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Investigating the Stability of Subglacial Lakes with a Full Stokes Ice-Sheet Model

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Investigating the Stability of Subglacial Lakes with a Full Stokes Ice-Sheet Model Journal of Glaciology, Vol. 54, No. 185, 2008 353 Investigating the stability of subglacial lakes with a full Stokes ice-sheet model Frank PATTYN Laboratoire de Glaciologie, Faculte´ des Sciences, CP160/03, Universite´ Libre de Bruxelles, B-1050 Brussels, Belgium E-mail: [email protected] ABSTRACT. Despite the large amount of subglacial lakes present underneath the East Antarctic ice sheet and the melt processes involved, the hydrology beneath the ice sheet is poorly understood. Changes in subglacial potential gradients may lead to subglacial lake outbursts, discharging excess water through a subglacial drainage system underneath the ice sheet. Such processes can eventually lead to an increase in ice flow. In this paper, a full Stokes numerical ice-sheet model was employed which takes into account the ice flow over subglacial water bodies in hydrostatic equilibrium with the overlying ice. Sensitivity experiments were carried out for small perturbations in ice flow and basal melt rate as a function of ice thickness, general surface slope, ice viscosity and lake size, in order to investigate their influence on the subglacial potential gradient and the impact on subglacial lake drainage. Experiments clearly demon- strate that small changes in surface slope are sufficient to start and sustain episodic subglacial drainage events. Lake drainage can therefore be regarded as a common feature of the subglacial hydrological system and may influence, to a large extent, the present and future behavior of large ice sheets. INTRODUCTION similar magnitude, but underneath an ice stream (Whillans More than 145 subglacial lakes have been identified under- Ice Stream, West Antarctica, just upstream of the grounding neath the Antarctic ice sheet, mainly by analyzing airborne line). In this case, a surface lowering of 9 m is observed over ∼ 3 radio-echo sounding profiles. Subglacial lakes are charac- Lake Engelhardt, corresponding to a discharge of 1.25 km 3 −1 terized by a strong basal reflector, a constant echo strength over a period of 18 months at a rate of 40 m s . The events (corroborating a smooth surface) and a flat surface compared are therefore similar in timing and magnitude. to the surroundings with a slope around ten times, and in The above-mentioned observations stem from rapid the opposite direction to, the lake surface slope (Siegert and changes at the surface from which the drainage events are others, 2005). The ice column above a subglacial lake is inferred. Direct observation of such subglacial events, and therefore in hydrostatic equilibrium. Most lakes lie under a hence knowledge of their triggering mechanisms, is lack- thick ice cover of >3500 m and are therefore situated close ing. However, one way to investigate possible mechanisms to ice divides. Although subglacial lakes are usually small leading to rapid subglacial drainage is through ice-sheet (<20 km in length), the largest, Vostok Subglacial Lake, con- modeling in which ice flow across and interactions with a tains 5400±1600 km3 of water (Studinger and others, 2004). subglacial lake are taken into account. A full Stokes ice-sheet Assuming an average water depth of 1000 m for large lakes model, taking into account all stress components acting on and 100 m for shallow lakes, the known volume of water in the ice, is probably the most suitable tool for this (e.g. Mart´ın Antarctic subglacial lakes is ∼22 000 km3 or approximately and others, 2003; Zwinger and others, 2007). Therefore, such 25% of the global water in surface lakes (Subglacial Antarc- a model is developed and coupled to the subglacial lake en- tic Lake Environments (SALE) Workshop report, http://salepo. vironment to investigate the stability of subglacial lakes and tamu.edu/saleworkshop2006). This is equivalent to a uniform their drainage through a sensitivity analysis. It is shown that sheet of water ∼1 m thick if spread out beneath the entire the coupled ice-sheet/lake system, due to hydrostatic equilib- Antarctic ice sheet. rium, exhibits a feedback mechanism that leads to periodic Subglacial lakes are defined as relatively closed and stable lake flushing on decadal timescales. environments with long residence times and slow circula- tions (Kapitsa and others, 1996; Siegert and others, 2001; Bell and others, 2002). Such a huge amount of subglacial melt- MODEL DESCRIPTION water would pose no threat to the stability of the ice sheet In the ice-flow model, the different surfaces are expressed if it were not moving through a hydrological network. This with a right-handed (0, x, y, z) Cartesian coordinate system view has recently been challenged through the observation as depicted in Figure 1. The ice-surface and ice-bottom Carte- of rapid subglacial lake drainage events (Gray and others, sian representations are defined as z = S(x, y, t)andz = 2005; Wingham and others, 2006; Fricker and others, 2007). Bice(x, y, t), respectively, from which it follows that ice thick- − The observations by Wingham and others (2006) suggest the ness equals H(x, y, t)=S(x, y, t) Bice(x, y, t). The under- rapid transfer of subglacial lake water and the periodic flush- lying bedrock is defined by z = B(x, y). For ice resting on ing of subglacial lakes connected to other lakes that conse- the bedrock it follows that Bice(x, y, t)=B(x, y). When a > quently fill through a hydrological network. For the draining subglacial lake is present, however, Bice(x, y, t) B(x, y) lake (Adventure Trench Lake), they observe a surface lower- and the water depth of the lake is defined by W (x, y, t)= ∼ 3 − ing of 4 m, corresponding to a water discharge of 1.8 km Bice(x, y, t) B(x, y). − over a period of 16 months at a rate of 50 m3 s 1.Thean- The model approach is based on continuum modeling alysis by Fricker and others (2007) points to another event of and encompasses balance laws of mass and momentum, 354 Pattyn: Stability of subglacial lakes equations further reduced to two equations in the horizontal direction (see below). Rheological non-linearities are ignored, which is an ac- ceptable approximation as long as the stress perturbations remain small enough for the flow law to be linearized around the mean stress field (Gudmundsson, 2003). A few experi- ments were performed with values of n = 3, i.e. the expo- nent in Glen’s flow law (ε ˙ = Aτ n). Although the timing of drainage events was slightly different in these cases, due to the different parameter settings, the overall sensitivity of the system to parameter perturbations was the same. Fig. 1. Ice-sheet surfaces expressed in a Cartesian coordinate system. For a linear flow law relating deviatoric stresses to strain rates, the effective viscosity in Equation (3) equals 1 η = . (10) extended with a constitutive equation. Treating ice as an in- 2A compressible fluid with constant density, the equation for The flow rate factor A is temperature-dependent (Paterson, conservation of mass implies that 1994). For this study, temperature–flow coupling is not taken ∂v ∂vy ∂v into account and ice is considered to be at the pressure- ε˙ =0 or x + + z =0, (1) ii ∂x ∂y ∂z melting point at the base all the way (a prerequisite for having i subglacial lakes). Nevertheless, the flow parameter A is used where (vx , vy , vz ) denote the respective x, y, z coordinates of as a major sensitivity parameter to investigate a large range ε the velocity vector v and ˙ii are the normal strain rates. Ice of ice viscosities. In reality, however, thermomechanical incompressibility implies that the stress tensor must be split coupling would allow for changes in viscosity over the ver- into a deviatoric part and an isotropic pressure P , i.e. tical, hence influencing the vertical profile of the velocity − . field, with stiff ice on top of a more deformable ice layer. T = T P I (2) This would certainly result in different timings of subglacial The constitutive equation for ice then links deviatoric stresses lake drainage but not alter the trends in sensitivity of the to strain rates: system to parameter perturbations, explored later. η T =2 e˙ ,(3)Velocity field where T and e˙ are the deviatoric stress and strain-rate tensor, Combining the force-balance Equations (5–7) with the con- η respectively, and is the effective viscosity. stitutive Equation (3) and replacing the isotropic pressure by Equation (8) leads to Stokes’ equations ∂ ∂ ∂ ∂τ Neglecting acceleration terms, the linear momentum bal- zz 4ηε˙xx +2ηε˙yy + 2ηε˙xy + (2ηε˙xz ) = − ance is written as: ∂x ∂y ∂z ∂x ρ − ρ (11) div T + iceg =divT grad P + iceg =0, (4) and −2 ρ where g is the gravitational acceleration (9.81 m s )and ice ∂ ∂ ∂ ∂τ −3 ηε ηε ηε ηε − zz . is the ice density (910 kg m ). Since only acceleration due ∂ 4 ˙yy +2 ˙xx + ∂ 2 ˙xy + ∂ 2 ˙yz = ∂ to gravity in the vertical is considered, y x z y (12) ∂τ ∂τ ∂P xy ∂τ xx − + + xz =0, (5) ∂x ∂x ∂y ∂z Strain rates are related to velocity gradients by definition, so ∂τ ∂τ ∂ ∂τ ∂ ∂ yx yy − P yz ε 1 vi vj . ∂ + ∂ ∂ + ∂ =0, (6) ˙ij = ∂ + ∂ (13) x y y z 2 xj xi ∂τ ∂τ ∂τ ∂ zx zy zz − P ρ . Combining Equation (13) with Equations (11) and (12) leads ∂ + ∂ + ∂ ∂ = iceg (7) x y z z to an expression for the horizontal velocity field, which is By definition, the isotropic pressure is equal to P = presented in Appendix A. An expression for the vertical vel- − / τ ocity vz is obtained through vertical integration of the in- 1 3 i ii . Making use of Equation (2), we derive compressibility condition (1).
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