Description: Global mean sea level during the mid-Pliocene Epoch, when CO〈sub〉2〈/sub〉 and temperatures were above those of the present day, was significantly higher as a result of reduced global ice sheet coverage. However, the extent to which ice sheets responded to Pliocene warmth remains in question, owing to high levels of uncertainty in proxy-based sea-level reconstructions and solid Earth dynamic models that have been compared with only a limited set of data constraints. Here, we present a global dataset of 11 wavecut scarps that formed over successive Pliocene sea-level oscillations and occur today at elevations varying from ~6 to 109 m above sea level. The present-day elevations of these features have been identified using a combination of high-resolution digital elevation models and field mapping. Using the MATLAB interface, TerraceM, we project the cliff and platform surfaces to determine the elevation of the scarp toe, which often is buried under meters of talus. We correct the scarp elevations for glacial isostatic adjustment and find that this process alone cannot explain observed differences in paleoshoreline elevations. We next calculate the signal associated with mantle dynamic topography by back-advecting the present-day three-dimensional buoyancy structure of the mantle and calculating the difference in radial surface stresses over the last 3 Myr using the convection code ASPECT. We include a wide range of present-day mantle structures (temperature and viscosity) constrained by seismic tomography models, geodynamic observations, and laboratory experiments. Finally, we explore to what extent models can reproduce the different shoreline observations and deformation along them.

Description: As ice sheets load Earth's surface, they produce ice-marginal depressions which, when filled with meltwater, become proglacial lakes. We include self-consistently evolving proglacial lakes in a glacial isostatic adjustment (GIA) model and apply it to the Laurentide ice sheet over the last glacial cycle. We find that the locations of modeled lakes and the timing of their disappearance is consistent with the geological record. Lake loads can deflect topography by 〉10 m, and volumes collectively approach 30–45 cm global mean sea-level equivalent. GIA increases deglaciation-phase lake volume up to five-fold and average along-ice-margin depth ≤90 m compared to glaciation-phase ice volume analogs—differences driven by changes in the position and size of the peripheral bulge. Since ice-marginal lake depth affects grounding-line outflow, GIA-modulated proglacial lake depths could affect ice-sheet mass loss. Indeed, we find that Laurentide ice-margin retreat rate sometimes correlates with proglacial lake presence, indicating that proglacial lakes aid glacial collapse.

Description: Sea-level projections depend on models calibrated with constraints of ice sheet sensitivity to past warm climate conditions. The early Pliocene Epoch is one important target for sea-level reconstructions since interglacial global mean temperatures were around 4 ˚C warmer than today. Paleoshorelines serve as measures of ancient sea level and ice volume but are deformed due to processes such as glacial isostatic adjustment (GIA) and mantle dynamic topography (DT). Along the southeastern passive margin of Argentina, three paleoshorelines date to early Pliocene times (4.8 to 5.5 Ma), and their variable present-day elevations (36 to 180 m) reflect a unique topographic deformation signature. We use a mantle convection model to back-advect present-day buoyancy variations, including those that correspond to the Patagonian slab window. Varying the viscosity and initial mantle buoyancy structures allows us to compute a suite of predictions of DT change that, when compared to GIA-corrected shoreline elevations, makes it possible to identify the most likely DT change. Our simulations illuminate an interplay of upwelling asthenosphere through the Patagonian slab window and coincident downwelling of the subducted Nazca slab in the mantle transition zone. This flow leads to upwarping of the southern Patagonian foreland since early Pliocene times, in line with the observations. Our preferred model of DT change leads to an estimate of global mean sea level of 17.5 ± 6.4 m (1σ) in the early Pliocene Epoch. This result confirms that sea level was significantly higher than present and provides important constraint for ice sheet model calibration.

Description: The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO2) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO2 beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO2 record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO2 thresholds in biological and cryosphere evolution.