Metazoan/microbial biostalactites from present-day submarine caves in the Mediterranean Sea

Biostalactites formed by metazoan–microbialite associations from three submergedmarine caves in the Plemmirio Peninsula (south of Syracuse, IonianSea) are randomly distributed and show different sizes and morphologies, aswell as variations in surface roughness/smoothness. The biostalactites consist ofcrusts a few centimeters thick of small serpulids and other metazoans, associatedwith fine-grained carbonate; the larger ones often include a nucleus of serpulidtubes (Protula). The metazoans include mainly serpuloideans, sponges,bryozoans and foraminifers but microbial carbonates are also significant components.The composition of both the living communities and thanatocoenoseson the outer surfaces, as well as the composition and fabric of the internalframework, were analysed and used to reconstruct the history of the caves. Allof the identified sessile faunas mainly consist of cryptic and sciaphilic dwellersthat reflect cave conditions and their variations through time. The distributionpattern, composition and abundance of the present-day dwellers largely dependon the degree of roughness of the biostalactite surfaces and their positionswithin the caves. It has been suggested that the Protula specimens in the nucleirepresent pioneer populations that formed aggregates during the early cave colonizationphase, in response to relatively high food supply from seawaterinflow and intruding continental waters. By contrast, the outer metazoan–microbialite carbonates reflect more confined conditions in the caves caused byHolocene sea-level rises. Hypotheses are proposed for biostalactite growth, takinginto account information about the growth rates of some constituents, andevidence of dissolution effects. Similarities and differences between these biostalactitesand other Holocene deposits previously described from submarinecaves in the Mediterranean Sea and in tropical reefs are discussed.