Microbial Bioerosion of Erratic Sub-Fossil Nautilus Shells in a Karstic Cenote (Lifou, Loyalty Islands, New Caledonia)

Abstract

In a cenote formed in the limestone karst of Lifou (New Caledonia), more than 35 shells of Nautilus macromphalus were discovered in 35–40 m water depth. Seven shells were recovered and subsamples of two shells were used for a study on microbial bioerosion. Both shells were intensively bioeroded and SEM-analyses revealed a total of six ichnotaxa with a dominance of the cyanobacterial trace Scolecia filosa. Such a low diversity ichnocoenosis and the strong dominance of a single ichnotaxon indicate that the environment in the cenote was hostile for many euendolithic organisms. The unfavourable conditions were caused by (1) restricted water-mass exchange with the open ocean, limiting the chance of larvae being transported into the cenote; (2) intense fluctuations in fresh water influx in the karst system promoting traces of euryhaline euendoliths (Ichnoreticulina elegans, S. filosa, Scolecia serrata); and (3) very limited light availability, allowing the development of traces of organotrophs (S. serrata, Flagrichnus profundus) and the most effective among the photoautotrophs only (I. elegans, S. serrata, and ‘Conchocelis’-stages of bangiacean rhodophytes), indicating deep-euphotic to dysphotic conditions and leading to the observed low ichno-diversity. These results foster the knowledge on the environmental tolerance of microbioerosion trace makers, helping to draw conclusions regarding the characteristics of other extreme (palaeo)environments.