Reconciling single-chamber Mg/Ca with whole-shell delta O-18 in surface to deep-dwelling planktonic foraminifera from the Mozambique Channel

Most planktonic foraminifera migrate vertically through the water column during life, meeting a range of depth-related conditions as they grow and calcify. For reconstructing past ocean conditions from geochemical signals recorded in their shells, it is therefore necessary to know vertical habitat preferences. Species with a shallow habitat and limited vertical migration will reflect conditions of the surface mixed layer and short-term and mesoscale (i.e. seasonal) perturbations therein. Species spanning a wider range of depth habitats, however, will contain a more heterogeneous, intra-specimen variability (e.g. Mg / Ca and delta O-18), which is less for species calcifying below the thermocline. Obtained single-chamber Mg / Ca ratios are combined with single-specimen delta O-18 and delta C-13 of the surface-water inhabitant Globigerinoides ruber, the thermocline-dwelling Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, and the deep dweller Globorotalia scitula from the Mozambique Channel. Species-specific Mg / Ca, delta O-13 and delta O-18 data combined with a depth-resolved mass balance model confirm distinctive migration and calcification patterns for each species as a function of hydrography. Whereas single-specimen delta O-18 rarely reflects changes in depth habitat related to hydrography (e.g. temperature), measured Mg / Ca of the last chambers can only be explained by active migration in response to changes in temperature stratification. Foraminiferal geochemistry and modelled depth habitats shows that the single-chamber Mg / Ca and single shell delta O-18 are in agreement with each other and in line with the changes in hydrography induced by eddies.