A modern-like rate of climate change observed in the latest Paleocene

Understanding the pace of past carbon-cycle disruptions is essential for contextualizing today’s rapid warming. The Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma) is commonly invoked as an analogue for anthropogenic change, yet its comparatively protracted onset implies carbon release, warming, and acidification rates substantially slower than those observed today. In contrast, a short-lived 1–2‰ negative carbon isotope excursion immediately preceding the PETM, termed the pre-onset excursion (POE), has been reported from multiple sites, but its timing and duration remain controversial due to limited chronological control. Key questions therefore remain: How rapidly did climate and environmental change unfold during the POE, and can it provide a more appropriate rate analogue for near-future change? Here we analyze two high-sedimentation-rate paleo-shelf cores from the Mid-Atlantic Coastal Plain (Maryland, USA): South Dover Bridge (SDB) and Cambridge Dorchester Airport (Cam-Dor). High-resolution paleoclimate proxy time series from X-ray fluorescence (XRF) scanning are evaluated using spectral and tuning approaches, and astrochronologic robustness is assessed with statistical tests. Dominant stratigraphic cycles at ~10.5 m and ~2.0 m yield ratios consistent with short eccentricity (~100 kyr) and climatic precession (~20 kyr), implying a mean sedimentation rate of ~10 cm/kyr. The resulting astrochronology constrains the total duration of the POE to 6-8 kyr. Independent δ¹¹B constraints indicate that the POE was accompanied by measurable surface-ocean acidification of ~0.1–0.3 pH units. The inferred rate of pH decline during the POE is of the same order as the present, reinforcing the POE as a potential high-rate analogue and highlighting rapid recovery consistent with strong Earth-system buffering prior to full PETM feedback activation.