The record of the early Toarcian and early Aptian oceanic anoxic events in the Apenninic Carbonate Platform (Southern Italy)
Trecalli, Alberto (2011) The record of the early Toarcian and early Aptian oceanic anoxic events in the Apenninic Carbonate Platform (Southern Italy). [Tesi di dottorato] (Inedito)
Full text disponibile come:
About one third of the carbon dioxide released mainly from burning of fossil fuels is absorbed into the oceans where it reacts to form carbonic acid. As a result the pH of the ocean and the amount of carbonate ions decrease in a process called ocean acidification. Detrimental effects on calcifying organisms, which use carbonate minerals to build their protective shells and skeletons, have been documented in the laboratory and in the field. However, due to the spatio-temporal limits of experiments and field observations, the long-term impact on marine ecosystems and the adaptation potential of marine species are best investigated by looking at the geological record of past episodes of ocean acidification. Episodes of short-term massive injection of CO2 in the atmosphere-ocean system are witnessed by negative carbon isotope events (CIE) recorded by marine carbonates and by marine and continental organic matter. Paroxysmal volcanism and/or clathrates dissociation are generally invoked as the source of isotopically depleted excess CO2. High pCO2 is also held responsible for the dramatic increase of atmospheric and seawater temperature. During the Mesozoic some of these events record also the deposition of large amounts of organic carbon in epicontinental and oceanic basins, witnessing widespread marine anoxia. For this reason they are commonly referred to as Oceanic Anoxic Events (OAEs). The early Toarcian (Posidonienschiefer event, T-OAE, 183 Ma) and early Aptian events (Selli event, OAE1a, 120 Ma) represent two of the most severe and best documented episodes of sudden perturbation of the global carbon cycle and therefore have been chosen as subject of this thesis. There is overwhelming evidence for both the events that geologically rapid injection of CO2 into the ocean-atmosphere system caused abrupt global warming. Ocean acidification has also been proposed for both the events. Most of what we know about the record of the early Toarcian and early Aptian events has been revealed by the study of relatively deep-water marine sediments, deposited in epicontinental basins and shelves and in oceanic basins. Comparatively much less is known on the response of shallow water carbonate platforms, which represent the other “half” of the ocean, in terms of carbonate production and accumulation. The Apenninic Carbonate Platform (ACP) grew isolated from major continental landmasses at least since the Early Jurassic. It accumulated more than 4500 m of shallow water carbonates from the Late Triassic to the Late Cretaceous and was able to survive all the Mesozoic OAEs. Its sedimentary archive offer the unique opportunity of investigating the response to CO2-induced perturbations without the obvious drawback of habitat loss due to drowning. In order to study the response of the ACP to the Late Pliensbachian-Early Toarcian environmental changes, two classical outcrops of platform carbonates of the southern Apennines have been logged and sampled: Mercato San Severino, about 30 km northwest of Salerno, and Monte Sorgenza, about 7 Km east of Formia. Both sections consist of bioclastic limestones of the "Lithiotis member" of the "Palaeodasycladus Limestones Formation" overlain by oolitic limestones of the "Oolitic-oncolitic Limestones Formation". Paired records of 13Ccarb and 13Corg, total phosphorus content (P) and clay-mineral assemblages have been investigated and integrated with detailed microfacies analysis for both the studied sections. The most prominent features shown by the isotopic records are two sharp negative excursions with an intervening positive excursion. The first negative CIE occurs in the upper part of the “Lithiotis member” and is recorded only by the 13Corg curves with a shift of about 3-4‰. The second negative CIE starts in the last beds of the “Lithiotis member” and reaches the lowest values at the boundary with the oolitic limestones. This excursion is recorded by both curves but is distinctly larger in the 13Corg (4-5‰) than in the 13Ccarb curve (2-2.5‰). Chemostratigraphic correlation with the reference section of Peniche allows unprecedented high-resolution dating of the Early Jurassic platform carbonates of the southern Apennines. This correlation is used to explore the response of a resilient carbonate platform to the early Toarcian oceanic anoxic event. The first CIE has been associated to the Pliensbachian-Toarcian boundary event, while the second one to the early Toarcian OAE. In the ACP, the Lithiotis/Palaeodasycladus carbonate factory, so typical of all the Tethyan tropical carbonate platforms during the Pliensbachian, was wiped out at the onset of early Toarcian negative carbon isotope excursion, seemingly marking the definitive extinction of these massive biocalcifiers. Clay-minerals and P content of the ACP records increased weathering across the Pliensbachian-Toarcian boundary. Many carbonate platforms of the Peri-Tethyan domain responded to the shift of nutrient levels, associated with increased weathering and runoff, by either drowning or shifting to heterotrophic carbonate production. The ACP continued growing in shallow water with no significant shift in the composition of the carbonate factory. This is probably due to the fact that the ACP grew isolated from major continental blocks and seemingly distant from the Early Jurassic upwelling zones. For these reasons nutrient levels seemingly did not cross the threshold of ecological tolerance of the main carbonate producing biota. Moreover, the ACP was situated further from the Jurassic rifting axis than other carbonate platforms, which were progressively drowned during the Early Jurassic. Lower subsidence rate was most probably a significant factor explaining the resilience of the ACP to Early Toarcian palaeoenvironmental perturbations. The extinction of carbonate platform biocalcifiers is coeval with a biocalcification crisis of calcareous nannoplankton. The coincidence with the negative CIE, interpreted as the result of the massive injection of CO2 into the atmosphere-ocean system, is consistent with a scenario of ocean acidification at the onset of the T-OAE, which likely led to the demise of the Lithiotis/Palaeodasycladus carbonate factory. Clay-minerals and P content show no evidence of enhanced weathering in the ACP across the early Toarcian OAE. Therefore, enhanced nutrient levels were probably not the cause of the demise of the Lithiotis/Palaeodasycladus carbonate factory. This would further support the scenario of ocean acidification. In the ACP, and in other resilient platforms of the Tethyan ocean, the disappearance of the most prolific biocalcifiers coincide with a shift to chemical precipitation in the form of massive oolitic limestones. Similar to what observed for the Permian-Triassic boundary crisis, chemical precipitation took over on carbonate platforms as soon as ocean alkalinity recovered. The evolution recorded by the ACP across the T-OAE conforms to the expectations of a biogeochemical model for the marine geological signature of ocean acidification. Very prolific biocalcification by massive bivalves and calcareous algae in the “Lithiotis member” represents the pre-event steady state of the model. The abrupt demise of Lithiotis bivalves and Palaeodasycladus at the onset of the CIE corresponds to the “dissolution interval”. The oolitic limestones represent the “CaCO3 preservation overshoot”, marking the recovery of carbonate supersaturation driven by enhanced weathering. The Early Toarcian record of the southern Apennines could be relevant for research on present and future ocean acidification. The message is that the threat posed by rapid increasing pCO2 could be well beyond the potential of acclimation and evolutionary adaptation of marine biocalcifiers. For the early Aptian OAE, two Berramian-Aptian sections, cropping out at Monte Raggeto (about 7 km northwest of Caserta) and Monte Tobenna (about 8 km northeast of Salerno) have been logged and sampled. Detailed microfacies analysis and high-resolution carbon isotope analysis have been performed for both the sections. The integration of this study with the carbon isotope stratigraphy of three Barremian-Aptian successions studied in a previous thesis project, has allowed the proposal of a bio-chemostratigraphic model for the Late Barremian-Aptian interval. The correlation with the most complete Monte Raggeto section reveals previously undisclosed gaps in the other sections. This highlights the difficulties of applying carbon isotope stratigraphy to inherently incomplete carbonate platform sections. The most significant result of this study is the proposal of chemostratigraphically constrained biostratigraphic criteria for the individuation of the time-equivalent of the Selli event in central and southern Tethyan carbonate platforms. Moreover this study proposes a chemostratigraphically constrained chronostratigraphic calibration of some important biostratigraphic events that are widely used in the Barremian–Aptian biozonations of central and southern Tethyan carbonate platforms. The interval of decreasing 13C values preceding the C3 negative peak, which marks the onset of the Selli event, starts just above the LO of V. murgensis. The C4-C6 segments, which correspond in deep-water sections to the interval of black shales deposition, ends just below the first acme of S. dinarica. The latter roughly corresponds to the C7 segment of peak 13C values. The "Orbitolina level" marks the return the pre-excursion values at the end of the broad positive CIE associated with the OAE1a. Another valuable result is the definition of a biostratigraphic criterion to spike the Barremian-Aptian boundary. According to the present calibration, the boundary is very closely approximated by the first occurrence of V. murgensis and D. hahounerensis. In all the biostratigraphic schemes published so far for the ACP, and other central Tethyan platforms, the chronostratigraphic calibration was anchored to the ages established for selected taxa of orbitolinid foraminifera in the carbonate platform of the northern Tethyan margin. This study proposes the first chronostratigraphic calibration constrained by carbon and strontium isotope stratigraphy. Chemostratigraphy is being successfully applied to Cretaceous carbonate platforms. Its integration with biostratigraphy hold the promise of producing standard biozonations, based on larger foraminifera and calcareous algae, perfectly tied to the chronostratigraphic scale. This would open the possibility of fully exploiting the valuable archive of palaeoenvironmental changes preserved by Cretaceous carbonate platforms.
Solo per gli Amministratori dell'archivio: edita il record