Spatial biases in oxygen-based Phanerozoic seawater temperature reconstructions

• Mineral δ 18 O reflects temperature and composition of seawater (δ 18 O sw ). • We use climate models and the co-variation of δ 18 O sw and salinity. • We estimate spatial variability in δ 18 O sw during the Phanerozoic. • We quantify temperature reconstruction bias resulting from neglecting local δ 18 O sw . • We identify regions and periods of strongest temperature reconstruction bias. Stable oxygen isotopes (δ 18 O) are routinely used to reconstruct sea-surface temperatures (SSTs) in the geological past, with mineral δ 18 O values reflecting a combination of the temperature and oxygen isotope composition of seawater (δ 18 O sw ). Temporal variation of mean-ocean δ 18 O sw is usually accounted for following estimates of land-ice volume. Spatial variations in δ 18 O sw , however, are often neglected or corrected using calibrations derived from the present-day or recent past. Geochemical methods for constraining δ 18 O sw and isotope-enabled general circulation model (GCM) simulations are still technically challenging. This lack of constraints on ancient δ 18 O sw is a substantial source of uncertainty for SST reconstructions. Here we use the co-variation of δ 18 O sw and seawater salinity, together with GCM simulations of ocean salinity, to propose estimations of spatial variability in δ 18 O sw over the Phanerozoic. Sensitivity tests of the δ 18 O sw -salinity relationship and climate model, and comparison with results of isotope-enabled GCMs, suggest that our calculations are robust at first order. We show that continental configuration exerts a primary control on δ 18 O sw spatial variability. Complex ocean basin geometries in periods younger than 66 Ma lead to strong inter-basinal contrasts in δ 18 O sw . Latitudinal SST gradients may be steeper than previously suggested during most of the Mesozoic and Cenozoic. This work has limitations, with δ 18 O sw -salinity relationships being less reliable in both low-latitude epicontinental settings and high-latitude regions of deep-water formation. Whilst our calculations are limited use in correcting δ 18 O measurements for local δ 18 O sw , they identify the time slices and paleogeographical regions that should be prioritized for future work using isotope-enabled GCMs.