The Global Stratotype Section and Point (GSSP) of the Langhian Stage and the Middle Miocene Subseries

The Global Stratotype Section and Point (GSSP) of the base of Langhian Stage (base of the Middle Miocene Subseries) is defined at a level of 17.84 m in the “Lower La Vedova Beach” section in central Italy. This level marks the mid-point of the darker marly interval above “Megabed IV” which has been astronomically calibrated to the most prominent ~100-kyr eccentricity maximum, with an astronomical age of 15.981 Ma according to La2004 nominal solution, and of 15.978 Ma according to the La2011, in the 405-kyr maximum around 16.0 Ma. The GSSP level in the Lower La Vedova Beach section corresponds closely to the top of Chron C5Cn (at 15.795 m), which is considered the principal event for recognizing the boundary globally. This magnetic reversal with an astronomical age of 16.017 Ma (Turco et al., 2017) (15.97 Ma in Hilgen et al., 2012 ATNTS, 2012; 15.994 Ma in Raffi et al., 2020 in GTS2020) is preferred to the historical guiding criterion, the Praeorbulina datum, which has been complicated by taxonomic confusion and revision (Turco et al., 2011a), and hence is considered less suitable for recognizing the boundary. The Lower La Vedova Beach section is preferred to the St. Peter’s Pool section (in Malta) as it has an independent astronomical tuning and a better paleomagnetic signal. With the selected astrochronologic criterion close to an important magnetic reversal boundary we follow standard procedures developed by SNS to define Neogene GSSPs over the years. This procedure guarantees that the Langhian GSSP is directly incorporated in the integrated astronomically dated stratigraphic framework that nowadays underlies our standard GTS, while it is sufficiently close to the top of Chron C5Cn that the latter can be used for correlating the boundary time-stratigraphically on a global scale. In addition, a Standard Auxiliary Boundary Stratotype (SABS) is designated at Integrated Ocean Drilling Program (IODP) Site U1337, in the eastern equatorial Pacific, with the aim to directly link the open ocean benthic foraminiferal stable isotope record to the boundary definition. This level marks the mid-point of a darker interval that has been astronomically calibrated to the same prominent ~100-kyr eccentricity maximum. This level coincides with distinctive features in the stable isotope record, falls right in the middle of the Miocene Climatic Optimum (MCO) and is approximately one 405-kyr cycle older than the most dramatic oxygen isotope minimum dated astronomically at 15.6 Ma, marking the most extreme warming during the entire Miocene (Holbourn et al., 2013). It further corresponds closely to the top of Chron C5Cn, based on detailed cyclostratigraphic correlations to parallel Sites U1335 and U1336, which have an excellent magnetostratigraphy. Biostratigraphically, the Langhian GSSP falls in the lower part of the Mediterranean planktonic foraminiferal Subzone MMi4a (Di Stefano et al., 2008; Iaccarino et al., 2011; Lirer et al., 2019), delimited by the First Occurrence (FO) of Globigerinoides sicanus (3 apertures) at the base and the Beginning of the Paragloborotalia siakensis Acmea at the top, and calcareous nannofossil Subzone MNN4b (Di Stefano et al., 2008; 2023), defined by the Last Common Occurrence (LCO) of Helicosphaera ampliaperta and the Beginning of the Sphenolithus heteromorphus Paracme interval. With respect to (sub)tropical biozonations, the later can be used for correlating the boundary time-stratigraphically on a global scale. In addition, a Standard Auxiliary Boundary Stratotype (SABS) is designated at Integrated Ocean Drilling Program (IODP) Site U1337, in the eastern equatorial Pacific, with the aim to directly link the open ocean benthic foraminiferal stable isotope record to the boundary definition. This level marks the mid-point of a darker interval that has been astronomically calibrated to the same prominent ~100-kyr eccentricity maximum. This level coincides with distinctive features in the stable isotope record, falls right in the middle of the Miocene Climatic Optimum (MCO) and is approximately one 405-kyr cycle older than the most dramatic oxygen isotope minimum dated astronomically at 15.6 Ma, marking the most extreme warming during the entire Miocene (Holbourn et al., 2013). It further corresponds closely to the top of Chron C5Cn, based on detailed cyclostratigraphic correlations to parallel Sites U1335 and U1336, which have an excellent magnetostratigraphy. Biostratigraphically, the Langhian GSSP falls in the lower part of the Mediterranean planktonic foraminiferal Sub zone MMi4a (Di Stefano et al., 2008; Iaccarino et al., 2011; Lirer et al., 2019), delimited by the First Occurrence (FO) of Globigerinoides sicanus (3 apertures) at the base and the Beginning of the Paragloborotalia siakensis Acmea at the top, and calcareous nannofossil Subzone MNN4b (Di Stefano et al., 2008; 2023), defined by the Last Common Occurrence (LCO) of Helicosphaera ampliaperta and the Beginning of the Sphenolithus heteromorphus Paracme interval. With respect to (sub)tropical biozonations, the Langhian GSSP falls within the planktonic foraminiferal Zone M5 (Wade et al., 2011) and Zone N8 (Blow, 1969), defined by Base Praeorbulina sicana (B) and Base Orbulina suturalis, and at the top of the calcareous nannofossil Zone CNM6 (Backman et al., 2012), defined by Base S. heteromorphus and Base Discoaster signus (= D. petalosus), corresponding to the upper part of Zone NN4 (Martini, 1971)