbations in Bulgaria

The Early Toarcian was characterised by the eruption of the Karoo-Ferrar large igneous province (LIP), rapid global warming, significant perturbations in the global carbon cycle, the development of widespread anoxia known as the Early Toarcian Oceanic Anoxic Event (T-OAE) and a biotic crisis in the marine realm known as the Early Toarcian Mass Extinction (ETME). Despite the purported global nature of these environmental and biotic changes, the majority of records come from Western European sections, and remain particularly focused on settings in which the T-OAE was clearly expressed. Fewer studies focus on sections where the manifestation of ocean deoxygenation appears to have been considerably weaker, or even absent. We herein focus on Lower Jurassic successions of the Moesian Basin in the Balkan Mountains of the Balkan Mountains (Bulgaria) that were deposited on an open-ocean facing carbonate shelf. The Bulgarian δ13C and δ18O profiles show similar trends through the Lower Jurassic to coeval European sections, suggesting that seawater in the Moesian Basin was recording global palaeoclimatic and palaeoceanographic conditions during this time. Analysis of the carbon isotope record reveals a broad positive carbon isotope excursion (CIE) of 3‰ in δ13Cbel through the Early Toarcian, interrupted by a negative CIE of 3.5‰ recorded in organic carbon (Tenuicostatum–Falciferum zones). Progressive warming of seawater and increased influx of freshwater into the Moesian Basin through the Early Toarcian is recorded in δ18Obel isotopes. Such changes are attributed to the eruption of the Karoo-Ferrar LIP, reflected in the Moesian Basin by an enrichment in Mercury (Hg) recorded as a shift in sedimentary Hg/TOC values synchronous with the negative CIE. A biotic crisis is recorded amongst bivalves and considered to be part of the ETME, which is here recorded in Bulgaria for the first time. Although a significant loss amongst bivalves during the ETME often coincides with the spread of anoxia, this link is not clearly seen in Bulgaria as geochemical and sedimentological records do not support the prevalence of anoxic conditions. As such, oxygen deficiency cannot be considered a key driving mechanism for the ETME in this part of the ocean and other factors such as rapid warming may have been more important in this central Tethyan region.

Download Full-text

The early Toarcian extinction event and associated Pliensbachian-Toarcian palaeoenvironmental perturbations in Bulgaria

10.7287/peerj.preprints.27948 ◽

2019 ◽

Author(s):

Autumn Pugh ◽

Crispin TS Little ◽

Ivan P Savov ◽

Lubomir Metodiev ◽

Paul B Wignall ◽

...

Keyword(s):

Carbon Isotope ◽

Oxygen Deficiency ◽

Lower Jurassic ◽

Significant Loss ◽

Large Igneous Province ◽

Driving Mechanism ◽

Oceanic Anoxic Event ◽

Biotic Crisis ◽

Marine Realm ◽

Balkan Mountains

Download Full-text

The chemostratigraphy and environmental significance of the Marlstone and Junction Bed (Beacon Limestone, Toarcian, Lower Jurassic, Dorset, UK)

Geological Magazine ◽

10.1017/s0016756821000972 ◽

2021 ◽

pp. 1-15

Author(s):

Hugh C. Jenkyns ◽

Sophie Macfarlane

Keyword(s):

Carbon Isotope ◽

Oxygen Isotopes ◽

Lower Jurassic ◽

Carbon And Oxygen Isotopes ◽

Oceanic Anoxic Event ◽

Nodular Limestone ◽

Anoxic Event ◽

Reference Curves ◽

High Degree ◽

Organic Rich Sediment

Abstract Two fallen blocks of the Marlstone and stratigraphically overlying Junction Bed sampled on the beach below Doghouse Cliff in Dorset, UK (Wessex Basin) have been examined for carbon and oxygen isotopes of bulk carbonate as well as for strontium, carbon and oxygen isotopes and Mg:Ca ratios in the contained belemnites. The sequence, which contains most of the Toarcian zones and subzones within a metre or less of grey to yellow to pink, red and brown fossil-rich nodular limestone, is extremely condensed and lithologically similar to pelagic red limestones of the Tethyan Jurassic that are locally mineralized with Fe-Mn oxyhydroxides (e.g., Rosso Ammonitico). Strontium-isotope ratios of the contained belemnites are compatible with existing reference curves and both blocks show a rise to more radiogenic values post-dating the Pliensbachian–Toarcian boundary. The high degree of correlation between the relatively negative carbon and oxygen isotopes of the bulk carbonate is compatible with significant diagenetic overprint, and contrasts with higher carbon-isotope values in coeval condensed coccolith-rich limestones elsewhere. Evidence for the characteristic signature of the Toarcian Oceanic Anoxic Event, as represented by organic-rich sediment, is absent, possibly owing to a stratigraphic gap. Both blocks exhibit abrupt carbon-isotope shifts to lower values, one of which could represent the limbs of an incompletely recorded negative excursion associated with the Toarcian Oceanic Anoxic Event. That the Toarcian Oceanic Anoxic Event was also a significant hyperthermal is illustrated in both blocks by a drop in oxygen-isotope values and rise in Mg:Ca ratios of belemnites close to the base of the Junction Bed in the lowest part of the serpentinum zone.

Download Full-text

Major palaeoenvironmental upheavals in both marine and terrestrial realms during the Toarcian oceanic anoxic event: a multi-proxy approach

10.5194/egusphere-egu2020-21187 ◽

2020 ◽

Author(s):

Francesca Galasso ◽

Susanne FeistBurkhardt ◽

Annette Schmid- Röhl ◽

Stefano Benasconi ◽

Elke Schneebeli-Hermann

Keyword(s):

Carbon Isotope ◽

Seawater Temperature ◽

Black Shales ◽

Land Plants ◽

Open Pit ◽

Large Igneous Province ◽

Isotopic Study ◽

Data Set ◽

Oceanic Anoxic Event ◽

Anoxic Event

<p>The Toarcian oceanic anoxic event (TOAE) ~183 Ma is not only associated with oceanic anoxia and rapid seawater temperature increase but also with a marine mass extinction event. These biotic and environmental upheavals are linked to the emplacement of the Karoo-Ferrar large igneous province. Negative carbon excursions and widespread deposition of black shales are typical for Toarcian sedimentary successions.</p><p>The occurrence and growth of dinoflagellates is influenced by environmental factors like oxygen content, salinity, temperature and nutrient availability. For land plants, changes in dominance structure of ecosystems reflected in spore pollen assemblages can be indicative of ecological disturbance. Thus species composition (and morphology) of dinoflagellates and land plants can be used to understand major environmental perturbations.</p><p>An extensively studied TOAE section is the former Rohrbach Zement quarry at Dotternhausen (today Larfarge-Holcim) with comprehensive data of carbon isotope analyses, total organic and inorganic carbon content and rock eval analysis.<br>The Dotternhausen quarry is not accessible anymore but a new open pit in Dormettingen ~2 km NW of Dotternhausen offers excellent outcrop conditions. Litho- and biostratigraphy of the new section is well documented and calibrated to the old Dotternhausen section on subzone levels. Comparison of the two sites showed that sedimentology, geochemistry and faunal data are laterally constant. <br>Palynological analysis of 59 outcrop samples from the Dormettingen section yielded an excellent quantitative data set of the Early Toarcian Posidonienschiefer sediments. Here we provide a high-resolution, multi-proxy study of this section including chemostratigraphy, particulate organic matter and palynology in order to understand the environmental conditions during the TOAE.</p><p>Carbon isotopic study reveals a negative excursion during the TOAE, varying between -33.49&#8240; and -26.5&#8240;, with a negative shift in the Falciferum Zone (Elengatum, Exeratum and Elegans Subzone) concurrent with the dinoflagellate "blackout". &#160;The vegetation shows significant changes from a mixed assemblage of pollen and spores in the lower part of the section, to exclusively spore-bearing during the negative carbon isotope excursion. The isotopic signal, the marine dinoflagellate &#8220;blackout&#8221; and the changes in terrestrial vegetation indicate/document major palaeoenvironmental upheavals in both the marine and terrestrial realms.</p>

Download Full-text

Record of the Toarcian oceanic anoxic event in the Grands Causses Basin (southern France) and its implications for vertebrate preservation

10.5194/egusphere-egu21-11835 ◽

2021 ◽

Author(s):

Brahimsamba Bomou ◽

Guillaume Suan ◽

Jan Schlögl ◽

Anne-Sabine Grosjean ◽

Baptiste Suchéras-Marx ◽

...

Keyword(s):

Soft Tissues ◽

Clay Mineralogy ◽

Lower Jurassic ◽

Large Igneous Province ◽

Volcanic Emissions ◽

Oceanic Anoxic Event ◽

Environmental Perturbations ◽

Anoxic Event ◽

Posidonia Shale ◽

Rock Eval

<p>Paleontological excavations realized by our group in Toarcian shales (Lower Jurassic) of the Grands Causses Basin in Roqueredonde (H&#233;rault, France), yielded several specimens of marine vertebrates. The newly discovered specimens are partly or entirely preserved in anatomical connection and include a partial ichthyosaur skeleton with soft tissues, and a 4&#160;m-long thalattosuchian longirostrine marine crocodile. A multi-proxy approach has been developed (XRD-bulk and clay mineralogy, Rock-Eval pyrolysis, phosphorus and mercury contents) in order to replace these findings in a well-defined temporal and paleoenvironmental context, and hence constrain the factors that led to their remarkable preservation. The fossiliferous succession exposes a 3&#160;m-thick upper Pliensbachian interval of marl and nodular carbonate beds, overlain by a 3&#160;m-thick interval of lower Toarcian laminated shales and limestone beds. Our high-resolution ammonite biostratigraphy, combined with inorganic and organic carbon isotope chemostratigraphy, shows that the fossiliferous Toarcian strata were deposited at a time of global warming and major carbon cycle perturbation known as the Toarcian Oceanic Anoxic Event (T-OAE). The studied succession shows several similarities with the classical coeval fossiliferous levels of the Posidonia Shale in SW Germany, including high organic matter and hydrocarbon contents as well as extremely reduced sedimentation rates. These results indicate that the unusual richness in well-preserved vertebrates of the studied site can be explained by a combination of warming-induced, low salinity and stratified waters, prolonged seafloor anoxia and reduced dilution by low carbonate and terrigenous input due to rapid sea-level rise. Our results also reveal a significant peak in mercury at the base of the T-OAE interval, consistent with that recorded in several coeval sections (e.g. Portugal, Morocco, Argentina, Chile). This mercury anomaly, most likely resulting from intense volcanic activity Karoo-Ferrar large igneous province, suggests that widespread exceptional vertebrate preservation during the T-OAE was initiated by a suite of severe environmental perturbations ultimately triggered by intense volcanic emissions.</p>

Download Full-text

Carbon-isotope record and palaeoenvironmental changes during the early Toarcian oceanic anoxic event in shallow-marine carbonates of the Adriatic Carbonate Platform in Croatia

Geological Magazine ◽

10.1017/s0016756813000083 ◽

2013 ◽

Vol 150 (6) ◽

pp. 1085-1102 ◽

Cited By ~ 18

Author(s):

NADIA SABATINO ◽

IGOR VLAHOVIĆ ◽

HUGH C. JENKYNS ◽

GIOVANNA SCOPELLITI ◽

RODOLFO NERI ◽

...

Keyword(s):

Shallow Water ◽

Carbon Isotope ◽

Carbonate Platform ◽

Lower Jurassic ◽

Positive Trend ◽

Oceanic Anoxic Event ◽

Anoxic Event ◽

Isotope Record ◽

The Impact ◽

Palaeoenvironmental Changes

AbstractGeochemical (δ13C, δ18O and Mn) compositions of Lower Jurassic shallow-water carbonates cropping out in Croatia were analyzed to elucidate the impact of the early Toarcian oceanic anoxic event (T-OAE) on the Adriatic Carbonate Platform (AdCP). The bulk-rock carbon-isotope records through the studied sections (Velebit-A, Velebit-B and Gornje Jelenje) are characterized by two significant excursions: (i) an initial positive trend interrupted by a pronounced negative shift (c. 2.5‰) that is followed by (ii) an increasing trend of positive values (up to 4.5‰). A comparison with δ13C trends obtained from well-calibrated sections from other localities in Europe shows that the overall character of the early Toarcian negative excursion is clearly reproduced in the curves derived from Croatian shallow-water deposits, which helps to date the sequences and reinforces the global character of the carbon-cycle perturbation. Lower Jurassic sedimentary successions in the studied area show a gradual deepening trend corresponding to deposition of the Toarcian spotted limestones. Assuming that the distinctive negative excursion in the carbon-isotope curves is synchronous across the AdCP, the contact between the spotted limestones and the underlying beds rich in lithiotid bivalves appears to be diachronous within the study area. The Mn record through the Croatian Velebit-A section and, in particular, the rise in concentration (up to 100 ppm) coinciding with the beginning of the δ13Ccarb positive shift, reflects a change in the redox conditions in seawater that allowed diagenetic incorporation of reduced manganese into the calcite structure of the carbonate sediment during the onset of the T-OAE.

Download Full-text

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

Geological Society London Special Publications ◽

10.1144/sp514-2021-16 ◽

2021 ◽

pp. SP514-2021-16

Author(s):

Brahimsamba Bomou ◽

Guillaume Suan ◽

Jan Schlögl ◽

Anne-Sabine Grosjean ◽

Baptiste Suchéras-Marx ◽

...

Keyword(s):

Oxygen Deficiency ◽

Causal Link ◽

Large Igneous Province ◽

Content Type ◽

Oceanic Anoxic Event ◽

Carbon Burial ◽

Carbon Isotope Excursion ◽

Supplementary Material ◽

Marine Vertebrates ◽

Rock Eval

AbstractThe Early Jurassic was marked by several episodes of rapid climate changes 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 realised in Toarcian strata near Lodève (Hérault, S France) have yielded several specimens of marine vertebrates and abundant invertebrate fauna. We have developed a multi-proxy approach (ammonite biostratigraphy, XRD-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 (CIE), 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 kyr, 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 due to reduced dilution by carbonate and detrital input.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5509789

Download Full-text