Foraminiferal record and environmental changes during the deposition of the Early-Middle Pleistocene sapropels in southern Italy

A foraminiferal investigation documents the palacoenvironmental changes across the sapropel units associated with insolation-cycles 90 and 86 in the Early-Middle Pleistocene IM/Fosso 5 Agosto section (Basilicata, southern Italy). Three benthic foraminiferal fossil associations (Associations A, B and C), identified by performing Hierarchical Cluster Analysis on the quantitative data, can be considered diagnostic of the palaeoenvironmental conditions before, during and after the sapropel intervals. They indicate a gradual decrease of bottom water oxygenation prior to the onset of sapropel deposition and a rapid return to stable environmental conditions at the end of the sapropel events. The constant presence of benthic foraminifera throughout the sapropels implies that the bottom environment was not totally anoxic. The high-resolution distributional pattern of planktonic and benthic foraminifera within the sapropel associated with i-cycle 90 documents a surface water cooling with consequent rapid re-oxygenation of bottom waters that subdivides this interval in two phases. Both of them are characterized by an early subphase with warm climate conditions, a decrease of salinity in surficial water layers and dominated by the low oxygen tolerant benthic species Globobulimina affinis. The late subphase shows thermal stratification during summer and a relatively deep and oxygenated mixed layer during winter. In this subphase, the contemporaneous replacement of G. affinis, the "less opportunistic, but more resistant to low oxygen conditions" species with Bolivina alata, Bolivina dilatata and Brizalina spathulata, the "less resistant to low oxygen conditions" species, indicates the relative oxygen improvement in sediment pore water. The frequent changes detected in the sapropel-associated faunal composition underline the unstable nature of bottom water oxygen content. (C) 2004 Elsevier B.V. All rights reserved.