Orbital Pacing of the East Asian Summer Monsoon across the Middle Miocene Climate Transition: Evidence from the Pearl River Mouth Basin

The Miocene epoch witnessed Earth's last major transition from a “Greenhouse” to an “Icehouse” state, serving as a pivotal interval for understanding climate dynamics under varying CO₂ levels. The peak warmth of the Miocene Climatic Optimum (~17-14.7 Ma) was abruptly terminated by the Middle Miocene Climate Transition (MMCT, ~14 Ma), characterized by a significant atmospheric CO₂ drop, global sea-level fall, and the establishment of a permanent East Antarctic Ice Sheet (EAIS). However, how sensitive hydroclimate systems like the East Asian Summer Monsoon (EASM) responded to this profound high-latitude cryosphere expansion remains unresolved. The northern South China Sea, specifically the Pearl River Mouth Basin (PRMB), provides an ideal natural laboratory to address this. Here, natural gamma ray (GR) logs from continuous marine sequences serve as robust proxies for EASM-driven continental weathering and terrigenous input. We present a high-resolution (~1.5 kyr) GR-based hydroclimate record from PRMB drill core LW9 (~23.2 to 9.5 Ma), anchored by a robust astronomical timescale that integrates a refined biostratigraphic framework with both manual and AstroGeoFit-automated orbital tuning. Our spectral analysis reveals a fundamental regime shift in the orbital pacing of the EASM at ~14 Ma. Prior to the MMCT (23.2-14 Ma), the hydroclimate was overwhelmingly controlled by low-latitude insolation, characterized by strong precession (~20 kyr), long-eccentricity (405 kyr), and a distinct half-precession (~10 kyr) cycle. During this “Greenhouse” phase, the EASM was relatively decoupled from Antarctic ice volume. However, following the EAIS expansion at ~14 Ma, the monsoon pacing shifted to become dominated by 100-kyr eccentricity and 41-kyr obliquity cycles, mirroring global deep-sea benthic δ¹⁸O records. Concurrently, the amplitude of GR fluctuations and total sediment flux dramatically increased. These findings suggest that the MMCT significantly influenced Asian monsoon dynamics. Following the MMCT, the EASM exhibited an enhanced coupling with high-latitude Antarctic ice volume fluctuations. This strengthened connection was likely mediated by glacio-eustatic sea-level changes and intensified continental weathering. Ultimately, this study highlights the shifting balance between high- and low-latitude forcings, demonstrating how the emergence of the “icehouse” world modulated the sensitivity and orbital pacing of the EASM.