THE TRIASSIC-JURASSIC BOUNDARY EVENT: SEARCHING FOR THE MECHANISM

The mass extinction that marks the end of the Triassic is one of the five largest extinction events of the Phanerozoic, wiping out about 20% of the families in the marine realm. On land, this event claimed most non-dinosaurian archosaurs, most therapsids, and the last large amphibians. These extinctions took place against a backdrop of long-term climate change, rapid fluctuations of sea level, and catastrophes with both terrestrial and extraterrestrial causes.

Aridification of much of Pangaea during the Late Triassic is well documented and undoubtedly played a decisive role in Late Triassic evolutionary trends, but the abruptness of the extinction dictates against causality by this mechanism. A strong regressive-transgressive couplet in the stratigraphic record potentially explains the marine extinctions; sea-level fall that resulted in a loss of habitat was followed by rapid rise that may have led to marine anoxia. This explanation suffers from the inability to explain nearly synchronous losses in terrestrial biota. Bolide impact has been invoked previously to explain this catastrophe. However, dating of the best-candidate impact structure, the Manicouagan crater, and sedimentary evidence of the impact event place the impact well before the Triassic-Jurassic boundary.

The current view is that this event resulted from eruptions of the flood basalts of the Central Atlantic Magmatic Province (CAMP). Frequently cited is the sudden increase in atmospheric CO2 from outgassing during widespread eruptions, resulting in intense global warming. However, careful measurement of the carbon-isotope composition of pedogenic calcite from calcareous palaeosols of Late Triassic to Early Jurassic age fails to document any substantial change in palaeo-pCO2 across the Triassic-Jurassic boundary. Suggestions of large increases in Early Jurassic palaeo-pCO2 from CAMP eruptions appear to be based on overestimates of the CO2 contribution from this magmatism. Alternative mechanisms for extinction related to volcanism remain to be explored more fully, particularly cooling and acidification of the atmosphere and oceans from SO2 aerosols, and rapid sea-level change driven by plume-driven thermal doming and collapse.