Résumé
We have collected 41 sediment samples from the depths of about 2000 m and 4000 m in the Nankai Trough area using the submersible Nautile. Study of these samples has enabled recognition of two main preservation states of planktonic and benthic foraminiferal tests: dissolution in all the samples and superimposed encrustation in some of them. The distributional pattern of planktonic foraminifera reveals a strong rate of dissolution of the tests. Among the benthic foraminifers, the aragonitic species Hoeglundina elegans shows, below 2000 m, a corrosion of test surfaces, removing parts of growth layers, and breakage of last chambers. On the basis of pore water chemistry the high rates of dissolution are related to unusual aragonite undersaturation in pore water at the sediment-water interface and to expulsion of pore water in open sea water. Furthermore, a peculiar etching of the whole test of 11. elegans and encrusted calcitic tests are observed, but only in the samples collected near fluid venting manifestations (clams, worms, and "shell concretions"), suggesting a possible relationship between diagenesis and fluids.