Planktic foraminiferal diversity as indicator of ocean environments

Abstract

The combination of the number of species (simple diversity), an information function (Shannon diversity) and a measure of equitability is used to characterize modern ocean environments based on planktic foraminiferal faunas. Regional environmental conditions can be recognized as deviations from the global latitudinal trend. Simple diversity (number of species) offers relatively little resolution itself, because it is sensitive to the addition of rare species. Variable conditions, such as the highly productive spring bloom and upwelling, are characterized by relatively low diversity and equitability values. Relatively high diversity and a lesser increase in equitability are found in mixing zones of adjacent water masses. In extreme conditions diversity is low, whereas equitability is intermediate to high. Modern patterns can be applied to fossil sections, to reconstruct regional paleoceanographic conditions. Thus relatively low Shannon diversity with low equitability in Maastrichtian heterohelicid faunas from El Kef, Tunisia, is interpreted as indicating variable conditions. An increase in simple diversity mostly traces evolution of the heterohelicids as a group, which diversified during the Maastrichtian. The number of species through time is a function of available niches and as such related to oceanic circulation on a global scale. Therefore, when applying modern patterns to fossil sections, a distinction has to be made between global diversity trends and regional environmental change. The correlation between Cenozoic stable isotope record and number of planktic foraminiferal species suggest indeed that simple diversity registers changes in global circulation. Maximum number of species occurred in the Maastrichtian, the Middle Eocene and the early Middle Miocene. In these three instances the maxima are found after the start of a cooling trend and are interpreted as transition periods between two modes of ocean circulation, causing the temporary coexistence of old and new species.