THE INFLUENCE OF TRIASSIC ATMOSPHERIC CO2 IN THE EVOLUTION AND TAPHOCENOSIS OF A PALEOVERTEBRATE ASSEMBLAGE: AN EXAMPLE FROM THE ISCHIGUALASTO FORMATION, NW ARGENTINA

CARINA COLOMBI; ISABEL MONTAÑEZ; PORTER, AMANDA; MC ELWAYN, J.

Congreso; IV International Paleontological Congress; 2014

The carbon balance (incoming and losses) between different carbon reservoirs is regulated by the carbon cycle and affects all life processes as well as the preservation or biotic material. In this study we present the results of a preliminary study of the Upper Triassic Ischigualasto Formation, Ischigualasto Basin, showing how carbon perturbations of the paleoatmosphere, evidenced by paleosol geochemistry and paleofloral stomatal frequency, influenced the vertebrate biota and taphocenocis. Carbonate nodules of Calcisols permit the estimation of the paleoatmospheric CO2 over a 300 m succession recording the period 231 to 229Ma. Initial CO2 concentrations are high (~1000 ppmv ±300 ppmv) falling abruptly within a half million years to half the original levels. This perturbation is remarkably coeval with the most fossiliferous level (paleovertebrate bone-beds) of the Ischigualasto Formation. Following the CO2 minima, concentrations once again increase to 900 ppmv ±300 ppmv and subsequently gradually decrease to near present-day levels at around 229Ma. This process is accompanied with a gradual decrease in the amount of preserved vertebrates, culminating in the boundary between Scaphonix-Herrerasaurus-Exaeretodon biozone and Exaeretodon biozone of the Ischigualasto Formation. For the interval 229 to 227 Ma, near the top of the Ischigualasto succession (300 to 550 m), paleoatmospheric CO2 concentrations are estimated using cuticle stomatal frequency. Despite a tentative calibration of stomatal frequency to atmospheric CO2, the data define a sharp increase in CO2 followed by a clear trend of gradually decreasing values. This gradual decrease is coeval with an important reduction in diversity characterized by the extinction of common vertebrate-groups as well as by a significant decrease in the abundance of preserved specimens. The latter includes the transition between the Exaeretodon and Jachaleria Ischigualasto biozones, where most of the paleovertebrates disappear, leaving only one representative, more related with the fauna of the overlying Los Colorados Formation than the Ischigualasto itself. Based on these results, we propose that, in general terms, (1) the inferred decrease in CO2 and consequent surface temperature is contemporaneous with the observed decrease in the amount of preserved vertebrates, and (2) large-scale perturbations in atmospheric CO2 were mechanistically linked to important changes in the biota and taphocenosis recorded by paleofaunistic changes and/or paleovertebrate bonebeds.