GLACIAL ISOSTATIC ADJUSTMENT AND COASTLINE MODELLING Glenn Milne Dept of Geological Sciences University of Durham, UK

Outline General GIA • What is GIA? • Key observables • General model components • Constraining model parameters (2) Modelling coastline evolution • General idea • Predicting GIA-induced sea-level change • Example predictions

Oxygen Isotope Record 18O/16O(sample) - 18O/16O(standard) d18O=1000 x

GLACIAL ISOSTATIC ADJUSTMENT Surface Mass Redistribution Earth Earth Response Relative sea level Geopotential Rotation vector 3D solid surface deformation Model Surface load + Rotational potential Rheological Earth model Better understanding of GIA process Constraints on Earth rheology Constraints on surface mass redistribution

Ice history and earth rheology are the key inputs GIA MODEL Earth Forcing Earth Rheology Impulse response formalism Linear Maxwell rheology 1D structure Rotational potential Surface loading Euler equations Ice Other? Interdisciplinary approach Ice dammed lakes Sediment redistribution Ocean Sea-level equation Ice history and earth rheology are the key inputs

Constraining Model Parameters Largest uncertainties associated with ice sheet histories and earth rheology Near-field data give best constraints on local ice histories Near-field and far-field data can be effectively used to constrain earth viscosity structure Far-field sea-level data give best constraints on integrated ice melt signal Both forward and inverse modelling techniques are used

Modelling Coastline Evolution (Associated with GIA) Position of coastline is influenced by rising/falling relative sea level AND advancing or retreating marine-based ice

Modelling Coastline Evolution Driven by GIA-Induced Sea-Level Changes Choose optimal model parameters and predict changes in relative sea level for period of interest Compute palaeotopography via the relation Accuracy of prediction will depend on accuracy of the present-day topography data set and the accuracy of the relative sea-level prediction GIA model does not include tectonic motions or sediment flux (associated with marine or fluvial processes)

Eustatic Sea-Level Change Mass conservation Earth is rigid and non-rotating Ice and water have no mass

Glaciation-Induced Sea-Level Change S(q,f,t) = G(q,f,t) – R(q,f,t) + HG(t) Geoid perturbation, G(q,f,t) geopotential perturbed directly by surface mass redistribution and changing rotational potential and indirectly by earth deformation caused by these forcings Solid surface perturbation, R(q,f,t) vertical earth deformation associated with surface mass redistribution and changing rotational potential Surface mass conservation, HG(t) VOW(t) = G(q,f,t) – R(q,f,t)  + HG(t)AO HG(t) = VOW(t) AO-1 – AO-1 G(q,f,t) – R(q,f,t)  “eustatic” “syphoning”

Sea-Level Model ● Original theory published by Farrell and Clark (1976). ● Theory extended to include: (1) Time-dependent shorelines (Johnston 1993; Peltier 1994; Milne et al. 1999). (2) Glaciation-induced perturbations to Earth rotation (Han and Wahr 1989; Bills and James 1996; Milne and Mitrovica 1996; 1998). (3) The influence of marine-based ice sheets (Milne 1998; Peltier 1998).

Some Comments on the Sea-Level Algorithm Computing ∆G and ∆R requires knowledge of the sea-level change since this is a key component of surface load. Iterative process required at each time step in computation. Sea loading in given by C(θ,Φ,t) S(θ,Φ,t). Continent function can only be determined when RSL is known. Iterative process required over each glacial cycle. High spatial resolution computations are computationally intensive (CPU and disk space).

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