Correlation between extinction pattern and δ13C fluctuations across the Triassic Jurassic boundary in shallow water settings: a proxy for the present day acidification processes

Abstract

With the aim to assess the possible influence of the carbon variations on the benthic communities across the Triassic Jurassic boundary we have carried out sedimentological, biostratigraphic and stable isotope studies on an about 220 m thick peritidal section cropping out in Northwestern Sicily. The subtidal facies of the lower and middle part of this succession show the common occurrence of large and thick megalodontids and a benthic foraminiferal assemblage with Triasina hantkeni and Aulotuortids that supports a Rhaetian age. In the lower part of the succession, a significative biotic perturbation is evidenced by a sudden reduction of the size, shell thickness and abundance of the large megalodontids. This “Lilliput effect” corresponds to a first severe negative shift of the carbon curve that match a perturbation recorded worldwide known as “initial CIE”. The “Lilliput effect” does not seems to have any influence on the microbenthic community (e.g. foraminifers and calcareous algae). Upward, a second negative excursion spans a large stratigraphic interval and consists of at least 4 subtrends. This negative excursion is well comparable to the “main CIE” as defined worldwide in several sections (Hesselbo et al., 2007). In our section, the lower part of the main CIE corresponds to the disappearance of the megalodontids, while the upper part of the main CIE records the LO of the microbenthic communities and, in particular of the Triasina hantkeni assemblage. Upward, concurrently to a gradual positive trend of the carbon curve (ca. +1‰) a gradual recovery of the benthic communities occurs, as witnessed by oligospecific assemblages of the problematic calcareous alga Thaumatoporella parvovesiculifera and, subsequently, by the appearance of benthic foraminifers such as Siphovalvulina sp. The obtained carbon signature is comparable to the negative CIE recorded during Late Triassic in many other sections of the world as a consequence of the high CO2 rates related to CAMP volcanism. Even if the correlation between extinction events and negative shift of the δ13C curve is a few constrained, this study adds new data on the effect of the acidification process on carbon productivity and benthic communities and allow to make a comparison with the present day acidification processes that are involving the carbonate calcifiers organisms.