Global controls on phosphatization of fossils during the toarcian oceanic anoxic event

Konservat-Lagerstätten—deposits with exceptionally preserved fossils—vary in abundance across geographic and stratigraphic space due to paleoenvironmental heterogeneity. While oceanic anoxic events (OAEs) may have promoted preservation of marine lagerstätten, the environmental controls on their taphonomy remain unclear. Here, we provide new data on the mineralization of fossils in three Lower Jurassic Lagerstätten—Strawberry Bank (UK), Ya Ha Tinda (Canada), and Posidonia Shale (Germany) —and test the hypothesis that they were preserved under similar conditions. Biostratigraphy indicates that all three Lagerstätten were deposited during the Toarcian OAE (TOAE), and scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) show that each deposit contains a variety of taxa preserved as phosphatized skeletons and tissues. Thus, despite their geographic and paleoenvironmental differences, all of these Lagerstätten were deposited in settings conducive to phosphatization, indicating that the TOAE fostered exceptional preservation in marine settings around the world. Phosphatization may have been fueled by phosphate delivery from climatically-driven sea level change and continental weathering, with anoxic basins acting as phosphorus traps.

Fossilized leftover falls as sources of palaeoecological data: a ‘pabulite’ comprising a crustacean, a belemnite and a vertebrate from the Early Jurassic Posidonia Shale

Especially in Lagerstätten with exceptionally preserved fossils, we can sometimes recognize fossilized remains of meals of animals. We suggest the term leftover fall for the event and the term pabulite for the fossilized meal when it never entered the digestive tract (difference to regurgitalites). Usually, pabulites are incomplete organismal remains and show traces of the predation. Pabulites have a great potential to inform about predation as well as anatomical detail, which is invisible otherwise. Here, we document a pabulite comprising the belemnite Passaloteuthis laevigata from the Toarcian of the Holzmaden region. Most of its soft parts are missing while the arm crown is one of the best preserved that is known. Its arms embrace an exuvia of a crustacean. We suggest that the belemnite represents the remnant of the food of a predatory fish such as the shark Hybodus.

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

<p>Paleontological excavations realized by our group in Toarcian shales (Lower Jurassic) of the Grands Causses Basin in Roqueredonde (Hérault, 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 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 m-thick upper Pliensbachian interval of marl and nodular carbonate beds, overlain by a 3 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>

Biostratigraphic constraints of the Early Toarcian Oceanic Anoxic Event: new data from calcareous nannofossil investigations of Boreal and Tethyan sections

Calcareous nannofossil biostratigraphy was carried out in Upper Pliensbachian – Lower Toarcian sediments, which cover the Toarcian Oceanic Anoxic Event (T-OAE) interval. In particular, semiquantitative analyses were performed on a total of 156 samples in the composite Sogno Core (Lombardy Basin, Southern Alps) representing a pelagic Tethyan section. Quantitative investigations were applied to additional 168 samples across the Amaltheenton Fm. and Posidonienschiefer Fm., from two cores of the Boreal Realm (Lower Saxony Basin, northern Germany). Primary and secondary events of the Tethyan and Boreal zonations were recognized, allowing the identifications of the NJT5, NJT6 nannofossil Zones for the Sogno Core and the NJ5, NJ6, NJ7 Zones for the German sections, respectively. The sequence of nannofossil biohorizons is generally consistent with data available for various areas at lower and higher latitudes, confirming their reproducibility and reliability for intra and inter-regional correlations. Geochemistry evidences the presence of the negative C isotopic excursion across the “Fish Level” black shale interval expression of the T-OAE in the Sogno Core. The same anomaly is recorded in the German successions at the base of the Posidonia Shale witnessing the passage from well oxygenated to predominantly anoxic conditions. Our results show that the T-OAE C isotopic excursion recorded in the Sogno Core is excellently constrained by the first occurrence (FO) of Carinolithus superbus at the onset and the last occurrence (LO) of Mitrolithus jansae at the end. A significant decrease in abundance and size of Schizosphaerella punctulata (the “S. punctulata crisis”) and an abundance drop of M. jansae further characterise the T-OAE perturbation. Only S. punctulata shows a recovery at the end of the T-OAE, while M. jansae barely survived the palaeoenviromental stress and disappeared soon after its termination. The extreme rareness of S. punctulata and the absence of M. jansae in the Boreal Realm prevent the recognition of the “S. punctulata crisis” and the M. jansae decline. Our study reveals the LO of Biscutum finchii together with the FO of C. superbus as an additional event approximating the onset of the C isotopic excursion exclusively in the German successions. Further events, such as the LOs of Biscutum grandis, Crepidolithus granulatus and Parhabdolithus liasicus are detected within the C isotopic anomaly exclusively in the German sections. Nannofossil biostratigraphy permits the effective dating and correlating of Early Jurassic major palaeoceanographic events and particularly of the T-OAE which are of a great importance to derive a definitive model for the Posidonia Shale deposition.