The sedimentary facies of Posidonia oceanica seagrass meadows from the central Mediterranean Sea

Abstract

Sedimentary facies of seven Posidonia oceanica meadows of western Mediterranean Sea were investigated. Five meadows are located in the Tyrrhenian coast, two are placed in the western coast of Sardinia and Corsica. These meadows develop on soft and hard substrates, often forming ``mattes'', in areas characterized by different oceanography, morphology, and terrigenous inputs produced by coastal erosion and fluvial runoff. A total of five sedimentary facies have been recognized ranging from pure terrigenous to bioclastic: terrigenous sand to gravelly sand, bioclastic sands, skeletal gravelly sands, mixed siliciclastic--carbonate sands, well to moderately sorted skeletal siliciclastic sands. All of the sedimentary facies associated with P. oceanica are in the sand grain size. The gravelly fraction is generally subordinated and variable, whereas the muddy fraction is generally low. The very low frequencies of the muddy fraction can be attributed to re-suspension processes and to the lack of carbonate mud production. The rate of epiphytic carbonate production obtained by two of the investigated meadows averages 400 g m−2 year−1. This value is in the range of temperate Mediterranean as well as of tropical and subtropical seagrasses. The epiphytic carbonate production plus the calcareous biota living on seagrass substrate contributes to form mixed siliciclastic--carbonate sediments of the nearshore environment of the Mediterranean. Lastly, the carbonate production associated with seagrass was derived by biota belonging to the heterozoan assemblage, where aphotic organisms are dominant, together with oligophotic biota such as coralline algae and symbiont-bearing foraminifera. Consequently, in the well-illuminated seagrass settings, the prevalent skeletal assemblages is represented by the heterozoan association while the components of the photozoan assemblages are absent or subordinate. This a key point for the paleoenvironmental reconstruction of the photic zone in the fossil record. Because the skeletal components of many seagrass dwellers greatly contribute to the carbonate sediment production of photic shallow-water environments, the seagrass meadows became substantial places of carbonate production and C (organic and inorganic) sequestration during the Cenozoic

The seagrass skeletal assemblage from modern to fossil and from tropical to temperate: Insight from Maldivian and Mediterranean examples

“…On the contrary, the calcifying coralline red algae are diffuse from high latitudes to the tropical belt (Wray, ; Riosmena‐Rodríguez, ). In the Mediterranean, sea floor coralline algae dominate the skeletal fraction in the seagrass environment (Mateu‐Vicens et al ., ; Gaglianone et al ., ), but also in the intertidal zone forming the ‘ trottoir algaire ’, a boundstone dominated by encrusting red algae (Pérès & Picard, ; Betzler et al ., ) and basinward of the lower limit of the P . oceanica meadows (Basso et al ., ), in the oligophotic zone ( sensu Pomar, ).…”

“…The western Mediterranean is characterized by temperate carbonates mixing with a siliciclastic input (Betzler et al ., ; Brandano & Civitelli, ), which along the Tyrrhenian coast can be very high, forming up to 90% of seagrass substrate sediment (Gaglianone et al ., ). For this reason, the selected sites are Ponza Island and Alghero Bay, which receive a reduced terrigenous contribution.…”

“…The seminal work by P er es & Picard (1964) presented the main compositional characteristics of carbonate benthic communities occurring on the Mediterranean sea floor. The shallow euphotic zone (infralittoral stage) is dominated by Posidonia meadows that develop on clastic, bioclastic and, less frequently, rocky substrates down to 40 m of water depth (Forn os & Ahr, 1997; Gaglianone et al, 2017). The oligophotic zone (circalittoral stage) is dominated by a red algal dominated carbonate production forming free-living branches (m€ aerl) and rhodoliths, while in the aphotic zone, at water depths below 90 m, mixed carbonate-terrigenous sand and silt occurs (Carannante et al, 1988;Betzler et al, 1997;Forn os & Ahr, 1997Basso, 1998;Brandano & Civitelli, 2007).…”

“…On the contrary, the calcifying coralline red algae are diffuse from high latitudes to the tropical belt (Wray, 1977;Riosmena-Rodr ıguez, 2017). In the Mediterranean, sea floor coralline algae dominate the skeletal fraction in the seagrass environment (Mateu-Vicens et al, 2012;Gaglianone et al, 2017), but also in the intertidal zone forming the 'trottoir algaire', a boundstone dominated by encrusting red algae (P er es & Picard, 1964;Betzler et al, 1997) and basinward of the lower limit of the P. oceanica meadows , in the oligophotic zone (sensu Pomar, 2001). Different from the two Modern cases, the Eocene examples show a clear dominance of foraminifera in the skeletal assemblage, with mean abundances above 50% in the Ypresian of Oman and higher (68%) in the Lutetian of Salento ( Fig.…”

“…Hence, GA-foralgal is exclusive in seas where the temperature regime allows Halimeda to calcify, in seagrass meadows characterized by a low-density shoot that ensures good light conditions on the substrate (Table 9). In contrast, the RA-foralgal forms in seagrass meadows where plant species have very long leaves (for example, P. oceanica), and create complex structures that increase the canopy and even produce oligophotic conditions on the substrate, in tropical to subtropical, and temperate seawater in modern and fossil examples (Tables 8 and 9; Modern, Mediterranean: Betzler et al, 1997;Forn os & Ahr, 1997;Mateu-Vicens et al, 2010Gaglianone et al, 2017; fossil examples; Oligocene, Australia: Riordan et al, 2012;Upper Miocene Balears, Mateu-Vicens et al, 2008;Oligocene, Malta: Brandano et al, 2009a,b, 2016Oligocene, Lessini Shelf: Bassi & Nebelsick, 2010;Nebelsick et al, 2013). To distinguish between tropical and temperate RA-foralgal assemblages, a detailed taxonomic analysis of skeletal components such as foraminifera and red algae is necessary (cf.…”