WHAT CONTROLLED BENTHIC FORAMINIFERAL ASSEMBLAGE CHANGES IN THE CRETACEOUS POLAR SEA?

A complete Polar Cretaceous paleoenvironmental history is exposed in the Sverdrup Basin near Glacier Fiord, Axel Heiberg Island, Nunavut. The sedimentary record reveals biotic responses to changes in: a) paleoclimate; b) sea-level fluctuations with resulting facies changes; c) a changing carbon cycle with at least three oceanic anoxic events; d) frequent volcanic eruptions delivering ash beds to the seafloor; e) variable benthic oxygen conditions, and f) phases of tectonically driven basin restriction. The Polar Sea was connected with the Western Interior Sea throughout Aptian to Maastrichtian time, and both basins responded in a similar manner to global transgressive-regressive cycles. Carbon isotope stratigraphy documents the OAE 1a, OAE 1b and OAE 2 events. The response of benthic foraminifera to this multitude of paleoenvironmental factors are investigated here. Benthic assemblages are generally dominated by agglutinated taxa, whereas calcareous species are associated with alkaline conditions of the Late Aptian cold snap that also produced glendonites. Foraminiferal morphotypes are a response to variable benthic oxygen and substrate conditions. Foraminiferal habitats range through time from distal to proximal shelf settings to deltaic/restricted brackish conditions. The Aptian to Campanian succession is marked by sequence boundaries within the Upper Aptian, at the Albian/Cenomanian transition and within the middle/upper Cenomanian. These exposure surfaces caused faunal turnovers. During the global Cenomanian temperature maximum this locality was a restricted basin under the strong influence of freshwater which resulted in a complete foraminiferal loss. During the OAE 2 increased freshwater runoff under warm, humid conditions caused at times water column stratification. Benthic foraminiferal communities responded with either absence or with a limited abundance of small morphotypes. These polar latitudes supported vegetation that added abundant terrestrially derived organic matter to marine shelf systems.