Spatial biases in oxygen-based Phanerozoic seawater temperature reconstructions

Stable oxygen isotopes (δ¹⁸O) are routinely used to reconstruct sea-surface temperatures (SSTs) in the geological past, with mineral δ¹⁸O values reflecting a combination of the temperature and oxygen isotope composition of seawater (δ¹⁸O_sw). Temporal variation of mean-ocean δ¹⁸O_sw is usually accounted for following estimates of land-ice volume. Spatial variations in δ¹⁸O_sw, however, are often neglected or corrected using calibrations derived from the present-day or recent past. Geochemical methods for constraining δ¹⁸O_sw and isotope-enabled general circulation model (GCM) simulations are still technically challenging. This lack of constraints on ancient δ¹⁸O_sw is a substantial source of uncertainty for SST reconstructions. Here we use the co-variation of δ¹⁸O_sw and seawater salinity, together with GCM simulations of ocean salinity, to propose estimations of spatial variability in δ¹⁸O_sw over the Phanerozoic. Sensitivity tests of the δ¹⁸O_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 δ¹⁸O_sw spatial variability. Complex ocean basin geometries in periods younger than 66 Ma lead to strong inter-basinal contrasts in δ¹⁸O_sw. Latitudinal SST gradients may be steeper than previously suggested during most of the Mesozoic and Cenozoic. This work has limitations, with δ¹⁸O_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 δ¹⁸O measurements for local δ¹⁸O_sw, they identify the time slices and paleogeographical regions that should be prioritized for future work using isotope-enabled GCMs.