THE ROLE OF GEOCHRONOLOGY IN LINKING FLOOD BASALTS TO MASS EXTINCTIONS

Geochronology has indisputably played a major role in the circumstantial linkage between flood basalt volcanism and/or large igneous province (LIP) emplacement, and environmental crises including mass extinctions. Although the broad connection is clear, ambiguities remain in the details. Climate perturbations triggered by magmatic volatiles, chiefly C and S species, derived from either mantle sources or degassing of crustal reservoirs, are generally envisaged as the mechanism for this linkage. Significant progress has been made in refining the eruptive (most straightforwardly by ⁴⁰Ar/³⁹Ar methods) and intrusive (best dated by U/Pb methods, where applicable) histories of LIPs and both have contributed to improved understanding of the phenomenon. Nonetheless, individual massive eruptive events occur on much briefer timescales than can be resolved by any applicable dating method, and our chronology of the eruption derived atmospheric volatile input does not have the 2 – 3 ka temporal resolution of some climate proxy records nor the < 1 ka time-scale for response of the coupled ocean-atmosphere Earth system. Moreover, simple eruption chronology is not necessarily a faithful proxy for volatile emission chronology, given that passive degassing of magma and/or crustal reservoirs may be significantly decoupled from eruptions, in which case the chronology of intrusions may be most relevant to the timing and tempo of atmospheric volatile loading. Unfortunately, the intrusive history of an LIP can only be determined to the extent that all relevant intrusions can be sampled and/or accounted for. Thus regardless of how precisely we can date the extrusive or intrusive components of LIPs, detailed understanding of extinction mechanisms awaits improved constraints on when, by what mechanism, and how fast the associated volatile emissions occur. Fortunately, ongoing studies of LIP volatile cargoes and their evolution hold promise for clarifying the picture.