Hydroclimatic thresholds and state-dependent vegetation responses in the Mediterranean during the intensification of Northern Hemisphere glaciation

The Piacenzian–Gelasian transition represents a key phase in the intensification of Northern Hemisphere glaciation and the reorganisation of Mediterranean hydroclimate. Here we present a high-resolution terrestrial palynological record from the Monte San Nicola Gelasian Global boundary Stratotype Section and Point (GSSP), southern Italy, integrated with astrochronology and marine multiproxy records between ~2.7 and 2.5 Ma. The record indicates that vegetation change culminated in a threshold-like reorganisation at MIS 104 following earlier precursor fluctuations. Prior to this transition, ecosystems remained relatively buffered, with variability expressed mainly through changes in forest composition. After MIS 104, vegetation responses became increas- ingly state-dependent, with recurrent landscape opening, steppe expansion and enhanced hydroclimatic sea- sonality. Spectral analyses reveal a hierarchical organisation of orbital forcing, with obliquity pacing vegetation structure, while precession primarily modulated hydroclimatic variability. However, precession-driven forcing was only intermittently translated into ecological change, indicating that vegetation response depended on background climate conditions. At shorter timescales, suborbital variability became increasingly expressed after MIS 104, with strong amplification during MIS 100, consistent with the increasing climate instability docu- mented in coeval marine records. Comparison of terrestrial and marine variability reveals a coupled but non- equivalent land–sea response, indicating that vegetation dynamics were governed primarily by effective mois- ture and its seasonal distribution rather than temperature. The palynological record also provides the first high- resolution terrestrial characterisation of the Gelasian GSSP section, linking the formal chronostratigraphic boundary to orbitally paced vegetation reorganisation and documenting the increasing sensitivity of Mediter- ranean ecosystems to hydroclimatic instability during the intensification of Northern Hemisphere glaciation.