Dynamics of benthic marine communities across the Early-Middle Pleistocene boundary in the Mediterranean region (Valle di Manche, Southern Italy): Biotic and stratigraphic implications

Abstract

The Valle di Manche (VdM) section (Calabria, Southern Italy), a candidate to host the Middle Pleistocene Global Stratotype Section and Point, offers the opportunity to investigate the ostracod turnover along a continuous, tens m-thick (ca. 45 m) shelf succession of Early-Middle Pleistocene age (MISs 22- 18), and compare it against other palaeoenvironmental (i.e., molluscs) and palaeoclimate (Uvigerina peregrina δ 18O values) proxies. High-resolution (ca. 1 sample/meter) ostracod fauna quantitative data, coupled with gradient analyses (Dentrended Correspondence Analysis-DCA and nonmetric MultiDimensional Scaling-nMDS), document a strong relationship between changes in faunal composition and lithofacies vertical stacking patterns The comparison between the mollusk- and ostracod-derived ordination data demonstrates that the meio- and macro-faunal turnover are guided by a common complex gradient: bathymetry. The integrated ostracod-mollusc gradient analysis also provides trend in water depths along the section, highlighting to what extent such multivariate approach can improve the palaeoenvironmental and sequence stratigraphic interpretation of ancient shallow marine successions. When plotted stratigraphically, ordination major axis sample scores reveal two increasing-decreasing patterns in palaeowater depth that fit well with the T-R cycles previously identified. Palaeobathymetric estimates combined with the vertical distribution of key ostracod groups (i.e., epiphytic taxa on sandy substrates vs. deep-sea mud lover taxa) allow refining depositional trends, stratal stacking patters and position of previously not well resolved sequence stratigraphic surfaces within each T-R cycle (e.g., Transgressive Surface-TS). Indeed, two rapid increases in water depth values mark the TSs that separate progradational (LST) from retrogradational (TST) stratal stacking patterns of shelf deposits. The TSs, which underline fine-grained successions dominated by deep-sea mud lover taxa, are invariably constrained to the inception of interglacials MIS 21 and MIS 19, identified within the VdM section by δ 18O values. Within both the VdM T-R cycles, the deepest conditions (ca. 140 m of water depth) are invariably identified within the Neopycnodonte unit, slightly above of the lightest δ18O intervals. The overlying decreasing bathymetric trend, coupled with shifts in ostracod ecological groups, allows to identify in the bryozoans lithofacies the highstand+falling stage system tracts, also tracked by a progressively heavier δ18O record. More stable palaeobathymetric conditions (around 40-45 m of water depth) characterize the overlying silt-sand deposits (lithofacies C3-D) dominated by epiphytic species and showing the heaviest δ18O values.