Biochronology and evolutionary implications of Late Neogene California margin planktonic foraminiferal events

The biochronology of eight events (first or last occurrences) among species of the planktonic foraminifer Neogloboquadrina plexus have been examined i n six Pliocene to Quaternary deep-sea sequences drilled during ODP Leg 167 off the coast of California. The sites form a meridional transect along the California margin from 31 degrees N to 41 degrees N, covering an area unde r the direct influence of the California Current and associated upwelling s ystem. Using age models based on combination of calcareous nannofossil and radiolarian data and magnetostratigraphic chron boundaries, ages were assig ned to the events, thus allowing investigations of their spatial and tempor al distribution. Three of the events have been identified as clearly diachr onous; two of these seem to represent examples of latitudinal immigrations. The evolutionary activity in the neogloboquadrinid clade on the California margin appears centered around 2.1 Ma, with most of the events occurring b etween 2.5 and 1 Ma. This interval was a time of major climate change. No e vents have occurred after 0.7 Ma supporting the notion that the extreme cli mate variations of the Late Pleistocene inhibited speciation. Assignment of ages to events defining boundaries of the California margin (CM) zones of Kennett and others has enabled us to compare this zonation with the standar d planktonic foraminiferal zonal schemes. Seven of the neogloboquadrinid ev ents occurred during the last 2.5 myr, suggesting that the time resolution of the CM zones (including the coiling dominance zones of Lagoe and Thompso n) during this interval is at least twice that of the standard planktonic f oraminiferal zonation. The remaining Pliocene is divided only into two CM z ones compared to six standard planktonic foraminiferal zones. One of the Pl eistocene neogloboquadrinids is formally described as a new species. (C) 20 00 Elsevier Science Ltd. All rights reserved.