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DIMORPHISM, SHELL Mg/Ca RATIOS AND STABLE ISOTOPE CONTENT IN SPECIES OF BOLIVINA (BENTHIC FORAMINIFERA) IN THE GULF OF CALIFORNIA, MEXICO

Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA.

¹ Correspondence author. E-mail: rdouglas{at}usc.edu

The stable isotope and shell Mg/Ca composition in the tests of dimorphic generations (microspheric, megalospheric) and polymorphs of low oxygen species of Bolivina from the Gulf of California were measured to evaluate their potential use as environmental proxies. In B. argentea Cushman, megalospheric tests are always more abundant than microspheric tests though the dimorphic ratio is relatively constant with water depth. Distinct morphovariants develop in the species with decreasing benthic oxygen concentrations. The species is in isotopic equilibrium except for lowest oxygen morphovariants, which have slightly lighter ¹³C values than the typical form. The shell Mg/Ca ratio of live specimens is higher than in dead specimens, and the species shows no trend with temperature. In B. subadvena and B. interjuncta bicostata, megalospheric tests predominate in higher oxygen environments, and the dimorphic ratio varies with water depth. Megalospheric generations of B. subadvena, which are concentrated in the organic-rich benthic boundary above the sediment-water interface, appear to grow rapidly to a smaller size and tend to have higher shell Mg/Ca and lighter ¹³C content than do the tests of the microspheric generation, although the differences are generally small. Microspheric forms are most numerous at the sediment-water interface and produce larger, slower growing adults that are more nearly in equilibrium with bottom water chemistry. B. argentea and B. subadvena are useful proxies in reconstructing ocean ventilation and paleoproductivity, but the species life strategies need to be considered in interpreting their shell geochemistry.