Pleistocene benthic foraminifera bioevents in the Central Arctic Ocean: stratigraphic and paleoceanographic implications

Benthic foraminifera show distinct temporal and spatial distribution patterns in the Central Arctic Ocean (CAO) demonstrating their potential to provide robust age constraints and to address paleoceanographic change in the Pleistocene. Several benthic foraminifera bioevents have been previously reported from the Pleistocene that are critically evaluated here by studying three sediment cores from the Mendeleev and Lomonosov ridges and analysing published data sets. Based on these data bioevents are defined by using absolute abundances of species in the >63 µm grain-size fraction, whereas relative abundances are considered not reliable because taphonomic processes such as disintegration and/or dissolution overprint the original assemblage composition. Bioevents are correlated to lithological horizons, and linked to marine isotope stages (MIS) based on available independent stratigraphic data. Three calcareous bioevents can be defined in the Middle Pleistocene: (1) the highest common occurrence of Bolivina arctica (∼MIS 9), (2) the lowest common occurrence of Oridorsalis umbonatus (∼MIS 7), and (3) the acme of Bulimina aculeata (∼MIS 7) in water depths of less than ∼2000 m. The lowest common occurrence of Oridorsalis umbonatus is coeval with the base of the acme of Bulimina aculeata at shallow sites. Since the number of radiometric and biostratigraphic ages is limited, the proposed correlation of bioevents to marine isotope stages should be considered provisional. Further benthic foraminifera bioevents may be useful for stratigraphic correlation on a regional to supra-regional scale but require evaluation of previous taxonomic identifications and additional sediment core studies. The extinct agglutinated species Haplophragmoides obscurus disappeared at the Lomonosov Ridge in the Middle Pleistocene but the complex taxonomy and the few data on the occurrence in Arctic sediment cores currently prohibits the application as biostratigraphic marker. The assemblage turnover from agglutinated to calcareous benthic foraminifera occurred close to the first downcore change of normal to reverse magnetic polarity at the Lomonosov Ridge and Morris Jesup Rise and might be a synchronous event in the eastern Arctic Ocean in middle Pleistocene sediments older than MIS 11. However, this fundamental change in assemblage composition is time-transgressive across the Arctic Ocean because it occurred in the Amerasian Basin in the Early Pleistocene. The bioevents in the CAO are caused by a complex interplay of various biological processes. Apart from B. arctica and H. obscurus that likely evolved in the Arctic Ocean, B. aculeata and O. umbonatus must have invaded the Arctic Ocean from subpolar latitudes. Since an unrestricted exchange of intermediate to deep-water masses with subpolar latitudes is only facilitated through the Fram Strait and the Barents Sea, these intermediate to deep-water species had to be transported as juvenile specimens (propagules) by Atlantic Water to CAO sites during time periods favourable for their propagation. The maximum reachable location for settlement within the Arctic Ocean depended on the species, the local environmental conditions, and the strength of Atlantic Water advection. Environmental conditions, in particular the availability of food, played then a major role for the successful colonization at a particular site, not only for the invading species but also for the species endemic to the CAO (H. obscurus, B. arctica). These sites must have faced a high (H. obscurus, B. arctica, O. umbonatus) or significantly higher particulate organic carbon export to the sea floor than today (B. aculeata). Such environmental conditions must have occurred basin-wide to trigger the synchronous and coincident changes in assemblage compositions. Moreover, external forcing may have triggered environmental change. A massive discharge of detrital dolomite-rich ice-rafted debris might have induced the abrupt collapse of a Bolivina arctica dominated fauna and imminent disappearance of Haplophragmoides obscurus. The most conspicuous change in the environment is expressed in the turnover from a predominance of agglutinated to calcareous benthic foraminifera which was either caused by a fundamental change in food supply and its quality or by corrosive bottom waters. In general, due to selective dissolution of thin-shelled epifaunal taxa, assemblages are enriched in robust epifaunal and/or infaunal calcareous species, or may consist only of an agglutinated taphocoenosis.