Ecostratigraphy of benthic foraminifera for interpreting Arctic record of Early Toarcian biotic crisis (Northern Siberia, Russia)
Abstract The present contribution focusses on ecostratigraphic fluctuations of foraminiferal assemblages occurring during the benthic biotic crisis corresponding to the Early Toarcian anoxic event. We aim at understanding the response of benthic biota to the environmental changes that occurred during the crisis in a high latitude setting and compare with previous data from total organic carbon (TOC) and δ13C from previous works. The Kelimyar River section exposes a Lower Jurassic marine succession made up of siltstones, sandy siltstones (uppermost Pliensbachian) and black shales (Lower Toarcian) deposited close to the Early Jurassic North Pole in a relatively deep continental shelf. The Pliensbachian sediments are represented by a foraminiferal assemblage dominated by epifauna, mainly Trochammina, but showing a progressive diminution in diversity. This suggests an unfavourable microhabitat for infaunal forms, probably involving salinity fluctuations. Although the opportunist Trochammina tolerated salinity fluctuations, the decreasing proportions of aragonitic and calcitic hyaline foraminifera indicate the progressive deterioration of the benthic environment. A context of relative sea-level fall in the Arctic palaeobasin during the margaritatus Zone and the beginning of the viligaensis ammonite Zone related to an abrupt climatic cooling would explain these environmental changes (including salinity fluctuations) that lead to decreased diversity and even the disappearance of calcitic and aragonitic forms. The debut of the falciferum Zone (Lower Toarcian) is characterised by laminated black shales, high total organic carbon, a negative carbon isotopic excursion, and an abrupt decrease in the foraminiferal abundance and diversity. These features indicate oxygen-restricted conditions in the sea-bottom. Only opportunist Trochammina shows a rapid increase in abundance after the debut of the black shales, congruent with the relation of this genus with high TOC and a poorly oxygenated sediment water interface. Trochammina was able to colonise the sea-bottom after the development of hypoxic conditions in the Kelimyar River section. These adverse conditions occurred just after the negative carbon isotopic excursion and maximum values of TOC and correlate with the climatic warming that occurred in the Arctic palaeobasins referred to by previous authors. Detailed analysis of the sea-bottom recolonisation points to a foraminiferal replacement in which distinct foraminiferal morphogroups reach their optimum values. Finally, despite certain differences, the benthic foraminiferal assemblages from Kelimyar River section are comparable with those from the North African palaeomargin in the Ratnek El Kahla section (Algeria), thus illustrating an equivalent response of benthic foraminiferal assemblages to the Early Toarcian biotic crisis. In both cases the composition of assemblages, diversity and abundance of foraminifera were sensitive to restricted oxygen conditions, and opportunist species did colonize the sea bottom after this biotic crisis.