This paper presents results from fracture toughness experiments performed on ice from the Brunt Ice Shelf, which include samples from firn, refrozen meltwater, and brine-saturated layers. The experiments reveal differences in fracture toughness between the layers, which have implications for the growth of rifts and crevasses through different ice shelf materials.

Measurements of fracture toughness in ice and snow are rare, and extending the existing data record to include a wider range of ice shelf materials is an extremely valuable contribution to the literature. The results presented here will be invaluable to inform more physically realistic models of ice shelf fracture and iceberg calving. The paper is well written and clearly presented, with well chosen figures and a thoughtful discussion of the results. I have only a few minor alterations to suggest:

Figure 1: I assume the red square in panel a is the site of Halley station, but this should be mentioned in the caption. If possible, it would also be nice to indicate in panel b the path of the GPR line shown in panel c.

Lines 123-126: I don’t think you mention what the full ice thickness is at site S2, and it would be interesting to know how it compares to the 37m core length.

Section 3: On first read through it took me a minute to understand the terminology you use to label samples from different core sections (e.g. line 143, “core 35”). I’d suggest either using the terminology “core section” rather than core, or adding a sentence in section 3.1 along the lines of “Each core section was numbered sequentially on retrieval, and we use this number to identify core sections in the text (e.g. core XX)”.

Figure 4: It would help readability if you define the variables here (i.e. “T and R are the thickness and radius of the sample, a is the notch length, and S is the distance between the rollers”).

Line 181: “as quickly as possible” is a little vague – it would be useful to give an indication for how quickly this is, and why speed was important.

Figure 6:  The text in some of the panels has ended up quite small and hard to read. It might help to move the colorbar, or even to separate some of the panels into a different figure. In panel a, I wasn’t clear whether there was a difference between the points colored deep blue and those colored in grey – are they all cases with no melt?

Line 224 The sentence “brine infiltration reduces KIc by 14% - 34% relative to density-matched, brine-free meteoric ice” might usefully include the qualification “at the same temperature”.

Section 5: I think it would be useful for the discussion to briefly address the effect of ice temperature and rheology on fracture propagation. Both brine infiltration and refreezing of surface meltwater are likely to increase the temperature of the ice column, and the results from Hulbe et al. (2010) suggest that areas of soft ice (such as areas of warmer ice) can reduce stress intensity at a fracture tip and reduce rift growth.

Lastly, this isn’t a suggestion to make any change to the paper, but your results made me think of Julian Scott’s paper where they found that drilling through a melt layer within the firn triggered crevasse formation – indicating that the melt layer had higher fracture toughness and had essentially been holding that area of fir together (reference below).

Hulbe CL, LeDoux C, Cruikshank K. Propagation of long fractures in the Ronne Ice Shelf, Antarctica, investigated using a numerical model of fracture propagation. Journal of Glaciology. 2010; 56(197):459-472. doi:10.3189/002214310792447743

Scott JBT, Smith AM, Bingham RG, Vaughan DG. Crevasses triggered on Pine Island Glacier, West Antarctica, by drilling through an exceptional melt layer. Annals of Glaciology. 2010; 51(55):65-70. doi:10.3189/172756410791392763

This manuscript describes a suite of experiments on samples of ice core from the Brunt Ice Shelf, examining their fracture toughness via three-point bending tests. Specifically, it examines the impact of refrozen melt and brine infiltration on the fracture toughness of natural ice. This is an interesting and worthwhile study, as previous studies have focused only on cold, dense ice, or pure laboratory-made ice, and models which incorporate calving typically assume a homogeneous value for Mode I fracture toughness. The authors show that in fact heterogeneity in the ice properties, particularly caused by melt and brine infiltration, can significantly alter the fracture toughness of the ice. 

Overall it's a great paper, very easy to read and with nice results which are well presented. I have a few general comments which could be worth an additional couple of lines in the discussion to take it a bit further, but I leave it to the authors to decide if it would be over-interpreting the results to do so. All of the specific comments are very minor.

General comments:

As you mention in the introduction that most models assume a certain fixed value for fracture toughness, it would be nice to revisit this in the discussion: do you have a recommendation for how your results could be incorporated into a model? If there must be a fixed value for the whole shelf, would you change it based on these results? Or if it could be varied, what observable parameter could be used to do this?

Again, in terms of generalising your results: the Brunt Ice Shelf is quite unique in its structure. Would you expect to see the same effect on e.g. Larsen C, or is the proportion of melt to brine/ geometry of the suture zones too different?

The impact of temperature on fracture toughness is interesting, and strikes me as a little unintuitive: at higher temperatures, I would have assumed the ice would be more inclined to deform in a ductile way rather than fracture. 

Do we know if the ice temperatures are higher in the brine saturated/melange areas, relative to the meteoric ice blocks? Do you think it would make a difference to the results if your experimental temperatures were more constrained?

Specific comments:

line 31: 'centimetre thick' sounds like they are exactly a centimetre... -> centimetre scale?

Fig 1: I assume the bright surface at ~37m depth is the top of the infiltrating brine? It's worth labelling this just to be clear.

Fig 2: 'The spread in density from core samples is reflective of the melt within the cores': I'm not sure exactly what this means, are you referring to the lab-measured densities specifically? Or just generally the melt layers impact the density? Is the bigger spread in lab measurements because the measured sections were smaller?

Line 43: is core 35 from 35m depth? (Oh I see from figure 2 that it's ~29m. For clarity, it would help to have the depth in the text, as the core numbers are a little confusing. Same on lines 50, 59, etc)

Line 47: 'isotropic crystal fabrics', is this based on eyeballing the thin section under cross-polars? That's a fair enough way to do it, just worth clarifying.

line 71: how was surface area of melt measured? Did you take a thin section of the fracture surface, or was it visually obvious?

line 73: is this a custom-made test system, or something off-the-shelf? What is the accuracy of the load and displacement measurements?

Figure 4: I'm a little unclear on the purpose of the rollers at the sides, since it looks like the ice is sitting on the grey bits on the inside? Or is the full weight of the ice on the rollers?

line 187: I think this should be a colon instead of semicolon

 

line 218: missing space before reference