Fish otoliths in superficial sediments of the Mediterranean Sea

Otoliths represent a significant biogenic carbonate component in marine sediments that may provide valuable information for paleoenvironmental and biogeographic reconstructions. In spite of their importance, relatively little is still known about the taxonomic composition, abundance and early taphonomic characteristics of recent otolith death-assemblages, which would add to their value to interpret situations in the geological record. Here we present data on the distribution of fish otoliths from bottom sediments collected in the central Mediterranean Sea ranging in depth from 51 to 3300 m. The preservation of otoliths ranges from fresh semi-translucent (white) specimens to dull-coloured (dark) ones, although whitish specimens are predominant across all the samples. This diversity in lustre and colour and at times texture reflects the degree of early taphonomic processes undergone by these aragonitic bodies post-mortem under submarine conditions, never being exposed to diagenetic processes on-land. In general, a correlation with depth is observed, with best preservation observed in otoliths sampled at depths < 500 m, while more degraded specimens occur deeper. In the upper depth range (< 500 m), a substantial number of benthic and benthopelagic taxa is counted with respect to mesopelagic taxa, which prevail from 500 down to 3300 m. The taxonomic composition and relative abundance of each taxon of otolith death-assemblages at various depths conform well to the distribution of related Mediterranean modern fish communities. The occurrence of pre-modern subfossil taxa in the death-assemblages is evidenced at some bathyal sites by the overwhelming presence of many highly-degraded (worn, chalky, opaque and patinated) otoliths and locally extinct species. This is the case of Protomyctophum arcticum, a mesopelagic myctophid absent in the modern Mediterranean Basin that represents an Atlantic Pleistocene ‘cold guest’ fish in the Pleistocene of this basin.