Reconciling single-chamber Mg / Ca with whole-shell δ18O in surface to deep-dwelling planktonic foraminifera from the Mozambique Channel

• Published in: Biogeosciences 2015-04, Vol.12 (8), p.2411-2429
• Main Authors: Steinhardt, J, Cléroux, C, de Nooijer, L J, G-J Brummer, Zahn, R, Ganssen, G, G-J Reichart
• Format: Article
• Language: English
• Subjects: Calcification; Calcium; Chemistry; Depth; Ecological distribution; Eddies; Foraminifera; Fossil Foraminifera; Geochemistry; Habitat preferences; Habitat selection; Habitats; Hydrography; Magnesium; Mass balance; Mixed layer; Ocean circulation; Ratios; Salinity; Sediments; Short term; Species; Stratification; Surface mixed layer; Surface water; Temperature; Thermocline; Vertical migration; Vertical migrations; Water circulation; Water column; Water depth
• ISSN: 1726-4170; 1726-4189
• DOI: 10.5194/bg-12-2411-2015

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 δ18O), which is less for species calcifying below the thermocline. Obtained single-chamber Mg / Ca ratios are combined with single-specimen δ18O and δ13C 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, δ13C andδ18O 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 δ18O 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 δ18O are in agreement with each other and in line with the changes in hydrography induced by eddies.