Threshold Response of Millennial Climate Variability to Orbital Forcing and Glacial Boundary Conditions Across the Intensification of Northern Hemisphere Glaciation

The intensification of Northern Hemisphere Glaciation (iNHG, ~2.7-2.4 Ma) marks a major climatic shift from the relatively warm and stable mid-Pliocene climate to the high-amplitude glacial–interglacial cycles of the early Pleistocene. Recent geochemical evidence from IODP Site U1385 on the Iberian Margin suggests that the iNHG was also associated with the emergence of millennial climate variability (MCV), an intrinsic feature of Quaternary glacial periods. These millennial-scale cooling events (stadials) first appear as isolated precursor events since MIS G6 (~2.7 Ma), and recur as multiple events within glacials from MIS 100 (~2.5 Ma) onward. This stepwise evolution raises the question of how MCV, nested within glacial cycles, responds to orbital forcing and the evolving climate background state (e.g. long-term cooling and CO₂ decline). Here, we explore this using new high-resolution planktic and benthic foraminiferal oxygen isotope records and X-Ray Fluorescence (XRF)-derived Zr/Sr ratios from Site U1385, spanning ~3-1.8 Ma and encompassing the iNHG.

Orbital-phase analyses indicate that obliquity strongly governs the timing of glacial terminations and inceptions across the study interval. Precession, together with obliquity, modulates the rate of ice-volume change and the associated shapes of glacial cycles. Within glacial periods, MCV exhibits strongly state-dependent, threshold-like behaviour, with stadials preferentially occurring under lower obliquity and higher benthic δ¹⁸O values. The number of stadial events within individual glacial cycles appears to increase with both the duration and intensity of glacial periods beyond a benthic δ¹⁸O threshold, and is further modulated by the magnitude of the corresponding obliquity minimum. Together, these results suggest that sufficiently intense and long-lived glacials provide the background conditions under which multiple stadial events can be sustained, thereby offering a conceptual basis for the observed MIS G6-100 shift from isolated precursors to persistent millennial-scale oscillations. This pattern is consistent with the development of marine-terminating ice-sheet margins as Northern Hemisphere glaciation intensified, potentially enabling iceberg calving and associated MCV to persist from MIS 100 onward.