The Neogene–Quaternary boundary at its type locality, Monte San Nicola, Sicily, southern Italy: X-ray computed tomography and ichnofabric signals of the sapropelic Nicola bed

• Published in: Marine and petroleum geology 2023-12, Vol.158, p.106552, Article 106552
• Main Authors: Radmacher, Wiesława, Head, Martin J., Uchman, Alfred, Mikołajczak, Mateusz, Lempart-Drozd, Małgorzata, Kaczmarczyk, Grzegorz Piotr, Wałach, Daniel
• Format: Article
• Language: English
• Subjects: Field methods; Laminite; Pleistocene and gelasian GSSP; Sedimentology; Trace fossils
• ISSN: 0264-8172
• DOI: 10.1016/j.marpetgeo.2023.106552

A new low-cost method is described for obtaining narrow blocks of the friable Nicola bed, the local marker for the Neogene–Quaternary system (Pliocene–Pleistocene series) boundary at the type section at Monte San Nicola, southern Sicily. The Global boundary Stratotype Section and Point (GSSP), which also defines the Gelasian Stage and is dated at 2.58 Ma, is placed at the base of a marly layer immediately overlying the sapropelic microlaminated Nicola bed. Sedimentary structures and ichnofabrics revealed by X-ray computed tomography (CT) and polished surface observations show the rapid onset of anoxia marking the base of the Nicola bed, followed by progressive dysoxia indicated by the trace fossil Chondrites isp. alone. The top of the Nicola bed is bioturbated over an interval of several centimeters, which obscures precise recognition of the GSSP horizon. The overlying marl is characterised by the presence of Chondrites isp. accompanied by Planolites isp. and Thalassinoides isp. indicating a return to higher oxygen levels in the bottom waters, although a brief return to poor oxygenation is interpreted from the temporary absence/rarity of Chondrites isp. Trichichnus isp. is prevalent throughout the interval immediately above the Nicola bed, and its lowest extent revealed by CT may serve at least locally as an independent marker for the GSSP level. In general, we recommend the careful study of any sapropel before placing a GSSP at its upper or lower boundary or midpoint. •The Neogene/Quaternary boundary, ∼2.58 Ma, at the type section at Monte San Nicola is revisited.•A new field method is described for collecting friable and microfractured rock blocks.•The sapropelic Nicola bed has been subjected to ichnofabric analyses using polished sections and X-ray computed tomography.•Observations reveal a shift from oxic to anoxic conditions at the onset of Nicola bed deposition followed by improving oxygen levels.•The Nicola bed top is bioturbated, obscuring precise recognition of the GSSP level.