Upper Tortonian skeletal assemblages of phosphatic hardgrounds record upwelling pulses and internal-wave processes (Latium–Abruzzi Platform, Italy)

Miocene hardgrounds of the circum-Mediterranean region are widely interpreted as records of upwelling of nutrient-rich water, commonly associated with phosphatization along platform margins. On the Latium–Abruzzi Platform, the Sirente Mt. and Tornimparte successions document two distinct phosphatic intervals: in situ authigenic hardgrounds and reworked phosphate-rich deposits, respectively. At Tornimparte, phosphorite beds are interpreted as the product of gravity flows triggered by internal-wave breaking along a density-stratified outer ramp. Erosion of a phosphatization zone generated lithoclasts that were transported basinward and redeposited chaotically, as supported by taphonomic evidence and CT-scan analyses of macrofossil-bearing blocks. The in situ deposits of Sirente Mt. represent fossil rhodolith beds, dominated by boxwork morphotypes, and composed chiefly of coralline algae of the genus Mesophyllum , with subordinate Lithothamnion , encrusting bryozoans, acervulinids, and serpulids. Their loose internal structure, multispecific encrustations, and fine sediment infill indicate low-energy conditions and relative substrate stability. Taxonomic composition, absence of shallow-water coral taxa, and dominance of thin hapalidialean thalli suggest development on a deep tropical oligophotic ramp (ca. 70–100 m). Growth-rate contrasts between slow-growing corallines and faster bryozoans, together with alternating algal- and bryozoan-dominated layers observed in rhodoliths, imply multiannual fluctuations in nutrient supply rather than simple seasonal forcing. Upwelling-driven nutrient enrichment likely enhanced phytoplankton productivity, turbidity, and pCO₂, periodically favoring suspension feeders over coralline algae in a context of intense space competition. These cyclical ecological shifts recorded in rhodoliths may reflect interannual to decadal variability in wind-driven upwelling intensity, consistent with broader Tortonian paleoceanographic perturbations. Collectively, sedimentological, compositional, and taphonomic evidence supports a dynamic outer-ramp system modulated by recurrent upwelling pulses and possible internal-wave processes.