IODP REVEALS THE CENOZOIC HISTORY OF SEA-LEVEL, CLIMATE, AND CO2 CHANGES

Sea-level history reflects the Earth’s thermal and cryospheric evolution and the behavior and operation of the climate system under ice-free and glaciated conditions. IODP has provided constraints on the timing, magnitudes, and rates of sea-level change, with coring of: 1) carbonate platforms and atolls (Exp 310 Tahiti; Exp 325, Great Barrier Reef); 2) continental margins (Legs 150, 150X, 174A, 174AX, & Exp 313, New Jersey; Exp 317 New Zealand); and 3) deep sea sections that provide continuous δ¹⁸O and Mg/Ca proxy records for ice-volume changes. Comparisons between Cenozoic sea-level estimates from backstripping the mid-Atlantic U.S. with those from deep Pacific Ocean δ¹⁸O-Mg/Ca records show similar timings and amplitudes reflecting Global Mean Sea Level changes (GMSL). Peak warmth, peak sea levels, high CO₂ (>1000 ppm), and ice-free conditions occurred in the Hothouse Early Eocene (55-48 Ma), with GMSL falls of ~15 m reflecting growth and decay of a small East Antarctic Ice Sheet (EAIS) or other mechanisms. Cool greenhouse (Paleocene, Middle to Late Eocene) sea-level changes of 15-25 m were caused by growth and decay of small (~1/3 of modern) EAIS ice sheets with ice-free interglacials. During the largely unipolar Icehouse of the Oligocene-Early Miocene, the EAIS was not permanently developed, with periods of large-scale growth and deglaciation associated with ~50 m sea-level fluctuations. During the Miocene Climate Optimum (MCO; ~17-15 Ma) ice-volume changes were small (<20 m), with ice-free conditions likely attained. Development of a permanent EAIS occurred as 3 steps during the Middle Miocene Climate Transition (MMCT; 14.8-12.8 Ma). During the Middle to Late Miocene the EAIS was relatively stable with ~20-30 m Myr-scale sea-level variations. During the Pliocene Climate Optimum (PCO, ca. 3-4 Ma), sea levels peaked at 12-20 m above present, requiring loss of ice in Greenland, West Antarctica, and portions of the EAIS, despite CO₂ levels similar to 2019. Large lowerings (60-130 m) were restricted to the past 2.7 Myr, associated with continental-scale Northern Hemisphere ice sheets. The last glacial maximum stands as the largest sea-level lowering (~130 m) of the Mesozoic-Cenozoic. Rates of sea-level rise during the last deglaciation (ca. 20-10 ka) reached 40 mm/yr, the fastest GMSL rise yet measured in the Cenozoic record.