Spiroplectinata, key benthic foraminifer genus for palaeoceanographic reconstruction of the Albian Lower Saxony Basin

This paper is focused on Albian Spiroplectinata, a morphologically unique, benthic, agglutinated foraminifer genus, and its application for reconstructing changes in the water masses within the Lower Saxony Basin. A new interpretation of microhabitat preferences and feeding habit for Spiroplectinata is proposed and discussed. Their functional morphology, composition of their tests, and their relationships with adherent foraminifera suggest that at least adult individuals of Spiroplectinata were epibenthic and were either suspension or deposit feeders. The suspension-feeding mode is favoured because this genus is associated with adherent suspension-feeding foraminifera and it does not correlate with other epibenthic deposit feeders. Spiroplectinata constructed an elongated uniserial terminal part, which probably facilitated an erect position to spread a reticulopodium well above the sediment/water interface. The distribution of this foraminifer can, therefore, be used here as a sensitive indicator for the existence of bottom currents. Four (or five) periods of intensified bottom water circulation around the Early/Middle Albian boundary (S-1), Middle (S-2), late Middle to earliest Late Albian (S-3, probably represents two intervals) and early Late Albian (S-4) are reconstructed and well marked by maxima of Spiroplectinata annectens, S. complanata, and Spiroplectinata sp. A. ectens is known from strata in the Western Tethys (i.e. in the Late Aptian and Early Albian) that are older than those in the ‘Boreal’ realm. Its first appearance in the Lower Saxony Basin occurs in the late Early Albian, followed by the first maximum just above the Early/Middle Albian boundary, and probably represents the main immigration event into the Lower Saxony Basin. This supports the idea that the deep water in the late Early to early Late Albian ‘Boreal’ epicontinental sea during ‘the Spiroplectinata intervals’ came from the Tethys and not from the polar regions. If one interprets the plankton/benthos foraminifera ratio as reflecting sea-level changes, it implies that sea-level rises may have been responsible for opening and/or deepening of ‘Boreal’/Tethyan gateways that activated extensive exchange of water masses. This late Early Albian and early Late Albian ‘invasion’ of Tethyan water can be correlated with a general global warming, and shifting of climate zones.