Climate-Tectonic Forcing of East Asian Monsoon Variability and Sedimentary Processes in the Northern South China Sea between the Pliocene–Pleistocene

The Pliocene-Pleistocene interval (5.3–0.01 Ma) is an important timeframe for understanding the interplay among tectonics, monsoon variability, and global climate transitions. Marine sediments from IODP Sites U1499A and U1501C in the northern South China Sea (NSCS) provide an excellent record for reconstructing these links. This study integrates bulk geochemistry, stable isotope, and biomarker records to determine the evolution of the Asian monsoon and its controlling mechanisms. During the Early Pliocene (~5.3-4.3 Ma), CaCO₃ concentrations were moderately low (5-12%); however, site U1501C maintained consistently higher carbonate deposition due to its outer margin high setting, which favoured carbonate preservation and reduced clastic dilution. Lower TOC (0.1-0.25%), relatively enriched δ¹³C_carb (0.5-1.2‰) and δ¹⁸O_carb values of -2.2 to -0.8‰, signifying warm surface waters and reduced global ice. Enriched δ¹³C_carb further supports high marine productivity under higher sea levels. While δ¹³Corg (-24 to -26‰) and elevated C/N ratios (~10-25), CPI (~0.70-0.85) and Pr/Ph (~1.5-2.0), indicate predominantly marine organic matter (OM). Thus, δ¹³C_org, C/N, biomarker patterns, and Chemical Index Alteration (CIA) collectively indicate humid conditions with enhanced monsoon runoff. Geochemical proxies (Si/Al, Ca/Ti, Rb/Sr, Ti/Zr, Fe/K) suggest moderate to high chemical weathering, suboxic to oxic bottom-water conditions and relatively stable tectonic settings. Transitioning into the Mid-Pliocene Warm Period (MPWP) (~4.3-3.6 Ma), strengthened East Asian Summer Monsoon (EASM) activity enhanced terrigenous supply. Carbonate content remained high at U1501C, unlike U1499A, where dissolution and sea-level fluctuations reduced carbonate preservation under suboxic bottom-water conditions.  Positive δ¹³C_carb (0.3-1.5‰) and more negative δ¹⁸O_carb (-2.4 to -1.2‰) records warm, high-productivity conditions consistent with the MPWP. δ¹³C_org (-25 to -27‰), C/N (~10-20), CPI (~0.75-0.90) and Pr/Ph (~1.2-1.8) indicate a mixed marine-terrestrial OM source owing to intensified monsoon. A slight decline in CIA (~64) and higher TOC (0.2-0.6%) signals the onset of cooler, and less humid conditions toward the late Pliocene. Between ~3.6-3.0 Ma, increased δ¹³C_carb (0.4-1.3‰), depleted δ¹⁸O_carb (-2.0 to -1.0‰), increased TOC (0.1-0.6%), and fluctuating CaCO₃ (5-20%) reflect a cooling trend accompanied by strengthening of the East Asian Winter Monsoon (EAWM). The δ¹⁸O_carb enrichment marks progressive global cooling associated with early Northern Hemisphere Glaciation. Since the Early Pleistocene (~2.5 Ma), obliquity and eccentricity-driven glacial-interglacial cycles reorganised the monsoon system. Increased CaCO₃ (5-25%), enriched δ¹³C_carb (0.5-2.2‰) and stable TOC (0.3-1.2%)  reflect intensified terrestrial input during wetter interglacial phases, while δ¹⁸O_carb values (-1.8 to -0.5‰) follow global cooling and expanding ice volume. Since ~0.8 Ma, the Himalayan-Tibetan uplift was responsible for enhanced physical erosion and reduced CIA (<28). An increased TOC (0.4-1.2%), strong carbonate preservation (15-45%) and enhanced marine productivity reflect colder glacial climates and a strengthened EAWM.