High resolution 3D images of sediment cores as powerful tool for exploring foraminiferal microhabitats

Abstract

Benthic foraminifera are marine protists largely used as bioindicators and proxies of paleo- environments. Epifaunal species are supposed to live at or above the sediment surface and are therefore used as proxies for bottom water conditions, while infaunal inhabit the sediment column, thus tracing porewater chemistry. Traditional analytical methods based on core slicing, however, have a low resolution that does not allow to precisely characterise the preferential microhabitat(s) of indicator species.

In this study we performed microtomographic analyses on an experimental sediment core, to observe the life-position of living foraminifera of two surface-dwelling species Ammonia confertitesta and Haynesina germanica, reported both as epifaunal or shallow infaunal. The images we obtained offered for the first time the possibility to observe each individual in 3D space with a numerical resolution of 13 μm/voxel.

The results revealed that the two species are never located above or at the sediment surface and have their preferential microhabitats in a sub-superficial sediment layer constrained in the 0–500 μm interval below the surface. Rapid decrease of abundances below this layer suggests that their microhabitat could be even more specific than previously thought.

μCT-scan of sediment cores is also a valuable tool to obtain high-resolution information about foraminiferal ecology. The described method is useful to assess the effective microhabitat of all foraminiferal species that are usually used as proxies for paleorecords, to ensure that the information we can obtain from them is attributable to bottom water or to porewater conditions at a specific sediment depth.