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Late Triassic dinosaur tracks from Penarth, south Wales
Primary v. carbonate production in the Toarcian, a case study from the Llanbedr (Mochras Farm) borehole, Wales
Abstract The leading hypothesis for the Toarcian oceanic anoxic event (T-OAE; c. 183 Ma) and the associated negative C-isotope excursion is the massive release of 12 C favouring greenhouse conditions and continental weathering. The nutrient delivery to shallow basins supported productivity and, because of O 2 consumption by organic-matter respiration, anoxia development. However, several studies have shown that calcareous nannoplankton experienced a decrease during the T-OAE. Nannofossil fluxes measured in the Llanbedr (Mochras Farm) borehole, Wales, UK, were the highest prior to the negative C-isotope excursion, along with high amounts of taxa indicative of nutrient-rich environments (Biscutaceae). Such conditions attest to high productivity. Fluxes show the lowest values in the core of the event, along with a size decrease of Schizosphaerella and a peak in Calyculaceae. The recovery of nannofossil fluxes and Schizosphaerella size occurred concomitant with the return of C-isotopes to more positive values. Concomitantly, deep dwellers ( Crepidolithus crassus ) dominated, indicating a recovery of the photic-zone productivity. These observations demonstrate that the cascade of environmental responses to the initial perturbation was more complex than previously considered. In spite of elevated nutrient delivery to epicontinental basins in the early Toarcian, carbonate and primary productions of nannoplankton were depressed in the core the T-OAE, probably because of prolonged thermohaline seawater stratification.
The palaeoenvironmental context of Toarcian vertebrate-yielding shales of southern France (Hérault)
Abstract The 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.
Abstract We report new ichthyosaur material excavated in lower Toarcian levels of the LafargeHolcim Val d'Azergues quarry in Beaujolais, SE France. A partially articulated skull and a smaller, unprepared but likely subcomplete skeleton preserved in a carbonate concretion are identified as stenopterygiids, a family of wide European distribution during the Early Jurassic. These specimens are among the finest preserved Toarcian exemplars known from Europe and, in one of them, soft tissue preservation is suspected. Their state of preservation is attributed to the combination of prolonged anoxic conditions near the water–sediment interface and early carbonate cementation resulting from the activity of sulfate-reducing bacteria. We also present carbon and strontium isotope values obtained from the study site that allow detailed temporal comparisons with other Toarcian vertebrate-yielding sites and environmental perturbations associated with the Toarcian Oceanic Anoxic Event (T-OAE). These comparisons suggest that the relatively high abundance and good preservation state of Toarcian vertebrates was favoured by a prolonged period of low bottom water oxygenation and accumulation rates. The environmental conditions that prevailed during the T-OAE were probably responsible for the extensive nature of Lagerstätte-type deposits with exceptional preservation of marine organisms. Testing whether the T-OAE had a biological impact on marine vertebrates requires a precise chemostratigraphic context of the fossil record spanning the Pliensbachian–Toarcian interval.
Ocean acidification during the early Toarcian extinction event: Evidence from boron isotopes in brachiopods
ABSTRACT During the Paleocene–Eocene, the Earth experienced the warmest conditions of the Cenozoic and reduced equator-to-pole temperature gradients. Compared to low- and mid-latitude sites, Paleogene environmental changes are less documented in Arctic regions, although such high latitude areas constitute a critical domain to constrain global climate changes. Floral and faunal assemblages indicative of exceptionally warm and humid conditions during the late Paleocene–early Eocene have been reported in several localities around the Arctic Ocean. Additional studies are required to ascertain the effects of Paleocene–Eocene global environmental changes on western Arctic regions. Here we present multiproxy data from early Eocene deltaic plain sediments of the Mackenzie Delta (Canada). This environment is characterized by littoral forest, including swamp, showing that the mangrove Avicennia grew in Arctic Canada near 75°N under air temperatures averaging 21–22 °C annually and 10–14 °C in winter and with precipitation of 1200–1400 mm/yr. Kaolinite contents are high (up to 75% of clay assemblages), as under a modern subtropical climate. The Avicennia pollens recently found in the New Siberian Islands and in Arctic Canada imply that warm and wet conditions were widespread along the Arctic coast during the early Eocene. It also suggests a marine connection between the Arctic Basin and the mid-latitude oceans. We propose that an oceanic current must have connected the Arctic Basin to the Atlantic and/or Pacific and that an internal current developed in the Arctic Basin since the early Eocene.
Abstract The New Siberian Islands are affected by a number of Mesozoic tectonic events. The oldest event (D1a) is characterized by NW-directed thrusting within the South Anyui Suture Zone combined with north–south-trending sinistral strike-slip in the foreland during the Early Cretaceous. This compressional deformation was followed by dextral transpression along north–south-trending faults, which resulted in NE–SW shortening in the Kotelny Fold Zone (D1b). The dextral deformation can be related to a north–south-trending boundary fault zone west of the New Siberian Islands, which probably represented the Laptev Sea segment of the Amerasia Basin Transform Fault in pre-Aptian–Albian times. The presence of a transform fault west of the islands may be an explanation for the long and narrow sliver of continental lithosphere of the Lomonosov Ridge and the sudden termination of the South Anyui Suture Zone against the present Laptev Sea Rift System. The intrusion of magmatic rocks 114 myr ago was followed by NW–SE-trending sinistral strike-slip faults of unknown origin (D2). In the Late Cretaceous–Paleocene, east–west extension (D3) west of the New Siberian Islands initiated the development of the Laptev Sea Rift System, which continues until today and is largely related to the development of the Eurasian Basin.