Paleoecology of late miocene mediterranean foraminifera

Abstract

Qualitative and quantitative analyses were made of the benthic and planktonic foraminiferal associations derived from about 400 samples of the Cretan and Sicilian Upper Miocene (Tortonian and Messinian). The quantitative data were analyzed by means ofR- and Q-mode computer programs, which resulted in clusterings of benthic and of planktonic foraminiferal species and the grouping of the samples. Oxygen and carbon isotope data were collected from the foraminiferal carbonate of some 150 samples. The combination of these data with the data from sedimentological analyses and with paleogeographic reconstructions made it possible to arrive at fairly refined estimates of the Late Miocene Mediterranean paleo-oceanography. The foraminiferal distribution and abundance patterns were compared with the paleo-oceanographic interpretation in order to trace the Late Miocene preference and tolerance patterns of the foraminiferal species. Nutrient abundance and, to a lesser extent, salinity were probably the main parameters controlling the Late Miocene distribution patterns. Temperature, type of substrate, depth and pH apparently played only a subordinate role. The various existing models of the Messinian salinity crisis are discussed. The roles of structural evolution, climate and oceanographic factors are considered. Special attention is paid to the genesis of the laminated and diatomaceous sediments. It is concluded that the laminated sediments are the result of changes in the inflow-outflow balance of the Atlantic and Mediterranean waters. The diatomites are ascribed to the same processes but with the additional characteristic that they were formed in the western Mediterranean, against shoals and landmasses, during periods of upwelling. It is suggested that the laminated sediments were deposited during eustatic sea-levellowerings. The ecology and paleoecology of planktonic and benthic foraminifera are discussed extensively. It is concluded that the distribution of both groups of foraminifera is mainly dependent on the distribution of water-masses; each water-mass contains a characteristic faunal association. The abundance of both groups of foraminifera is mainly linked to nutrient abundance, whereas physical/chemical factors are of lesser importance. A calibration of the Late Miocene distribution and abundance patterns and those described from Recent environments suggests that the niches of many foraminiferal species, in particular benthic taxa, changed considerably during the last 5 to 6 m.y. This implies that the Recent distribution patterns cannot simply be applied for the interpretation of fossil associations and for the reconstruction of paleo-environmental conditions. A tentative model is presented which might account for the inferred changes in habitats.