Environmental controls of size distribution of modern planktonic foraminifera in the equatorial Indian ocean: A calibration study
Résumé
Palaeoceanographic studies often rely on microfossil species abundance changes, with little
consideration for species traits (e.g. size) that could be related to environmental changes. We
hypothesize that whole-assemblage and species-specific planktonic foraminifera (PF) test
size could be good predictors of environmental variables, and we test this using an Equatorial
Indian Ocean (EIO) core-top sample set (62 viable samples). We use an automated imaging
and sorting system (MiSo) to identify PF species, analyze morphology and quantify
fragmentation using machine learning techniques. Machine accuracy was confirmed by
comparisons with human classifiers. Data for 25 mean annual environmental parameters
were extracted from modern databases and, through Exploratory Factor Analysis and
regression models, we investigate the potential of PF size, at the assemblage and species
level, for reconstructing oceanographic parameters in the Indian Ocean. Within our tropical
dataset, we find that SST is not a significant driver of assemblage size, although thermoclinedwelling species Globorotalia inflata and Globorotalia truncatulinoides show a significant
relationship with temperature. Our analyses indicate that deep carbonate ion concentration
and core depth may be important factors influencing PF size, especially in species that are
large-sized or bear calcite crusts such as Globigerinoides conglobatus, Globorotalia
menardii, and Neogloboquadrina dutertrei. We propose that PF population size could
potentially be useful to reconstruct bottom water carbonate concentrations and sea surface
temperature. This approach will be tested on a new downcore record from the Arabian sea
(ODP Site 722) during key Pleistocene glacial-interglacial transitions, where existing sea
surface temperature and other paleo-reconstructions will allow meaningful comparisons.