Pendant bioconstructions cemented by microbial carbonate in submerged marine caves (Holocene, SE Sicily)

Abstract

Unusual pendant bioconstructions occur within two submerged caves in the Plemmirio Marine Protected Area, south of Syracuse (SE Sicily). These cm- to dm-sized bioconstructions project downward from the cave roofs. The main framework builders are serpulids belonging to the genera Protula, Semivermilia and Josephella. Encrusting bryozoans, corals, hydrozoans and sponges are subordinately present, whereas bivalves and brachiopods are overall rare. These faunal associations are characteristic of the studied caves, but only in some places they form small bioconstructions that are cemented by microbial carbonates made up by autochthonous, peloidal to clotted peloidal and aphanitic micrite. The autochthonous micrite stabilizes the pendant bioconstructions, herein referred to as biostalactites. The microbial carbonate reveals high contents of bacterial lipid biomarkers. Among the most abundant compounds are mono-O-alkyl glycerol ethers (MAGEs) and branched fatty acids (10-Me-C16:0; iso- and anteiso-C15:0 and -C17:0), interpreted as biomarkers of sulfate-reducing bacteria. Other compounds preserved in the autochthonous carbonates include lipids derived from marine zoo- and phytoplankton (brassicasterol, dinosterol and monounsaturated short-chain fatty acids), unspecified marine bacteria (saturated and monounsaturated short-chain fatty acids) and land plants (long-chain fatty acids, long-chain alcohols, stigmasterol, sitosterol and campesterol). The observed lipid biomarker signatures are remarkably similar to those of post-glacial reefal microbialites, where microbialite formation in cavities of the coral framework was also mediated by sulfate-reducing bacteria. In the submerged caves of Plemmirio Marine Protected Area, serpulid colonies apparently occur preferentially where freshwater is seeping through crevices in the limestone. The consequent nutrification is believed to favor serpulid aggregation and the growth of other skeletal (i.e., bryozoans and corals) and soft-bodied organisms (i.e., sponges and ascidiae). The resultant accumulation of biomass and its taphonomy provide niches for sulfate-reducing bacteria, which induce carbonate precipitation cementing and stabilizing the biostalactites. The finding of pendant, cemented bioconstructions in the Mazzere and Granchi caves of the Plemmirio Marine Protected Area reveals that sulfate-reducing bacteria are involved in microbialite formation in various cryptic environments. The formation of such peloidal to clotted peloidal microcrystalline carbonates in cryptic ecosystems is a significant factor for the stabilization of different kinds of bioconstructions, spanning from small biostalactites to large reefs.