HITS AND MISSES: WHY SOME LARGE IMPACTS AND LIPS CAUSE MASS EXTINCTION AND OTHERS DON'T

In detail, neither bolide impacts nor major episodes of flood basalt effusion (LIPs—large igneous provinces) can be tied to most biotic mass extinctions, although the Cretaceous/Tertiary Boundary (KTB) event is a well-known exception. In some cases this may be the result of insufficient data for event ages or, for example, impact craters, if on the seafloor, remain unrecognized or have been subducted. Likewise, there are many significant impact craters and LIPs that, in terms of timing, do not appear to correspond to a biotic mass extinction event. Examples are the late Triassic Manicouagan impact group (214 Ma), the Eocene Chesapeake Bay and Popigai craters (35.7 Ma) and the huge LIP associated with construction of the Ontong-Java Plateau (118 Ma). A number of researchers have hypothesized that the severity of mass extinctions, and the extent of implied global change accompanying them, is in direct relation to impact or LIP size (crater size or volume of basalt extruded). This is not the critical factor, however, inasmuch as some of the largest known events had only minor influence on biotic extinction.

Our analysis of the extant record indicates that, although size of impact or LIP is a factor in potential mass extinction, the main factor that determines the scale of effects of such an event is the type of terrain hit by the bolide or through which the LIP is extruded. We argue that volatile (sulfur dioxide, carbon dioxide, methane) fluxes associated with impact or LIP events are the major consideration in extent of global change and related extinctions. Large impacts or LIPs on basaltic oceanic crust and/or in shield terrains are likely accompanied by relatively low volatile fluxes, whereas similarly sized impacts on thick carbonate-evaporite terrains (e.g., the KTB event) and/or LIPs intruded and extruded through thick black shale, coal or sulfate evaporite sequences (e.g., the Permian/Triassic Siberian Traps) would have much greater potential to wreak global environmental change and cause mass extinction.