| Summary: | Planktonic foraminifera are widespread in the global ocean, consist of both cosmopolitan and endemic taxa, have a very long evolutionary history and are most sensitive to environmental change in the ocean-atmosphere system. A detailed and quantitative documentation in terms of species composition, variation of shell size and abundance of the planktonic foraminifera across the Selli Level equivalent is presented from the Cismon core (Italy, southern Alps). The Selli Level is a regional marker-bed consisting of laminated black shales rich in organic matter, alternated with radiolarian silts and with a poor carbonate content, and it has been regarded as the sedimentary expression of the OAE1a (Oceanic Anoxic Event 1a). The OAE1a is marked by a pronounced negative carbon isotope excursion preceding the δ13C increase registered in marine pelagic and shallow-water carbonates, marine organic matter, and terrestrial higher-plant materials. Planktonic foraminiferal analyses from the Cismon core were conducted on both washed residues (110 samples) and thin sections (50 samples). Species richness and shell size measurements (maximum diameter of species) were performed on the washed residues and absolute abundances were obtained from the thin sections (number of specimens per square unit). The planktonic foraminiferal assemblage is composed by pseudo-planispiral forms (Leupoldinids) with elongate chambers and ampullae, planispiral forms (Globigerinelloidids) and trochospiral forms (Hedbergellids, Lilliputianellids and Gorbachikellids) with globular and elongate chambers. The planispiral and pseudo-planispiral species are easily recognizable thanks to their morphology (e.g. Globigerinelloides duboisi, G. paragottisi and G. aptiensis). On the contrary, trochospiral species characterized by having from 4 to 9 chambers in the last whorl, a low to very high trochospire and globular to elongate chambers (e.g. Hedbergella tuschepsensis, H. sigali, H. occulta, H. infracretacea, H. aptiana and Lilliputianella globulifera) are ...
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