Latest Cenomanian—earliest Turonian low-oxygen tolerant benthonic foraminifera: a case-study from the Sergipe basin (N.E. Brazil) and the western Anglo-Paris basin (southern England)

Distinctive sedimentological, geochemical, isotopic and micropalaeontological response characterise the nature of sediments deposited under hypoxic/anoxic bottom conditions during the latest Cenomanian to earliest Turonian of most of the world's basins. The composition of the biota is a direct response to overall environmental conditions. The recognition of distribution patterns of low-oxygen tolerant microfauna is, therefore, of primary importance in evaluating and reconstructing the regional palaeoceanographic settings where such events took place. A case-study is presented from two coeval stratigraphic sequences, the Sergipe basin (northeastern Brazil) and the western Anglo-Paris basin (southern England). The associated benthonic foraminiferal assemblages from oxygen depleted environments consist mostly of “opportunistic”, r-selected species. Basic patterns in community trophic structures appear to have primarily dictated the composition of the biota. The assemblages from outer shelf to upper slope oxygen depleted environments are commonly characterised by: (1) low-diversity; (2) limited intraspecific variation; (3) predominance (c. 40–60% of the total foraminiferal assemblage) of epifaunal, deposit feeding, plano/concavo-convex, trochospiral, calcareous-hyaline morphotypes (gavelinellids, conorboidids, rosalinids and bagginids); (4) variable numbers (c. 10–30%) of infaunal, bacterial/detrital scavengers, tapered and elongate morphotypes (turrilinids, bolivinids, buliminellids, fursenkoinids, polymorphinids and caucasinids); (5) epifaunal/shallow infaunal, deposit feeding, lenticular morphotypes (coiled vaginulinids) (c. 5–19%); (6) variable numbers (c. 10–30%) of agglutinated specimens (lituolids, discamminids, eggerellids, textulariids, trochamminids, ammodiscids, tritaxiids and valvulinids); and (7) dominance (about 60–80%) of minute (c. 100–200 μm in maximum diameter, width or length) calcareous-hyaline taxa during widespread dysaerobic/quasi-anaerobic episodes. Highly oxygen depleted bottom conditions are marked by the absence of benthonic microfauna. Minor taxonomic differences between the two studied areas are mostly due to palaeobiogeographic provincialism.