STABLE ISOTOPIC SIGNALS AND PHOTOSYMBIOSIS IN LATE PALEOCENE PLANKTICFORAMINIFERA

Late Paleocene planktic foraminifera exhibit strong positive correlati ons between carbon isotopic values and test mass, but negative correla tions between oxygen isotopic values and test mass. Based on analogy w ith modern taxa, these trends are probably ecotypic and may or may not apply to an ontogenetic series. Among Acarinina and Morozovella speci es, the magnitude and direction of these trends resemble those of mode rn planktic foraminifera with dinoflagellate photosymbionts. This is c onsistent with current models of photosymbiosis and calcification in p lanktic foraminifera and suggests that Acarinina and Morozovella relie d heavily on photosymbionts for nutrition. Acarinina and Morozovella s pecies resemble modern, strongly photosymbiotic taxa in their associat ion with low and mid latitude nearsurface water masses. However, their test morphologies differ greatly from those of extant taxa that bear obligate photosymbionts. Earliest Paleocene taxa that exhibit a simila r paleohabitat association and similar size-related isotopic trends ar e characterized by still different test morphologies. These comparison s suggest that (1) throughout geologic time, strong reliance on photos ymbiont activity has been closely linked to habitat, but not to test m orphology; (2) photosymbiosis has been a common and convergently evolv ed strategy of planktic foraminifera over geologic time, and (3) moder n relationships between planktic foraminiferal test morphology and pho tosymbiont dependence are largely an artifact of geologically recent p hylogenetic relationships and shared ecologic strategies. Intersite co mparison suggests that the stable isotopic signals of narrowly constra ined size fractions of a late Paleocene Acarinina or Morozovella speci es can be used to reconstruct the magnitude and direction of relative variation in equilibrium stable isotopic values throughout its geograp hic and temporal range. This is supported by analogy with extant photo symbiotic taxa. However, since photosynthetic depletion of C-12 leaves C-13-enriched HCO3- for calcification, the carbon isotopic values of Acarinina and Morozovella tests may have been consistently greater tha n paleoseawater values. Failure to account for this effect could lead to overestimation of late Paleocene marine productivity.