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Larsen C Ice Shelf – Marine Ice Formation in a Suture Zone and its Influence on the Dynamics of the Ice Shelf

The main source of ice for ice shelves is the glaciers that flow into them. Supercooled water can rise and freeze into suture zones downstream from promontories that separate these glaciers, which leads to the accretion of marine ice. Marine ice bodies have been observed in several ice shelves in Antarctica, but not much is known of the geometry, accretion rate, or influence of these ice bodies on ice dynamics. In this study, Jansen et al. investigate marine ice in a suture zone downstream from the Joerg Peninsula in the southern sector of the Larsen C Ice Shelf, Antarctic Peninsula. They present the data from ground penetrating radar which they based their inference on of the base of the meteoric ice and, combined with GPS data and assuming hydrostatic equilibrium; estimate the thickness of marine ice within a suture zone. They have shown that the Joerg Peninsula suture zone contains a layer of marine ice that increases in thickness alongflow from about 140 m – 180 m over a distance of 20 km, which implies an average basal accretion rate in the area of the study of about 0.5 m per annum. They examined the impact of this observed marine ice on the dynamics of the ice shelf by modelling the suture zone within an ice flow model. It is shown by the results, which replicate surface velocities that that have been observed and strain rates, that the ice of the suture zone, that is warmer and therefore softer, channels shear deformation. Decoupling of neighbouring flow units that have different flow velocities is enabled by this, while the structural integrity of the ice shelf is maintained.

Ice formed by the freezing of sea water, marine ice, has been found accreted to the base of several ice shelves around Antarctica (Thyssen, 1988; Morgan, 1972; Craven et al., 2009; Khazendar et al., 2009; Holland et al., 2001; Neal, 1979; Khazendar et al., 2001; Tison et al., 2001; Tison et al., 1998). The accretion process is an essential part of the ice-ocean interaction in the cavity, as well as of the mass balance of some ice shelves. The layer of marine ice extends all the way to the calving front in the case of the Amery Ice Shelf (Craven et al., 2009), and sometimes is detected in icebergs calved from the ice front (Warren et al., 1993), while most of the marine ice layer is lost at the Ronne Ice Shelf as a result of enhanced basal melting in the vicinity of the calving front (Oerter et al., 1992). The deposition of frazzle ice crystals at the base of an ice shelf is the main source of marine ice. Enhanced melting at the grounding line where the ice draft is usually at a maximum as a result of the pressure dependency of the melting point of the ice. The fresh supercooled meltwater is more buoyant that the surrounding water, therefore flowing upwards along the base of the ice shelf. As the supercooled meltwater rises it generates frazil ice which accumulates where the overlying ice is thinner and the water pressure is reduced, and marine ice may be generated by compaction (Lewis & Perkin, 1986; Bombosch & Jenkins, 1995). At the melting point of seawater the accumulated ice is formed and is therefore warm compared with the meteoric ice above (e.g., Paterson, 1994; Craven el al., 2009). In large areas where the draft is lower and the subsurface topography is smooth this accretion process can be active if there is a sufficient supply of supercooled meltwater, such as occurs in the central part of the Ronne Ice Shelf (Thyssen, 1988). Marine ice that originated from frazil ice crystal deposition has also been observed in the rift systems of ice shelves, where it can contribute to what is referred to as an “ice mélange”, which contains debris from the shoulders of the rift (e.g. MacAyeal et al., 1998; Tison et al., 2001; Khazendar et al., 2001; Khazendar & Jenkins, 2003; Pattyn et al., 2012).

It has been demonstrated (Holland et al., 2009) that marine ice is contained in the Larsen C Ice Shelf concentrated in narrow zones that originate downstream from promontories. The boundaries of ice shelf flow units, coming from inlets that feed the central part of the ice shelf, are delineated by these so-called suture zones. The smooth surface of suture zones compared with the neighbouring flow units which are mostly fractured at their margins (Luckman et al., 2012; McGrath et al., 2012), cause the suture zones to stand out on satellite imagery. The results obtained by Holland et al. (2009) were based on the hypothesis that in airborne ice penetrating radar surveys the lack of basal reflections indicates the presence of marine ice at the base of the ice shelves. An explanation of the lack of reflections from the interface between the marine ice and ocean can be the high dielectric absorption of the saline ice (Thyssen, 1988). As is observed in ice cores from the Ronne Ice Shelf, the transition from marine ice to ocean water may be on a spectrum-spanning water that contains slushy ice, frazil ice and fully consolidated ice (Treverrow et al., 2010; Craven et al., 2009; Oerter et al., 1992). These elongated zones play an important role in the stability of ice shelves, though the extent and proportion of mass of the marine ice bodies in the Larsen C Ice Shelf are smaller than those in the Ronne Ice Shelf or the Amery Ice Shelf, as they appear to prevent the propagation of lateral rifts (Holland et al., 2009; Glasser et al., 2009).

In this study Jansen et al. investigated a suture zone in the Larsen C Ice shelf in order to understand better the extent, thickness and likely accumulation rates of marine ice in such areas, as well as to examine the influence it has on the flow regime. They present geophysical field data in a 20 km x 20 km grid that was sampled at a 4 km grid spacing at the origin of the suture zone. Included in these data are profiles from Ground Penetrating Radar (GPR) along flow and across flow in order to map the basal boundary of the meteoric layer, and differential Global Position System (dGPS) data to measure the surface elevation along these profiles. They were able to map the extent of a marine ice layer in the suture zone of the Joerg Peninsula, and determine its thickness by combining the 2 datasets, dGPS and GPR, and assuming that the ice shelf is in hydrostatic equilibrium. They used the ice thickness data of the meteoric and marine ice, which was newly derived, as boundary conditions for a numerical ice shelf model, so as to quantify the influence of this marine ice layer on the flow regime downstream of the Joerg Peninsula.

Conclusions

The detailed geometry of a marine ice body within a suture zone near the Joerg Peninsula has been revealed by a geophysical field survey on the Larsen C Ice Shelf. The data obtained by Jansen et al. that the thickness of the marine ice requires rates of basal accretion that are significantly higher than the rates of freezing that were predicted previously by ice-ocean interaction modelling for the area. Strong evidence was also found that indicated that the basal accumulation of marine ice is ongoing over the 20 km survey region, as much as 35 km from the grounding line. The rate of freezing amounts to 0.5 m per annum when averaged over the 20 km long section of the ice shelf that was covered by the survey, though the amount of freezing can reach up to 2 m per annum in some areas. The rate of thickening that is due to basal and surface accumulation along the flow line is in good agreement with the ice thickness profile that had been derived from measurement of elevation independent from the data of Jansen et al.

Around the Joerg Peninsula suture zone numerical modelling of the dynamics of the ice has revealed that warm marine ice can provide a soft coupling between adjacent ice shelf flow units, enabling the lateral velocity gradients between them that have been observed. Suture zones that contain marine ice can lead to relaxation within the neighbouring flow units and prevent or reduce the propagation and growth of shear crevasses, and thereby stabilise the ice shelf in this region, by channeling shear deformation and thereby dissipating shear stress without fracturing.

Sources & Further reading

1. Jansen, D., A. Luckman, B. Kulessa, P. R. Holland and E. C. King (2013). "Marine ice formation in a suture zone on the Larsen C Ice Shelf and its influence on ice shelf dynamics." Journal of Geophysical Research: Earth Surface 118(3): 1628-1640.