Characterization of Halimeda Bioherms of the Pre-Evaporitic Messinian of the Salento Peninsula (Southern Italy)

Calcareous green algae bioherms have recently gained significant attention in the scientific community, as they serve as valuable stratigraphic and paleoecological archives, comparable to the most studied coral reefs. Currently, the genus Halimeda, which is very abundant in tropical settings and common in temperate ones, contributes significantly to the production of sediments rich in CaCO₃. This is due to a rapid calcification process that occurs in its peculiar internal structure. Halimeda bioherms represent a key case because of their non-continuous distribution in the stratigraphic record and in the Modern oceans. In fact, the presence of the green alga Halimeda is widely documented in present-day reefs throughout the entire tropical belt, although extensive bioherms are limited to areas such as the Indonesian K-Bank, the Caribbean, and particularly along the Australian Great Barrier Reef (GBR), where they reach remarkable extensions exceeding 6000 km². Discontinuous distribution is also noted during the Miocene, with Halimeda bioherms reported only in three main localities within the Mediterranean basin, all dating to the pre-evaporitic Messinian. In this study, we present data obtained from detailed stratigraphic and sedimentological analyses of two small Halimeda bioherms (with a maximum thickness of 5 to 6 m), located in the southernmost portion of the Apulia Carbonate Platform (Salento Peninsula, Italy). Field observations, coupled with thin sections analysis show that despite these two sections being ascribed to different stratigraphic intervals, both record the same facies, with an alternation of shallowing-deepening cycles. While it is documented that different Halimeda species currently inhabiting carbonate systems respond variably to factors such as light, temperature, and salinity, the input of large quantities of nutrients via upwelling currents has recently become the most widely accepted hypothesis to support the occurrence of wide bioherms such as along the GBR. We infer that the occurrence of Salento Messinian Halimeda-rich facies could be related to upwelling of cool, nutrient-rich waters, possibly enhanced by the occurrence of internal waves (IWs). Such processes are well documented in the Salento Peninsula and in many areas of the Mediterranean throughout the Tortonian and likely still acting during the early Messinian.