Ocean acidification as a planetary signal linking Earth system memory to deep-time lithosphere–ocean geochemical interactions

Ocean acidification (OA) is widely recognized as a defining chemical signature of the Anthropocene ocean, yet it is still predominantly framed as a near-surface ecological stressor. Here, we advance a planetary-scale reinterpretation of OA as an information-bearing signal that links rapid anthropogenic carbon forcing to deep-time lithosphere–ocean geochemical interactions and long-lived Earth system memory. Observations since the late 20 th century document a persistent, spatially heterogeneous decline in global surface-ocean pH, corresponding to a ~30–35% increase in hydrogen ion concentration, driven by sustained air–sea CO 2 uptake. Projected pH declines of 0.3–0.4 units by 2100 under high-emission scenarios imply rates of change at least an order of magnitude faster than most natural variations recorded in the geological archive. We synthesize carbonate system thermodynamics, carbonate compensation depth dynamics, sedimentary proxy evidence, and Earth system modeling to demonstrate how small, rapid surface perturbations propagate vertically, trigger widespread carbonate undersaturation, and imprint durable stratigraphic and geochemical signatures through sediment dissolution, altered burial fluxes, and delayed alkalinity restoration. Comparison with hyperthermal events, particularly the Paleocene–Eocene Thermal Maximum, reveals that the uniqueness of modern OA lies not in magnitude alone but in an unprecedented rate mismatch that overwhelms geological buffering mechanisms. Furthermore, we identify critical knowledge gaps regarding irreversibility, sedimentary signal emergence, and threshold behavior under rapid forcing. Overall, this study reframes OA as a planetary-scale diagnostic of whole-Earth system disequilibrium, revealing its fundamental significance for evaluating long-term carbon cycle stability, Earth system resilience, and the enduring geological legacy of anthropogenic emissions beyond human timescales.