STATIONARY OR MOBILE? RECENT ADVANCES IN STABLE ISOTOPIC DEPTH-ECOLOGY AND PALEOCEANOGRAPHIC APPLICATION OF MID-CRETACEOUS PLANKTONIC FORAMINIFERA

Stable oxygen and carbon analysis of fossil planktonic foraminifera has been and remains a key tool in surveying oceanographic properties in the geologic past. In particular, parallel analysis of surface- and deep-dwelling taxa from the same samples is used to infer vertical water-column structures through time. Significant assumptions, often implicit, in such studies are that the depth-related ecological distribution of extinct planktonic foraminifera is adequately known (via comparison with modern morphological counterparts, paleobiogeographic distributions, and/or isotopic ecological ranking), and that each taxonomic group occupied the same ecological position through time and serves as a "stationary" recorder of paleoenvironmental conditions at a particular depth.

These views have recently been challenged by our multispecies isotopic studies for the uppermost Albian-middle Cenomanian of the western North Atlantic (Ando et al., 2009, 2010). We found that the relationships between the test morphology and paleoecology of ancient planktonic foraminifera do not parallel those seen in extant morphologically comparative taxa, implying that morphology is a poor predictor of the planktonic paleoecology. Further, across the Albian/Cenomanian boundary, some taxa underwent dramatic depth-habitat reorganization (i.e., rapid habitat changes from thermocline to mixed-layer and vice versa, without major speciation). Such dynamic changes in the ecological preferences within planktonic foraminiferal assemblages should be a primary consideration in paleoceanographic interpretation of stable isotope data. The key to documenting such evolutionary paleoecological dynamics is to maximally utilize the coeval fine, coccolith-rich fraction data. Noteworthy is the fact that our data (unpublished) support highly flexible ecological behavior of a simple Hedbergella species, and its wide purely paleoceanographic application in previous studies should be viewed with great caution.