The effects of the Late Pliensbachian-Early Toarcian climate changes on the carbonate factory of the Apenninic Platform

A. Trecalli (*), J. Spangenberg (**), T. Aadatte (***), K. B. Föllmi (***) & M. Parente (*) 

(*) Dipartimento di Scienze della Terra, Università di Napoli “Federico II”, 80138-Napoli, Italy.

albertotrecalli@gmail.com
(**) Institut de minéralogie et géochimie, Université de Lausanne, 1015-Lausanne, Switzerland
(***) Institut de géologie et paléontologie, Université de Lausanne, 1015-Lausanne, Switzerland

Abstract 

The Late Pliensbachian-Early Toarcian was characterized by a transition from semi-arid and relatively cool to humid and warm greenhouse climatic conditions.
Across the same time interval, the global sedimentary archive contains evidence of two severe perturbations of the global carbon cycle, witnessed by prominent carbon isotope excursions (CIE) in marine carbonates and continental and marine organic matter. The first one straddles the Pliensbachian-Toarcian boundary, the second occurred at the end of the tenuicostatum ammonite zone (ca 183 Ma) and is associated with the early Toarcian oceanic anoxic event (T-OAE). The CIEs are associated with abrupt climatic change, ocean acidification, widespread oceanic anoxia and major episodes of extinction and/or accelerated biotic turnover.
Most of what we know about the T-OAE comes from the record of relatively deep-water successions, much less is known of the response of shallow-water neritic environments. While many carbonate platforms in the Tethyan realm were drowned during the late Early Jurassic, the Apenninic Carbonate Platform (ACP) continued growing in shallow-water: its geological record contains an important archive of the response of shallow water carbonate platform ecosystems to global warming and ocean acidifi cation.
We report the results of a high-resolution sedimentologic and geochemical study of two carbonate platform successions cropping out in the southern Apennines (Italy).
We performed a detailed facies analysis and produced a detailed record of the carbon-isotope ratio of carbonate and organic matter. Total phosphorus was investigated to trace changes in nutrient input and availability while the composition of clay minerals assemblages was used as a proxy of climatic change.
Chemostratigraphic correlation with basinal reference δ13C curves allows unprecedented high-resolution dating of our sections and the precise correlation of carbonate platform biostratigraphy to standard ammonite zones. In the fi rst part of the studied sequences, the most prolifi c carbonate producers are the large bivalves of the “Lithiotis” facies and the green alga Palaeodasycladus mediterraneus: both occur in rock-forming abundance, witnessing optimal environmental conditions. The Lithiotis/Palaeodasycladus carbonate factory survived in the ACP across the
Pliensbachian-Toarcian boundary interval, when the increased frequency and thickness of pelitic interlayers, the increase of kaolinite and phosphorous content indicate enhanced weathering and nutrient supply. The demise of these massive biocalcifiers coincides with the onset of the early Toarcian CIE. The rising branch of the early Toarcian CIE corresponds to a massive accumulation of ooids.
The occurrence of oolitic limestones, abruptly replacing biogenic facies dominated by large bivalves and calcareous algae, is a well known feature of all the Tethyan carbonate platforms that were not drowned during the T-OAE. In absence of the prolific biocalcifiers, wiped out by extinction, the chemical precipitation in the form of oolites, assisted or not by microbial activity, could represent the only effective way to buffer the post-acidification rebound of alkalinity in the Toarcian shallow ocean.

KEY WORDS: Pliensbachian-Toarcian, Toarcian oceanic anoxic event, chemostratigraphy, Apenninic Carbonate Platform, stable isotopes.