The authors present an interesting study about the amino acid racemization extent in the mollusc Arctica islandica. They have investigated the three microstructural layers and analysed the amino acid concentration, composition, and racemisation of the inter- and intre-crystalline fractions. The results obtained here contribute to increase the knowledge of amino acid racemisation and the processes that may affect D/L values. In my view, the this study is of general interest. Therefore, it is suitable to be published in EGUsphere after moderate revision.

In my view the authors should amplify the discussion especially Sections 3.2 and 3.3 regarding the use of more samples.

In section 3.2 the authors used a modern sample and an early Pleistocene sample to test the impact of bleaching. In my view the authors should also use:

- Not just a sample of each of them (modern and early Pleistocene), but more shells to be statistically reliable.

- They should also use medieval and Holocene samples.

In section 3.3 The authors performed the experiment using samples from the three layers of the same modern shell. In my view they should use more than one shell.

I also suggest extending the experiment beyond 48 hours.

I recommend redrawing Figure 11 based on the comments provided below.

Other suggestions:

In some parts of the text, the authors use Asx and Glx, while in others Asp and Glu.

Section 3.1 Mineral diagenesis

I suggest changing the title of this section, since one of the most important conclusions is about the procedure for sampling the shells, which is not a diagenetic process.

Line 190: The authors state that aragonite is the polymorph of CaCO3 that forms the shells of A. islandica. Is it valid for all three layers?

Line 190: The authors state that aragonite is the polymorph of CaCO3 that makes up the A. islandica shells. Is it valid for the three layers?

Lines 196: medieval or post-medieval?

Lines 198-199. Also in post-medieval and mid-Holocene shells there is a small calcite peak.

Line 203. Regarding the process to remove the periostracum and isolate and sample the required layers, it is also possible to use HCl and not just drill.

Line 210. So, the presence of calcite in specimens up to Early Pleistocene is only due to the drilling procedure?

Section 3.2 Impact of bleaching

The authors used a modern sample and an Early Pleistocene sample to test the impact of bleaching. In my view, authors should use both:

- Not only one sample of each of them (modern and early Pleistocene), but more shells to be statistically realiable.

- They should also use medieval and Holocene samples.

Line 215: There is and initial sharp decrease.

Line 218-219: with only one sample?

Line 227: with only one sample?

Line 289: Which of the three layers should be used?

Section 3.3 Closed system behaviour

The authors conducted the experiment using samples from all three layers of the same modern shell. In my opinion, they should use more than one shell.

Line 325: the samples were exposed to high temperatures in hydrous conditions…under N2 atmosphere?

Line 325: The authors heated the samples at 140ºC for 8, 24, 48h. Why not more times, up to 1 week or 10 days?

Section 3.4 Ontogenetic trends in modern and subfossil AAG

The authors sampled eight shells from modern, post-medieval and Holocene times. They must indicate whether the samples come from the same layer (and which of the three layers was used) or come from the entire shell by mixing the three layers.

Were the samples taken from the same part of the shell? apex? margin?

Lines 395-396. The authors indicate that the eight samples with ages spanning between 100 yr and 400 yr, but 3 shells were modern, 2 shells were from post-medieval times and 3 from the Holocene.

Line 401: cursive for Porites.

I suggest redrawing Figure 11 and the caption should be explained better.

Various patterns were used to represent shell values from the Holocene and medieval and modern samples. They should be the same. In fact, they showed the D/L values for Holocene samples (delete them), but not for the other shells.

In the medieval sample plots the values were identified as ic 1 and ic2, but in the modern and Holocene sample plots this is not shown.

They should explain and unify the nomenclature of P1, P2, P3; M1, M2, M3; and ic1, ic2.

Section 3.6 IcPD AAG framewotk for A. islandica

Line 470: were the samples taken from the apex?

Section 3.7 AAG rangefinding of undated shells

The authors should explain whether the analysis comes from the same layer or from the entire shell, and whether from the same part of the shell or form different parts.

Of course, that AAG is based on D/L values, but I suggest that you also consider the evolution of the concentration and mainly the percentages of amino acids in the discussion.

For more recent samples (modern and post-medieval), they must consider the “age at death” of the shell, as they can live more than 500 years.

This paper provides important new information on protein diagenesis in the mollusk species Arctica islandica. The paper is based on analyses of recent and fossil specimens from Northwestern European waters and includes experiments with bleaching, heating experiments as well as XRD analyses. The scope of the paper, as well as the presentation of the findings, is clearly presented and scientific sound. The paper is well suited for the journal and I will recommend it to be published. In a minor revision I will ask the authors to address the following point:

1.    A new and higher quality location map should be included. The sample locations should be identified with a number on the map.

2.    Each sample should be identified by numbers/sample siets in the table.

3.    A scetch of a cross section illustrating the growth of time with should be included. This will illustrate the way the inner layer(which is growing through the life time is making up most of the section  when approaching the umbo).

4.    Some places I do not follow the authors definition on when a plateau is reached in their experiments. For some of the data it seem like the plateau is reached between the first (initial) and second analytical points, and this should be  stated in text.

5.    It would be good if the authors somewhere present  some of their finding on a geological time line.

6.    Also be clear if recent shells are found with a living animal inside.