Ecological replacement of Valanginian agglutinated foraminifera during a maximum flooding event in the Boreal realm (Spitsbergen)

Data from agglutinated benthic foraminiferal assemblages and geochemical proxies make it possible to characterise different order sea-level dynamics —from a geological scale third-order transgressive–regressive cycle to a minor-scale, ecological range, maximum flooding-initial regressive phase—in part of the Valanginian from Spitsbergen. The transgressive–regressive cycle starts with abundance (foram/100 g) of opportunist epifaunal forms related to deep waters and low productivity conditions on oxygenated sea-bed. The increase of these foraminifera, together with increased condensation and an impoverishment of trophic resources, is associated with the maximum flooding phase. The beginning of the subsequent regression shows a progressive diminution in foraminiferal abundance as a result of the greater sedimentation rate, the mounting proportions of deep infaunal forms, and the diminution of genera of deep-water environments. This initial regressive phase is associated with more arid climatic conditions (abrupt increase of eolian detrital proxies), and high palaeoproductivity (increase of Ba/Al, Sr/Al and P/Ti ratios and TOC), determining a diminution in the oxygenation (high values in Co/Al, Ni/Al and Mo/Al ratios and hydrogen index). After the initial productivity bloom, the foraminiferal abundance decreases, whereas diversity increases, favoured by the stability of environmental conditions (no fluctuations in palaeoproductivity, detrital and redox proxies), in turn related to deceleration of sea-level fall. The peaks of maximum foraminiferal abundance (optimum) of each morphogroup during the transition from maximum flooding to the beginning of regression allow us to identify an ecological replacement inter- and intra-microhabitats. In the different phases of ecological replacement, the dominant morphogroups are distributed in distinct microhabitats (epifaunal, shallow infaunal and deep infaunal) as a result of competition.

► Benthic foraminifera and geochemical proxies characterise sea-level changes. ► Trophic resources and redox conditions controlled the foraminiferal assemblages. ► Transition from maximum flooding to initial regressive phase present ecological replacement. ► Dominant morphogroups are distributed in distinct microhabitats as a result of competition.