Effects of Past Global Change on Life (1995)
Chapter: ABSTRACT
3
Global Change Leading to Biodiversity Crisis in a Greenhouse World: The Cenomanian-Turonian (Cretaceous) Mass Extinction
Earle G. Kauffman
University of Colorado
ABSTRACT
The Cenomanian-Turonian (C-T) mass extinction occurred during a peak global greenhouse interval, with eustatic sea-level elevated nearly 300 m above present stand; atmospheric CO2 at least four times present levels; and global warm, more equable climates reflecting low thermal gradients from pole to equator, and from the top to the bottom of world oceans. Despite development of an oceanic anoxic event (OAE II), marine diversity was at a Cretaceous high just prior to the extinction interval; many lineages had evolved narrow adaptive ranges over millions of years under greenhouse conditions. Marine biotas were thus extinction prone. Tropical reef ecosystems experienced widespread extinction beginning near the early-middle Cenomanian boundary; major lower Cretaceous lineages of reef-building rudistid bivalves were largely extinct by middle-late Cenomanian time. Within 520,000 yr of the C-T boundary, nontropical late Cenomanian biotas experienced 45-75% species extinction, depending on the group, through a series of discrete, ecologically graded, short-term events, or steps, beginning with subtropical and warm temperate stenotopic biotas, and terminating with more broadly adapted cool temperate biotas. These extinction events were closely linked or coeval with abrupt, large-scale perturbations in the ocean-climate system, as evidenced by major fluctuations in trace elements (including Ir), stable isotopes, and organic carbon values; the rate and magnitude of these chemical and thermal perturbations progressively exceeded the adaptive ranges of various components of the marine ecosystem as the effects of late Cenomanian environmental perturbations became compounded through time. Two possible catalysts for these abrupt environmental changes are (1) expansion of the oceanic oxygen minima zone(s) to intersect both the deep ocean floor, and deeper continental shelf and epicontinental sea habitats, initiating trace element advection and chemical stirring of the oceans; and (2) oceanic impacts of meteorites and/or comets as part of the Cenomanian impact shower. Evidence is presented for both hypotheses, and a multicausal explanation for C-T mass extinction is
