Reappraisal of the Angerman River decay time estimate and its application to determine uncertainty in Earth viscosity structure

Relative sea level histories from previously glaciated areas have been used to study Earth rheology and ice sheet evolution during the last glacial cycle. The analysis of postglacial decay times has been used to place estimates on Earth viscosity structure that are relatively independent of uncertainty in the local ice history. Reconstructed sea levels from Angermanland, Sweden have been commonly adopted for this purpose. We have assessed and compiled an updated relative sea level curve for this region, combining both varve-dated and radiocarbon dated index points. We fitted an exponential curve to the observations, taking into account estimates of eustatic sea level rise, elevation uncertainties as well as the geographical spread of the data sites to arrive at a decay time range (2 sigma) of 4.2-4.9 kyr for the whole record length (0-8 kyr) and 4.2-6.2 when 0-7 kyr fits are included. We computed model decay times using a large suite of over 900 ice and earth model combinations based on over 400 three-layer Earth viscosity models and more than 30 ice history reconstructions. Based on these extensive results, we confirm that decay time estimates are relatively independent of the regional ice model (at least within the range of ice chronology uncertainties) and so this data parametrization provides a relatively robust measure of Earth viscosity structure. We find that the observational constraints listed above are satisfied by 29 (8 kyr record) and 52 (7 and 8 kyr record) of the viscosity models considered. These subsets define uncertainty ranges in upper and lower mantle viscosity that are interdependent (Fig. 5). Consistent with previous analyses, we find that the observational decay time estimate does not provide useful constraints on model lithospheric thickness (within the range explored; 46-146 km).