Stable carbon and oxygen isotope ratios of Miocene to Recent foraminifera from the Rio Grande Rise, South Atlantic

All species are most abundant within subtropical-temperate watermasses throughout their history. The near stasis in distribution within the thermocline and the subtropical convergence suggests the major morphological changes in Globorotalia (Globoconella) may have occurred through habitat subdivision rather than by vicariant shifts into new watermasses.

At the Rio Grande Rise, in the South Atlantic, modern G. inflata is 0.66-0.84‰ more positive for delta18O than the most enriched coexisting Globigerinoides sacculifer and probably grows in the mid thermocline deeper than 325 m. All extinct globoconellid species have mean delta18O ratios 0.5-0.8‰ more positive than Globigerinoides trilobus and G. sacculifer and probably lived within the thermocline as well. Major events in skeletal evolution are poorly correlated with changes in delta18O in this group.