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The seagrass skeletal assemblage from modern to fossil and from tropical to temperate


Seagrasses are marine angiosperms that form extensive submarine meadows in the photic zone where carbonate producing biota dwell as epiphytic on the leaves or as infaunal forms, constituting prolific carbonate sediment factories. As seagrasses have a low preservation potential and records of exceptionally well preserved and plant material from marine settings are rare, these paleoenvironments are difficult to identify in the rock record. Consequently, sedimentological and paleontological proxies are the main indicators of the presence of seagrass-dominated ecosystems. In this work we investigate the skeletal assemblage of Modern (Maldivian and Central Mediterranean) and fossil (Eocene, Apula and Oman Carbonate Platforms and Oligocene, Malta Platform) seagrass examples to characterize the skeletal assemblage of modern and fossil seagrasses. Two main types of grains, calcareous algae and foraminifers, constitute around the 50% of the bioclastic sediment in both tropical Maldivian and temperate Mediterranean scenarios, However, in the tropical setting they are represented by the green algae (Halimeda), while in the Mediterranean by corallinacean red algae. In contrast, in the Eocene examples, the foraminifers are the most conspicuous group, and the green algae are also abundant. The opposite occurs in the Maltese Chattian, which is dominated by coralline algae (mean 42%) and the foraminifers are still abundant. We suggest the use of the term foralgal to identify the seagrass skeletal assemblage. To discriminate between red algae and green algae dominance, we propose to introduce the prefix GA (green algae) and RA (red algae). The investigated examples provide evidence that the GA-foralgal is typical of tropical, not excessively dense seagrass meadows, characterized by a well-illuminated substrate to support the development and calcification of the Halimeda thalus. Contrarily, the RA-foragal is typical of high density tropical to subtropical seagrass meadows which create

very dense to create oligophotic conditions in the seafloor or of temperate settings where Halimeda cannot calcify.