Macrobenthic community structure in a high-productivity region: Upper Campanian Mishash Formation (Israel)

Abstract

A paleoecological study of molluscan fossils from the Upper Campanian Mishash Formation of southern Israel was undertaken to provide new insights into the inference of bottom-oxygen levels, food availability and bottom water energy on high-productivity seafloors. Quantitative data for molluscan analysis were collected from each of the main lithologies that comprises the high-productivity facies-mosaic of the Mishash sea, i.e., organic-rich carbonates, thin-porcelanite and -chert beds, and loosely- to densely-packed shell beds. Diagenesis has obliterated faunal remains in carbonate layers. Pollution studies in modern environments have demonstrated the close connection between stressful conditions – such as organic load, toxicity and water and sediment aeration – and benthic community structure, as expressed through species richness, evenness, taxonomic composition, body sizes, and trace fossil assemblages. These connections and behavior patterns were investigated in the varied benthic fossil assemblages of the Phosphate Member of the Mishash Formation. Bivalves and gastropods dominated the macrofauna in all rock types but in varying abundance and taxonomic compositions, changing from diverse assemblages (15–22 species per sample) to less rich ones (5–7 species) and even barren intervals. This range of abundance neatly describes the lateral mosaic and heterogeneous nature of the upwelling environment of the Upper Campanian and is comparable to the mosaic of sub-facies that characterize modern upwelling facies-belts today (e.g., in the Benguela upwelling system). The variations shown for the macrobenthic assemblages within the Mishash Formation high-productivity regime are associated with proximity to the high-productivity center or to periods of intensified upwelling. The inferred, variable oxygen levels and organic-load changes point up how dynamic these regions really are. This is expressed in community structure and composition, which vary between two extremes: The low-oxygen biofacies 1 is characterized by a smaller range of shell sizes, infaunal deposit feeders and low species richness, while the high-oxygen biofacies 3 is characterized by greater ranges in shell size, infaunal but also epifaunal suspension feeders and higher species richness. The chert and porcelanite beds exhibit a variety of sub-environments and seem to represent an intermediate facies that bridges the two extremes of biofacies 1 and 3. The above ecological criteria may provide evidence of the original carbonate environment for the thin-chert and -porcelanite layers. The chert or porcelanite samples that contain a moderately even and rich assemblage of species intermediate to the values of the two end-member facies are taken to represent a shallow-water carbonate marine environment marginal to the upwelling center, whereas layers of chert and porcelanite containing very low species richness and evenness are taken to characterize primary depositional environments proximal to the paleo-upwelling cells.