Abstract
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.
Highlights
Most planktonic foraminifera inhabit the upper 200 m of the water column, with the exceptions of some species living as deep as 1000 m (e.g. Hemleben et al, 1989)
The δ18Occ values are most depleted for G. ruber, somewhat more enriched in comparison to G. ruber for P. obliquiloculata and N. dutertrei, and most enriched in G. scitula (Table 1, Figs. 2 and 3)
G. ruber (−2.57 ± 0.04 ‰; SD: ±0.24 ‰), N. dutertrei and P. obliquiloculata record negative δ18Occ values between −1.53 ± 0.03 ‰ (standard deviation (SD): ±0.42 ‰) and −1.13 ± 0.04 ‰ (SD: ±0.24 ‰); more noticeable positive values are found for G. scitula with 1.47 ± 0.14 ‰ (SD: ±0.87 ‰; Fig. 3)
Summary
Most planktonic foraminifera inhabit the upper 200 m of the water column, with the exceptions of some species living as deep as 1000 m (e.g. Hemleben et al, 1989). Most planktonic foraminifera inhabit the upper 200 m of the water column, with the exceptions of some species living as deep as 1000 m The average depth habitat of individual species and the range of water depths at which they are found reflect their ecology (e.g. feeding behaviour), ontogeny and seasonal preferences. Stable oxygen isotope values (δ18O) and Mg / Ca ratios (Shackleton, 1974; Fairbanks et al, 1980; Ortiz et al, 1996; Elderfield and Ganssen, 2000) have been used to reconstruct upper water column conditions using species with a known depth range Previous studies revealed major Mg / Ca heterogeneity within foraminiferal shells J. Steinhardt et al.: Reconciling single-chamber Mg / Ca with whole-shell δ18O
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