The role of benthic foraminiferal assemblages in the postglacial coastal evolution: a regional model from the Tyrrhenian basin (Mediterranean).

The landscape evolution of the late Quaternary Tyrrhenian coast during the Holocene was controlled by the rates of postglacial sea-level rise and fluvial input. In this study the microfaunal data coming from some significant Italian coastal sites were considered in order to elaborate an evolutionary model valid at regional level, by mean of benthic foraminiferal assemblages. The coastal depositional system is characterized by a lowstand phase, which developed during the eustatic sea-level fall between about 120 and 30-26 kyr BP. During the late lowstand phase, which is characterized by stillstand and slow eustatic sea-level rise, lagoon and marsh associations occurred. These paralic environments are very sensitive to coastline variations, and provide one of the most sensitive tools for studying sea level change. One of their main characteristics is the degree of marine influence that depends on climatic conditions and on the coastline morphology and evolution. Foraminiferal assemblages represent one of the most important microfaunistic component of the paralic environments for their high adaptability also in euryhaline conditions. Salinity seems to be the most significant factor controlling the foraminiferal distribution in these environment although other parameters like organic and oxygen content could have a great influence on the frequencies of the taxa and on the assemblage composition. Two assemblages were recognized distinguished on the base of different confined conditions. The first one (Ammonia parkinsoniana, Ammonia tepida, Haynesina depressula and Haynesina germanica assemblage) is characterized by very low -Fisher index (mainly < 5) and it can be related to very strong restricted conditions that can be found in environments as the inner lagoon. The second assemblage (Ammonia parkinsoniana, Ammonia tepida, Aubignyna perlucida and Elphidium poeyanum assemblage) shows a minor oligotipic degree (-Fisher index between 5 and 10) and indicates brackish-water conditions, but with a lower level of environmental stress (probably an outer lagoon with periodic connections with the sea. During the transgressive phase coastal-shelf sedimentation took place and the littoral area was interested by a barrier-lagoon system, consisting of small brackish lagoon delimited towards the sea by sandy bars. The paralic assemblages were quickly replaced by normal marine shallow waters associations testifying the sea level rise. This conditions are indicated by Ammonia spp. and Elphidium spp. assemblage typical of infralittoral zone with sandy bottoms (-Fisher index > 10). Locally this infralittoral environment was colonized by Posidonia oceanica meadows as suggested by high frequencies of epiphytic taxa (miliolids, -Fisher index > 10). During the subsequent highstand phase and sea-level stabilization the foraminiferal assemblages reflect the current microfaunal distribution and the establishment of modern water circulation pattern (Globocassidulina subglobosa, Melonis pompilioides and Reussella spinulosa assemblage with -Fisher index > 10). After the sea-level stabilization, locally the coastal progradation and flood events lead to freshwater marsh episodes followed by backshore environments. On the whole, five main characteristic benthic foraminiferal assemblages are recognized in the Holocene cores: two indicate lagoonal or, in general, confined environments, two are related to marine infralittoral zone (0-50 mwd) and, finally, one is characteristic of the upper circalittoral zone (50-100 mwd).