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 internalwave processes.