Impact of sea-level variations on transitional settings: micropaleontological evidences from the Holocene succession of core 240 S6 (Po coastal plain)

Transitional environments are extremely sensitive to environmental changes, both of natural and anthropogenic origin. Nevertheless, abundant human activities and natural resources are hosted in back barrier to coastal settings. Among the major threatening factors that pose at risk the existence of such environments (exploitation of natural resources, heavy metal and organic pollution, etc.), the recent sea-level rise is one of the most pressing and already acting from low to high latitudes (Horton et al., 2018). Sediment successions represent a great archive where to study the response of transitional environments to the Holocene sea-level rise. The role of microfossils as proxies for paleoenvironmental reconstructions is widely recognize, also within back barrier to coastal settings, where benthic foraminifers and ostracod can provide abundant insights on many (paleo- )environmental parameters, such as salinity, organic matter and sediment inputs, oxygen conditions, and degree of confinement (e.g., Athersuch et al., 1989; Murray, 2006). Even though benthic foraminifers are abundant in brackish to marine conditions, ostracods are also present within freshwater environments; moreover, the two groups seem to be driven by distinct (paleo- )environmental drivers, which make them excellent proxies in transitional settings (Barbieri et al., 2018). Well-developed transitional sediment successions of Holocene age are recorded locally beneath the modern Po coastal plain and their microfossil content allowed to obtain detailed paleoenvironmental reconstructions, also in relationship with drainage network changes (Dinelli et al., 2013). The micropaleontological record (benthic foraminifers, ostracods) of the back barrier to coastal sediment succession of core 240 S6 is presented here. The main aim is getting insights on the evolution of transitional settings under the influence of the increasing sea level during the Holocene transgression, in relationship with local (autogenic) factors, such as subsidence or river inputs. Core 240 S6 was drilled in the Po coastal plain, ca. 5 km afar from the modern coastline (Figure 1), through a wire-line perforation system that guarantees a high percentage of recovery (>90%). Sedimentology and stratigraphy of the core are reported in Cibin et al. (2005). Here, we summarize 16 the qualitative benthic foraminifer and ostracod content of 44 samples analyzed from 24.50 to 10.10 m core depth, where back-barrier to coastal sediments are recorded. Sediments devoid of benthic foraminifers and ostracods or including scattered, reworked individuals are present in the remaining intervals. Samples were treated using the standard procedure for micropaleontological samples previously adopted in other reference work of the Po Plain area (e.g., Rossi and Vaiani, 2008). Residues were observed under a binocular microscope to determine the microfossil content, following the original taxonomic descriptions and reference works focused on the Mediterranean area. Within the lower portion of the analyzed succession, from 24.50 to 17.20 m core depth, benthic foraminifers and ostracod indicate the presence of thin, alternated swamp and lagoon sediment intervals. Specifically, rare transported benthic foraminifers associated to abundant valves of the limneticoligohaline Ilyocypris decipiens, indicative of a swamp paleoenvironment, are followed up to 22.60 m core depth by a brackish water microfauna dominated by the euryhaline ostracod Cyprideis torosa and the oligo-mesohaline Candona neglecta, along with rare brackish water benthic foraminifers, mostly Ammonia tepida, Aubignyna perlucida and Haynesina germanica. Such a microfossil assemblage, indicative of inner lagoon, is overlaid up to 21.80 m by ostracod assemblages with dominant C. torosa and P. albicans associated to oligo-mesohaline taxa such as Candona candida, Candona neglecta and Loxoconcha elliptica that are indicative of a swamp paleoenvironment. The ostracod assemblage from 21.80 to 21.40 m core depth indicates the development of inner to central lagoon conditions, as also supported by the abundance of the brackish water benthic foraminifers A. tepida, A. parkinsoniana and H. germanica in the upper portion of the interval. Swamp and lagoon microfossil assemblages are developed, again, in the 21.40-18.90 and 18.90-17.20 m core depth. Overall, three swamp-lagoon intervals are recorded and connected with the Holocene sea-level rise and the subsequent progradation during short phases of sea-level stillstand, in accordance with the step-like retrogradation of coastal systems recorded throughout the Po Plain sediment succession (Amorosi et al., 2017). Nevertheless, local factors such as elevation with respect to the sea level and morphology of the paleo-lagoonal basin could have influenced the development of swamp-lagoon alternations. A few km inland, alternations of coastal plain, swamp and lagoon deposits (Dinelli et al., 2013) represent the inland portion of the transitional system, but it seems difficult to establish a precise correlation between each interval due to the high facies variability of transitional settings. Upcore, between 17.20-13.00 m core depth, ostracod assemblages with abundant brackish-water species (C. torosa, Loxoconcha elliptica, and Leptocythere ex gr. castanea) mostly associated to the brackish to marine benthic foraminifers A. tepida, A. parkinsoniana, H. germanica and Cribroelphidium oceanense and other inner shelf species indicate the development of an outer lagoon paleoenvironment. The maximum transgression is recorded with the onset of a shoreface paleoenvironment, where both microfossil groups are characterized by diversified communities with 17 marine taxa (e.g., Ammonia beccarii, Cribroelphidium spp., Elphidium spp., Pontocythere turbida, Semicytherura spp.). During this last phase, the sea level acceleration drove the passage towards outer lagoon and shoreface conditions, whereas a few km inland a thick lagoon succession suggests that widespread flooding occurred (Dinelli et al., 2013). Upcore, beach ridge sands and coastal plain sediments with rare, transported marine microfossils mark the onset of regressive conditions. Our study case highlights the complex response of coastal systems to sea-level changes. Under natural conditions, the balance between eustatic conditions and river input points determined a fragile equilibrium of transitional (paleo-)environments. Nevertheless, under reduced sediment input or rapid increase of the sea-level, retreat of paralic and coastal settings induce strong (paleo-)environmental changes.