ASSESSING ABUNDANCE CHANGES IN SKELETAL MARINE ORGANISMS ACROSS THE END-TRIASSIC MASS EXTINCTION THROUGH COMPOSITIONAL ANALYSIS OF LIMESTONE

Mass extinction events significantly alter marine ecosystems along many dimensions, including taxonomic diversity, ecosystem structure, body mass distributions, and population abundances. Among these properties, changes in abundance, which are critical to understanding energy flow in ecosystems, have remained the most difficult to assess. Here we constrain the effect of end-Triassic mass extinction on the abundances of shelled organisms in marine ecosystems as well as the trajectory of subsequent recovery through the compositional analysis of carbonate rocks. We analyzed 72 samples from a 320 m-thick stratigraphic section located in the southern Apennines. The section spans the Upper Triassic (Rhaetian) to Lower Jurassic (Hettangian-Pliensbachian) and was deposited in an intertidal to subtidal environment throughout the study interval. The overall skeletal abundance decreased from 11% in the Rhaetian to 4 % in the Hettangian-early Sinemurian before increasing to 13% in the late Sinemurian-Pliensbachian strata. These results are consistent with data from ichnology and biodiversity across the end-Triassic extinction in indicating a marked decrease in animal diversity and activity. The mean local animal skeletal content (1%) remained relatively unchanged across the Rhaetian-Hettangian boundary and increased significantly, to 6% in the Pliensbachian strata. The algal skeletal content decreases from 10% to 5% across the extinction horizon and maintains a similar percentage in the late-Sinemurian-early Pliensbachian strata. In contrast, the abundance of foraminifera declined from 2% to 0% across the extinction boundary before increasing to 3% in the Pliensbachian strata. In this environment, animal skeletal content was low but did not change across the extinction horizon. However, both algae and foraminifera exhibit substantial declines, consistent with an effect of mass extinction on the absolute abundances of these taxa in the platform-top environment. The local dominance of skeletal algae in Hettangian strata versus their near-total absence from Lower Triassic strata suggests the existence of important, currently unknown, differences in the type or magnitude of environmental stress between the end-Permian and end-Triassic mass extinctions.