Oscillations of global sea-level elevation during the Paleogene correspond to 1.2-Myr amplitude modulation of orbital obliquity cycles

Abstract

The mechanisms involved in the formation of 1- to 2-Myr-long third-order stratigraphic sequences through the Phanerozoic have been extensively debated, but the main underlying forcings remain uncertain. In this study, we investigated third-order sequences of Paleocene-Eocene age in the East China Sea Shelf Basin, identifying a prominent ∼1.2-Myr periodicity in the 405-kyr-tuned gamma ray profile of the WP-1 borehole. We infer that this ∼1.2-Myr cycle, which corresponds to long-period amplitude modulation of obliquity, was the main driver of third-order sea-level changes and sequence development. This modulation entails variation in the range of obliquity amplitude between a maximum of 2.4° (i.e. 24.5°–22.1°) and a minimum of 0.3° (i.e. 23.3°–23.0°) as a result of interactions among the three primary parameters of Earth's orbit (i.e., eccentricity, obliquity, and precession). We anchored our floating astronomical time scale (ATS) to the T42 seismic horizon, a major sequence boundary between the Lin Feng and Mingyuefeng formations that corresponds to the top of Nannofossil Zone NP8 (reported age of 57.7 Ma). This yields a fixed ATS for the late Paleocene-to-middle Eocene interval spanning 60.84 to 48.71 Ma. Finally, we evaluated our findings relative to the third-order sequence stratigraphy of the East China Sea Shelf Basin and to records of early Cenozoic eustasy and global events.