AFTER the end: reassembly of marine communities during the Early Triassic recovery

Informations

Funding country

France

Acronym

AFTER

URL

-

Start date

11/1/2013

End date

-

Budget

280,000 EUR

Fundings

Abstract

The Permian-Triassic boundary (~252 Myr) is marked by the largest mass extinction of the Phanerozoic. Recovery from this devastating event is usually assumed to be a long delayed process, spanning at least the entire Early Triassic (~5 myr). Indeed, the crisis aftermath is generally portrayed as a time of high ecological stress characterized by large-scale fluctuations of the global carbon cycle and harsh marine conditions, including a combination of ocean acidification, euxinia, and fluctuating productivity. This suggests tight links between these environmental variations and the restructuring of ecosystems, but the actual driver(s) remain elusive. Associated to the delayed recovery observed mainly among benthic organisms, other Early Triassic global-scale paradigms were consensually admitted by the scientific community. The best known among them are a global anoxia/euxinia event, the “reef gap”, a “chert gap”, a “coal gap” and the “Lilliput effect”. Contrasting with these paradigms, recent analyses on nekto-pelagic taxa such as ammonoids and conodonts document an explosive rediversification. Moreover, metazoan reefs, supposed to re-establish during the Middle Triassic, were newly reported from the Early Triassic of western USA, suggesting a fast reef rebuilding wherever permitted by environmental conditions. These observations show that the western USA basin is a regional counterexample of commonly accepted Early Triassic global-scale paradigms of a slow and delayed recovery. The geological record of that region shows that the timing and the processes underlying the recovery are still poorly understood. This requires new studies integrating paleontology, geo- and bio-chronology, geochemistry and sedimentology at different geographical and temporal scales. Our multidisciplinary project focuses on the geological and fossil records of the western USA basin, an area where our team has already evidenced some of the most intriguing PT recovery patterns. Paradoxically, these patterns contrast with several paradigms that were themselves first described in this basin. Our priority is to first test the extent of validity of several Early Triassic paradigms, with a special emphasis on the Smithian and Spathian substages (corresponding to the major onset of the rediversification). This time interval is marked by several global extinction and origination pulses coeval with dramatic shifts in the sedimentological, geochemical and palynological records. Clearly, the relationships between biodiversity fluctuations and shifts in abiotic parameters need to be deciphered further. We will thus investigate the regional environmental perturbations during this crucial time interval (occurrence, magnitude, continuity) and then to test for (1) the spatial and temporal impacts of these fluctuations on the regional diversity, and (2) the influence of potential taphonomic biases on observed recovery patterns. We will first conduct intensive fieldwork on the numerous outcrops of the western USA basin that record proximal to distal marine environments. These original field data, obtained with controlled and replicated sampling protocols, will then be combined with innovative sedimentological and geochemical studies to establish a robust and high-resolution biochronological scheme, to test for potential environmental changes concomitant of biodiversity fluctuations, and to check to what extent the global-scale Early Triassic paradigms apply to the vast region investigated here. We will contrast the spatio-temporal differences in recovery between nekto-pelagic and benthic faunas and we shall infer the bias-corrected diversity dynamics. This will lead to a new basin-scale scenario for the Early Triassic recovery, illustrating the reassembly of local to regional marine communities after a mass extinction event. This could have direct implications on scenarios for the modern biosphere that is currently entering its sixth major extinction crisis.