New insights into Meltwater Pulse 1a from Empirical Sea-level Reconstructions and Numerical Modelling

Abstract

Understanding past sea-level dynamics is crucial for future predictions of sea-level change. A past analogue of a rapidly melting ice sheet can be found in the deglaciation of the Eurasian Ice Sheet (EIS) and its impact on global sea-level during periods of excessive ice sheet melting. Meltwater Pulse 1a (MWP1a) (14.65-14 ka BP) is a global event during which a warming period in the Northern Hemisphere led to accelerating melt of ice sheets, causing a global sea-level rise of 12-14 m over the stretch of ~340 years. The contribution from the EIS to MWP1a remains widely debated. This study offers new insights into MWP1a through an unique integration of relative sea- level (RSL) data from the southwestern coast of Norway with simulated RSL scenarios from numerical modelling.

First, three new sea-level index points (SLIPs) were constructed based on isolation basin analysis, contributing to the existing relative sea-level curve from Bokn, located along the coast of southwestern Norway. The marine limit of the area was found to coincide with the deglaciation of Bokn, around 15,560 ± 295 calibrated years before present (cal. BP) and was constrained to an altitude of 21.73 ± 0.78 m above present-day sea level (asl). Furthermore, the lake Søra Jøsenvatnet, which has a threshold altitude of 18.93 ± 0.34 m asl, was found to be isolated from the sea around 14,010 ± 175 cal. BP, contributing to a better understanding of local changes in RSL in a near-field site during MWP1a. The basin was found to be inundated by the sea again during the Younger Dryas (YD) transgression, an unique event for the southwestern coast of Norway, at 11,870 ± 145 cal. BP. The final isolation of Søra Jøsenvatnet could not be precisely determined due to disturbance of the sediments at the YD-Holocene transition.

Second, a numerical sea-level model was employed to explore the impact of different forcings on RSL changes in the near-field. A sensitivity analysis revealed high dependencies on the solid Earth rheology and the ice thickness history that the model is supplied with, indicating that simulated local RSL from a near-field site can change up to almost 50% with changing Earth rheology variables. The model experiments suggest a potential contribution of the EIS to MWP1a ranging from 1.27 to 7.25 m. Through integration with the updated empirical RSL curve from Bokn, the range was narrowed down to 4.17-5.58 m, supporting a larger contribution from the EIS as suggested in several recent studies. However, the estimate is highly limited by the large dependency on the ice thickness history and the solid Earth rheology for local RSL reconstructions in the study area. These findings show that high-accuracy ice thickness histories and a better understanding of the local solid Earth structures are crucial for simulating RSL in a near-field area such as Bokn. Further research, utilising a wider set of empirical data integrated with larger ensembles of simulations, is required to better constrain the EIS contribution to MWP1a.