Benthic foraminiferal evidence of sediment supply changes and fluvial drainage reorganization in Holocene deposits of the Po Delta, Italy

Abstract

Detailed benthic foraminiferal analyses performed on the Holocene subsurface deposits of modern Po Delta evidenced a complex palaeogeographic evolution. Hierarchical R- and Q-mode cluster analyses allowed to distinguish four assemblages indicative of different marine environments and sub-environments. Temporal and spatial distribution patterns of benthic foraminifers reflect changes in Po River discharge during delta evolution. The capability of foraminiferal assemblages to track nutrients and sediment supply oscillations permitted to recognize four main evolutionary phases (A–D), which took place after the maximum marine transgression (ca. 5500 cal yr BP). Prodelta and delta front deposits are accumulated in the western part of the study area between the first stages of progradation and the late Middle Ages (ca. 10th century AD); these are paralleled, in a more distal position, by open-marine sedimentation, replaced upward by prodelta sediments (phases A and B). Owing to the combined effect of a major avulsion (Rotta di Ficarolo) and the Adriatic circulation patterns, an organic-rich zone developed in the eastern part of the present Po Delta at the transition to the Modern Age (phase C), around 600 cal yr BP. For the fist time a fossil mud-belt, although its marginal fringe, along with its peculiar microfossil content ( Nonionella turgida assemblage), is recorded beneath a modern delta plain. During the recent past an outstanding rate of sedimentation affected the Po Delta, as clearly evidenced by the deposition of 14 meters fine-grained prodelta sediments between 600 and 380 cal yr BP. Prodelta deposits were finally replaced by delta front sands after the Porto Viro cut (ca. 350 cal yr BP), which marked the onset of Modern Delta (phase D).