Review of article: Radionuclide tracers reveal new Arctic pathways shaping water mass mixing and formation in Baffin Bay and the Labrador Sea

General Comments – paragraph evaluating overall quality of the article

This paper utilizes measurements of the artificial radionuclides ¹²⁹I and ²³⁶U from seawater in Baffin Bay and the Labrador Sea, in order to trace the origins and interactions of various water masses present in the region. The use of these artificial radionuclides as water mass tracers can provide new insights compared to previous tracer studies in the region that have employed nutrient concentrations (e.g. the nitrate to phosphate ratio) , stable oxygen isotopes, as well as temperature and salinity measurements. This study provides a substantial contribution to furthering knowledge of water mass formation and interactions between Arctic-outflow waters from the Canadian Arctic Archipelago and the subpolar North Atlantic.

One general comment that I have is that the calculation of mixing fractions is not explained very clearly. I think that simply stating that a “mixing fraction = length A/B” without clearly defining what A and B are (points? Line segments?), is not thorough enough for a scientific paper. I would be happy to see clearer language used around the calculation of mixing fractions (I provide an example sentence from Leist et al. (2024) that I think is more clear, in my other specific comments). As another option, providing some example equations within the methods section could also help. This clarity issue comes up again throughout the discussion section, and I have made specific comments below about where I think the text lacks clarity in regards to this.

A minor general comment that I have, throughout the manuscript, is that when referring to smaller passageways or water bodies in the Canadian Arctic, such as Davis Strait, Lancaster Sound, Nares Strait, and Baffin Bay, you do not need to include “the” before their names in sentences. However for Seas or Oceans, “the” should be included before their names (e.g., the Labrador Sea, the North Atlantic Ocean). The authors are currently inconsistent about this throughout the text, please check for consistency.

Specific comments:

Paragraph 2 of the Introduction (lines 32 to 38): I think it should be clarified here that the West Greenland boundary current system consists of two components: (1) the West Greenland Coastal Current (WGCC), which transports fresh and relatively warm PSW from the Arctic Ocean, and represents the continuation of Arctic outflow waters from the EGC, and (2) the West Greenland Current (WGC) which transports warm and salty WGIW at depth, and is a shelfbreak jet (see introduction of Huang et al. 2024 for details on this distinction). It is the WGC that bifurcates into two branches in the northern Labrador Sea, with the majority of the current being diverted westwards across the northern Labrador Sea, and only a limited amount continuing north into Baffin Bay. I suggest adding these details into your introduction.

Line 85-86: I would also suggest citing Bamber et al. (2018) “Land ice freshwater budget of the Arctic and North Atlantic Oceans: 1. Data, methods and results” JGR:Oceans

Line 165-167: I am a bit confused by this sentence – are both stations NS89 and NS79 represented by the orange square in northern Nares Strait in Fig. 1B? And the orange circles represent southern Nares Strait stations NS102 to NS114 and NS107? This could be phrased more clearly.

Last paragraph of section 2.2 (lines 190 – 203): Many of the water mass definitions in this paragraph do not match the defined ranges of conservative temperature (CT) and absolute salinity (SA) given in Table A1. Please make sure they match for consistency, or where they may differ provide some clarification as to why. For example, in Table A1 West Greenland Irminger water (WGIW) is defined as CT > 3.5 and SA < 34.2 but in this paragraph it is defined as CT > 4 and SA >34.7.

Line 211: I understand the concept of how mixing fractions are calculated along mixing lines between end-member values, but I don’t find it to be very clear from this sentence. Especially the “(fraction = length A/B) with A and B being end-member values”. Since this is not a real equation, just the concept of it that you are trying to convey. I looked at Leist et al. (2024) and I preferred this sentence from that paper, I find this more clear: “The water mass fraction is calculated by dividing the distance between the sample and one endmember by the total distance between the two endmembers”. I think this wording gives the reader a better idea about how to re-produce the method.

Line 212: In this sentence do you mean that end-members were calculated as the mean of sample values in a certain CT and SA range? This is stated a bit more clearly on lines 215-216, but I think it should be stated earlier in this first paragraph of section 2.3. And how exactly were the estimated fractions rounded? I think this sentence could be phrased more clearly.

Line 292-294: The part of this sentence that states “…with a clear influence from low-tracer Pacific water” didn’t make sense to me, because a Pacific water end-member is not shown in tracer space on Figure 4. It made sense once I saw the Pacific end-member in Figure 5 tough.

Section 4.1.1: As I was reading the first paragraph of this section, describing how the various end-member values were defined, I was wondering about the depth range used to define each end-member. For example, the NAC end-member and ISOW are both defined in the region east of the Reykjanes Ridge, but they have very different depth ranges (and T/S properties; Table A1). I see that the depth ranges for each end-member are listed on Figure 5B – perhaps just a mention in the text that the depth ranges relevant to each end-member can be found in Figure 5B would be nice.

Line 350: This sentence mentions a diamond with a black outline in Fig. 5B – but the symbol is missing from the figure.

Line 374: Here it is stated “fraction PSW-EGC = A/B, Fig. 6A, see section 2.3 for fraction calculation”. I can see that the capital letters for A and B are on Figure 6A, but it looks like A, C, and D represent the length of dashed arrows? And I assume that B is supposed to represent the entire length of the line between the NAC and PSW-EGC end-members? I would appreciate if the definitions of A, B, and other letters could be clearly defined in the text for the reader, so that there is no confusion about the calculation. I think this also needs to be clarified earlier in the methods section 2.3.

Lines 377-378: This sentence states “…consistent with estimates from Huang et al. (2024) for southern Baffin Bay (160 m out of 600 m)…”. I’d like some clarify on what these depths indicate – I believe you are referring to the thickness of the WGSW layer in the total water column, as was presented in Huang et al.? Please clarify what these depths are indicating.

Line 380: This is again in reference to my earlier comment that I would like it to be clearly stated what C and B represent.

Line 387: In this sentence the fraction of PSW-EGC = 1-D/B. Why is this formula for the fraction of PSW-EGC different from those at line 380 and 374?

Line 389: Similar to my earlier comment about depths stated from Huang et al. (2024) – what do these depths represent?

Section 4.3 title: Should this title say Baffin Bay Mode Water, instead of bottom water? Since BBMW is discussed in the final paragraph of this section. I would be interested to read more about the interpretation of the very low tracer concentrations of BBBW though.

Line 409: Again, could E and F be defined more clearly in the text? And would the fraction of AAW (CB) be equal to 1-E/F? I have the same comment for Lines 463, 467, 486, 488, 510, 511 and 532 in relation to Fig. 6B and 6C. Line 486 does provide a bit more clarification that “L” represents a line, maybe K should be a dashed arrow similar to G?

Line 481: Doesn’t cold Arctic water show the strongest WGSW influence? As evidenced by point “a” in the middle of the Northern Line (65% WGSW), as stated on line 462. Whereas the greatest WGSW contribution to Arctic water at point ”e” is 30%.

Line 509 and 525: This sentence states “Labrador Sea Water (red symbols Fig. 6C)…” I am unsure if you are referring to the points labelled as LSW or LS Surface in Fig/ 6C? To me, the LSW triangles appear an orange color, and LS Surface appears a dark orange.

Line 520-522: I believe another possible mechanism that might lead these two surface samples (indicated by “g” in Fig. 6C) to have relatively higher ²³⁶U (in the range of DSOW) is a contribution of cold AW. Rysgaard et al. (2020) showed evidence for a southward current along the southwest Greenland coast, which transports Baffin Bay Polar Water (referred to as cold AW in this manuscript) as far south as 64 degN latitude. I see that you already consider/discuss that Arctic water and cold Arctic water from the Labrador Current would only likely play a very small role (lines 523-524), but what if there is another southern transport on the east side of Davis Strait?

Technical corrections:

For the title of this article, it should end with “the Labrador Sea”.

Line 37-39: I found this sentence difficult to read as currently worded, I suggest changing it to: “In the northern Labrador Sea it bifurcates into two branches, with one continuing north into Baffin Bay, and another (larger) branch following the bathymetry of the Labrador Sea, turning westwards towards the Labrador Shelf.”

Line 42: The BIC current in Figure 1A appears light red in color to me, with the LSW appearing orange, and the NEADW yellow. The WGSW current is dark red.

Line 43: “transports fresh water of Arctic-origin” – Arctic should always be capitalized.

Line 48: Make it more clear that the depths listed in brackets are sill depths for each Arctic-outflow passageway

Line 50: “surface of Baffin Bay and along the BIC”

Line 60: should be “bathymetrically steared” not stirred.

Line 64: I think this line should read “…approximately 6-8% of LSW” to be more clear.

Line 116-117: I suggest changing some wording in this sentence to “…from the global fallout from nuclear weapons testing in the 1960s, and liquid releases from nuclear reprocessing plants in Sellafield (UK) and La Hague (France)…”

Line 184: Spelling should be “Zenodo” not Zenode.

Line 197: I believe the word “overlying” should be used here instead of “overflowing” to indicate that BBMW sits on top of BBBW.

Line 232: A range is given for ¹²⁹I , but ²³⁶U is stated separately for north and south. Could also present ²³⁶U values as a range, since the reader can see which location has the lower/higher value in Figure 2.

Line 281: Should have a bracket before “Fig. 4A and B indicated by black circles).”

Line 302: The Labrador Sea surface water (LS surface) symbols in Figure 4 appear as a dark orange to me, while the LSW appears as a lighter orange.

Line 305: Should reference Fig 6C here I believe.

Line 310: Sentence should end with “concentrations”, plural for both tracers.

Line 330: Spelling of “reprocessing”.

Line 366: This sentence should say “…with cooling and salinification in winter and freshening in summer”.

Line 390: I believe this sentence should say “…due to the limitation of the mixing of two endmembers in the tracer analysis.”

Line 454: This sentence should say “For WGSW, the mean and standard deviation were derived from…”

Line 455: This sentence should say “Cold Arctic Water generally has higher tracer concentrations…”

Line 465: Spelling of polynya.

Line 497: I suggest rephrasing the statement to be “Therefore, AAW originating from either Lancaster Sound or Nares Strait needs to be considered.”

Line 505: Sentence should read “In a previous study by Leist et al. (2024) using ¹²⁹I and ²³⁶U in the Labrador Sea…”

Line 535: This sentence refers to the 236U maximum on the mooring line – perhaps reference Fig. 3 to support this statement, and the “x” symbol is missing in the given ²³⁶U value here (scientific notation).

Line 557: The sentence that starts on this line, I would rephrase as “At the same time, our results show that West Greenland Shelf Waters feed into Arctic waters in central Baffin Bay, while cold Arctic Waters largely originate from Nares Strait and mixes with West Greenland shelf water.”

Line 560: Sentence starting on this line, I would re-phrase as “South of Davis Strait, our results show that Transition Water…”. I would also consider mentioning in this sentence that TrW might also provide a contribution to NEADW.

Technical corrections on Figures:

Figure 1: In this figure caption I would state “Appendix Table A1”, similar to the caption for Figure 6. Also, perhaps clearly state that ocean current names are stated in black text, while specific water mass names are in colors.

Figure 3: In panel (B) BBBW is labeled, but then in panel (F) BBMW is labeled in the same place. I am assuming this label should be the same between the two panels? Also the caption for Figure 3 should read “²³⁶U concentrations (E-H)” and “The water masses are based on Curry et al…”

Figure 5: Check spelling of Canada Basin in the label on the top left of Fig. 5B.

Figure A1: Please indicate where the data for this plot was obtained – would also be helpful to reference the data source in the text somewhere between lines 115 – 120. Also, spelling of “input” in the figure caption is incorrect.

Figure A3: The lower-case theta symbol on the y-axis of panel A usually represents potential temperature, and conservative temperature is represented by a capital theta symbol. Should the figure caption state potential temperature instead of conservative temperature? Another suggestion for this figure is to add shaded ranges of the CT/SA bounds of Transition water, to clearly highlight how long these measurements fall within the definition of that water mass.

Figure A4: The figure caption should be adjusted to state that “Section plots of Absolute Salinity (A-D) and Conservative Temperature (E-H), along the Northern Line (A,E), Mooring Line (B-F), Northern Labrador Sea Line (C,G), and AR7W Line (D,H). Many of the panels were referenced incorrectly in the figure caption, please check.

In this paper, Leist et al. combine two artificial radionuclides (¹²⁹I and ²³⁶U) with conservative seawater properties (temperature and salinity) to characterize water mass origin and distribution in Baffin Bay, Davis Strait and the Labrador Sea. The study describes an interesting and comprehensive dataset and provides novel insights into our current understanding of water mass mixing and circulation in Baffin Bay and the Labrador Sea.

General comments:

This manuscript describes a new tracer combination for the study region. Mixing processes and source water attributions, however, have previously been reported for the various passages draining into the Baffin Bay (Nares Strait, Jones Sound, Lancaster Sound), as well as Baffin Bay and Davis Strait based on a range of different tracers (e.g., δ¹⁸O, nitrate-to-phosphate ratios, carbonate chemistry, nitrate isotope ratios). Yet this current manuscript provides limited reference to this previous literature (beyond Huang et al., 2024), particularly in Nares Strait and the CAA. I think this study would benefit from a broader discussion of the literature and how their new estimates quantitively compare with previous findings from different tracers (e.g., Jones 2003, Muenchow et al., 2007; Lehmann et al., 2022; Shadwick et al., 2011; Sherwood et al., 2021; Azetsu-Scott et al., 2012).

While I do understand the concept of how the mixing fractions were derived, I find the description not very intuitive and difficult to follow. I would suggest adding actual equations or specific examples to increase clarity. Also, while described in the methods section, I was surprised that the mixing fractions were not discussed in the results. I suggest to briefly discuss the derived fractions in the results and add them to a summary table either to be included in the main text or the appendix.

Specific comments:

Line 43: Change to Arctic origin (capitalize).

Line 50: Correct to ‘… are present primarily at the surface of Baffin Bay…’.

Lines 82-84: Consider referencing some more recent studies (later than 2011) to discuss ‘recent’ events.   

Line 90: ‘Despite Davis Strait’s critical role as a changing freshwater source to the subpolar North Atlantic (Shan et al., 2024),…’; This is a bit of a confusing sentence. Consider rewriting.

Line 105: This argument comes out of the blue, given that no description has been given on nutrient or carbon dynamics and the potential influence of different water masses. Either elaborate or remove.

Line 115: Add space ‘…seawater(Christl…’.

Lines 159-164: Add description of symbols to figure caption. Also consider adding depth range of profiles at different locations, similar to description of NS102 and TCA.

Lines 207-220: Consider adding equation for mixing model to section 2.3 to increase clarity.

Lines 224-225: ‘The results of ¹²⁹I and…’; I’d suggest moving this sentence to the methods sections (2.1).

Line 350: Symbol indicating tracer-free data depicted in Fig 6, but not visible in Fig. 5B as suggested here. Please add.

Line 358: The authors switch between ‘Canada Basin’ and ‘Canadian Basin’ (mentioned here). Stay consistent and refer to Canada Basin throughout the manuscript (and figures).

Line 377-384: Previous studies have discussed potential cascading events along the WGC, leading to the sinking and intrusion of shallow WGSW to > 300 m depth in Baffin Bay (e.g., Marson et al. 2017; Lehmann et al., 2019). I’m curious if such a process aligns with the observation of a significant contribution of WGSW and tracer-free water discussed here.   

Lines 405-420: The authors indicate that none of the previous studies considered the pathway of Atlantic-origin water through the CAA, while providing a somewhat speculative explanation of how Atlantic water from the Canada Basin may contribute to different water masses in Baffin Bay. The propagation of Pacific- and Atlantic-origin waters through the CAA, including corresponding mixing fractions and proposed mixing mechanisms, have previously been described based on more detailed sampling throughout the CAA (e.g., Lehmann et al., 2022 and references therein). Similarly, contributions of Atlantic (and Pacific) water have also been described for the Nares Strait outflow into Baffin Bay (e.g., Muenchow et al., 2007). In addition, previous studies have also described significant recirculation of northern Baffin Bay and Nares Strait water within Lancaster Sound (i.e., westward flow towards western CAA in northern section of Lancaster Sound (e.g., Colombo et al., 2019; Lehmann et al., 2022). This recirculation potentially contributes both Atlantic and Pacific water entering Baffin Bay through Nares Strait and Jones Sound to the Lancaster Sound. The manuscript would benefit from a more detailed discussion of these previous findings, and how the novel results presented in this current study fit into this broader context.

Line 439-420: The contribution of Canada Basin Atlantic water to BBMW and BBBW (through Nares Strait) has previously been discussed by e.g., Rudels et al. (2004) and Bailey (1956).  

Lines 446: ‘The Radionuclide tracers’; no capital R.

Lines 450: Please elaborate on why ‘Transition Water mix’ is considered a distinct water mass rather than a mix between two.

Figure 1: I suggest adding a legend (e.g., different symbols in 1B) and additional details to figure caption to better explain the map. For example, it is not clear what the difference is between solid and dashed arrows, and what the blue star in 1A refers to.

Figure 2 caption: This should be ‘central Baffin Bay’ in both cases.

Figure 3 caption: ‘The water masses are based on…’. Please correct.

Figure 4: Solid symbols are color-coded based on water masses. I understand this is based on ranges in T/S listed in Table A1? I would suggest adding a few more details to the figure caption describing the water mass attribution. Also, the grey star symbols in A) do they fall outside of defined water mass ranges or why are they not color-coded? If so, please state that in the caption. The grey crosses in B) are not referenced in either the legend or the caption. Please add details.

Figure 5: Symbol for ‘Tracer free’ not visible. Also, correct figure caption to ‘Figure 5. A) Location of each endmember…’, and ‘Arctic Atlantic Water in Canada Basin’.

Figure 6: I find it confusing how the data were split between the subplots (e.g., density/depth range versus region?). Also, the angle of dotted lines seems somewhat random. Please add additional details to the caption for clarification.

Table A1: Values for T and S don’t align with values stated in paragraph 2.2 on water mass classification (Lines 190-200; e.g., WGIW, AW, cold AW, NEADW,…). Please double-check.

Table A2: It would be useful to add depth or density ranges for each endmember, either here or elsewhere in the manuscript.

Figure A1. ¹²⁹I and ²³⁶U are given in units of at/kg throughout the rest of the manuscript. I suggest adapting the units here to stay consistent with the rest of the manuscript.