Late Pliocene to Early Pleistocene Mediterranean mud-dwelling foraminifera : influence of a changing environment on community structure and evolution

Abstract

Seven sections on the Italian mainland are correlated and combined into two extensive stratigraphic records, together embracing the Upper Pliocene and the lower half of the Pleistocene. The benthic foraminiferal faunas derived from these sections are analyzed both quantitatively and qualitatively. Numerical variations of some species are interpreted in terms of bottom water temperature changes; others are assumed to reflect a salinity signal. Patently obvious, however, is the dominating role of oxygen shortages in the benthic environment which determines the faunal fluctuations for the most part. Thereby, the severity of the deoxygenation events turns out to increase in time. This trend culminates in a repeated sterilization of the bottom environment, especially from 2.0 Myr onwards. This paper treats the causes of the Plio-Pleistocene oxygen crises as well as their impact on the foraminiferal communities and the development of individual species. The anoxic facies are considered the ultimate result of overall climatic deteriorations which interfere with local oceanographical and environmental circumstances. They have a profound but momentary effect on the numerical composition of the foraminiferal associations. However, the anoxia also gave rise to some specific morphological alterations which are evidently of a permanent character. This relation between ecological events and phylogeny of benthic foraminiferal species is explained in terms of population dynamics and modern evolutionary theory.