A NEW RECORD OF THE TOARCIAN OCEAN ANOXIC EVENT FROM WESTERN PANTHALASSA AND THE UTILITY OF GEOCHEMICAL DATA FOR CONSTRAINING THE PROVENANCE OF EX SITU FOSSILS

The Early Jurassic Toarcian Ocean Anoxic Event (~183 Ma; T-OAE) was a major interval of global environmental perturbation during the Mesozoic Era, and it occurred when many modern vertebrate clades rapidly diversified. In western Panthalassa, the T-OAE was first recognized from the shallow marine siliciclastic strata of the Nishinakayama Formation (NF) at Sakuraguchidani in Shimonoseki, Japan. The NF is a key biotic archive of this time preserving abundant fossils of ammonoids, plants, and rarer vertebrates, including actinopterygians and reptiles. Among these key vertebrates are partial skeletons of a turtle and a crocodylomorph that were collected ex situ from the river Eragawa ~1 km from Sakuraguchidani, so their stratigraphic provenance is unknown, in a section where the stratigraphic position of the T-OAE is unclear.

We measured a 173 m section of the NF exposed in ten discrete outcrops near Eragawa and measured δ¹³C_org and TOC in 127 samples to establish the location of the negative carbon isotope excursion associated with the T-OAE globally and previously recognized at Sakuraguchidani. The lowest 14-meter interval of our section is characterized by a steady increase in δ¹³C_org from -26‰ to -24.8‰, which we correlate with the recovery interval of the T-OAE. Above that, δ¹³C_org ranges from 24.8‰ to -22.4‰ without any clear directional trends. We also used X-ray fluorescence (XRF) to measure elemental abundances in samples from each outcrop as well as matrix from the fossil vertebrate slabs (in addition to δ¹³C_org and TOC from the slabs). All geochemical data from the measured section and fossil matrix were then subjected to principal component and linear discriminant analyses to explore statistical support for the stratigraphic provenance of each fossil sample based on its geochemical profile. These analyses constrain the likely provenance of the turtle to a ~5-meter interval in the recovery phase of the T-OAE. The provenance of the crocodylomorph is more poorly constrained but likely above the T-OAE interval. These vertebrate fossils’ stratigraphic position mean they are critical for understanding changes in western Panthalassa marine ecosystems immediately after the T-OAE, and show the utility of geochemical data for clarifying the provenance of ex situ fossils lacking precise stratigraphic information.