Extinction patterns in Paleogene benthic foraminiferal faunas; relationship to climate and sea level

Five cycles of first and last appearances are discernible in Paleogene benthic foraminiferal faunas from the Gulf Coast states of Texas, Louisiana, Mississippi, and Alabama. These cycles were apparently caused by episodic increases in extinction rates followed by longer periods of recovery. In most respects they resemble cyclic patterns observed by other workers in Gulf Coast Paleogene molluscan faunas. Comparison of these cyclic patterns with global and local climatic trends shows an association between the increased extinction rate at two (Middle and Late Eocene) cycle boundaries and periods of climatic deterioration. Likewise, periods of climatic stability, indicated by stable Gulf Coast sporomorph diversity, coincide with lower than average extinction rates among benthic foraminifers. Comparison of the cyclic patterns with Gulf Coast sea level curves shows an association of increased extinction rates at three (Paleocene, Early Eocene, Middle Eocene) cycle boundaries with significant, widespread, rapid sea level falls that produced type 1 unconformities. Local and/or slow sea level falls do not appear to have effected extinction rates. Calculations using a species survivorship curve suggest that the time necessary for an average faunal assemblage to reach equilibrium following an extinction event is longer than the duration of any Paleogene cycle. Thus, Gulf Coast Paleogene benthic foraminiferal faunas were rarely if ever at equilibrium.