Greenland ice cores have revealed large, rapidly switching, millennial-scale climate oscillations (Dansgaard-Oeschger [D-O] cycles) during the last glacial episode. These climate changes were of sufficient magnitude and rapidity to potentially cause major changes in the global biosphere. Roy et al. (1996) predicted biotic responses to these rapid climate changes. They recognized that such rapid, frequent climate changes must have repeatedly disrupted communities, exerting major control on species ecology and evolution. Furthermore, they suggested that absence of late Quaternary major extinctions and speciations indicates adaptation of the biosphere to this unsteady climate condition. Until now, however, these hypotheses have not been adequately tested. Our data is the first high-resolution (~50 yr increment) record of large-scale biotic change associated with these climate switches.

Paleoceanographic evidence indicates rapid late Quaternary oscillations in the strength and distribution of the California margin oxygen-minimum zone (OMZ) including its intersection with Borderland basins. Broad-scale biotic changes on the margin occurred in response to fundamental changes in ocean circulation affecting ventilation of Pacific Upper Intermediate Water. These were driven by changes in thermohaline circulation associated with global climate change (D-O cycles). Effects of the biotic changes are amplified in the Borderland province, especially Santa Barbara Basin. Major switches in benthic foraminiferal assemblages are representative of the broader benthic biota. Specific benthic foraminiferal assemblages were associated with extremely weekened OMZ during cool intervals and greatly expanded OMZ during warm intervals.

This record has significant implications relative to the evolutionary adaptation of late Quaternary marine benthic faunas. Switches related to D-O cycles in upper PIW probably extended over broad areas of the Pacific Ocean. These switches occurred within decades and represent episodes of extreme environmental variability that had the potential to cause extinctions and speciations in the benthic biota. Yet no extinctions or speciations have been observed at any of the abrupt stadial-interstadial boundaries in Site 893A. Instead, our record shows a long-term complex equilibrium in which individual benthic foraminiferal species consistently regained dominance during the species' optimal environmental state. This finding implies that the benthic foraminiferal community was fluid and species were well adapted to exploit rapid switches between specific environmental conditions by rapid migration from refugia. Moreover, the repetitive sequencing of dysoxic taxa during interstadials indicates these species were well adapted to and able to exploit the variable OMZ habitat. Northeast Pacific upper-continental-margin benthic ecosystems appear to be both resilient and robust in response to rapid and often extreme environmental changes.

Our observation of late Quaternary benthic foraminiferal evolutionary species stability in the face of large-scale climate change was predicted by Roy et al. (1996). As species shifted individually with changing environments, communities were formed and broken up within ~1000 yr. Such instability of communities prevents long-term isolation and genetic differentiation.

The global extent of late Pleistocene millennial-scale climate oscillations and the paleoecological record from Santa Barbara Basin suggest that broad segments of the global biosphere must be well adapted to these extremely rapid climate changes (Roy et al., 1996). Although these climate changes must have influenced the terrestrial as well as the marine biosphere, most terrestrial chronologies are inadequate to test this hypothesis. Although benthic foraminifera, with short generations (months to years), have become adapted to these switches, organisms with longer generations (e.g., trees) should have been especially vulnerable.

The biota are almost certainly linked to the evolution of this highly fluctuating system, perhaps when the 100 kyr climate cycle developed at ~700 Ka. Because the biota has become highly specialized to such changes in ventilation any major change in the state of ventilation could lead to extinctions. Changes in oceanic thermohaline circulation can fundamentally affect the evolutionary development of benthic biota associated with low oxygen environments. Thus, modification of the general state of thermohaline circulation has the potential to create drastic changes in marine biota as periodically seen during the Phanerozoic.

Roy, K., Valentine, J. W., Jablonski, D., and Kidwell, S. M., 1996, Scales of climatic variability and time averaging in Pleistocene biotas: Implications for ecology and evolution: Trends in Ecology and Evolution, v. 11, p. 458-463.