Applying faunal indices for palaeoenvironmental reconstructions with benthic foraminifera: a case from a Holocene succession of the Po Delta, Italy.

In the last decades, the depositional history of the Po River coastal plain has been studied using different environmental proxies. In this work we first ever attempt to interpret the palaeoenvironmental evolution of a shallow marine succession from this area (core 205-S9, collected about 10 km south of the modern Po Delta) using several diversity indices, bivariate analysis and Murray’s Ternary Plot. We also tried to find out the potentials and limits of these tools in reconstructing palaeoenvironments. The analysed shallow marine succession was formed in response to the last phases of sea-level rise (ca. 8 kyr BP) and the Holocene regression resulted from the high-stand coastal progradation. In this part of the core, Murray ternary plot and Bivariate plot of Fisher’s alpha index and Shannon-Wiener index show two distinct clusters highlighting high percentage of calcareous porcelaneous forms in the lower part and high percentage of calcareous hyaline taxa in the middle and upper portions. Fischer’s α index and species richness values provide evidence for three intervals: the lower is highly diversified while in the middle the value gets decreases and in the upper the value increases but not as high as lower part. Evenness (E) and Dominance (D) indices show that the lower part is characterised by a homogeneous, highly diversified and evenly distributed fauna. In contrast, the middle part shows a strong dominance of few species with low faunal diversity with some variations in the upper part. The ratio of epifaunal and infaunal species defined oxygenated bottom water condition with low organic matter in the lower part, and high organic matter with relatively low oxygen concentration in the upper part. All these indices clearly discriminate between a lower and a middle-upper portion of the sediment succession. The integration with the distribution of selected species allows to attribute the lower interval to a shallow marine environment with low fluvial influence (transgressive barrier and offshore transition), whereas middle and upper parts record an increase of fluvial influence (prodelta). Therefore, the applied indices can be considered as good proxies for recognition of deltaic conditions and identification of changes in riverine influence within shallow marine deposits. Furthermore, most indices highlight variations in foraminiferal distribution within the prodelta. Combining foraminiferal distribution and indices values, these variations can be attributed to unsteady freshwater influx.