Trophic regimes of benthic foraminiferal assemblages in Lower Jurassic shallow water carbonates from northeastern Italy (Calcari Grigi, Trento Platform, Venetian Prealps)

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Benthic foraminiferal distribution patterns in the Lower Jurassic Rotzo Member (Trento Platform, northeastern Italy) are analysed in terms of trophic gradients to provide a better understanding of the benthic communities during the time of the initial diversification of Lower Jurassic large foraminifera in the Southern Alps. The Rotzo Member deposited on the Trento Platform records a complex mosaic of distinct shallow water ecosystems. The changes in the foraminiferal assemblages of the Rotzo Member are considered to be primarily a reflection of gradients in trophic resources and oxygen levels. The comparative analysis of benthic foraminiferal assemblages (characteristic taxa, number of species, test size, reproduction strategy), the associated sediments (lithofacies) and the macrofossil and microfossil content allow three trophic regimes to be identified during the time of deposition of the Rotzo Member: (a) eutrophic, (b) mesotrophic, and (c) oligotrophic. (a) The eutrophic regime, marked by episodically dysaerobic bottom conditions and high organic matter supply, occurs stratigraphically below and within the Orbitopsella spp. assemblage (Sinemurian pro parte-Carixian). Gradual change in the benthic foraminiferal assemblage may have been caused by a combination of reduced nutrient supply and increased bottom-water circulation which led to a mesotrophic regime (b). The latter, which has been recognised in the basal and middle part of the Rotzo Member alternating with the eutrophic assemblage, is characterised by a foraminiferal assemblage dominated by Orbitopsella sp. A major foraminiferal faunal turnover across the Carixian/Domerian boundary, corresponding to the disappearance of Orbitopsella spp., indicates a significant environmental change. An oligotrophic regime (c) became established towards the upper part of the Rotzo Member. New taxa appeared and diversity of the foraminiferal faunas was higher than in the previous ecosystems. This part of the member, characterised by the maximum lateral and vertical development of large Lithiotis bivalve mounds, reflects low and more stable nutrient supply, which provided optimal conditions for the more specialised benthic communities. Evolution of these benthic foraminiferal faunas from the Late Pliensbachian onwards was obliterated by events that led to the drowning of the Trento Platform, which is marked by a regional unconformity.