DEGASSING OF CAMP BASALTS AND END-TRIASSIC EXTINCTIONS: CAUSE AND EFFECT?

The end-Triassic extinctions are popularly referred to as one of the “big five” Phanerozoic extinction events. Although careful examination of the paleontological record indicates that this “event” was neither as sudden nor severe as generally supposed, significant environmental turnover occurred during the Late Triassic. Evidence for bolide impact is no more than circumstantial at this time. Other possible causes include sea-level change, which is well documented for the late Rhaetian, and degassing during eruptions of the Central Atlantic Magmatic Province (CAMP) basalts.

Although the actual dimensions of the CAMP are as yet speculative, eruptions of as much as 2 million cubic kilometers of basalt occurred over a period of 100,000s to millions of years, concurrent with latest Triassic biotic turnover. Basaltic magmas are commonly sulfur rich; values of over 1500 ppm are cited as typical and so total sulfur emissions to the atmosphere from CAMP eruptions were possibly 6 X 10E18 g. Sulfur emitted as SO2 during the CAMP eruptions may have been injected into the stratosphere, driven upward convectively by the heat of the eruptions. Considered as a series of brief eruptive pulses, individual episodes could have injected 10E14 to 10E16 g of elemental sulfur to the stratosphere. Global cooling resulting from stratospheric conversion of SO2 to H2SO4 aerosols and the consequent increase in atmospheric opacity likely resulted in prolonged episodes of cooling with individual eruptive episodes potentially capable of causing global cooling of 1 to 7 degrees C.

This hypothesis is consistent with the palynological record of prolonged cooling at the Triassic/Jurassic boundary. Data showing a decrease in stomatal indices at this boundary have been misinterpreted to suggest increasing atmospheric CO2 rather than increased sulfate. Other effects of sulfur degassing include surface water acidification on a wide regional, if not global, scale. In concert with sea-level change, volcanic sulfur emissions forced Late Triassic biotic turnover.