Morphological, geochemical, and ecological differences of the extant menardiform planktonic foraminifera Globorotalia menardii and Globorotalia cultrata

Taxonomic consistency is the prerequisite for any foraminiferal study. In particular, interpretation of planktonic foraminiferal geochemical data requires consistent selection of monospecific tests due to varying habitats of different species. In order to exemplify the impact of slightly varying morphological characteristics on geochemical signatures, we investigated three basic shape dimensions, test weights, stable isotope values, and Mg/Ca ratios of Globorotalia menardii and Globorotalia cultrata, which are often grouped or referred to as synonyms. Along with visual distinction, encrusted G. menardii is distinguished from shiny G. cultrata by its on average doubled test weight, when referring to the same maximum diameter. More elongated and flattened G. cultrata calcifies within the deeper mixed layer, which applies also for the initial calcification of G. menardii. Final encrustration of G. menardii after vertical migration is suggested to take place close to the bottom of the seasonal thermocline. Combined Mg/Ca ratios versus calcification–temperature calibration provides a reliable tool for monospecific thermal reconstructions for the seasonal thermocline (G. menardii) and the deeper mixed layer (G. cultrata).