Rhodolith Beds in a Shifting World: A Palaeontological Perspective

The occurrence of rhodolith beds in the stratigraphic record from the Cretaceous to the Pleistocene was analysed from published papers. Most data refer to low–mid latitude records of rhodolith beds described in the Tethyan–Paratethyan–Mediterranean domain. The first putative rhodolith beds are from Albian (uppermost Lower Cretaceous) deposits. These rhodolith beds are made up mostly of unattached loose branching corallines as well as of nodular structures. From the Coniacian (Upper Cretaceous) to the Langhian (Middle Miocene), abundance of rhodolith beds shows a generally increasing fluctuating trend with two significant expansions in the Priabonian (late Eocene) and during the Aquitanian–Langhian (Early-Middle Miocene). After the Langhian maximum, rhodolith beds sharply declined to a minimum in the Zanclean (Early Pliocene). During the Pleistocene, they recovered to values similar to those reached in the Langhian. The general increase in rhodolith beds up the Langhian maximum correlates well with global temperature and pCO₂ declines and with an ocean pH increase. The tectonic activity leading to important palaeogeographic changes in the Tethyan–Parathetyan–Mediterranean realm might account for the Serravallian–Zanclean downfall of rhodolith-dominated deposits. The Cretaceous–Pleistocene record of rhodolith beds shows that these ecosystems withstood successfully a highly changing world. The rapid acclimation of particular taxa to environmental changes and the variable reaction of taxa distributed at different water depths can be crucial to understand their success. In this regard, it would be interesting to analyse how different taxa in modern deep rhodolith beds respond to changing oceanic conditions.