Mesozoic Tethyan climate evolution and preconditioning for marine deoxygenation

The Mesozoic Tethys Ocean hosted widespread organic-rich deposition. However, proxy records come from only a few sites, and previous simulations have focused on individual oceanic anoxic events (OAEs) or a few time slices. Here, using 19 equilibrium palaeoclimate simulations with the fully coupled Community Earth System Model (CESM), we characterize the long-term evolution of the Tethys Ocean from 250 to 70 Ma. Tethyan warm-pool sea surface temperature was highest in the Triassic (~33 °C), decreased to a Jurassic minimum (~30 °C), and recovered in the Cretaceous (~32.5 °C), while the equator-to-mid-latitude SST gradient varied broadly in antiphase. As Pangaea broke up, the surface circulation fragmented from a relatively coherent large-scale structure into local cells, while the decline in signed regional-mean precipitation seasonality reflected both a modest weakening of local seasonal amplitude and stronger spatial cancellation. Ideal age indicates intermediate and deep waters in the Cretaceous were generally renewed more slowly, forming a background state more susceptible to oxygen depletion. The slower renewal of Tethyan intermediate and deep waters is associated with stronger upper-ocean stratification and is further conditioned by changes in basin connectivity and water-mass pathways. The global Eulerian overturning circulation varies non-monotonically and is therefore treated as a background diagnostic rather than a direct proxy for regional Tethyan ventilation. Furthermore, modeled upwelling in the Tethys Ocean was generally strong in the Cretaceous, which, together with sluggish circulation, created conditions more prone to anoxia than in the Triassic and Jurassic. Our findings provide a basis for understanding the spatiotemporal pattern of Tethyan anoxia.