Guillem Mateu-Vicens

Mixed carbonate-siliclastic sediments and benthic foraminiferal assemblages from Posidonia oceanica seagrass meadows of the central Tyrrhenian continental shelf (Latium, Italy)

Sedimentological and micropaleontological characterisation of bottom sediments from two Latium continental platform areas (central Tyrrhenian Sea) was carried out. The two studied areas are Santa Marinella in northern Latium and Ponza Island in southern Latium (Pontine Archipelago), characterised by the presence of Posidonia oceanica seagrass, but with different sediment inputs. In the Santa Marinella area, where seagrass ends at 15 m water depth, there are both siliciclastic sediments from fluvial run-off and carbonate lithoclasts originated by coastal erosion. In the Pontine Archipelago, in which P. oceanica meadows end at 35-40 mwd, terrigenous clasts originate only from coastal erosion and, consequently, carbonate production prevails. On the whole, grain size analysis shows a strong predominance of sandy fraction, whereas mud and gravel are subordinate. The very low frequencies of muddy fraction can be attributable to re-suspension processes and the lack of aragonitic components that produce mud-sized particles. The better sorting of Santa Marinella seagrass substrate with respect to Ponza Island is related to the origin of terrigenous sediments, which are mainly provided by rivers and so are previously sorted. Facies analysis was performed using component analyses, grain size percentage, sorting and carbonate content, complemented with the observations obtained from artificially-hardened sediments and, finally, tested with a Q-mode cluster analysis. Consequently, 4 sedimentary facies have been recognised. Facies F1, limited to the Santa Marinella area (7.5-13.5 mwd), is characterised by sand-sized wellsorted sediments, and dominated by the terrigenous fractions. Facies F2 represents the shallowest seagrass environment (5-7 mwd), and it is restricted to the Ponza area. In this environment the sedimentation is still dominated by terrigenous components, while the carbonate sediment contribution is due principally to red algae. Facies F3 has the widest water depth interval (8-34 mwd), and principally occurs in the Ponza site, while only two samples are from the Santa Marinella. The terrigenous fraction is conspicuous, however there is an increase of carbonate sediment produced mainly by the epiphytes (foraminifera, bryozoans) and, subordinately, by benthic organisms (foraminifera, molluscs). Facies F4, occurring in the Ponza site (17-29 mwd), is the most carbonate rich facies with a carbonate sediment produced by epiphytic (foraminifera, bryozoans, serpulids, red algae) as well as benthic (foraminifera, red algae, molluscs) biota. Q-mode cluster analysis performed on the most abundant benthic foraminiferal species led to the identification of 4 assemblages, dominated by epiphytic foraminifera. The A1 assemblage was found only in the Ponza site (5-12 mwd) and is dominated by Lobatula lobatula, Peneroplis pertusus and Miliolinella subrotunda. The A2 assemblage (10-34.5 mwd), characterised by L. lobatula and Rosalina bradyi, is mainly limited to Ponza Island, with only one sample from Santa Marinella. The B1 (7.5-9 mwd) and B2 (8-13.5 mwd) assemblages were found only in the Santa Marinella area and are dominated by L. lobatula, R. bradyi and Conorbella hexacamerata and by Asterigerinata mamilla and R. bradyi, respectively. The calculated diversity indices (α-Fisher and Shannon index) are very similar in both areas, whereas the dominance percentage is higher in the Santa Marinella site. The lack of the symbiont-bearing foraminifer P. pertusus in the Santa Marinella samples may be linked to less favourable environmental conditions. In this area, the prevalent siliciclastic sedimentation and the water turbidity, which decreases the light penetration, are the limiting factors also for the seagrass growth. The comparison between the foraminiferal assemblages of two areas may be used to estimate the health of present-day and fossil seagrass.

Rhodolith-rich lithofacies of the Porto Badisco Calcarenites(upper Chattian, Salento, southern Italy)

This study describes the rhodolith-rich lithofacies of the Porto Badisco Calcarenites, an upper Chattian rhodalgal/larger foraminiferaldominated unit exposed in the southern part of the Apulia Carbonate Platform (Salento Peninsula, Italy). The lensoid rhodolith-rich lithosome at the base of the studied section is made of rhodolith rudstone and floatstone which infill a channel-like depression inherited from the substrate. Changes in the texture of the rhodolith facies and in the inner structure of the rhodoliths reflect variations in submarine current velocity across the section of the channel. Inherited topography controlled both the locus and the mode of rhodolith accumulation. The rhodolith lithosome is bounded by a flat surface above which rhodoliths are notably absent and beds are tabular. This implies that rhodolith accumulation ended as soon as substrate topography was completely levelled off. The taxonomic composition of the red algal and larger foraminiferal assemblages suggests that the rhodolith-rich unit deposited in the oligophotic zone. The abundance of tropical genera among the coralline algae (Lithoporella and Sporolithon) and the high diversity of larger foraminiferal assemblages indicate that the deposition of the Porto Badisco Calcarenites took place in the warm waters of the tropical to subtropical zone

Evaluating the Role of Seagrass in Cenozoic CO2 Variations

Marine seagrass angiosperms play an important role in carbon sequestration, removing carbon dioxide from the atmosphere and binding it as organic matter. Carbon is stored in the plants themselves, but also in the sediments both in inorganic and organic forms. The inorganic component is represented by carbonates produced by calcareous organisms living as epiphytes on seagrass leaves and rhizomes. In this paper, we find that the rate of seagrass epiphyte production (leaves and rhizomes), averages 400 g m-2 yr-1, as result of seagrass sampling at seven localities along the Mediterranean coasts, and related laboratory analysis. Seagrasses have appeared in the Late Cretaceous, becoming a place of remarkable carbonate production and C sequestration during the whole Cenozoic era. Here, we explore the potential contribution of seagrass as C sink on the atmospheric CO2 decrease by measuring changes in seagrass extent, which is directly associated with variations in the global coastal length associated with plate tectonics. We claim that global seagrass distribution significantly affected the atmospheric composition, particularly at the Eocene-Oligocene boundary, when the CO2 concentration fell to 400 ppm, i.e. the approximate value of current atmospheric CO2.

Response: Commentary: Evaluating the Role of Seagrass in Cenozoic CO2 Variations

Dipartimento Scienze della Terra, Università Roma La Sapienza, Rome, Italy, 2 Istituto di Geologia Ambientale e Geoingegneria (CNR), Sez. Sapienza, Dipartimento Scienze della Terra, Università Roma La Sapienza, Rome, Italy, Càtedra Guillem Colom Casasnovas, Universitat de les Illes Balears, Palma de Mallorca, Spain, 4 Laboratorio de Zoología, Departament de Biologia, Universitat de les Illes Balears, Palma de Mallorca, Spain