Relative sea level change in west Greenland during the last millennium

Relative sea level (RSL) data provide important long-term (century to millennial-scale) constraints on ice load history in Greenland. In this paper we present the results of a litho-, bio- and chronostratigraphic study designed to reconstruct RSL during the last millennium from salt marsh deposits recovered from a field site near to the town of Sisimiut, west Greenland. The stratigraphy at three marshes typically records an upwards transition from freshwater to salt marsh deposits. We use a quantitative (transfer function) and subjective model to reconstruct palaeomarsh elevation and changes in mean tide level (MTL) from 16 sediment profiles from these marshes. These palaeomarsh elevations are placed in a chronological framework established by 18 radiocarbon dated index points. Both models yield similar results and show MTL rose from −0.60 ± 0.20 m at c. 600 cal a BP to reach −0.10 ± 0.20 m at c. 400 cal a BP. After this time, MTL remained close to present (±0.20 m) until the present day although low sedimentation rates limit the resolution of our reconstructions during this interval. The initial rise in RSL can be explained by the dominance of non-Greenland processes, notably the collapse of the Laurentide forebulge, over local (Greenland) solid Earth uplift caused by postglacial ice unloading. This is despite some reloading of the crust that occurred during the neoglacial expansion of the Greenland Ice Sheet in this part of west Greenland. The slow-down in RSL at 400 cal a BP does not record either a change in the rate of Laurentide forebulge collapse or a change in eustatic sea level. We argue instead that this slow-down records the effects of a sustained reduction in local (Greenland) ice mass that persists over most of the past 400 years. The latter interval is widely acknowledged as a period of generally cooler than present conditions associated with the later stages of the Little Ice Age. During this period, field evidence suggests that in many areas the ice sheet had reached its maximum late Holocene extent. It is not obvious at this stage how to reconcile an expanding ice sheet with a reduction in ice load during this interval although we hypothesise it could reflect one or more of; i) a change in ice sheet dynamics; ii) reduced mass accumulation caused by cold and dry conditions, and; iii) a lagged response to earlier periods of climate warming