Glacial isostatic adjustment and sea-level change – State of the art report Glacial isostatic adjustment and sea-level change Technical Report TR-09-11 Glacial isostatic adjustment and sea-level change State of the art report Pippa Whitehouse, Durham University April 2009 Svensk Kärnbränslehantering AB Swedish Nuclear Fuel and Waste Management Co Box 250, SE-101 24 Stockholm Phone +46 8 459 84 00 TR-09-11 ISSN 1404-0344 CM Gruppen AB, Bromma, 2009 Tänd ett lager: P, R eller TR. Glacial isostatic adjustment and sea-level change State of the art report Pippa Whitehouse, Durham University April 2009 This report concerns a study which was conducted for SKB. The conclusions and viewpoints presented in the report are those of the author and do not necessarily coincide with those of the client. A pdf version of this document can be downloaded from www.skb.se. Preface This document contains information on the process of glacial isostatic adjustment (GIA) and how this affects sea-level and shore-line displacement, and the methods which are employed by researchers to study and understand these processes. The information will be used in e.g. the report “Climate and climate-related issues for the safety assessment SR-Site”. Stockholm, April 2009 Jens-Ove Näslund Person in charge of the SKB climate programme Contents 1 Introduction 7 1.1 Structure and purpose of this report 7 1.2 A brief introduction to GIA 7 1.2.1 Overview/general description 7 1.2.2 Governing factors 8 1.2.3 Observations of glacial isostatic adjustment 9 1.2.4 Time scales 9 2 Glacial isostatic adjustment systems 11 2.1 Introduction 11 2.2 The solid Earth 11 2.2.1 General description 11 2.2.2 Governing parameters and system dynamics 12 2.2.3 Representation in GIA models 13 2.3 The hydrosphere 14 2.3.1 General description 14 2.3.2 Governing parameters and system dynamics 14 2.3.3 Representation in GIA models 15 2.4 The cryosphere 16 2.4.1 General description 16 2.4.2 Governing parameters and system dynamics 16 2.4.3 Representation in GIA models 17 3 Governing equations 19 3.1 Introduction 19 3.2 Key definitions 19 3.3 Derivation of the sea-level equation 21 3.3.1 Conservation of momentum, mass, and ocean geometry 21 3.3.2 Perturbations to sea-level on a rigid Earth 22 3.3.3 Perturbations to sea-level on an elastic/viscoelastic Earth 23 3.3.4 Reconstruction of palaeotopography and shoreline migration 26 3.4 Physical processes contributing to relative sea-level change 26 3.4.1 Sea-level change due to deformation of the ocean’s bounding surfaces 27 3.4.2 Near-field relative sea-level changes 29 3.4.3 Far-field relative sea-level changes 30 3.5 Modifications to the sea-level equation 32 3.5.1 Shoreline migration 32 3.5.2 Treatment of floating and marine-grounded ice 34 3.5.3 Rotational effects 37 3.5.4 3-D Earth structure 38 3.5.5 Relative importance of modifications to the sea-level equation 38 3.6 Outputs of the sea-level equation 39 4 State-of-the-art GIA models 41 4.1 Introduction 41 4.2 Modelling inputs to the sea-level equation 41 4.2.1 Earth structure and rheology 42 4.2.2 Ice-loading 43 4.2.3 Ice sheet modelling 43 4.3 Methods for solving the sea-level equation 45 4.3.1 Solution domain and geometry 45 4.3.2 Normal mode and spectral methods 46 4.3.3 Finite element/finite volume methods 47 4.3.4 Other solution methods 48 4.3.5 Solutions with a time-dependent ocean function 48 5 4.3.6 Using GIA modelling to refine model inputs 49 4.4 State-of-the-art GIA models 50 4.4.1 Peltier 50 4.4.2 Mitrovica and Milne 51 4.4.3 Lambeck 54 4.4.4 Wu 55 4.4.5 Kaufmann 57 4.4.6 German Research Centre for Geosciences 58 4.4.7 Zhong, Paulson and Wahr 59 4.4.8 Sabadini, Spada et al. 60 4.4.9 Vermeersen 61 4.4.10 Fjeldskaar 61 4.4.11 Implications of GIA modelling for ice sheet history, global ice volumes and eustatic sea-level change 61 4.4.12 Implications of GIA modelling for Earth structure 62 4.5 Modelling advanced GIA processes 62 4.5.1 Shoreline migration 62 4.5.2 Floating and marine-grounded ice 63 4.5.3 Rotational effects 63 4.5.4 3-D Earth structure 64 4.6 Data sets used to constrain GIA models 74 4.6.1 Relative sea-level data 74 4.6.2 Relaxation-time spectrum 76 4.6.3 Geodetic data 76 4.6.4 Gravity data 78 4.6.5 Estimating rotational parameters 78 4.6.6 Regional stress indicators 79 4.7 Accuracy of solutions to the sea-level equation 79 4.8 Shortcomings of current GIA models 79 4.8.1 Sediment redistribution 79 4.8.2 Ice-dammed lakes 80 4.8.3 Non-GIA causes of relative sea-level change 81 4.9 Applications of GIA modelling 82 4.9.1 Constraining GIA model inputs 82 4.9.2 Climate 83 4.9.3 Interpretation of the sedimentary record 83 4.9.4 Sea-level change 83 4.9.5 Correcting gravity data 85 4.9.6 Shoreline migration 85 4.9.7 Predicting seismic/volcanic activity 85 4.10 Final remarks 85 References 87 Appendix 1 Acronyms and selected abbreviations 105 6 1 Introduction 1.1 Structure and purpose of this report This report outlines the physics of glacial isostatic adjustment (GIA), how this affects sea-level, and the methods which are employed by researchers to study and understand these processes. The report describes the scientific background into the processes and methods presented in /SKB 2006, section 3.3/. The purpose of this report is to provide a reference document for people who require a more in-depth understanding of GIA processes than is presented in the SKB report. The key components of the GIA system are described, and this is followed by a concise description of the processes that take place within and between these components during a glacial cycle. The report contains 4 chapters: • “Introduction” (chapter 1) • “GIA systems” (chapter 2) • “Governing equations” (chapter 3) • “State-of-the-art GIA models” (chapter 4) Chapter 2, “GIA systems”, describes the three main systems which are involved in the GIA process; the solid Earth, the hydrosphere and the cryosphere. The various parameters which govern the behaviour of these systems, and must be known in order to model GIA processes, are defined. Chapter 3, “Governing equations”, lays out the physics of GIA and derives the equations which must be solved to determine the redistribution of water over the surface of the Earth, and the solid Earth response. Secondary processes, such as ocean syphoning, are also described. The driving forces behind glacial cycles are briefly discussed. The methods used to solve these equations are laid out in chapter 4, “State-of-the-art GIA models”. In this chapter, the different approaches used by different groups of researchers are discussed, as are the relative accuracy of the methods. Recent improvements to the theory are described, as are current shortcomings of the models. The various data sets used to calibrate and verify the accuracy of the modelling are also briefly described in this chapter. In the past few years advances in computational speed have enabled researchers to develop models which attempt to account for the effects 3-D Earth structure upon GIA processes. The preliminary results from this research are also discussed within chapter 4. 1.2 A brief introduction to GIA 1.2.1 Overview/general description Glacial isostatic adjustment (GIA) is the response of the solid Earth to mass redistribution during a glacial cycle. For the past ~700,000 years the Earth’s climate has followed a cycle of alternating glacial and interglacial conditions, with a periodicity on the order of 100,000 years. During a glacial period the lower temperatures result in the growth of ice sheets at higher latitudes, and consequently water is removed from the oceans and relative sea-level falls. Conversely, during interglacial conditions several of these ice sheets have melted, water is returned to the oceans, and relative sea-level rises. The movement of water over the surface of the Earth, both as water and as ice, during a glacial cycle acts as a load upon the lithosphere. The Earth deforms in response to this force; subsiding under the load of an ice sheet or full oceanic basin, and rebounding once the ice sheets melt or water is removed from the oceanic basins. The deformation that takes place is isostatic. Isostasy refers to a concept whereby deformation takes place in an attempt to return the Earth to a state of equilibrium. In isostatic equilibrium the elevation of a column of the Earth’s crust is a consequence of its density profile, which defines how that column ‘floats’ upon the Earth’s mantle. The process of glacial isostatic adjustment refers to isostatic deformation related to ice and water loading during a glacial cycle. 7 There are many feedbacks within a glacial cycle, relating both to GIA and relative sea-level change, and the climate system. However, feedbacks relating to the climate system are beyond the scope of this report, and are not discussed in detail further.