Palynological Constraints on the Gelasian GSSP: Terrestrial Markers of Hydroclimatic Thresholds and Variability at Monte San Nicola Type Section

The Gelasian Global boundary Stratotype Section and Point (GSSP) at Monte San Nicola (MSNt-s) in Sicily (southern Italy) formally defines the base of the Quaternary (~2.58 Ma) and represents a key reference section for evaluating the stratigraphic expression of climate variability across the Piacenzian–Gelasian transition. Its astronomically calibrated succession records the progressive intensification of Northern Hemisphere glaciation and the emergence of higher-amplitude climate variability. As part of the GELSTRAT programme, promoted by the Subcommission on Quaternary Stratigraphy (SQS-ICS), we present a palynological record from the MSNt-s, directly integrated with previously published multiproxy data. This setting combines a well-constrained marine framework with high-resolution terrestrial records, providing an integrated archive for the characterisation of the Piacenzian–Gelasian transition. The pollen record resolves terrestrial ecosystem dynamics at both orbital and sub-millennial scales and provides an independent stratigraphic signal across the interval spanning MIS G4 to MIS 99. At orbital timescales, vegetation variability is structured by obliquity-paced boundary conditions, while precession primarily modulates hydroclimatic variability and terrestrial input. A major ecological reorganisation at MIS 104 marks the transition from a buffered to a state-dependent terrestrial system. At shorter timescales, millennial- to submillennial-scale variability becomes progressively amplified and culminates during MIS 100 in a structured stadial-interstadial succession. Distinct palynological features closely bracket the Nicola bed (Sapropel A5) and contribute to the stratigraphic characterisation of the GSSP, located at the base of the overlying marls. A pronounced but short-lived expansion of thermophilous forest immediately precedes the boundary, followed by a sharp decline in palynomorph concentrations and the expansion of steppe and xerophytic taxa, marking a rapid shift toward more seasonal and moisture-limited conditions. This reorganisation is anticipated by a marked increase in Cupressoideae, reflecting enhanced hydroclimatic seasonality and reduced moisture reliability. The combined expression of these signals between MIS 104 and MIS 103 defines a coherent terrestrial response across the interval encompassing the Gauss–Matuyama geomagnetic reversal and provides a consistent environmental characterisation of the GSSP. The MSNt-s can therefore be regarded as an integrated stratigraphic archive in which terrestrial, marine and cyclostratigraphic signals are jointly expressed, enabling the characterisation of a major climate transition in the early Quaternary.