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Alegret, Ortiz & Kaminski (eds.), 2012. Ninth International Workshop on Agglutinated Foraminifera, Abstract Volume
The record of Toarcian Oceanic Anoxic Event by agglutinated foraminiferal
morphogroups from Northern Siberia
Matías REOLID 1 , Boris L. NIKITENKO 2 and Larissa A. GLINSKIKH 2
1 Departmento de Geología, Universidad de Jaén, Campus Las Lagunillas sn, 23009 Jaén, Spain.
2 Institute of Petroleum Geology and Geophysics, Siberian Branch Russian Academy of Sciences, Ac.
Koptyg av. 3 Novosibirsk 90, 630090, Russia.
e-mail: mreolid@ujaen.es
The Early Toarcian Oceanic Anoxic Event (T-OAE) was one of the most important
environmental changes of the Mesozoic, resulting in a mass extinction event of benthic
groups in marine ecosystems (Wignall et al., 2005). Typically, the T-OAE is characterized by
the record of organic-rich sediments associated with a negative excursion in δ 13 C coincident
with a sea-level rise (from Jenkyns and Clayton 1997 to Suan et al., 2011). The main aspects
of the T-OAE on foraminiferal assemblages are well documented in low latitudes of Western
Tethys domain but few data are available from high latitudes. The analysis of foraminiferal
life-habits is a useful tool for interpreting oxygenation and nutrient availability. This research
is focused on the analysis of foraminiferal assemblages for interpreting the incidence of the T-
OAE in benthic habitats from North Siberia.
The Kelimyar River section exposes a Pliensbachian-Toarcian marine succession (28.2
m-thick) composed by siltstones and sandy siltstones deposited close to the Early Jurassic
North Pole in a relatively deep shelf environment (middle sublittoral zone) (Nikitenko, 2008).
The upper Pliensbachian corresponds to the Kyra Formation and the Toarcian correspond to
Kelimyar Formation. The succession is characterised by a condensed antiquum Biozone
comprising few centimeters. The beginning of the falciferum Biozone is represented by the
Kurung Member with finely laminated black shales and high values of total organic carbon
(TOC, 6 wt.%) and a negative carbon isotopic excursion. The upper part of the Kelimyar
Formation has TOC values < 1wt.% and higher values of δ 13 CTOC (Suan et al., 2011).
The foraminiferal assemblages consist of benthic forms belonging to the suborders
Textulariina, Lagenina and Robertinina. The foraminifera are mainly agglutinated and
secondarily calcareous-perforated test forms, comprising 30 genera (18 agglutinated, 11
calcitic hyaline and 1 aragonitic hyaline). Eight morphogroups have been differentiated.
MG-A: forms with tubular unilocular shells and inferred epifaunal suspension-feeding habit
(Hyperammina and Jaculella).
MG-B: globular unilocular shells, including epifaunal, passive depositivorous (Lagenammina,
Tolypammina and Saccammina).
MG-C: elongated, subcylindrical, multilocular species, with infaunal detritivorous scavengers
(Ammobaculites, Bulbobaculites, Reophax, Spiroplectammina and Verneuilinoides).
MG-D: spiral multilocular species living as detritivorous epifauna (Ammoglobigerina,
Evolutinella, Kutsevella, Recurvoides and Trochammina).
MG-E: flattened taxa with proloculus and a spiral or irregular chamber, adapted to an
epifaunal to phytal, herbivorous and detritivorous (Ammodiscus, Glomospira and
Glomospirella).
MG-G: aragonitic taxa with a plano-convex trochospiral test, being seaweed epifaunal grazing
herbivores (Conorboides).
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