Cyclostratigraphy and Astronomical Tuning of the Middle Eocene Terrestrial Successions in the Bohai Bay Basin, Eastern China

The Eocene is an enigmatic and interesting period due to the critical thermal events during the gradual global cooling, which are demonstrated in the marine realm but are still less reported in the terrestrial basin. In this study, a continuous 404 m interval of Middle Eocene terrestrial succession is cored in the Well Fanye 1 in Shengli Oilfield, Bohai Bay Basin, eastern China. It provides a unique record to better clarify the terrestrial sedimentary responses to astronomically forced climate changes, especially the Middle Eocene Climatic Optimum (MECO). Here, a high-resolution cyclostratigraphic framework from the lower 3th member (Es3l) to the upper 4th member (Es4u) of the Shahejie Formation (Middle Eocene) is established by using magnetic susceptibility (MS) as a palaeoenvironmental proxy, as well as lithologic and magnetostratigraphic data. Power spectral and Fast Fourier Transform (FFT) spectrogram analysis of the MS series reveal that the wavelength ratio of stratigraphic cycles is 20: 5: 2: 1, consistent with the period ratio of astronomical cycles. A high-resolution astronomical time scale (ATS) for the Well Fanye1 is established by tuning the 405-kyr MS curves to the La2010d astronomical solution based on the magnetic stratigraphic framework. Compared with the geomagnetic polarity timescale in 2012 (GPTS2012), we found that the Bartonian/Lutetian boundary was estimated at the core depth of 3259.9m with age of 41.2 Ma, and the boundaries of C18n2n/C18r and C18r/C19n are respectively at 3164.01 m and 3260.47 m. The durations of Es3l and Es4u are identified to be 1.974 and 1.954 Ma, respectively, with the boundary of 41.109±0.2 Ma. The terrestrial record across the MECO event is presented and the duration is estimated as 500 kyr, which is consistent with the marine records and indicates the seemingly primary role of MECO event in shaping Asian paleoenvironments.