Searching for eustasy in deglacial sea-level histories

Perturbations to the Earth's gravity field and solid surface associated with glacial isostatic adjustment (GIA) cause the total (observable) sea-level change to depart from the eustatic curve, which is defined as a spatially uniform height shift of the ocean surface to accommodate any mass gained/lost from grounded ice. In this study we apply a state-of-the-art model of GIA-induced sea-level change to quantify the magnitude and spatial form of this departure at the global scale for a range of model parameters with an aim to identify regions that are well suited to obtain accurate and precise estimates of eustatic sea level (and therefore past grounded ice volume). In general, our results indicate that eustatic sea level is not a directly measurable quantity and so must be estimated by subtracting a model-derived estimate of non-eustatic contributions from observations. In this regard, we use our results to isolate regions where this procedure can be applied with optimal accuracy and precision. That is, where (1) the GIA predictions are relatively insensitive to plausible ranges in input parameters and (2) where the non-eustatic contribution is small (i.e., the predicted sea level closely approximates the eustatic value). We present maps that can be employed by the field community to identify areas where sea-level reconstructions would be well suited to arrive at robust estimates of eustatic sea level. Note that sea-level changes associated with tectonic motion and changes in ocean water temperature/salinity, which also lead to departures from eustasy, are not considered in this analysis.