Foraminifera and sediments cored from the bottom of the Faanu Mudugau Blue Hole, Ari Atoll, Maldives: Diversity, taphonomy and environmental reconstruction of an inhabitable substrate

The Maldives Archipelago is located above the largest carbonate platforms on Earth in the Indian Ocean. The Faanu Mudugau Blue Hole, located in the Ari Atoll (Maldives), is the only one so far discovered in the Indian Ocean and has several peculiarities.

We provide for the first time a comprehensive study of the benthic foraminiferal assemblages recovered from a push core collected from the deepest part of the blue hole; a detailed taxonomy allowed the reconstruction of both abundance and diversity of foraminifera through the recovered core. Despite the acidic bottom conditions and an increase in hydrogen sulfide concentration, the benthic foraminifera of the Faanu Mudugau Blue Hole are well preserved and mostly unaltered. The systematic study gives evidence that benthic foraminifera, retrieved at the bottom of the blue hole at ∼80 m depth, are instead typical of the lagoon in a coral reef environment. The most abundant taxon is the genus Amphistegina, a symbiont-bearing larger benthic foraminifer that commonly lives on macroalgae on well-irradiated seafloors. The predominance of hyaline taxa based on the triplot and the evaluation of the Foram Index, which indicate a good oxygenation condition in the bottom waters, suggest that benthic foraminifera of the Faanu Mudugau Blue Hole have all been transported.

Sediment granulometry, the taphonomic characteristics of foraminiferal shells, and the physical-chemical parameters of the water column confirm the hypothesis that all retrieved benthic foraminifera are transported within the blue hole from the nearby coral reef lagoon, and neither any of the individuals nor any of the taxa identified are living on the blue hole seafloor, making this environment inhospitable for protists. Transport is primarily triggered by sea surface currents, mostly related to seasonal monsoons and tides, which are typical of the Maldivian Archipelago.