Shell size and abundance of Aptian planktonic foraminifera across the Selli Level (Cismon Core)

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 usually more difficult as the number of the chambers is not always a discriminant feature for the identification at species level. Based on absolute abundance data, diversity and shell size measurements of the planktonic foraminifera, three intervals have been identified across the Selli Level: 1) the interval below the Selli Level (31.29-23.67 m) is characterized by a moderate abundance and diversity of planktonic foraminifera. The assemblage includes several hedbergellids, few leupoldinids and globigerinelloidids. Shell size ranges from 77-120 μm; 2) in the interval within the Selli Level (23.67-18-62 m) planktonic foraminifera are rare, except at the base and top of the interval where planktonic foraminifera show the highest abundance recorded in the studied section. Shell size ranges from 75-300 μm; 3) in the interval above the Selli Level (18.62-7.96 m) planktonic foraminifera are abundant and highly diversified being the assemblage composed by hedbergellids, clavate hedbergellids, leupoldinids and globigerinelloidids. Shell size ranges from 105 to 428 μm. In general, absolute abundance of planktonic foraminifera and species shell size progressively increase from the base to the top of the studied stratigraphic interval.