Globocassidulina subglobosa (Brady); environmental control of species abundance and specimen test size

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Abstract
In the late Pliocene, during the onset of Northern Hemisphere glaciation, assemblages of benthic foraminifera underwent marked changes at DSDP-IPOD Site 548, located in the northeast Atlantic Ocean. In particular, initially dominant Globocassidulina subglobosa (Brady) was reduced to a minor assemblage component. Parallel to this trend, the average size of individuals in the G. subglobosa population was also reduced. To discover what caused the changes in G. subglobosa abundance and test size, species percent and size were compared to independent indicators of environmental change, including benthic foraminiferal oxygen and carbon isotope values, sediment grain-size distribution and carbonate content, and the abundance of planktonic foraminiferal species. These variables were used to quantify changes in bottom water characteristics, substrate and food supply. Reductions in species abundance and specimen size coincided with two different paleoenvironmental events. One event was a shift in isotopic values indicative of bottom water physicochemical changes at about 2.9 Ma; the other event was a permanent shift in planktonic assemblage composition, beginning about 2.6 Ma, which indicates an increased flux of organic matter to the sea bea. The populations of G. subglobosa have distinct specimen-size distributions. Where the average size is large, the size distribution is bimodal, with peaks at 200-300 .mu.m and 500-600 .mu.m. When the mean size is small, the size distribution is unimodal, with a peak between 200 and 300 .mu.m. The bimodal size distribution of G. subglobosa populations at Site 548 may result from a bi-phasic life cycle, in which the larger specimens have larger proloculi.