A Revised Estimate of Early Pliocene Global Mean Sea Level Using Geodynamic Models of the Patagonian Slab Window

Abstract

Paleoshorelines serve as measures of ancient sea level and ice volume but are affected bysolid Earth deformation including processes such as glacial isostatic adjustment (GIA) and mantle dynamictopography (DT). The early Pliocene Epoch is an important target for sea-level reconstructions as it containsinformation about the stability of ice sheets during a climate warmer than today. Along the southeastern passivemargin of Argentina, three paleoshorelines date to early Pliocene times (4.8–5.5 Ma), and their variable present-day elevations (36–180 m) reflect a unique topographic deformation signature. We use a mantle convectionmodel to back-advect present-day buoyancy variations, including those that correspond to the Patagonian slabwindow. Varying the viscosity and initial tomography-derived mantle buoyancy structures allows us to computea suite of predictions of DT change that, when compared to GIA-corrected shoreline elevations, makes itpossible to identify both the most likely convection parameters and the most likely DT change. Our simulationsilluminate an interplay of upwelling asthenosphere through the Patagonian slab window and coincidentdownwelling of the subducted Nazca slab in the mantle transition zone. This flow leads to differentialupwarping of the southern Patagonian foreland since early Pliocene times, in line with the observations. Usingour most likely DT change leads to an estimate of global mean sea level of 17.5 ± 6.4 m (1σ) in the earlyPliocene Epoch. This confirms that sea level was significantly higher than present and can be used to calibrateice sheet models