GEOCHEMICAL EVIDENCE DOES NOT SUPPORT PROLONGED MARINE ENVIRONMENTAL STRESS IN EARLIEST GRIESBACHIAN MARINE ENVIRONMENTS OF MID-LATITUDINAL EASTERN PANTHALASSA

It is generally accepted that marine ecological complexity varied between regions, environments, and geologic time following the Permo-Triassic mass extinction. Still lacking, though, is a consensus on the mechanisms of the ensuing taxonomic radiation and ecologic restructuring. Benthic marine conditions, such as anoxia, have been habitually hypothesized to explain occurrences of Early Triassic paleocommunities with low ecological complexity. However, emerging studies indicate an absence of environmental degradation even when ecological complexity remains low.

Herein we tested the hypothesis that functionally non-diverse paleocommunities were inhibited by adverse marine conditions in strata deposited during the early aftermath of the Permo-Triassic mass extinction. Bottommost dark siltstone of the Griesbachian-age Dinwoody Formation exposed in the Tendoy Range, Montana, USA were chosen for this study as paleoecology and geochemistry have not been examined here jointly. We found that these strata contain no vertical bioturbation and a fauna solely of Claraia. Carbon and nitrogen stable isotopes of sedimentary organic matter suggest prolonged intervals of similar ecologic structure and relatively oxic conditions in the water column. A shift in ecologic structure and environmental conditions is inferred from an abrupt change in N isotope data as well as two positive isotope excursions (PIE) midsection; the N trend persists to the end of the studied succession. The observed PIEs are unlike typical PIEs that are sustained over kyr timescales, in that they represent rapid departures in isotopic composition for a single bed, followed by a return to similar isotopic composition in overlying beds.

Our results suggest that low functional diversity in the earliest Early Triassic does not correspond with low oxygen marine conditions. These findings challenge current Early Triassic paradigms and raise questions about other environmental mechanisms and ecologic interactions.