Century-Scale Relative Sea-Level Changes in West Greenland - Unravelling the Contributions from the Cryosphere and the Ocean

This paper interprets high resolution relative sea-level (RSL) reconstructions obtained from salt-marsh deposits in Greenland in terms of possible contributions from local ice mass balance, density-related variations in sea-level and contributions from cryospheric sources outside of Greenland. The RSL trends extracted from the marshes are significantly different to millennial-scale trends determined from isolation basins in their respective areas. The main aim of this study is to determine whether these differences can be used to constrain spatial and temporal changes in the history of the ice sheet during the past millennium. At sites in the west of Greenland near Aasiaat (68.6N,-52.6W) and Sisimuit (66.1N,-53.6W), the salt marsh data show that a major slowdown in sea-level rise from ~2.5-3mm/yr to ~0.5mm/yr occurred around 1500-1600 AD, with no significant departure from a 0.5mm/yr trend thereafter. Substantial contributions from oceanographic changes and cryospheric sources outside of Greenland are ruled out as dominant drivers of this departure from the observed millennial-scale trend. Model sensitivity tests suggest that the slowdown in RSL is most likely the combined contribution of dynamic-related ice loss from Jakobshavn Isbrae and a continuing viscous earth response to mass loss during a period of elevated temperatures spanning approximately 1000-1400AD, known as the Medieval Climatic Optimum (MCO). Marginal ice loss beginning around 1500AD and continuing to present is deemed an unlikely scenario to drive the RSL slowdown since this is incompatible with climatic conditions of the Little Ice Age (LIA). In the South of Greenland, near Nanortalik (60N,-44.7W), a similar slowdown in sea-level is observed. However, this occurs approximately 200 years later compared to Sisimuit and Aasiaat. During the period from 1500-1800AD, RSL trends at Nanortalik are around ~2-3mm/yr higher than the background millennial-scale trend (+1.1mm/yr). This is compatible with expected growth of the ice sheet during the LIA. In summary, the RSL data from these three sites highlight important regional differences in sea level over the last millennium. These data also indicate that the ice sheet has experienced significant changes in the past few centuries that most current ice models, calibrated to millennial-scale RSL data, do not incorporate. The presentation will consider this implication of this omission with regard to the interpretation of GPS-derived rates of vertical land motion and gravity changes from GRACE.