Astronomical Forcing of late oligocene to early Miocene Paleoclimate: A case study from the Northern South China Sea

Abstract

Astronomical forcing, such as Earth's orbital obliquity, has played a crucial role in climate evolution throughout geological history. The Qiongdongnan Basin in the South China Sea provides valuable geological records for reconstructing the paleoenvironmental conditions of the western Pacific during the late Oligocene to early Miocene. In this study, we conducted cyclostratigraphic and paleoclimatic analyses using high-resolution geophysical well-logging data, planktic foraminifera, and sporopollen from Well Ls33a. The power ratio accumulation (PRA) method indicates that among various geophysical logging parameters, natural gamma-ray (GR) data exhibit the highest sensitivity. Time series analysis reveals astronomical signals in the GR data, which were further validated through objective statistical methods such as TimeOpt, COCO, and PRA. By calibrating the interpreted 405-kyr eccentricity cycle, we developed a floating astronomical timescale spanning approximately 7.32 million years. Subsequently, based on the preliminary chronological framework derived from planktic foraminiferal biostratigraphy (∼28.4 Ma–21.12 Ma) and the La2010d theoretical astronomical solution, we established an absolute astronomical timescale for the 3547–3934 m interval of Well Ls33a, covering 28.55–21.23 Ma. Within this astronomical timeframe, we reconstructed sea-level fluctuations in the study area from the late Oligocene to early Miocene using the sedimentary noise model. Notably, the sea-level variations near the Oligocene-Miocene boundary exhibit a weak response to astronomical forcing. Further application of recurrence quantification analysis (RQA) reveals nonlinear characteristics in the region's climate evolution. By integrating sporopollen data, we classified the climate history into three distinct stages. Over long timescales, astronomical forcing is the primary driver of global climate change. However, local sedimentary and environmental variations introduce strong nonlinearities, leading to phase shifts in response to astronomical forcing. Finally, we propose a simplified model to elucidate the mechanisms by which Earth's orbital obliquity influences climate system dynamics, providing new insights into the climate evolution of the late Oligocene to early Miocene.