Amisor ice-coring sites

The hot-water drill system used in the AMISOR program has been described previously (Reference CravenCraven and others, 2004, Reference Craven2005). By exchanging the drill stem for a specialized coring attachment (similar to that described by Reference Engelhardt, Kamb and BolseyEngelhardt and others, 2000) ice cores can be retrieved from targeted depths within the borehole. Drilling within the same borehole is recommenced by reverting back to the drill stem. The nominal core diameter is 100 mm, but may be reduced to ˜60mm in sections of a core where there has been a mismatch between the coring attachment vertical velocity and the drill system water temperature. Cores vary from well-formed cylindrical samples over 1m long to 0.2 m lengths with tapered ends. Figure 1 indicates the four locations on the Amery Ice Shelf where boreholes have been drilled (to date) as part of the AMISOR project.

Boreholes drilled at the AM02 (69.71° S, 72.64° E) and AM03 (70.56° S, 72.39° E) sites during the 2000/01 and 2005/06 austral summers, respectively, are located within regions of net basal melting and ice cores were not retrieved. Table 1 summarizes the location, depth and ice type of cores recovered from the Amery Ice Shelf. Throughout this paper, AMISOR ice cores are identified using a label combining the borehole name and the core depth (e.g. AM01-240 represents a core from 240 m depth at the AM01 site). Where multiple cores were obtained from the same approximate depth via successive coring attempts a suffix (e.g. a, b, c,…) is added.

Table 1.

Location, depth and character of ice cores retrieved from the Amery Ice Shelf using a hot-water drill, as part of the AMISOR program. Indicated depths correspond to the top of core sections. Samples from the thermally drilled G1 ice core analysed as part of the current work are also listed. The AM01 and AM01b sites are separated by 1.6 km, due to bulk ice-shelf flow between the 2001/ 02 and 2003/04 field seasons. Locations where more than one core was obtained from the same approximate depth due to successive coring attempts are marked with an asterisk

The AM01 drilling site (2001/02) was situated in the same geographic location as G1, where a continuous 315 m ice core was thermally drilled in 1968. This site is located nearly along the centre line of the Amery Ice Shelf, ˜100 km from the shelf front. The AM04 site (2005/06) was 68 km upstream from AM01, on the same flowline (Fig. 2). The site marked JP, near Jetty Peninsula on Figure 2, is located 88.1 km upstream of AM04, along the AM01-AM04 flowline and indicates the start of the marine ice accretion zone (Reference Craven, Allison, Fricker and WarnerCraven and others, 2009). Downstream of the AM01 site, along the same flowline, the site marked LT (due to its proximity to the Amery Ice Shelf ‘Loose Tooth’ rift system;Reference Fricker, Popov, Allison and YoungFricker and others, 2001) is identified by Reference Craven, Allison, Fricker and WarnerCraven and others (2009) as a location of zero net basal melt or accretion.

At AM01 the hot-water drill coring barrel was used to retrieve ice cores at 240 m depth within the meteoric ice and depths of 290, 360 and 390 m within the marine ice layer (Reference CravenCraven and others, 2004). Figure 3 provides a schematic illustration of the ice-shelf thickness profile and structure along the AM04-AM01 flowline and indicates a horizon within the marine ice layer, referred to as the hydraulic connection depth. Marine ice below this depth has a sufficient concentration of interconnected brine-filled pores and channels to maintain a hydraulic connection to the underlying ocean (Reference CravenCraven and others, 2004). At AM01 the measured total ice thickness is 479 m, with a hydraulic connection noted at a drill depth of 376m, suggesting the lower 103 m of marine ice is permeable (Reference CravenCraven and others, 2004). The AM01-390 marine ice core is the only ice core retrieved from within this permeable layer.

Fig. 3.

Schematic Amery Ice Shelf thickness profile along the AM04-AM01 flowline. The hydraulic connection depth was detected during hot-water drilling operations (after Reference Craven, Allison, Fricker and WarnerCraven and others, 2009).

In the 2003/04 austral summer a further borehole, AM01b, was drilled at the AM01 field site, which had moved 1.6 km northeast with the bulk ice-shelf flow since the 2001/02 drilling season. A meteoric ice core was retrieved from 255 m depth. Digital video footage from the AM01b borehole verifies the meteoric/marine ice transition at 276 m (Reference CravenCraven and others, 2004, Reference Eicken, Oerter, Miller, Graf and Kipfstuhl2005). Attempts were made to retrieve a core across the meteoric/marine ice transition, but the inherent difficulty in precisely controlling the hot-water coring barrel depth resulted in two cores from ˜280 m depth, just below the meteoric/marine ice transition. Marine ice samples from ˜280 m depth in the geographically colocated G1 core have been analysed for comparison with the corresponding AM01b samples.

Upstream, at the AM04 site, a single meteoric ice core was retrieved from 350 m. Marine ice cores were retrieved at 50 m intervals from 400, 450 and 500 m depth. The total ice- shelf thickness, of 603 m, at AM04 was measured using the borehole video system and a borehole caliper tool. Reference Craven, Allison, Fricker and WarnerCraven and others (2009) report the meteoric/marine ice interface was detected at 396 m from video observations, giving a marine ice layer thickness of 207 m. A hydraulic connection depth of 530-535 m was registered during drilling.

Temperature profiles from thermistor strings left to freeze into the Amery Ice Shelf at the AM01 and AM04 drill sites (Fig. 4) indicate that below the hydraulic connection depth marine ice is nearly isothermal at the in situ freezing point. The thickness of the ice above the permeable marine ice close-off depth strongly influences the gradient of the temperature profile (Reference Craven, Allison, Fricker and WarnerCraven and others, 2009).

Fig. 4.

Temperature profiles at the refrozen AM01 and AM04 boreholes measured by in situ thermistor strings. Data are from Reference Craven, Allison, Fricker and WarnerCraven and others (2009). Temperatures from the G1 borehole (black curve) are shown for comparison with the current AM01 data.

Observed flow at the AM01 and AM04 borehole sites

Figure 5 shows the computed distribution of Amery Ice Shelf vertical strain rates and shear strain rates transverse to the local flow direction, calculated using the inferometric synthetic aperture radar (InSAR)-derived surface velocities of Reference Young and HylandYoung and Hyland (2002). Strain-rate tensors for the AM01 and AM04 borehole sites (Fig. 5) are given in Equations (1) and (2). Local orthogonal (right-handed) coordinate systems are defined with the x-direction tangent to the flowline, a transverse y-direction and the vertical z-direction normal to the x-y plane. As noted previously, the AM04 site is ˜68km upstream from AM01 along the same flowline.

Fig. 5.

Amery Ice Shelf strain rates calculated using the InSAR-derived surface velocities of Reference Young and HylandYoung and Hyland (2002). The AM01 and AM04 boreholes and the approximate flowline passing through the two sites are indicated. (a) Vertical strain rates. Negative values (blue) indicate compression. (b) Shear strain rates, , are aligned to the local flow direction. As Amery Ice Shelf velocities are highest near the centre of the shelf and decrease towards the margins, the sign of the transverse shear strain rate, , changes across the shelf. Horizontal and vertical axes indicate distance from the geographic South Pole.

Equation (1) indicates nearly two-dimensional flow, dominated by longitudinal extension and vertical compression, with no appreciable simple shear at the AM01 site, which is similar to the deformation described by Reference Budd, Smith, Wishart and OuraBudd and others (1967) for the geographically colocated G1 core site. The flow situation upstream at AM04, which is closer to the western margin of the shelf, is complex and differs from AM01. Figure 5 and Equation (2) indicate a significant transverse shear deformation, E_xy , relative to the local flow direction due to velocity gradients across the shelf. Longitudinal extension and vertical compression strain rates are approximately equivalent at AM04. Reference Budd, Smith, Wishart and OuraBudd and others (1967) report a similar flow situation of two-dimensional compression and extension, combined with transverse shear for a location towards the margin of the Amery Ice Shelf, west of the G1/AM01 site. Furthermore, upstream of AM04 the flowline passes through a region along the ice-shelf margin dominated by transverse shear, E _xy.

Equations (1) and (2) indicate that as ice flows northwards from AM04 to AM01 within the Amery Ice Shelf it accelerates and diverges, leading to significant longitudinal and (relatively lower) transverse strain rates. Due to the incompressibility of ice, there is a corresponding vertical strain rate, resulting in thinning of the ice shelf. Analysis of mass balance along the AM04-AM01 flowline by Reference Craven, Allison, Fricker and WarnerCraven and others (2009) indicates that the 207 m marine ice layer at AM04 would be reduced to 113.9m by strain thinning as it flows to AM01. As the measured marine ice thickness at AM01 is 203 m, an additional 89.1 m of ice must be added between AM04 and AM01. Because ice accumulated upstream of AM01 is also subject to strain thinning, the total accumulation between AM04 and AM01 is estimated to be 118.1 m, with an average annual accumulation rate of 1.16 ma^-1 (Reference Craven, Allison, Fricker and WarnerCraven and others, 2009). Thus, nearly half of the marine ice layer present at the AM01 site is accreted downstream of AM04.

Ice Crystal orientation fabrics and texture

All crystal orientation fabric data presented in this text were measured using a Russell-Head Instruments G50 automated ice crystal fabric analyser and are presented in a horizontal reference frame (lower hemisphere projection). Data were obtained from either horizontal or vertical thin sections. Where possible, horizontal thin sections were obtained from the top or bottom of well-formed cores. For cores with tapered ends and those with a large grain size relative to the core diameter, vertical thin sections were obtained from a chord section cut along the long axis of the cores. For crystal orientations obtained from vertical thin sections, the c-axis data have been rotated into the horizontal reference frame. To allow direct comparison with historical records (e.g. Reference Wakahama and KuroiwaWakahama, 1974) reported grain sizes, g_a , are the arithmetic mean grain area determined from the total thin section area and number of grains, N. Histograms of c-axis colatitudes (e.g. Fig. 6a) include a solid black curve to indicate the frequency of c-axis colatitudes in an ideal isotropic distribution.

Fig. 6.

Crystal orientation fabric and texture of meteoric ice from 255m depth at the G1/AM01 site. (a) AM01b-255c vertical thin section data. N = 81 crystals, mean grain area, g _a = 34:9mm2. Schmidt plot of c-axis orientations (top), colatitude frequency histogram (middle) and radial histogram of c-axis azimuths (bottom). (b) G1-255 site crystal orientation fabric data reproduced from Reference Wakahama and KuroiwaWakahama (1974). (c) Russell-Head Instruments G-50 automated fabric analyser photomicrograph of the AM01b-255c vertical thin section viewed between orthogonal-plane polarizing filters.

Meteoric ice samples AM01-240 and AM01b-255 display multiple-maxima crystal orientation fabrics and coarsegrained textures (Fig. 6). Multiple-maxima fabrics are characterized by low misorientation angles between adjacent or nearly adjacent grains, leading to the formation of several distinct poles of alignment. Within ice sheets such fabric patterns are typically encountered below the high shear layer (e.g. Reference Wakahama and KuroiwaWakahama, 1974;Reference Gow and WilliamsonGow and Williamson, 1976;Reference Tison, Thorsteinsson, Lorrain and KipfstuhlTison and others, 1994; Reference LiLi, 1995;Reference GowGow and others, 1997;Reference DiPrinzio, Wilen, Alley, Fitzpatrick, Spencer and GowDiPrinzio and others, 2005) in relatively warm basal ice which has undergone a stress relaxation as a result of topographically induced flow stagnation (Reference Russell-Head and BuddRussell-Head and Budd, 1979; Reference Budd and Rowden-RichBudd and Rowden-Rich, 1985). All meteoric ice core samples described in this paper are from below the layer of local accumulation and thus contain material that has entered the Amery Ice Shelf in the region of the Southern Prince Charles Mountains. As the G1 and AM01/AM01b cores were drilled at the same geographic location, thin sections obtained from corresponding depths can be directly compared. The multiple-maxima crystal orientation fabric reported by Reference Wakahama and KuroiwaWakahama (1974) for the G1 core at 255 m depth (Fig. 6) is similar to that measured for the AM01b- 255c core. Grain sizes of ˜-100mm² for AM01/AM01b meteoric ice samples (Fig. 7) agree well with original measurements of the G1 core by Reference Wakahama and KuroiwaWakahama (1974). Due to the large mean grain size of meteoric ice at AM01 and AM04 relative to the core diameters of ˜-100mm, the number of grains included in thin sections is typically restricted to N ≈ 50—100. Such small grain counts per thin section partially account for the variability in meteoric ice mean grain-size estimates (Fig. 7).

Fig. 7.

Variation of grain size with depth for Amery Ice Shelf cores. The data of Reference Wakahama and KuroiwaWakahama (1974) for the G1 core have been plotted to allow comparison with grain sizes measured for several thin sections from the AM01/AM01b and AM04 ice cores.

Multiple-maxima crystal orientation fabrics similar to those observed in meteoric ice samples from the AM01/ AM01b site (Fig. 6) have been reported previously for the Ross (Reference GowGow, 1963) and the Ronne Ice Shelves (Reference Eicken, Oerter, Miller, Graf and KipfstuhlEicken and others, 1994). Whilst there is no azimuthal orientation control relative to the ice-shelf flow direction for the hot- water drilled cores, it is apparent that the angular distribution of the maxima differs in the Amery, Ross and Ronne Ice Shelf meteoric ice crystal orientation fabrics. Reference BuddBudd (1972) suggests such differences arise from dissimilar flow histories at each site. Meteoric ice crystal orientation fabrics from AM04-350 (Fig. 8) were characterized by a less distinct multiple-maxima pattern compared with the AM01 site. In addition to the four-maxima, Figure 8 indicates the presence of opposing clusters of near-horizontal c-axes in the AM04- 350 crystal orientation fabric. These clusters are attributed to differences in flow conditions at AM01 and AM04, which are discussed in the following subsection.

Fig. 8.

AM04 site meteoric ice crystal orientation fabric from the AM04-350 core. Vertical thin section. Schmidt equal-area c-axis plot (top left), histogram of c-axis colatitudes, ϕ(top right) and radial histogram of c-axis azimuths (bottom left).

Relationship between meteoric ice crystal orientation fabrics and ice-shelf dynamics

In the Amery Ice Shelf meteoric ice layer, which originates from grounded continental ice flowing into the shelf, the observed multiple-maxima crystal orientation fabrics and large mean grain areas are characteristic of zones in large ice masses that have undergone stress relaxation and recrystallization (e.g. Reference Budd and JackaBudd and Jacka, 1989). Laboratory stress- relaxation experiments conducted by Reference GaoGao (1989) demonstrate that for a sample in tertiary creep with an anisotropic crystal orientation fabric compatible with the applied stress configuration, a gradual reduction in stress under isothermal conditions leads to the development of multiple-maxima crystal orientation fabrics and larger mean grain sizes, similar to that observed in the AM01 meteoric ice.

Reference Budd and JackaBudd and Jacka (1989) provide tabulated octahedral shear stress/octahedral shear strain-rate data for isotropic polycrystalline ice for temperatures from -0.02°C to -50.0°C, based on experimental observations. Using these data, along with AM01 and AM04 strain rates (Equations (1) and (2)) and temperature profiles (Fig. 4), the deviatoric stresses driving deformation at AM01 and AM04 are estimated to be t_o ≈ 0.05 MPa. This value agrees with estimates of stresses in the Ronne Ice Shelf by Reference Eicken, Oerter, Miller, Graf and KipfstuhlEicken and others (1994). Significant basal melting of ungrounded meteoric ice entering the Amery Ice Shelf makes estimating the past maximum grounded ice depth of AM01 and AM04 meteoric ice samples difficult. Reference AllisonAllison (1979) indicates that basal shear stresses in grounded ice entering the southern region of the Amery Ice Shelf may be as high as t ≈ 0.20 MPa. It is therefore reasonable to expect that the deviatoric stresses driving deformation of the grounded ice are higher than those experienced by the same ice once afloat as part of the Amery Ice Shelf where the basal shear stress has decreased to zero. This reduction in driving stresses results in considerable relaxation of highly strained microstructures, enabling significant grain growth and fabric modification in the meteoric ice as it flows downstream through the AM04 and AM01 sites.

The AM04 meteoric ice multiple-maxima fabrics are more complex than those from AM01. The individual maxima are less distinct and the opposing clusters of nearhorizontal axes may indicate some compatibility with the transverse xy-shear component of the stress configuration evident at AM04 and increasingly upstream. Without azimuthal control of the ice cores, this interpretation is somewhat speculative. The difficulty in interpreting the AM04 fabric pattern reflects the complex stress configuration driving deformation up to AM04 and the influence of flow history, stress relaxation and grain growth on meteoric ice fabric evolution within the Amery Ice Shelf.

Crystal orientation fabrics and texture

Digital video footage of the AM01b and AM04 boreholes (Reference CravenCraven and others, 2005) indicates the lowest 40-50 m of the permeable but consolidated marine ice layer is characterized by regions of bonded platelets and interstitial sea-water-filled cavities. Whilst the presence of platelet ice at the ice-shelf base indicates a marine ice accretion mechanism, Reference Craven, Allison, Fricker and WarnerCraven and others (2009) note that no unattached platelets were observed within the borehole during drilling, suggesting active accretion was not occurring locally during the short observation period. This is attributed to the highly seasonal cycles of temperature, salinity and flow within the Amery Ice Shelf cavity, which are thought to produce corresponding variability in marine ice accretion (Reference Leffanue and CravenLeffanue and Craven, 2004).

Insoluble particulate and brine inclusions

Particulate inclusions, present variously in linear, planar and random dispersions, have been observed in all Amery Ice Shelf marine ice cores from the G1, AM01/AM01b and AM04 borehole sites (Reference Wakahama and KuroiwaWakahama, 1974;Reference CravenCraven and others, 2004, Reference Craven2005, Reference Craven, Allison, Fricker and Warner2009). Reference Roberts, Craven, Cai, Allison and NashRoberts and others (2007) report that particulate inclusions from the AM01 cores comprised approximately two-thirds lithogenic and one-third biogenic material, although it is reasonable to expect these proportions to vary throughout the marine ice layer due to spatial and temporal variability in the accretion process and biological activity beneath the Amery Ice Shelf and in Prydz Bay, north of the ice shelf. Inclusions range in size from pm- scale to ˜1 mm particulates (Reference Roberts, Craven, Cai, Allison and NashRoberts and others, 2007), of which the latter could be clearly observed with the naked eye. Similar linear and planar distributions of particulate inclusions have been described by Reference Eicken, Oerter, Miller, Graf and KipfstuhlEicken and others (1994) for regions of the Ronne Ice Shelf marine ice layer in close proximity to the meteoric/marine ice transition.

The size and concentration of brine inclusions visibly increases with depth in the AM01 and AM04 marine ice cores (Reference Craven, Allison, Fricker and WarnerCraven and others, 2009). Below the hydraulic connection depth, brine inclusions tend to be large, often interconnected vertical or sub-vertical channels and pores (Fig. 10). Above the hydraulic connection depth, interconnections between pores are closed off by continued consolidation of the marine ice layer, but many inclusions retain a sinuous structure (Fig. 13). In older marine ice, closer to the meteoric/marine ice interface, brine inclusions become increasingly isolated and spherical. Analysis of the brine distribution in Ronne Ice Shelf marine ice cores by Reference Moore, Reid and KipfstuhlMoore and others (1994), including scanning electron microscope examination, revealed that in addition to discrete inclusions such as those observed in the Amery Ice Shelf marine ice cores, brine was also present along grain boundaries. Reference CravenCraven and others (2005) confirm the brine concentration increases with depth at AM01, with total core salinities of 0.03% at the meteoric/marine ice transition increasing to 0.56% at 390 m. Total salinities at the Amery Ice Shelf meteoric/marine ice transition are thus comparable to the upper regions of the Ronne Ice Shelf marine ice layer (Reference Eicken, Oerter, Miller, Graf and KipfstuhlEicken and others, 1994). For the comparatively thinner marine ice layer in the Hells Gate Ice Shelf, Reference Tison, Ronveaux and LorrainTison and others (1993, Reference Tison, Lorrain, Bouzette, Dini, Bondesan, Stievenard and Jeffries1998) report salinities of 0.2-0.5% for banded marine ice similar to that observed at AM01-390. For vertically banded marine ice from the Nansen Ice Shelf, Reference Khazendar, Tison, Stenni, Dini and BondesanKhazendar and others (2001) report salinities largely within the range 0.035-0.10%.

Fig. 13.

Brine inclusions in the Amery Ice Shelf marine ice core AM04-450a (i.e. 80m above the hydraulic connection depth). Composite image created from superimposed transmitted plain and plane-polarized light images from the same core section. The dark inset corresponds to the transmitted plane-polarized component of the image. Elongated sinuous inclusions extending over several centimetres are evident. Scale is indicated by millimetre gradations at the base of the image.

Examination of marine ice cores and thin sections from the G1, AM01/AM01b and AM04 boreholes between orthogonal plane-polarizing filters and in plain light has confirmed the distribution of insoluble particulate and brine inclusions along grain boundaries and triple junctions (Fig. 14). A small proportion of intragranular inclusions were also observed.

Fig. 14.

Orientation-dependent Russell-Head Instruments G50 fabric analyser artificial-colour image of the AM04-450a marine ice core. The location of linear distributions of particulates along the grain boundaries of a remnant platelet are indicated. Regions of smaller, equiaxed grains contain particulates distributed randomly along grain boundaries and triple junctions. Marine ice at AM04-450 is estimated to have undergone a minimum postdeposition octahedral shear strain of e _o = 50%. Grain-boundary pinning during deformation due to high particulate inclusion contents is proposed as the cause of local restrictions in grain size: regions of comparatively smaller grains coincide with locally high levels of impurities.

For meteoric ice cores there are considerable data indicating the influence of insoluble and soluble particulate inclusions on restricting grain size and preserving highly anisotropic crystal orientation fabrics (e.g. Byrd Station: Reference Gow and WilliamsonGow and Williamson, 1976;Law Dome: Reference Li, Jacka and MorganLi and others, 1998; Siple Dome: Reference Gow and MeeseGow and Meese, 2007). Applying the theory of Reference Alley, Perepezko and BentleyAlley and others (1986) for polycrystalline ice grain growth in the presence of insoluble microparticles, Reference Eicken, Oerter, Miller, Graf and KipfstuhlEicken and others (1994) conclude the inclusion concentration of the Ronne Ice Shelf marine ice cores is sufficient to inhibit grain growth (boundary migration) due to particle drag. Based on the observation of particulate and brine inclusions in Amery Ice Shelf marine ice cores and the similarity in Ronne and Amery Ice Shelf marine ice mean grain areas and total salinities, it is reasonable to assume that particulate inclusions similarly restrict grain size and fabric development in Amery Ice Shelf marine ice (Reference Eicken, Oerter, Miller, Graf and KipfstuhlEicken and others, 1994;Reference Moore, Reid and KipfstuhlMoore and others, 1994;Reference CravenCraven and others, 2005).

Relationship between marine ice crystal orientation fabrics and ice-shelf dynamics

In general, differences in the AM01 and AM04 marine and meteoric ice crystal orientation fabrics and textures can be related to accretion processes and their influence on fabric and texture. Frazil ice platelets accreted to the base of the Amery Ice Shelf accumulate in an initially strain-free state. During the early stages of marine ice layer accumulation the only stress acting on the platelets is the buoyancy-derived deviatoric stress. With continued platelet accretion, consolidation is driven by compaction aided by freezing of entrained sea water and grain growth. Unlike Amery Ice Shelf meteoric ice, the deviatoric stress acting on marine ice during accretion and consolidation steadily increases to an estimated maximum value of r _o ≈ 0.05 MPa, so the marine ice does not undergo stress-relaxation-induced grain growth and fabric modification.

The strongly anisotropic single-pole crystal orientation fabric of the AM01-390 marine ice core is assumed to reflect the process of marine ice layer formation by platelet accretion. Based on fabric observations at AM01 from below the hydraulic connection depth and borehole video footage of Reference CravenCraven and others (2005), which indicates ordered layering of platelets, it is possible that a vertical or near-vertical pole of orientations is characteristic of newly accreted Amery Ice Shelf marine ice.

If newly accreted marine ice is characterized by a distinct single pole of orientations and a rectangular banded texture with a mean grain size of ˜10mm² (e.g. AM01-390; Fig. 11), a mechanism must exist for the development of vertical large circle girdle crystal orientation fabrics and reduced mean grain sizes of 4-6 mm² observed in most older marine ice above the hydraulic connection depth (Figs 9 and 12). During deformation the high content of insoluble particulate inclusions in the marine ice may contribute to the pinning of mobile grain boundaries (e.g. Reference Alley, Perepezko and BentleyAlley and others, 1986). In combination with microstructural modification processes typically associated with recrystallization during deformation (e.g. grain nucleation and polygonization), grain boundary pinning may lead to the observed reduction in grain size and retard the rate of fabric evolution. Furthermore, evolution of the observed large circle girdle marine ice crystal orientation fabrics is most likely influenced by the initially highly anisotropic fabric of marine ice as accreted and the complex multi-component stress configurations that exist and evolve along the AM01- AM04 flowline (e.g. Reference Kamb, Heard, Borg, Carter and RaleighKamb, 1972).

Marine ice samples AM01-280a and AM04-400 are both from ˜5 m below the marine/meteoric ice interface and are representative of the oldest marine ice in the Amery Ice Shelf, deposited at the southern limit of the marine ice accretion zone. The total post-deposition strain for marine ice just below the meteoric/marine ice interface at AM01 is estimated to be e ₀ ˜ 100%. As the AM01 and AM04 boreholes are located on approximately the same flowline, the AM01-280a (and G1-280) crystal orientation fabrics can be considered to have evolved from the AM04-400 fabric observed 100 km upstream. Clusters of c-axis orientations superimposed on the AM04 vertical large circle girdle fabric patterns (Fig. 12) are interpreted as a remnant of the highly anisotropic crystal orientation fabrics associated with newly accreted marine ice. The variable orientation of the clusters in AM04 marine ice suggests variability, over small spatial scales, in the mean orientation of newly accreted platelets. The high in situ strain accumulated downstream of AM04 towards AM01 and the absence of a superimposed cluster in the AM01-280a and G1-280 fabrics indicates microstructural evolution along the flowline is sufficient to overprint the original pole of platelet orientations. It is hoped this aspect of marine ice fabric evolution can be verified through collection of additional ice cores from various depths along the AM04-AM01 flowline in future AMISOR campaigns.

Whilst the concentration of particulate inclusions in the Amery Ice Shelf marine ice was not directly measured, visual inspection reveals a significantly lower particulate and brine inclusion concentration in the AM01b-280b core, which was obtained ˜1 m below the AM01b-280a sample. The cause of a reduced inclusion content in the AM01b-280b core is uncertain, but temporal differences in thermohaline circulation and ISW production may influence the degree of supercooling and thus the size of frazil ice platelets which are subsequently accreted on the ice-shelf base (e.g. Reference Jenkins and BomboschJenkins and Bombosch, 1995;Reference Leffanue and CravenLeffanue and Craven, 2004). Larger platelets may have a reduced capacity to scavenge particulates from the water column, thus limiting the concentration of inclusions incorporated into the marine ice layer. Figure 9 illustrates the significant differences in the AM01b-280a and AM01b-280b crystal orientation fabrics and mean grain area. As previously discussed, the high concentration of particulate inclusions strongly inhibits post-accretion grain growth and influences fabric development. Conversely, the lower inclusion content of the AM01b-280b core allows increased grain growth and fabric development;the mean grain area of g _a ≈ 50 mm² at AM01-280b is comparable to the overlying meteoric ice. Based on the data of Reference Jacka and LiJacka and Li (1994), g _a = 50 mm² is the approximate steady-state grain area for inclusion-free ice at the temperature and estimated stress conditions at 255-280 m depth at the AM01 site (i.e. r _o ≈ 0.05 MPa and temperatures between -7 and -10°C). The small circle girdle crystal orientation fabric of the AM01b-280b marine ice sample indicates some clustering of orientations, but is generally similar to the patterns observed in meteoric ice where compression-dominated stress configurations exist (e.g. Reference Budd and JackaBudd and Jacka, 1989). Differences in the AM01b-255 meteoric ice and AM01-280b marine ice crystal orientation fabrics are related to their strain histories;unlike meteoric ice entering the Amery Ice Shelf, marine ice does not undergo a stress relaxation.

In addition to differing strain histories and temperature profiles, the contrasting crystal orientation fabrics and textures of meteoric and marine ice in the Amery Ice Shelf are suggestive of rheological differences arising from the presence of insoluble particulate and brine inclusions within the marine ice. In modelling the dynamics of the Ronne Ice Shelf, Reference Lange and MacAyealLange and MacAyeal (1988) introduced a marine ice softening parameter of ˜10% in order to reproduce observed velocities across the shelf front. This softening is required to account for the temperature-dependent reduction in marine ice layer viscosity, which is nearly isothermal with the sub-shelf ocean. In addition to the influence of basal marine ice layers, recent work (e.g. Reference Khazendar, Rignot and LarourKhazendar and others, 2007, 2009;Reference Humbert, Kleiner, Mohrholz, Oelke, Greve and LangeHumbert and others, 2009) illustrates how the rheology of the ice melange within ice-shelf rifts, which can include a significant proportion of localized marine ice, plays a significant role in ice-shelf dynamics and stability. It is hoped laboratory deformation experiments on Amery Ice Shelf core samples under approximately in situ conditions will provide further insights into the rheological properties of both meteoric and marine ice layers and improve the accuracy of ice-shelf dynamic models.