Integrating benthic foraminifers and ostracods as tracers of deltaic dynamics: the case of the Po Delta

The sedimentary record of the Po Plain succession has great potential for high-resolution stratigraphic studies, due to high subsidence rates affecting the area, high sediment supply and the absence of significant tectonic deformation. Sedimentological, geochemical and micropaleontological data were used in the last years to reconstruct the Holocene stratigraphic architecture and its relation to sea-level oscillations and coastal evolution (Amorosi et al., 2008, 2019). Additionally, abundant geomorphological and historical data represent useful constraints to enhance Holocene paleoenvironmental reconstructions. A thick (up to 20 m) fine-grained, shallow-marine succession is present in the subsurface of the modern Po delta and coastal plain. This succession was formed in response to the early Holocene transgressive phase and the following mid-late Holocene normal regression, associated with widespread shoreline progradation. However, owing to lithological homogeneity, deltaic evolution is difficult to unravel on the basis of physical sedimentological characteristics alone. Benthic foraminifers and ostracods have been successfully adopted as paleoenvironmental proxies within the subsurface succession of the Po coastal plain during the last years (Campo et al., 2017), but a quantitative integrated analysis of both faunal groups is still lacking. The aim of this work was to test and compare the efficiency of benthic foraminifers and ostracods as proxies in marine, river-influenced sediment successions from three cores of the modern Po delta plain. The availability of a North Adriatic modern analogue database (Barbieri et al., 2019) and the application of statistical analyses helped find the most similar modern sites to each fossil assemblage in terms of faunal composition and therefore to quantitatively determine specific paleoenvironmental conditions. The abundant chronological constraints on the cored successions were used to link the micropaleontological record to distinct phases of deltaic evolution and to assess the extent to which benthic foraminifers and ostracods may successfully track sediment dynamics in deltaic settings. Here, we present the first results from microfossils of all cores, with the exception of ostracods from the northernmost core (187-S4), along with preliminary statistical analysis and interpretation. The three analyzed cores were collected in the southern part of the modern Po delta plain, following approximately a north-south transect, from Po di Pila to Po di Volano distributary channels. All cores include thick, fine-grained (silt-clay) sediment successions with abundant marine microfossils. In the lowermost portion of the marine study successions, benthic foraminifers are highly diversified, with relatively common Aubignyna perlucida, Porosononion granosum, Miliolids, Textularia spp., Buccella granulata, Rosalina bradyi and a variety of Cribroelphidium and Elphidium species. Statistical elaborations based on similarity measures were applied to one of the three cores, revealing that fossil assemblages from the lower interval are significantly similar to modern, North Adriatic inner-shelf assemblages. Conversely, the overlying assemblages are similar to those nowadays distributed at shallower depths updrift of the Po River mouth, where organic matter inputs from the Eastern Alpine rivers are negligible. Upwards, all cored intervals show a gradual increase in Ammonia tepida and Ammonia parkinsoniana, associated with taxa tolerant to organic matter, such as A. perlucida, P. granosum and C. poeyanum. Similar modern assemblages are characteristic of prodelta environments downdrift of the Po Delta, where sediment and organic matter inputs are moderate. Benthic foraminifers record the shift from an inner-shelf environment (23-40 m water depth) towards shallower (8-22 m water depth) prodelta conditions with increasing organic matter supply that can be linked to deltaic progradation. Unlike benthic foraminifers, ostracods from the lowermost portion of the study successions do not show significant changes. Assemblages include a high number of taxa, mostly represented by Semicytherura spp., Sagmatocythere napoliana, Sagmatocythere versicolor, Loxoconcha gr. rhomboidea and Callistocythere spp. Statistical analyses indicate that similar assemblages are nowadays distributed along the North Adriatic inner shelf. The upper portion of the study successions is invariably characterized by the sharp increase in the benthic foraminiferal species A. tepida and A. parkinsoniana, which dominate the assemblages (frequencies >60%), showing strong similarities with proximal prodelta assemblages subject to moderate or strong organic matter inputs. Within the same interval, statistical elaborations reveal that scattered samples with more abundant A. perlucida and P. granosum are related to a decrease in riverine organic matter inputs, likely resulting from autogenic shifts of river mouths. Ostracod assemblages show marked lateral changes between cores: Palmoconcha turbida and Pontocythere turbida are very abundant in the central (EM13) and southernmost (205-S9) cores, forming assemblages similar to those nowadays found in shallow prodelta areas subject to relatively low river inputs and predominant longshore transport. Conversely, ostracod assemblages are markedly distinct in the overlying strata: a near-shore, sandy-related assemblage characterized by Loxoconcha rubritincta, Cistacythereis emaciata, Xestoleberis spp. and Cytheretta spp. was recovered from core 205-S9; in core EM13, the dominance of the opportunistic Leptocythere ramosa indicates proximal prodelta conditions under high riverine inputs. At both sites, ostracods record the initial development of Middle Ages wave-influenced deltas. Later, different assemblages suggest distinct delta dynamics under proximal prodelta conditions: the southern assemblage is related to the progradation of the wave-influenced Po di Volano delta, whereas ostracods from the northern site record the fast progradation of the river-dominated Po di Goro mouth. Preliminary results from the study cores and their quantitative (statistical) comparison with the modern North Adriatic database suggest that benthic foraminifers and ostracods can be used effectively to reconstruct delta dynamics within a chronologically constrained framework. Benthic foraminifera proved to be extremely sensitive to organic matter inputs, tracking clear progradational and retrogradational trends in deltaic deposits. On the other hand, within proximal prodelta clays, where benthic foraminifers are monotonous, ostracods were decisive in highlighting predominant sediment dynamics that governed delta outbuilding.