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Glacial isostatic adjustment (GIA) simulations using earth models that vary viscoelastic structure with depth alone cannot simultaneously fit geographic trends in the elevation of marine isotope stage (MIS) 5a relative sea level (RSL) indicators across continental North America and the Caribbean and yield conflicting estimates of global mean sea level (GMSL). We present simulations with a GIA model that incorporates three-dimensional (3-D) variation in North American viscoelastic earth structure constructed by combining high-resolution seismic tomographic imaging with a new method for mapping this imaging into lateral variations in lithospheric thickness and mantle viscosity. We pair this earth model with a global ice history based on updated constraints on ice volume and geometry. The GIA prediction provides the first simultaneous reconciliation of MIS 5a North American and Caribbean RSL highstands and strengthens arguments that MIS 5a peak GMSL reached values close to that of the Last Interglacial. This result highlights the necessity of incorporating realistic 3-D earth structure into GIA predictions with continent-scale RSL data sets. 

Abstract. In this review we compile and document the elevation, indicative meaning, and chronology of marine isotope substage 5a and 5c sea level indicatorsfor 39 sites within three geographic regions: the North American Pacific coast, the North American Atlantic coast and the Caribbean, and theremaining globe. These relative sea level indicators, comprised of geomorphic indicators such as marine and coral reef terraces, eolianites, andsedimentary marine- and terrestrial-limiting facies, facilitate future investigation into marine isotope substage 5a and 5c interstadial paleo-sealevel reconstruction, glacial isostatic adjustment, and Quaternary tectonic deformation. The open-access database, presented in the format of theWorld Atlas of Last Interglacial Shorelines (WALIS) database, can be found at https://doi.org/10.5281/zenodo.5021306 (Thompson and Creveling, 2021).