The Triassic–Jurassic Boundary Event at the Paleo-Equator: Evidence for Global Change from Carbonate Sedimentology and Chemostratigraphy, Ras Al Khaimah, UAE.

Abstract:

The Triassic­–Jurassic (T–J) transition was a time of significant environmental perturbation typically associated with CAMP volcanism and is considered to be one of the six major extinction events in Earth’s history. This event was associated with large perturbations of the global carbon cycle, as recorded in the isotopic composition of marine carbonate and bulk organic carbon. This has been elsewhere associated with widespread ocean acidification and a major disruption in marine carbonate production in neritic and pelagic environments. A carbon-isotope record from a paleo-equatorial carbonate platform exposed in Ras Al Khaimah, United Arab Emirates (UAE), shows continuous shallow-water carbonate sedimentation across the Triassic­-Jurassic boundary, in contrast with other boundary sections from higher latitudes, where carbonate sedimentation is typically discontinuous. δ¹³C shows a 4‰ negative excursion in both bulk organic carbon and carbonate carbon followed by a positive excursion. The boundary in Ras Al Khaimah, is placed at the base of a thick oolite unit overlying limestones with abundant and diverse Rhaetian reefal biota. Deposition of fossiliferous limestones in this location persisted into the uppermost Rhaetian and through the initial negative carbon-isotope excursion. While characteristic late Triassic biota with originally aragonitic mineralogy disappears at the boundary, aragonite persists as the inferred original mineralogy of the earliest Jurassic ooids and occasional skeletal bioclasts. The evidence presented herein suggest equatorial seawater of Tethys appears to have remained aragonite-supersaturated across the boundary and extinction event in contrast to higher latitude depositional sequences raising questions about how widespread ocean acidification was and in what conditions it may be favoured.