Astronomically Tuned Gr Record Reveals Two-Stage Regional Response to the Oligocene–miocene Transition in the Northern South China Sea

Establishing a robust astrochronologic framework for nonstationary sedimentary records remains a major challenge in astrochronology, because conventional tuning methods often rely on the assumption of a constant sedimentation rate. Here, we apply AstroGeoFit, which integrates genetic optimization and Bayesian inference, to analyze a natural gamma-ray (GR) log from Well LSX in the Qiongdongnan Basin, northern South China Sea. Our objectives are to reconstruct a high-resolution age model with quantified uncertainties and to evaluate the regional sedimentary response to the Oligocene–Miocene Transition (OMT). The studied interval (3547–3934 m), which comprises the entire Lingshui Formation and the lowermost Sanya Formation, represents a relatively continuous but nonstationary, mudstone-dominated succession with minor siltstone interbeds. AstroGeoFit yields a smooth age-depth model with a total duration of 7.330 Myr (90% credible interval: −0.223/+0.135 Myr), consistent with previous independent tuning results. Reconstructed sedimentation rates range from 3.4 to 6.9 cm/kyr, with a mean of ~5.3 cm/kyr, confirming substantial depositional-rate variability throughout the interval rather than a constant sedimentation rate. The tuned GR record exhibits clear orbital forcing signals, including five eccentricity-band peaks and five climatic precession-band peaks. To anchor the floating astronomical timescale to absolute geological time, the reconstructed eccentricity signal was statistically matched to the La2010d astronomical solution. The optimal match yields ages of 20.226 Ma for the top of the studied interval and 27.555 Ma for its base, with strong agreement between the reconstructed and target eccentricity patterns (Pearson correlation coefficient, r = 0.91). This chronostratigraphic framework provides a robust basis for comparison with global climate proxy records, including CENOGRID benthic foraminiferal δ13C and δ18O, as well as reconstructions of global sea level, deep-sea temperature, and global ice volume. Within this calibrated temporal framework, the LSX GR record shows persistent coupling to global climate variability at the 405-kyr eccentricity scale, indicating that the regional sedimentary system remained tightly linked to long-period orbital climate forcing throughout the studied interval. Notably, the GR response across the OMT does not show a simple linear correspondence with global climate change. Instead, it displays a distinct two-stage pattern: an initial low-frequency inflection occurred nearly synchronously with the global OMT transition at ~23.21 Ma, whereas the primary structural reorganization of the GR record occurred later and was expressed in lower-frequency background variability. Furthermore, increased amplitudes in the ~100-kyr and ~23-kyr bands after the OMT suggest enhanced regional sensitivity to orbital-scale climate forcing. Our findings demonstrate that AstroGeoFit can effectively resolve complex well-log records with variable sedimentation rates and provide a reliable temporal framework for deciphering regional sedimentary responses to major global climate transitions.