Styles of Agglutination in Benthic Foraminifera from Modern Santa Barbara Basin Sediments and the Implications of Finding Fossil Analogs in Devonian and Mississippian Black Shales

Abstract

Agglutinated foraminifera are benthic organisms that in modern sediments have been described from marginal marine to bathyal environments. A number of modern taxa have adapted well to oxygen-deficient environments, but even those require at least some oxygen in order to persist at the seafloor. Agglutinated foraminifera in sediments from the central deep of the Santa Barbara Basin are typically multi-chambered and have walls that incorporate a variety of silicate minerals. Some taxa are quite selective with regard to mineral type and size, whereas others seem to incorporate a wide range of minerals and grain sizes. With deeper burial, these foraminifera tests collapse and give rise to mineral streaks that are distinct through comparatively narrow sorting and grain-type variability. Collapsed agglutinated foraminifera with mono-mineralic as well as mixed mineral walls have been observed in black shale samples of Devonian to Mississippian age, suggesting benthic assemblages comparable to those seen in modern and sub-recent Santa Barbara Basin muds. The latter thrive in a largely suboxic setting and suggest by analogy that extended bottom water anoxia, though frequently postulated, was not a requirement for the formation of Paleozoic black shales. Improved criteria for the recognition of benthic agglutinated foraminifera in the rock record should help to bring new perspectives to the ongoing debate about the origin of ancient black shale successions and accelerate the removal of simplistic models from the discussion.