Constraining Greenland Ice Sheet Mass Balance in the Last Millennia Using High Resolution Sea-Level Data

Relative sea-level (RSL) data provide the most powerful constraints on Greenland Ice Sheet (GIS) history available. In recent publications we have developed new millennial-scale RSL records using the "isolation basin" approach from west, south and southeast Greenland, significantly improving the precision of previous reconstructions. However, as awareness of the dynamic response of the GIS to climate has heightened, so we have shifted our focus to developing century- to decadal-scale RSL records from the last millennia that extend to the present day. Here we present a new RSL history for Nanortalik, a field site located on the southern tip of Greenland. Previous tidal observations from Nanortalik Harbour suggest RSL rise of c. 3.9 mm yr between AD 1885 and AD 1934, significantly faster than the late Holocene rate inferred from isolation basins of 1.5 to 2mm yr. We employ a new approach that combines thin salt marsh deposits with a diatom-based transfer function and 14C, 210Pb and 137Cs dating to reconstruct RSL on decadal to century-timescales since AD 1300. RSL since this time has risen c. 0.7 m at an average rate of 1.1 mm yr, less than both the late Holocene and tide gauge data suggest. We compare these results with RSL curves that span the same time interval from the Søndre Strømfjord and Disko Bugt areas of West Greenland. The latter sites record a slow-down in the rate of RSL change at c. AD 1600 that we hypothesise reflects a regional change in the mass balance history of the GIS at this time. Our work demonstrates significant regional-scale variability in RSL and, by inference, mass balance history of the western and southern sectors of the GIS that provide new constraints for ice sheet models.