MASS EXTINCTIONS: LESS SEVERE IN THE OCEAN THAN TRADITIONALLY PORTRAYED BUT STILL INTRIGUING

A mass extinction can be defined as an event in which a significant percentage of the members of multiple taxa died out on a global scale during a brief interval of time relative to that represented by the stage that records the event. Total rates of extinction calculated for stages and substages cannot reveal many relatively minor mass extinctions that occurred suddenly at the ends of formally recognized intervals. Numerous mass extinctions have been identified only within the past few decades. This is ironic because mass extinctions account for boundaries between many formally recognized stratigraphic intervals: early workers recognized sudden biotic turnovers but failed to identify many as representing biotic crises and subsequent recoveries.

Parallel global shifts of stable oxygen and carbon isotope ratios in seawater have characteristically accompanied marine mass extinctions. The former reveal that most of these crises have been associated with relatively sudden episodes of global climate change, and the latter generally reflect resultant changes along continental margins in both the volume of methane hydrates and the metabolic rates of benthic bacteria.

The traditional calculation of the magnitude of a marine mass extinction as the total number of extinctions for a stage or substage divided by the total number of taxa known for the entire interva errs in failing to exclude background extinctions: those that occurred before the terminal crisis. A new method allows for the estimation of the number of these background extinctions, the standing diversity at the time of a mass extinction, and the magnitude of the Signor-Lipps Effect. It turns out that most previous estimates have exaggerated the magnitudes of major marine mass extinctions at the genus level. A second new method eliminates a problem for the rarefaction method that results in overestimation of the magnitude of a mass extinction at the species level: clustering of extinctions within certain taxa. The new estimate for the terminal Permian event is of 82% for loss of marine species, which is much lower than the frequently quoted numbers of 90% and 95%.