ALBERT

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: 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 Laurentide ice sheet was the largest late Pleistocene ice mass and the largest contributor to Holocene pre-industrial sea-level rise. While glaciological dates suggest final ice sheet melting between 8 and 6 ka, inversion of sea-level data indicates deglaciation at ca. 7 ka. Here, we present new chronostratigraphic constraints on Laurentide ice sheet disappearance based on Holocene relative sea-level observations from the tectonically stable north coast of Java, Indonesia. Age-elevation data from the flat upper surfaces of 13 fossil intertidal corals (i.e., microatolls) indicate that the Java Sea experienced a relative sea level of 1.3 ± 0.7 m above present between 6.9 and 5.3 ka. To determine uncaptured relative sea-level trends within the observational uncertainties of this apparently constant highstand, we analyzed the internal structure of three sliced microatolls from the same site to produce a high-resolution data set. These data were used to statistically model relative sea-level rates and trends. Employing the data with the model provided evidence for a short-lived rise of relative sea level from 1.0 ± 0.3 m above present at 6.7 ± 0.1 ka to 1.9 ± 0.3 m above present at 6.4 ± 0.1 ka. The end of this rise likely represents the last input of meltwater from the vast Laurentide ice sheet, which, consequently, collapsed at least 400 yr later than assumed by some widely used models of glacial isostatic adjustment. Incorporating these new results into such predictive models will help to better understand the geographical variability of future sea-level rise as a result of global warming.