First early Toarcian B/Ga paleosalinity record from the eastern Tethyan Ocean: Implications for hydroclimate evolution

The Toarcian Oceanic Anoxic Event (T-OAE, or Jenkyns Event, ∼183 Ma) was a significant hyperthermal event that might have led to global carbon-cycle disruption, driven by a massive increase in isotopically light carbon emission into Earth's surface reservoirs. Remarkable paleoceanographic change and paleoenvironmental perturbations during the T-OAE are well recognized in many sedimentary basins. However, the spatio-temporal evolution of oceanic salinity during this time interval remains poorly constrained, which hampers our understanding of the hydrological condition of aqueous system in critical area. Here, high-resolution B/Ga analyses were performed on two typical sections (i.e. lagoonal Biluocuo section and shelfal Suobucha section) from distinct sedimentary facies in the Qiangtang Basin to trace Toarcian salinity variations. This study provides the first record of stratigraphic variations in bulk sediment B/Ga ratios during the early Toarcian from the eastern Tethyan Ocean. The B/Ga ratios indicate that the lagoonal area was characterized by prevalent hypersaline conditions, while the shelf area experienced freshwater to normal marine conditions. Pre-T-OAE trends suggest increasing shelf salinity and decreasing lagoonal salinity likely orchestrated by eustatic rise. A coeval decline in B/Ga ratios within the lagoonal and shelfal sections at the onset of the T-OAE reveals a progressive seawater freshening at this time. This basin-scale salinity decrease during the T-OAE is attributed to a climate-driven shift in regional hydrology. Global warming-induced intensification of hydrological cycle led to increased precipitation and runoff, reducing surface water salinity in the Tethys realm through elevated riverine freshwater input. This study advances our understanding of the spatial extent and scale of freshwater perturbations and provides new salinity evidence for an intensified hydrological cycle in response to global warming during the T-OAE.