Ostracod response to delta dynamics: an example from a Holocene succession of the Po Delta

The detailed stratigraphic frame of the late Quaternary Po Delta deposits represents an ideal setting to test the response of microfossil assemblages to short-time phases of delta evolution, strictly constrained in time and space. Benthic foraminiferal studies within the Po Delta subsurface succession have been extensively performed in the last years, whereas less attention has been paid to ostracods. Holocene benthic foraminiferal and ostracod assemblages were determined in the 21m-thick shallow-marine deposits of core EM13. According to radiocarbon dates, the lower part of this fine-grained succession was formed during the last stages of transgression and the following high-stand phase, when a series of cuspate deltas developed in the study area. In contrast, the upper interval accumulated after the Ficarolo avulsion (mid XII Century AD), which determined the rapid outbuilding of the supply-dominated Po Delta. Here we test the ostracod ability to detect these evolutionary phases, possibly recorded as palaeoenvironmental changes. High concentration of valves and high species richness are recorded in the lowermost part of the succession, where ostracod assemblages include abundant Semicytherura spp. and Loxoconcha spp, indicative of a shallow-marine environment with low fluvial influence. The overlying sediments include scarce ostracods mainly represented by Pontocythere turbida, with lower frequencies of Palmoconcha turbida. Such low-diversity assemblage, dominated by opportunistic species, shows the transition to more stressed environmental conditions, in accordance with a prodelta environment. Species turnover is also recorded in the uppermost part of the succession, where Leptocythere ramosa becomes strongly dominant. Its tolerance to salinity variations and organic matter input indicates a very shallow and proximal prodelta with high fluvial influence. Scattered, mainly juvenile, valves of Cyprideis torosa are encountered through the entire sediment succession and suggest transport from coastal brackish-water environments due to the fluvial input. The superposition of these ostracod assemblages reflects small-scale palaeoenvironmental variations within a shallow prodelta environment, related to different phases of activity of the Po Delta. The comparison with foraminiferal assemblages reinforces the palaeoenvironmental interpretation and documents the combined analysis of benthic foraminifers and ostracods as a high-resolution proxy to reconstruct delta dynamics.