Skeletal vs microbialite geobiological role in bioconstructions of confined marine environments

The dark and confined conditions of submarine caves allow the development of cryptic bioconstructions. They have been named “biostalactites” due to their distinctive growth from the ceiling and walls. Biostalactites resemble small scale “build-up” and have received increasing attention in recent years since they allow geobiological studies that could clarify the style of growth of the cryptic bioconstructions in the fossil record. The “lu Lampiùne” cave in Apulia represents an example of these natural laboratories where the presence of bioconstructions locally enhances biodiversity. Micromorphological observations, UV-epifluorescence and micro-Raman spectroscopy analyses were applied to investigate the internal structures and growth pattern of the oblique meter-long biostalactite. Two types of building engineers were detected: sessile skeletonized organisms and microbialites. These contribute to the formation of three boundstone frameworks: 1) core, represented by a skeletal-supported boundstone of large Protula tubes; 2) autochthonous micrite (microbialite)/skeletal boundstone on the downward-facing side; and 3) pure microbialite boundstone on the upward-facing side of the biostalactite. Complex taphonomic and early diagenetic processes suggest variability in seawater chemistry. Phases of carbonate deposition, indicated by skeletal/microbialite growth and early cement precipitation, alternate with phases of carbonate dissolution and the precipitation of ferromanganiferous coatings. The uniformity of the organisms, microstructures and biochemical signals, from the base to the growing tip of the biostalactite, suggests a uniform growth style from its inception to the present-day.