GLACIAL ISOSTATIC ADJUSTMENT AND COASTLINE MODELLING

Glenn Milne

Dept of Geological SciencesUniversity of Durham, UK

Outline

(1) 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

d18O=1000 x18O/16O(sample) - 18O/16O(standard)

18O/16O(standard)

Oxygen Isotope Record

GLACIAL ISOSTATIC ADJUSTMENT

Surface Mass Redistribution

Earth Earth Response

• Relative sea level• Geopotential• Rotation vector• 3D solid surface deformation

ModelSurface load + Rotational potential

Rheological Earth model

Better understanding of GIA process

Constraints on Earth rheology

Constraints on surface mass redistribution

GIA MODEL

Earth Forcing Earth Rheology

Rotational potential

Euler equations

Surface loading

Ice

Interdisciplinary approachOcean

Sea-level equation

Other?

Ice dammed lakes Sediment redistribution

Impulse response formalism Linear Maxwell rheology 1D structure

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

• 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)

j p jT(θ,φ,t )=T(θ,φ,t )-RSL(θ,φ,t )

• Choose optimal model parameters and predict changes in relative sea level for period of interest

• Compute palaeotopography via the relation

Eustatic Sea-Level Change

ice ice

water water

-ρ ΔV (t)ΔS(t)=

ρ A

• Mass conservation

• Earth is rigid and non-rotating

• Ice and water have no mass

Glaciation-Induced Sea-Level Change

S(t) = G(t) – R(t)

• Geoid perturbation, G(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(t) vertical earth deformation associated with surface mass redistribution and changing rotational potential

• Surface mass conservation, HG(t)

VOW(t) = G(t) – R(t) + HG(t)AO

HG(t) = VOW(t) AO-1 – AO

-1 G(t) – R(t)

“eustatic” “syphoning”

+ HG(t)

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).

20 kyr BP

10 kyr BP