Ontogeny and habitat of modern menardiiform planktonic foraminifera

• Published in: The Journal of Foraminiferal Research
• Main Authors: Schweitzer, P. N., Lohmann, G. P.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cushman Foundation for Foraminiferal Research 1991
• Subjects: North Atlantic; Planktonic foraminifera
• Online Access: https://oceanrep.geomar.de/id/eprint/33445/; https://oceanrep.geomar.de/id/eprint/33445/1/Schweitzer.pdf; https://doi.org/10.2113/gsjfr.21.4.332

Ontogeny is an important source of variability in morphology and stable-isotopic composition in planktonic foraminifera. Through careful morphological analysis the populations of Globorotalia menardii and G. tumida were studied in detail at a single locality, the Ceara Rise. The foraminiferal test is dominated by two process of growth: the accretion of chambers and the formation of an enveloping calcite crust. These are recognized through measurements of shell size, shape, and density. The populations are divided into groups according to their stage of chamber and crust development. For both Globorotalia menardii and G. tumida the measured isotopic composition of whole specimens indicates that the organisms grow in the upper 50 m of the water. The crust is emplaced at depths of between 50 and 100 m, assuming that the shell is precipitated in isotopic equilibrium with seawater o18O. Assuming the smaller specimens lacking the crust represent the early stages of larger crusted specimens, one can calculate the oxygen- and carbon-asotopac compositions of the calcite added by the two processes. Crust composition in Globorotalia tumida appears to be in equilibrium with seawater o180. In G. menardii the crust is lighter in o13C than the equilibrium values. Similar measures of isotopic composition from the Sierra Leone Rise and the Bermuda Rise support these findings. At the Bermuda Rise, the isotopic data suggest growth in shallow water during the summer months, when a seasonal thermocline is well developed. Assuming these species require a shallow pycnocline to thrive, a decrease in deep-water upwelling during lacial time could deepen the pycnocline, which would explain their absence from North Atlantic sediments of that age.