Enhanced Hydrological Cycle Linked to Climatic Warming Preceding Oceanic Anoxic Event 1a in the Eastern Tethys

The early Aptian Oceanic Anoxic Event 1a (OAE1a, or Selli Event; ∼120 Ma) represents a significant Early Cretaceous hyperthermal event linked to a massive influx of isotopically light carbon that perturbed the global carbon cycle. Although this event has been widely recognized worldwide, the paleoenvironmental and paleoceanographic processes remain less constrained, particularly in the Tibetan Plateau. In this study, total organic carbon and inorganic geochemical data from the lower Aptian Gucuo II section of the Gamba-Dingri Basin (Tibetan Plateau) were conducted to reconstruct hydroclimatic and paleoceanographic conditions preceding OAE1a. Elevated Rb/Sr and Al/Mg ratios reveal intensified continental weathering under a warming climate at this time. This warming episode is consistent with coeval records from the Atlantic and western Tethyan regions, indicating a supra‑regional climatic perturbation preceded OAE1a. The concurrent decline in oceanic salinity and increased terrigenous supply reflect a strengthened climate‑driven hydrological cycle, featured by enhanced precipitation and freshwater runoff. These conditions modulated elevated nutrient delivery and stimulated marine bioproductivity in the study area. Initially, a vigorous hydrological cycle likely generated oxygenated runoff, establishing oxic conditions despite elevated productivity. However, the progressive increase of nutrient enrichment and rising productivity enhanced organic matter remineralization, likely contributing to a shift to more reducing conditions during the late phase of climate warming. These outcomes suggest that climate-driven intensified hydrological cycle played a prominent role in regulating nutrient supply, redox evolution, and bioproductivity preceding OAE1a. This study highlights that preconditioning of marine ecosystem through the coupled climate-weathering-nutrient feedbacks, where represent a key triggering factor of widespread oceanic anoxia.