Intrabasinal conditions promoting the development of a biogenic carbonate sedimentary facies associated with the seagrass Posidonia oceanica.

This paper aims to elucidate the spatial relationships between the distribution and growth dynamics of Posidonia oceanica, the sedimentary depositional facies (carbonate vs. siliciclastic), and the hydrodynamic features of the Gulf of Oristano (Western Sardinia, Mediterranean sea), a complex depositional system characterized by multiple sources of sediments and a marked hydrodynamic gradient. Three depositional environments were identified: (i) a poorly vegetated sector characterized by muddy sediments derived from the river input; (ii) a sector colonized by P. oceanica meadows characterized by biogenic carbonate sediments derived from the sediment production associated with the seagrass ecosystem and (iii) a sector colonized by P. oceanica meadows characterized by coarse siliciclastic sediments, possibly relict sediments. The sedimentary depositional environments are heavily influenced by the spatial distribution of the wind wave energy. Biogenic carbonate reefs associated with P. oceanica meadows develop in sheltered areas characterized by low amplitude of waves generated by the main wind regime. In the exposed sectors, characterized by a higher wave height, the meadows colonize relict siliciclastic sediments, without promoting carbonate particle deposition. Posidonia oceanica meadows in sheltered areas, associated with biogenic sedimentary facies, exhibit higher rhizome growth rate values (1.1 and 1.2 cm year1 vs. 0.7 cm year1) and a lower percentage of horizontal shoots (1.1% and 4.1% vs. 18%) in comparison to P. oceanica meadows in exposed areas, associated with siliciclastic sedimentary facies. The former tend to develop in a vertical direction, thus contrasting the sediment deposition rate, the latter tend to expand laterally due to the absence of sediment deposition. These results highlight that wave amplitude is the intrabasinal factor which influences the deposition of biogenic sediments and the growth dynamics of P. oceanica meadows.