Estimating past continental ice volume from sea-level data

We predict sea-level change since the Last Glacial Maximum (LGM) at four far-field sites (Barbados, Bonaparte Gulf. Huon Peninsula and Tahiti) using a revised theoretical formalism. This formalism includes a time-varying shoreline geometry, an accurate treatment of sea-level variations near regions of ice retreat and the influence of glacial cycle perturbations in Earth rotation. We elucidate the physics of far-field sea-level change by de-constructing the predicted signal into spatially uniform versus spatially varying components, as well as isolating contributions due to the ice load, the ocean load and the rotational potential. We demonstrate that the sum of these three contributions plus the spatially uniform sea-level fall associated the with retreat of grounded marine ice sheets can produce a significant difference between predictions of sea-level change at far-field sites and the eustatic (or meltwater) curve associated with the adopted ice model. This difference is site and time dependent. For example, the total sea-level rise since the LGM predicted using our ice-earth model can be either smaller (e.g., Barbados) or larger (e.g., Tahiti) than the eustatic sea-level change. Finally, we review procedures that have been applied to estimate continental ice volume from far-field sea-level observations and apply these procedures to data from Barbados and Bonaparte Gulf. Applying an ice-earth model that is tuned to fit the Barbados data, we estimate a change in grounded ice volume from the LGM to the present of 43.5-51 X 10(6) km(3) based on Barbados data and an LGM ice volume estimate of 51 x 10(6) km(3) based on Bonaparte Gulf data. Our results for the Bonaparte Gulf data are consistent with the recent study by Yokoyama et al. (Nature 406 (2000) 713). These LGM ice volume estimates map into a eustatic (or meltwater) sea-level rise of 115-135m. Taking into account plausible variations in the adopted radial earth model introduces uncertainties in the range of +/-1.5 x 10(6) km(3) for ice volume estimates based on the Barbados data. (C) 2001 Elsevier Science Ltd. All rights reserved.