Ostracod assemblages from Holocene subsurface deposits of modern Po Delta: a palaeoenvironmental proxy record

Benthic ostracod analyses performed on a 40 m long core (Core 1) enabled a detailed facies characterization and palaeoenvironmental reconstruction of the Late Pleistocene-Holocene deposits buried beneath the modern Po Delta. On the basis of relative abundance categories and hierarchical cluster analyses carried out on a presence-absence matrix composed of 23 samples and 22 ostracod species, four ostracod assemblages were identified. Assemblages 1 (Palmoconcha turbida), 2 (Pontocythere turbida and Semicytherura acuticostata) and 3 (Semicytherura incongruens and Leptocythere ramosa) correspond to three different shallow marine subenvironments, whereas assemblage 4 (Pseudocandona albicans) is indicative of a freshwater-oligohaline setting. Core 1 ostracod distribution patterns pointed out that several palaeoenvironmental oscillations occurred during the last glacial-interglacial period in the study area. Above an alluvial plain succession completely barren in microfossils and assigned to the last glacial interval, a transgressive-regressive sequence of Holocene age was deposited. Organic-rich paludal deposits containing assemblage 4 developed during the first phase of transgression. Following the sea-level rising trend, marine sedimentation reached the core site around 6,000 cal. yr BP, as documented by the deposition of washover and transgressive barrier sands containing few valves of transported ostracods. A highly diversified shallow marine ostracod fauna (assemblage 2) was found within the overlying inner shelf deposits formed during the last transgressive-early regressive interval. Upwards, an abrupt change of ostracod fauna occurred at the transition to prodelta deposits. The lower prodelta succession is characterized by an alternate record of two marine fluvial-influenced assemblages (assemblages 1 and 3), suggesting the development of an unstable prodelta environment subject to frequent river discharge oscillations during the first phases of deltaic progradation. On the contrary, a very scarce ostracod fauna is found within the upper prodelta succession probably due to the outstanding rates of sedimentation that affected the study area after the Ficarolo avulsion (last ca. 600 cal. yr BP).