Ecostratigraphic trends of Jurassic agglutinated foraminiferal assemblages as a response to sea-level changes in shelf deposits of Svalbard (Norway)

Abstract

Foraminiferal analysis proves to be a useful tool for the interpretation of major factors controlling the eco-sedimentary environment. Changes in benthic foraminiferal features, including morphogroups, are mainly affected by variations in nutrient availability and oxygenation rate. The integration of foraminiferal data with total organic carbon (TOC) values, together with differentiation of oxygen restricted biofacies (ORB) based on sedimentological features and macroinvertebrate composition, will improve the interpretations derived. Upper Jurassic shelf deposits from Svalbard (Norway) are revealed to be particularly sensitive to the record of fluctuations in nutrients and oxygenation, showing two extreme cases in TOC content. High TOC concentrations are characterized by low values of the Shannon–Weaver Index ( H′) and α-diversity index in foraminifera, pointing to a low diversity and dominance of r-mode epifaunal foraminifera ( Trochammina). Moreover, differentiated ORB can be characterized by parallel lamination in shales, and an absence of macrobenthic invertebrates (including trace fossils). Unfavourable benthic conditions are interpreted, mainly related with a high consumption of oxygen by microbes in the sediment associated to the degradation of the organic matter. Low TOC values are related to higher values of diversity ( H′ and α-diversity index), and foraminiferal assemblages dominated by infaunal forms. The differentiated ORBs are characterized by coarse fabrics, scarcity or absence of parallel lamination, presence of trace fossils and abundance of macroinvertebrate shelly epifauna. This is related to a better pore water circulation and higher oxygenation degree, congruent with the comparatively more favourable conditions in the infaunal habitats. Ecostratigraphic trends along the studied section allow the differentiation of stratigraphic intervals reflecting variations in parameters related to the sea-level dynamics. Transgressive phases reveal upward trends in palaeoenvironmental parameters, characterized by: decreasing grain-size, and increasing in both TOC values and epifaunal foraminifera. Differentiated ORB reveals diminution in abundance and diversity of trace fossils and benthic shelly macroinvertebrates upwards. Maximum flooding intervals show the maintenance or accentuation of the previously registered features (the thinnest fabrics, and the highest values in TOC and in opportunistic epifaunal foraminifera), with ORB dominated by parallel lamination. A regressive phase is characterized by coarsening-upwards sediments, decreasing TOC values and increasing infaunal foraminifera. ORB reveals a diminution in parallel lamination together with an increase in trace fossil abundance and diversity, as well as in benthic shelly macroinvertebrates.