Dear Authors,
Please find below my second review of your manuscript titled "The changing composition of the Gulf of St. Lawrence inflow waters observed from transient tracer measurements." I appreciate your efforts in addressing some of the concerns raised in my initial review. Below, I offer my comments on this new version of the manuscript.
In my first review, I recommended a substantial re-structuring of the study, if not outright rejection. While I acknowledge that some changes have been made, I feel that the manuscript remains quite similar to the previous version and still requires further revisions. I am aware that the other reviewers recommended only minor revisions, which may influence the overall direction of your decision-making. However, in light of the current state of the manuscript, I would still suggest a more in-depth rework.
With that in mind, I recommend rejecting the manuscript at this stage, as I believe it still does not meet the high standards expected for peer-reviewed publications. Specifically, the manuscript contains several repetitions, some unresolved questions, and areas where further clarification or refinement is needed. I also have concerns about the overall contribution of the study to the literature. Below are my detailed comments.
*** What is this study about? ***
The primary contribution of this study is to analyze water masses using the mean age of water parcels, determined from transient tracers, in addition to temperature and salinity. While other studies have used non-conservative tracers (e.g., O2, nutrients) for similar analyses, the use of transient tracers offers a novel perspective. The scientific basis seems sound, and the methodological approach is valid.
*** What are the main findings? ***
The study appears to have one key finding: that the NACW fraction of the water entering the Laurentian Channel has not yet reached 100%, as previously believed, but is still increasing. This is effectively demonstrated in Figure 6b.
The authors' own response to my earlier comments highlights those key findings
"The main novelties of this study, extending existing GSL research, are: 1) The observed ongoing shift towards increased NACW contributions to the deep water, as of 2022, which had been observed to have fully happen prior to our analysis. 2) The use of transient tracers in water mass analysis, to include insights on circulation and mixing processes." [quote from their response]
*** Are these findings sufficient to justify a peer-review publication? ***
While the scientific results themselves are valuable, the discussion section remains rather thin. This is not unexpected, given the limited scope of the findings. In fact, there is considerable repetition in both the Results and Discussion sections. For instance, the following sentences essentially repeat the same idea (and are located within just a few lines on page 19):
L. 390: recent studies, such as Jutras et al. (2023b), suggest that since 2021 the deep water within the Laurentian Channel has been almost entirely composed of NACW.
L. 413: The LCW and NACW fractions estimated from earlier studies (e.g. Gilbert et al., 2005; Jutras et al., 2020, 2023b) reported ongoing variabilities in the composition of the entering deep water up to a complete disappearance of LCW in 2021.
L. 415: In Jutras et al. (2023b), LCW fractions at Cabot Strait were reported as 0 % in 2021.
L. 393: This highlights that the transition towards NACW dominance is ongoing rather than complete
L. 401: As of 2022, NACW influence is continuing to rise throughout the Laurentian Channel...
L. 405: ... the ongoing trend of increasing NACW influence remains identifiable.
L.415: Our observations, however, indicate that this transition is still ongoing as of 2022.
These are important points, but they say the same things: we thought we had 100% NACW in 2021, but this new study shows that it was not the case and that the NACW fraction is still increasing. These findings are repeated one more time in the conclusion.
So, are these findings sufficient to justify a peer-review publication? I honestly don't think so... I feel that the entire study could hold in 2 pages.
*** Concerns About the Robustness of the Findings ***
In addition to the relative lack of new findings, I am afraid that some of those findings are not robust and the authors did not address very surprising results. I will try to go through a few of them related to Issues with LCW Fractions
How can the LCW fraction be 100% at several stations in the western Gulf and estuary (Figure 5a)?
The authors acknowledge this as a potential issue (L. 336), but the explanation—uncertainty in fraction calculations—seems inadequate. The authors mentioned that their uncertainty on LCW fraction is 21% (thus 100%-21% = 79% LCW), but this still seems problematic. The largest LCW fraction observed was believed to be ~75% in 1930 and has decline since. How can the authors be comfortable with their results that re-write our understanding of the deep Laurentian channel waters given those surprising results?
In addition, how do the authors explain that the LCW fraction went from ~75% in 1930 to ~50% in 2000 (70 years); but then the same magnitude of change (~75% to ~50%) would have occurred in only 4 years between 2018 and 2022. Is this reasonable? Does it call into question previous research?
I would expect that such surprising results would be discuss further and, as mentioned earlier, I am afraid that those results may bring more confusion in the literature if the paper is published. Those results may be well related to mesoscale activity and the uneven mixing happening near Cabot Strait, but the current explanations are not convincing to me at this stage.
L. 444, the authors say "Despite increasing mean ages and declining LCW fractions from 2018 to 2022 [note: we are talking about huge changes, from 80% LCW to <40%; Figure 6b], DO levels remained relatively stable, with only a slight decrease. This suggests that variabilities in oxygen concentrations might not be directly tied to water mass changes alone and are likely influenced by additional factors"
-> This is also quite surprising given that it is well known that the LCW and NACW are different DO properties. Could this suggest instead that your method is not working properly?
*** Other Comments ***
L. 20: "The tracer data reveal an unexpected age distribution, with ‘older’ deep waters present near the Gulf’s entrance, whereas ‘younger’ water is found further inshore, contrary to the expected estuarine circulation pattern, which transports deep water inland (increasing age along the flow path)."
-> I already commented about this in my previous review, yet this mis-leading statement still remains. This is not unexpected if the ratio LCW:NACW is changing and maybe not even surprising. Stating that this is "contrary to expected estuarine circulation patterns" is extremely mis-leading and should be removed from the abstract.
To continue on this comment, while the mean age can be used in the water mass analysis, I would not present it as a result because of this confusion that it brings. I would focus on the LCW:NACW fraction instead.
L. 31: It describes the transport of surface waters to the interior of the ocean, *where the age of the water refers to* the time since a water parcel was last in contact with the atmosphere.
-> When one says "where ... refers to..." we understand that the concept has been introduced before, which is not the case here.
L. 42: By combining measurements of both tracers enables to resolve water mass formation, mixing, and circulation patterns, which are key to understanding distribution of biogeochemical properties, such as oxygen.
-> problem with this sentence.
L. 103: The cold (< 1 °C) and *saline* intermediate layer...
-> Is *saline* a good qualification for the CIL?
L. 183: We focused specifically on data collected after 2010 in the region near the mouth of the Laurentian Channel, where the deep water enters, while still representing LCW and NACW as distinct unmixed water masses (see Figure 4b).
-> The fact that LCW and NACW are un-mixed when they enter the Laurentian channel is news to me and goes against previous understanding that those waters are mixed outside the channel. For example: Brickman, D., Hebert, D., & Wang, Z. (2018). Mechanism for the recent ocean warming events on the Scotian Shelf of eastern Canada. Continental Shelf Research, 156, 11-22.
Section 3.4 - Deep water time proxy at Cabot Strait
"A *proxy time series* at Cabot Strait was constructed to better resolve temporal changes in mean age and water mass composition and potentially related water mass properties, such as oxygen."
-> A proxy of what?? the entire sections says nothing about what kind of proxy is this.
Are we talking about a *proxy for mean age at Cabot Strait* (e.g. Figure 6A)? Then just say it.
The same applies for L.384 where the authors mentioned "proxy time series" without explanation.
Table 1: Looking at the table, one can estimate that a 50:50 mix between NACW (~86 years) and LCW (~12 years) should lead to a ~50 years water mass. This seems clear from Figure 6 and in my view the repetition of figures for age and fraction is just a repetition of the same information (e.g. Figures 4a and 5a show mirrored observations).
L. 411: "therefore, the measurements are solely influenced by deep water entering through Cabot Strait"
-> Are you sure? Then how do you explain 100% LCW in the western part of the gulf?
L. 421: The limitation of relying on, e.g., oxygen as an indicator for water mass analysis are evident in our proxy timeseries at Cabot Strait (see Figure S2), where only minor overall changes are detected despite high variability.
-> It is not evident to me what the authors refer to.
L. 440: "However, this small apparent decrease should be treated with caution, as the time series is statistically insufficient to confirm a significant trend."
-> A time series cannot be "statistically insufficient".
L. 465: "The observations show a distinct pattern: near the Gulf’s entrance, a signal of older, warmer, more saline NACW, while further inshore along the Laurentian Channel, younger, colder, less saline LCW is evident."
-> We don't see LCW or NACW. It is always a mixture with different LCW:NACW ratios.
While the study presents interesting data, there remain significant concerns regarding the robustness of the findings, the redundancy in the manuscript, and the interpretation of certain results.
I hope these comments are helpful, and I am sorry that I cannot be more supportive for the moment.
Best regards. |
Review of Gerke et al.’s “The changing composition of the Gulf of St. Lawrence inflow waters observed from transient tracer measurements” (manuscript #: egusphere-2025-3999)
General comments:
The authors of this manuscript assess the mean ages of the waters in the Gulf of St Lawrence and suggest that there has been a gradual increase in the proportion of North Atlantic Central Waters from inshore areas to the entrance of the Gulf of St Lawrence is evidence of a shift towards deep waters dominated by North Atlantic Central Waters since 2022. They use transit-time distributions to derive the mean ages and then integrate them into a water mass analysis to analyze the water mass composition of the region. The authors use transient tracer measurements collected and described in Stevens et al. (2024) and use the same density surface to represent the core of the deep water inflow. CFC-12 has seen its maximum concentrations in the atmosphere, but SF6 continues to increase; CFC-12 is useful for water masses between 23-85 years in mean age whereas SF6 is useful for younger water masses and in helping to resolve the ambiguity in mean age estimates using the Inverse Gaussian transit-time distribution approach (e.g., Guo et al., 2025). Although, it seems like the authors here just relying on SF6 according to their Appendix B and back-calculate CFC-12, which makes me wonder what information the authors are getting from CFC-12. There may be data issues with regions lacking SF6 and in the regions where waters approach the time scale of CFC-12 beginning to be emitted, there are signal issues. I appreciate the trend analysis and other detailed efforts that went into this manuscript, especially to the end that the relative proportion of waters is changing, but the authors need to perform some additional analyses to convince me of their interpretation of the data beyond that. I suggest minor revisions. Specific comments are listed below:
Specific comments:
Line 36: You should add “abiotic” in front of “transient tracer concentrations” here; you could say “passive” but that would exclude radioactive transient tracers
Line 151: I’m not sure why a Delta/Gamma ratio of 1.2 was chosen; Ebser et al. (2018), which a co-author on your study was also a co-author on, found different ratios for different water masses (e.g., 0.5-0.6 where Labrador Sea Water dominates and 0.9 where North Atlantic Deep Water dominates). Please say more about Figure B1, which seems to be where 1.2 came from. Please also explain why you don’t consider using a different ratio of mean age to half-width for North Atlantic Central Waters as opposed to Labrador Current Waters.
Lines 167-168: According to Guo et al. (2025), your estimates of the mean age are likely biases wherever you only use CFC-12 and no SF6 so did you see any spatial discontinuities or other signs that your estimates were different where you have SF6 vs where you do not? You can evaluate the bias you would have in regions where you have SF6 measurements by doing the mean age estimation with both CFC-12 and SF6 and again with only CFC-12 to assess the bias. Analysis was done to corroborate the CFC-12 measurements with the back-calculated CFC-12 concentrations in Appendix B where there are SF6 data but it’s unclear to me what information CFC-12 is then providing.
Lines 174-177/Equations 1-4: Is the water in the Gulf of St Lawrence exclusively composed of LCW and NACW? There’s also the cold intermediate layer and surface/warm slope water, I thought. Also, while the mean ages of two IG TTD for LCW and NACW would linearly sum to a new mean age, the resulting TTD will not be IG. So are you assuming that the TTDs for LCW and NACW are not IG but their sum is IG (in which case the TTDs for LCW and NACW will still need to have their means linearly combine)? Or are you going to use a sum of two IGs as your TTD?
Lines 246-247: Is the sudden discontinuity in temperature and salinity at the eastern tip of Anticosti Island physical or actually due to the availability of SF6 on one side and lack of SF6 measurements on the other?
Figures 4-5: When I see waters with mean ages of 60+ years using the tracer-based constraints you have, there becomes a signal detection issue because of the very low concentrations of CFC-12 in its first couple of decades of being emitted and the fact that you used a backwards calculation to infer the CFC-12 concentrations from the mean ages that you got from SF6 measurements (lines 450-452). Your Figure 4d makes it look like this generally is reflected in your uncertainties, but your Figure 4b has mean ages of up to 100 years, which shouldn’t be detectable using CFC-12 and/or SF6. Also, your Figures 4a-b makes it look like waters are being ventilated after mixing with waters coming from the St Lawrence River in the western part of the Gulf of St Lawrence and there is a barrier for younger waters southeast of the Gulf of St Lawrence to get into the Gulf there through the Laurentian Channel, which leads to an increase in age as the waters reside for longer within the southeastern portion of the Gulf. Your interpretation is that the younger waters in the western portion of the Gulf are due to a higher portion of LCW mixing with the other waters there but is the mix of high and intermediate proportions of LCW shown in Figure 5 in the western portion of the Gulf with large variability over a small spatial distance due to potential data issues such as the ones I’ve pointed out in this comment and others? For example, you tend to have higher proportions of LCW where you don’t look like you have SF6 measurements in the western part of the Gulf.
Figure 6: I’m not sure what the purpose of showing the relative stability of the oxygen concentrations is here because oxygen concentrations can change due to respiration changes, which isn’t part of your analysis here. If you use your TTDs to calculate the preformed oxygen, on the other hand, then that may be worth showing. This figure, on the other hand, does show a trend in the variables that support your interpretation of the relative proportion of NACW vs LCW changing.
Lines 327-330: Where was this shift previously reported to be occurring, specifically?