ECOLOGIC RESTRUCTURING AFTER THE PERMO-TRIASSIC MASS EXTINCTION LINKED TO VARYING REDOX CONDITIONS: A GEOCHEMICAL AND PALEOECOLOGICAL CASE STUDY OF THE UPPER LOWER TO MIDDLE TRIASSIC OF THE WESTERN UNITED STATES

Aspects of Early Triassic paleocommunities have been interpreted as indicating that biotic recovery occurred very quickly (<500 Kyr), as well as much later during the Middle Triassic (5 Myr). Other studies indicate that Early Triassic biotic recovery varied with region and environment. Evidence shows episodic incursions of anoxic or euxinic deep water into shallow marine settings may have acted as an extinction mechanism. However, it is unclear how long incursions of anoxic deep water affected the continental shelves. Strata exposed in the western USA suggest that shallow marine environments were well-oxygenated during most of the Early Triassic, allowing for more rapid ecosystem recovery in proximal environments. In contrast, global geochemical proxies and sedimentary evidence from distal localities suggest that the deep ocean remained anoxic for the entirety of the Early Triassic and possibly into the Middle Triassic.

We report here some preliminary geochemical and paleoecological observations from a shallow to deep transect from the Lower and Middle Triassic of Nevada and California of the western US that highlight this paradox. Carbonate associated sulfate (CAS) abundance data are low overall, but averaged an order of magnitude lower in the distal versus proximal localities. Total sulfur (TS) abundances were uniformly low in all environments. Low [CAS] is consistent with a prolonged period of deep ocean anoxia, and the low TS abundances could have resulted from low availability of sulfate in an anoxic basin. Initial paleoecological observations from the same environmental transect also suggest a dichotomous response between the proximal and distal sections. Dark mudstone and siltstone deposited at the basin margin contain very low amounts of bioturbation and dense accumulations of benthic, low-oxygen-adapted invertebrates. Packstone and wackestone from shallower-water environments contain higher diversity faunas suggestive of biotic recovery.

Our findings hint that late Early Triassic to Middle Triassic faunas in nearshore versus deeper environments in eastern Panthalassa differed significantly. These initial results suggest that the relationship between localized and global redox changes and biotic recovery is more complex and nuanced than previously appreciated.