Articles | Volume 12, issue 8
https://doi.org/10.5194/bg-12-2411-2015
https://doi.org/10.5194/bg-12-2411-2015
Research article
 | 
24 Apr 2015
Research article |  | 24 Apr 2015

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

J. Steinhardt, C. Cléroux, L. J. de Nooijer, G.-J. Brummer, R. Zahn, G. Ganssen, and G.-J. Reichart

Abstract. 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 and δ18O 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.

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Short summary
In this paper we present, for the first time, results from single-chamber Mg/Ca analyses combined with single-shell δ18O and δ13C for four planktonic foraminiferal species from a sediment trap in the Mozambique Channel. Eddy-induced hydrographic variability is reflected in test carbonate chemistry of these different species. A species-specific depth-resolved mass balance model confirms distinctive migration and calcification patterns for each species as a function of hydrography.
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