Benthic diversity patterns in oxygenation gradients: an example from the Middle Jurassic of Switzerland

Abstract

Etter, W. 1995 11 30 Benthic diversity patterns in oxygenation gradients: an example from the Middle Jurassic of Switzerland. A high-resolution study of the lower Opalinum Clay (Aalenian, Middle Jurassic) of northern Switzerland revealed a pattern of macrobenthic diversity and abundance which does not conform with the dysaerobic-biofacies models currently in use. The upper dysoxic zone contains an association of moderate diversity and moderate abundance which, with increasing oxygen depletion, is replaced by an association dominated by a few peak opportunists (tiny epibenthic bivalves and presumably small annelids) at high abundances but very low diversity. In the lower dysoxic zone, abundance drops to very low levels, but diversity rises again to quite high values. The rather high diversity of the lower dysaerobic biofacies is explained in part by the presence of specialized chemosymbiontic species. The pattern documented in the lower Opalinum Clay has been calibrated by sedimentologic, taphonomic, ichnologic, and microfaunal evidence. It contradicts equilibrium models, which postulate gradual or even linear decreases in both abundance and diversity with increasing oxygen-depletion of the bottom water. The equation of rising diversity with improved oxygenation can be misleading and can yield erroneous results in the reconstruction of ancient bottom-water oxygenation. □Dysaerobic biofacies, high-resolution study, diversity, chemosymbionts, equilibrium and non-equilibrium models.