INVARIANT SIZES IN DEEP-SEA BENTHIC FORAMINIFERA AND OSTRACODES ACROSS THE EOCENE-OLIGOCENE BOUNDARY IN THE SOUTHERN OCEAN

The sizes of organisms are tightly linked to the physical environment via metabolic processes. The metabolic demand of an organism is determined by its overall size while the mass-specific metabolic rate of an organism is strongly influenced by ambient temperature—organism size should increase with decreasing in ambient temperature. The Eocene-Oligocene transition represents a period of dramatic global climate cooling marked by the initial growth of continental ice on Antarctica. In this study, we test the hypothesis that the sizes of deep-sea meiofauna increased at the Eocene-Oligocene boundary (33.9 Ma), reflecting suppressed metabolic rates driven by cooling seawater temperatures. We studied the morphological sizes of benthic foraminiferal tests (Cibicoides mundulus) and adult ostracode valves (Krithe sp.) picked from 10 deep-sea sediment samples spanning 37 to 32 Ma cored on the southern Kerguelen Plateau (Ocean Drilling Program Site 744A). The mean foraminiferal test area and mean area of adult ostracode valves show no morphological change with the ~1‰ increase in the Southern Ocean benthic foraminiferal oxygen isotope record across the Eocene-Oligocene boundary, corresponding to cooling temperatures and increasing continental ice volume. The identification of the adult growth stage in foraminiferal tests is not possible in the fossil record because no maximum size is required for either sexual or asexual reproduction in benthic foraminifera. Thus, in addition to mean foraminiferal test area, we also analyzed morphological trends in the upper quantile of test area and found no variation with increasing oxygen isotope values. Benthic foraminifera and ostracodes from Site 744A do not increase in mean or maximum size with temporal global cooling across the Eocene-Oligocene boundary, inconsistent with Bergmann’s rule. Instead stasis best describes the pattern of within-species morphological evolution in these two meiofaunal groups. Thus, the sizes of deep-sea benthic foraminifera and ostracodes in the Southern Ocean are detached from the metabolic effects imposed by temperature during the first significant cooling event of the Cenozoic.