Marine radiocarbon calibration requires an estimate of the reservoir offset from the marine calibration curve (ΔR).  These estimates can be based on ¹⁴C measurements of pre-nuclear weapons testing, known age shells, independently dated coral, or contemporaneous marine and terrestrial samples. Until now the ΔR values for coastal Greenland have been sparse.  The authors have significantly enlarged the dataset of known age shell measurements from coastal Greenland and neighboring regions of the Arctic.  They have carefully selected samples from museum specimens to ensure the mollusks were collected alive.  The effects of sea ice cover, water depth and mollusk feeding habits were investigated and discussed. Regional averages were calculated for zones based on “prevailing currents and water masses” although most of the zones have overlapping values.  The authors also compared ΔR values for a limited number of samples stored in ethanol to dry samples.  

Specific comments/questions:

Wet vs dry sample comparison:  This comparison is based on only 6 dry samples and 4 wet samples from one region (Suppl. Fig 2). This is a rather small dataset to reach the conclusion that dry samples are not reliably collected alive. It is difficult to tell which dry samples were used in the comparison but, of the 5 dry samples from Kaiser Frans Joseph Fjord, 4 were species with unknown feeding habits or deposit feeders.  It is well known, and also shown in this manuscript, that deposit feeders may incorporate older carbon from their environment. This comparison apparently forms the basis for one of the stated criteria for sample selection (line 412-413): ‘Museum sample storage: As the exact age of samples from “dry” collections is possibly unknown, only samples with soft tissue present, stored in “wet” collections, should be used for ΔR evaluation’.  Samples stored in ethanol may be ideal to ensure live collection but this criterion would exclude many of the existing values in the literature.  In some cases, the museum documentation is unambiguous about live collection but there are also other indications of whether “dry” bivalves in collections were most likely collected live or shortly after death.  These include fragile mollusks that would have been abraded if transported to a beach as well as those with residual ligament, muscle or periostracum (O’Connor et al. 2010).  In addition, some species have colours that are light sensitive so would be bleached if not collected alive and stored in the dark (Angulo et al. 2007).

The study also makes use samples from water depths that would not be considered surface ocean in general. The low ΔR values for these samples provide a very interesting and useful observation for these locations which are ‘characterized by convection and formation of North Atlantic Deep Water and Labrador Sea Water’.  The authors advise that: ‘When calibrating benthic dates from deeper sites one could therefore consider excluding extreme values obtained from surface ocean samples when making the choice of which reservoir correction to apply’.  This seems like valid advice for these regions however it should be noted that surface ocean ΔR values are not generally applicable for benthic dates in other regions where deep water can be very depleted in ¹⁴C.  Ideally one would have ΔR values from deep water samples to use for radiocarbon calibration of benthic samples but these are scarce in the literature.  Also is there an explanation for the low ΔR values for relatively deep samples in NW Greenland zone 5?  Is the West Greenland Current fed by Labrador Sea water?

Technical comments:

Line 18: ” Marine20, the most recent radiocarbon calibration curve”  Insert “marine” ahead of radiocarbon.

Line 19 and 74: ‘we introduce the term ΔR₁₃”.  This term has been previously introduced in Heaton et al. 2023.  I would suggest replacing ‘introduce’ with ‘use’

Line 51: ‘to a lesser extent, injection of 14C-depleted CO2 from the burning of fossil fuels’  Although this is a common perception and definitely true for reservoir ages relative to the atmosphere, for ΔR this is insignificant.  ΔR is the difference between the marine radiocarbon age and the marine calibration curve which is modelled with input from the atmosphere so includes the Suess effect.

Line 54: ‘tephrochronology (Pearce et al., 2017; Austin et al., 1995; Olsen et al., 2014), or paired marine/terrestrial dating’  ΔR values may also be determined by U-Th dated coral (e.g. Hua et al. 2015).

Line 57:  ‘Several hundred different studies were made to study the local reservoir age’.  Replace ‘were’ with ‘have been’.

Line 127: ‘the most commonly used value for the reservoir age correction (prior to publication of Marine20), ΔR = 0 14C years’ Since ΔR without a subscribe is defined earlier as relative to Marine20 ,it would be better if this written here as 'Rxx = 0 14C were xx =04, 09 or 13. 

Line 184: ‘Wet samples were placed in a drying oven at 40 °C for several days’ It would be worth stating that this is to remove any ethanol from the shell since contamination from the ethanol might be a concern.

Line 189: ‘milliQ water’  Trademark symbol needed

Line 239: ‘where ΔRi and σi are the mean value and uncertainty of calculated local reservoir age offset’.  Add ‘of sample i’ to clarify.

Line 243: ‘Where the subscript w indicates that the uncertainty is calculate using the error each ΔRi’  Change ‘error’ to ‘uncertainty’ and ‘is calculate’ to ‘is calculated’

Line 368: ‘no ΔR values higher than 50 years are found, and where ΔR values exceed 160 years,’  ΔR values should be given as ‘14C yrs’ rather than  ‘years’

Line 370: ‘there is also a positive correlation between sea-ice cover and reservoir age’.  Are the correlations significant?

Line 429: ‘these values remain only valid for the modern situation’  Insert ‘pre-bomb’ before modern because the values would not be valid for post-bomb samples. 

Fig. 1 caption: Need to define WGC, NFL, EGC.

Also. ‘Areas of deep convection in the Labrador Sea and north of Iceland are colored yellow’. These look light green on top of the blue background - perhaps 'shaded light green' would be better

Fig 2.  Given the results, is there justification for separate zones for the Greenland coastal waters since ΔR values overlap? 

Suppl. Fig 2. Sample numbers on Suppl. Fig 2 would be helpful for comparison of species and feeding habits

References:

Angulo, R. J., Reimer, P. J., De Souza, M. C., Scheel-Ybert, R., Tenório, M. C., Disaró, S. T. & Gaspar, M. D. 2007. A tentative determination of upwelling influence on the paleo-surficial marine water reservoir effect in southeastern Brazil. Radiocarbon, 49, 1-5.

Heaton, T. J., Bard, E., Bronk Ramsey, C., Butzin, M., Hatté, C., Hughen, K. A., Köhler, P. & Reimer, P. J. 2023. A response to community questions on the MARINE20 radiocarbon age calibration curve: marine reservoir ages and the calibration of 14c samples from the oceans. Radiocarbon, 65, 247-273.

Hua, Q., Webb, G. E., Zhao, J.-X., Nothdurft, L. D., Lybolt, M., Price, G. J. & Opdyke, B. N. 2015. Large variations in the Holocene marine radiocarbon reservoir effect reflect ocean circulation and climatic changes. Earth and Planetary Science Letters, 422, 33-44.

O'Connor, S., Ulm, S., Fallon, S. J., Barham, A. & Loch, I. 2010. Pre-bomb marine reservoir variability in the Kimberley region, Western Australia. Radiocarbon, 52, 1158-1165.

Pearce et al. present about 100 new marine radiocarbon (¹⁴C) reservoir ages (MRA) of coastal and shelf waters around Greenland, Baffin Island, Newfoundland, and Iceland. The data result from ¹⁴C measurements on pre-bomb molluscs retrieved from museums. The MRA results are binned to seven regions and discussed with respect to the global Marine20 ¹⁴C calibration curve in terms of the regional MRA correction, ∆R₂₀. The authors also discuss their ∆R₂₀ results in the light of specific factors such as sample depth, sea ice cover and feeding habits. 

The manuscript is well written, the presentation is clear, and the dataset is an important contribution to the MRA / ∆R data base. However, there are a few minor issues that should be addressed before publication in GChron (L = line):

L 30: The half-life of ¹⁴C has been slightly revised to 5700 years (e.g., Audi et al., 2003; Bé and Chechev, 2012; Kutschera, 2013)

L 124 "marine mammals": "marine" should be removed

Figure 1:

 (i) Add a depth scale (such as in Fig. 1 by Pieńkowski et al. 2022)

 (ii) "NFL", "WGC", and "EGC" should be also explained in the caption.

L 214-216 (and Figure 2): Is there a hard objective criterion to separate the three southernmost data points in East Greenland from region 7?

Figure 2: Explain "CS"

L 353: Explain "mwd"

Figure 3: Would it make sense to indicate the positions of the outliers in the inserted map?

L 376: Explain "mwd"

Figure 4:

 (i) As ∆R depends on the sea ice concentration, the coordinate axes should be swapped. The situation is different from Figure 3 where the independent variable (usually plotted along the horizontal axis) is depth (typically plotted in vertical direction).

 (ii) Can you quantify the trends, and are they significant? I wonder if the trends are still visible once the coordinate axes have been swapped.

 (iii) Would it make sense to indicate the position of the outlier in the inserted map?

 

References:

Audi, G., Bersillon, O., Blachot, J., and Wapstra, A. H.: The Nubase evaluation of nuclear and decay properties, Nuclear Physics A, 729, 3–128, https://doi.org/10.1016/j.nuclphysa.2003.11.001, 2003.

Bé, M.-M. and Chechev, V. P.: 14C - Comments on evaluation of decay data, Laboratoire National Henri Becquerel, Gif-sur-Yvette, http://www.lnhb.fr/nuclides/C-14_com.pdf, 2012.

Kutschera, W.: Applications of accelerator mass spectrometry, International Journal of Mass Spectrometry, 349–350, 203–218, https://doi.org/10.1016/j.ijms.2013.05.023, 2013.

This is a very nicely written paper presenting 92 new radiocarbon dates on pre-bomb mollusks collected from around Greenland (with the exception of its northern Arctic Ocean margin). In addition to the utility of these new dates for constraining regional reservoir corrections, I think the manuscript is timely in presenting a nice practical discussion (and examples) on the need to update reservoir corrections when using the new Marine20 calibration curve.

The comparisons of dR with water depth and sea ice coverage are interesting in highlighting patterns, although somewhat inconclusive in identifying a cause/explanation for the variability. I do not think this limits the scientific contribution made by the paper, and certainly sets the stage for future work needed to understand this variability. I believe this would require a considerable amount of work, and could potentially start with moving away from water depth and looking at the variability in Temperature-Salinity space to see if ages cluster in specific water masses. However, I do not think this is a necessary addition to this work, which very well suited for publication in Geochronology in its current form.

I do feel one aspect the paper is missing is a discussion on the limited, but rather informative data on Holocene dR values from the central Arctic Ocean. Specifically the inferred differences between the age of Pacific and Atlantic waters that are found in the interior Arctic, and should be impacting the age of surface waters(?) in northern Baffin Bay. For example, West et al (2022), Geochemistry, Geophysics, Geosystems (doi: 10.1029/2021GC010187) used tephra from the Aniakchak eruption circa 3.6 ka  in two cores from the Chukchi Sea - one at 50 m depth (Pacific water) and one at 120 m depth (likely Atlantic water) - to show that the dR (using Marine 20) for benthic foraminifera and mollusks at these sites was about 330 years for Pacific waters and 205 years for Atlantic waters. These seem to be somewhat consistent with the larger dR values in sections 3 and 4 from Northern Baffin Bay. It would be nice to see some discussion about the influence of Arctic outflow and the water masses involved on the dR values in Northern Baffin Bay. Currently these are described simply as 'outflow' from the Arctic, which could easily be expanded to detail the role and age of Pacific and Atlantic waters in this outflow.

Overall, I feel this is a great contribution that will provide significant support to future paleoceanographic work around Greenland.