Interactions between Ice Sheet Dynamics and Glacial Isostatic Adjustment

Abstract

The Antarctic ice sheet is a complex system highly influenced by global and local processes and characteristics including a varying bedrock elevation and structure of the solid Earth and a changing climate. Sea level rise has a high impact on society and the improvement of forecasts are vital to generate both adaptation and mitigation strategies. A recent comparison of 15 ice sheet models projected that the Antarctic Ice Sheet could contribute -7.8 to 30 centimeters of sea level rise between 2015 and 2100, meaning that sea level rise could increase a lot although the uncertainty is high. To better predict the future of the AIS, more accurate simulations of the evolution of the AIS are needed. The Antarctic ice sheet consist of three main components: grounded slow-moving ice, fast flowing ice streams or outlet glaciers and floating ice shelves. Over glacial-interglacial cycles, the evolution of an ice sheet is influenced by Glacial Isostatic Adjustment (GIA) via two negative feedback loops. First, vertical bedrock deformation due to a changing ice load alters ice-sheet surface elevation. Second, bedrock deformation will change the location of the grounding line of the ice sheet. GIA is mainly determined by the viscosity of the interior of the solid Earth which is radially and laterally varying. Underneath the Antarctic Ice Sheet (AIS), there are relatively low viscosities in West Antarctica and higher viscosities in East Antarctica, which affect the response time of the above-mentioned feedbacks. However, most ice-dynamic models do not consider lateral variations of viscosity in the upper mantle in GIA feedback loops when simulating the evolution of the AIS. The main research question of this study is: •What is the effect of the interaction between Glacial Isostatic Adjustment and ice sheet dynamics on the Antarctic Ice Sheet growth during the last glacial cycle? This study presents a new method to investigate 3D GIA feedback effects in detail at any chosen period during the last glacial cycle. The method is applied using ANICE and a 3D GIA FEM model. This led to the development of a fully coupled ice dynamic-3D GIA model with coupling timesteps of 1000 and 5000 years. Following the new method, the model computations alternate between the ice-sheet model, ANICE, and a 3D Finite Element Method model until convergence of the ice thickness occurs at each timestep. We simulate the evolution of the AIS from 120 000 years to 115 000 years before present, considering 1D and non-linear 3D rheologies. The results of the coupled model are discussed in detail for the period 120,000 years to 115,000 years before present with a focus on the Siple Coast and the Ross Ice Shelf. The maximum difference between the uncoupled deformation (iteration 1) and the coupled deformation (average between the last two iterations) for the period 120,000 to 115,000 years BP is 3 to 8 mm per year, depending on the viscosity of the upper mantle. The maximum difference in ice thickness at 115,000 years BP is 50 meters close the Ronne Ice Shelf and the Ross Ice Shelf. The grounding line position differs up to 80 meters when applying the coupling method compared to the uncoupled result. The increases in deformation using a 3D wet rheology with a grain size of 10 mm are highest at the Siple Coast, the Ronne Ice Shelf, and several other locations along the grounding line of the AIS. The results of this study emphasize the importance of the 3D GIA feedback effects when simulating the evolution of the AIS during the last glacial cycle. Therefore, the GIA feedback effects should be taken into account in future studies.