THE LAST 250 KYR BEFORE THE END-CRETACEOUS MASS EXTINCTION

The cause for the end-Cretaceous mass extinction (KTB) has long been attributed solely to the Chicxulub impact based on two major assumptions: (1) the Chicxulub impact trumped any earth-derived potential cause, and (2) Deccan volcanism played no significant role. Both of these assumptions have been challenged in recent years based on evidence that: (a) the Chicxulub impact predates the mass extinction, (b) the mass extinction occurred directly between massive Deccan eruptions and (c) U-Pb dating of Deccan lava flows revealed over 3200m of basalts accumulated during magnetochron C29r (736 ± 37 ky) with bulk eruptions over just 250 ky preceding the KTB. In 2015 these findings led the Berkeley impact group to revise the impact hypothesis by proposing that Chicxulub triggered the Deccan eruptions that led to the mass extinction, though caution that no evidence for this scenario exists to date.

Here we examine the environmental events during the 250 ky that preceded the mass extinction. Deccan volcanism occurred in three phases (C30n, C29r, C29n). We are concerned here with just the main phase-2 that erupted >1.1 million km³ basalts (80% of total by volume) depositing ~3000 m of stacked lava flows over a period of only 250 ky. Deccan phase-2 eruptions caused rapid though fluctuating global warming of 8°C on land and 4°C in the oceans sandwiched between hyperthermal warming at the base C29r and ending with a hyperthermal warming prior to the KTB.

On a global basis, planktic foraminiferal assemblages show increasingly high-stress environments in CF2-CF1 evident by dwarfed species, decreased diversity, blooms of the disaster opportunist Guembelitria cretacea, and near disappearance of large, robust species particularly in zone CF1 preceding the mass extinction. During the last 30-50 ky before the mass extinction, hyperthermal warming, decreased species diversity and abundance, shell dissolution and ocean acidification, low magnetic susceptibility and high mercury concentrations have all been linked to the most intense phase of Deccan volcanism. These data demonstrate that the KTB mass extinction was not an instantaneous event but occurred over a few thousand to several tens of thousands of years ending at the KTB. The Chicxulub impact can no longer be considered the sole cause, or even primary cause for this mass extinction event.