This paper examines spatial and temporal patterns in REE concentrations in water and sediments in the Mackenzie river delta. The work builds on previous work in the area by the group and presents a valuable addition to that dataset, highlighting a few interesting trends in the occurrences of some REE. The manuscript is well written, the graphics are of high quality, and the interpretation and discussion of the results are mostly solid. I believe the paper is suitable for the journal and can be published pending a few clarifications and corrections. My comments (in no particular order):

1. I am appreciative of the logistical challenges collecting these samples from remote regions. Did the authors collect duplicate samples (i.e., sampling duplicates rather than analytical duplicates) of the sediments or porewaters and was there an operational/sampling blank?
2. The authors state that porewaters were filtered with 0.15 micron PES, but do not explicitly explain what happened with the river water samples.
3. Sediment digestion appears (near)complete for Fe, Mn and most REE, but the MESS reference standard that was used only has REE concentration certified as ‘informational’ (i.e., total levels measured with NAA without uncertainty), and the reference standard has no Pr, Gd, Tb, Dy, Ho, Er, Tm concentrations certified. The concentrations of these elements are not reported in the paper, so the authors really only reported on a select number of REE. Can the authors clarify how total REE (sum of REE) levels were calculated? Also, the recovery for Lu appeared to be low. Could there have been an analytical bias toward the light REE?
4. Concentrations below the detection limit can be anywhere between zero and the detection limit, not necessarily half the LOD. In L156: how many values exactly were set to 0.5xLOD?
5. Are the indices in table 2 unitless?
6. My biggest concern with the interpretation of the data is the inferred control of Fe/Mn/DOC on REE patterns (causality?) and the lack of consideration of other variables therein. There has been previous work on these Mackenzie delta samples, so I think it would be helpful to the reader if the authors (briefly) discussed the general water chemistry. For instance, what were pH/ORP levels? What water types are these? Do the authors have any data on DIC (inorganic carbon/alkalinity; talked about a bit in L325-330)? Carbonate (and also phosphate) can be important ligands for REE. While DOC concentrations were significant, it is difficult to understand the importance of carbonate (and other) complexation without other water quality parameters. A water charge balance could be helpful here, and some insights into the prevailing redox conditions may help substantiate the claimed role of Fe/Mn oxyhydroxides.
7. Figure 6: clarify in the legend that this figure is for porewaters?
8. I have a bit of issue with the discussion around L275 and assigning mechanisms to PCA – the components themselves are not causal; they are just a way to reduce redundancy, and other parameters may be missing. A word of caution may be appropriate here.
9. The discussion in L298 – 304 about DOC properties and molecular weights is interesting. Would it be helpful to perform additional analyses (dialysis?) to further address this? There are other analyses the authors could perform (or at least recommend) to help address some of the remaining speculation and strengthen the evidence for a few of their arguments (e.g., investigate REE fractionation patterns).
10. It seems to me that the argumentation in/around L304-309 would only hold when all concentrations of all other ions that complex with these sites are constant (through competition – as mentioned in L330). So, were other (major) ion concentrations constant?

Reviewer Report Manuscript: Seasonal Contrast in Rare Earth Elements Concentration in Sediment of the Mackenzie Delta

Recommendation: Accept with Minor Revisions

This manuscript presents a compelling and well-executed investigation of seasonal contrasts in rare earth element (REE) concentrations in sediments, porewaters, and overlying waters across the Mackenzie River Delta. The nearly two-order-of-magnitude enrichment of dissolved ∑REE in winter porewaters relative to fall conditions represents a striking and scientifically important observation in Arctic fluvial–marine transition systems. While REE dynamics in Arctic freshwater and coastal environments have been examined previously (e.g., Johannesson and Lyons, 1995; Johannesson and Zhou, 1999; MacMillan et al., 2017), the magnitude of seasonal porewater enrichment documented here appears unusually large and is clearly demonstrated through a well-constrained dataset. I encourage the authors to temper any claim of being the first to report seasonal REE variability in Arctic systems, as related studies exist; however, they are justified in emphasizing the scale, clarity, and statistical robustness of the seasonal contrast observed in this deltaic transition zone. The integration of REE concentrations with dissolved organic carbon (DOC), chromophoric DOM indices (SUVA₂₅₄, HIX, BIX), Fe and Mn, and multivariate statistical analyses (PCA and MANOVA) provides a coherent and well-supported analytical framework. Overall, the manuscript is thoughtfully structured and scientifically sound, and I support publication following minor revisions aimed at strengthening mechanistic clarity, methodological transparency, and internal consistency.

The central result of winter porewater ∑REE concentrations averaging ~217 nmol L⁻¹ compared to ~3 nmol L⁻¹ in fall is robust and statistically well supported. The PCA explains 83% of the total variance in the first two components and clearly separates seasonal datasets, while MANOVA (Pillai’s trace = 0.883, p < 0.0001) confirms statistically distinct multivariate profiles between seasons. These analyses convincingly demonstrate that seasonal forcing dominates over spatial heterogeneity within the dataset. However, the discussion would benefit from a more explicit hierarchical framing of the mechanisms controlling winter enrichment. The manuscript invokes enhanced DOM accumulation, temperature-dependent carbonate complex stability, and Fe/Mn competition for ligands, yet these processes are presented largely qualitatively and without prioritization. A short integrative paragraph ranking the likely dominant control (for example, ligand abundance) relative to secondary modifiers (such as carbonate speciation shifts or competitive metal binding) would strengthen the mechanistic narrative and clarify the interpretation of seasonal drivers.

The treatment of DOM quantity versus quality represents one of the manuscript’s strongest conceptual contributions. Winter DOM is characterized by lower SUVA₂₅₄ and HIX values relative to fall, indicating lower aromaticity and humic character, yet dissolved REE concentrations are substantially higher in winter. The authors attribute this contrast to greater ligand abundance in winter overwhelming lower conditional binding strength relative to fall DOM. This interpretation is reasonable and consistent with established REE–organic complexation literature. Nevertheless, the argument would benefit from explicitly quantifying the seasonal DOC differences and briefly clarifying how increased ligand concentration can offset lower conditional stability constants. A concise conceptual framing of ligand concentration versus binding strength would render this interpretation more quantitatively grounded and prevent the explanation from appearing speculative.

Because the discussion references pKa values and logK differences in complexation strength, the Introduction would benefit from a short paragraph summarizing REE interactions with Fe oxyhydroxides, Mn oxides, and natural organic matter (including humic and fulvic substances). This would provide conceptual grounding for later references to binding site density and complex stability constants. Several prior studies have examined REE association with Fe–Mn phases and humic materials; briefly contextualizing this framework early in the manuscript would improve overall coherence and strengthen the mechanistic basis of the Discussion.

With respect to sedimentary REE dynamics in cold regions, the manuscript suggests seasonal contrasts are understudied. While this is largely accurate for deltaic Arctic porewater systems, the phrasing should acknowledge prior Arctic REE investigations while emphasizing that coupled sediment–porewater seasonal contrasts in fluvial–marine transition zones remain comparatively poorly constrained. This refinement would maintain accuracy while preserving the manuscript’s contribution. The study region description would benefit from inclusion of seasonal precipitation data or at least qualitative hydrological context. Seasonal REE concentrations may reflect dilution, flushing, organic matter mobilization, or restricted exchange processes, and precipitation variability could influence both DOM supply and REE mobility. Even a brief discussion of seasonal hydrologic regime would strengthen interpretation and situate the results within a broader environmental framework.

Several methodological clarifications are required to improve transparency and reproducibility. Figure 1 would benefit from inclusion of river pathways or bathymetric context to better visualize potential fluid flow paths between sampling locations and to contextualize spatial variability. Table 1 should include salinity and pH values for each site, particularly because carbonate complexation and DOM interactions are invoked as mechanistic controls. The manuscript should clarify why different sampling containers were used in fall and winter and whether field blanks and container blanks were conducted to assess contamination or adsorption effects. A supplementary table reporting certified reference material (CRM) recoveries, relative standard deviations, and measured concentrations is strongly recommended. Additionally, the manuscript should clarify how CRM analysis validates the porewater analytical method, as validation of solid CRMs does not necessarily confirm dissolved-phase matrix accuracy.

Further analytical clarity is needed regarding instrumentation. The Methods section suggests both ICP-OES and ICP-MS were used for major and trace elements, but it is not fully clear which analytes were measured on which instrument and why. This ambiguity could imply duplication unless clarified. The isotopes monitored for REE quantification should be explicitly reported. The authors should also indicate whether any matrix separation or column chemistry was performed prior to ICP-MS analysis, particularly given salinity variability across sites.

The statistical treatment would benefit from additional transparency. The approach to handling below-detection-limit data (half detection limit substitution) should be further justified, as PCA and ANOVA results can be sensitive to such treatments. The normalization procedure applied prior to PCA should be more clearly defined and supported with an appropriate methodological reference. The R packages used for PCA, MANOVA, and related analyses should be reported for reproducibility. It is also noteworthy that pH, salinity, REE anomalies (e.g., Ce/Ce*), and grouped LREE/HREE parameters were not included in the statistical analyses; inclusion or justification for exclusion of these variables would strengthen interpretation and potentially provide additional insight into seasonal fractionation processes.

In the Results section, Figure 2 requires clearer explanation regarding normalization procedures and the distinction between overlying water, porewater, and sediment data. The definition of “overlying water” should be explicitly stated (e.g., 1–2 cm above the sediment–water interface versus broader water column). Fe and Mn concentrations should be reported in the text alongside DOC and ∑REE values to maintain consistency in presentation. Figure 3 could potentially be examined across salinity or pH gradients to determine whether seasonality remains dominant when controlling for physicochemical gradients. Figures 4 and 5 would benefit from reconsideration. While Fe–Mn–REE correlations are well established, the rationale for selecting Ce and Nd as representative REEs should be explained. Alternatively, grouping by LREE, HREE, and ∑REE may better capture seasonal fractionation trends. Currently, panels 5b–d most directly support the DOM control argument; some redundancy in earlier figures could potentially be moved to supplementary material to streamline the narrative.

In the Discussion, the covariance of Fe–Mn and REE should be more explicitly justified mechanistically rather than inferred from correlation patterns. The vertical structure of overlying water chemistry and its transition into porewater should be more clearly described to support interpretations of sediment–water exchange processes. The section on DOM quantity versus REE complexation quality should move beyond stating that multiple factors contribute and instead more directly interpret what the dataset reveals regarding ligand availability relative to binding strength. The brief reference to complexation models and binding site density could either be removed or expanded slightly to avoid appearing cursory. The conclusion should more clearly synthesize Fe, Mn, DOM seasonality, and REE mobility into a unified mechanistic summary that reinforces the manuscript’s broader significance.

In conclusion, this manuscript provides an important and well-supported contribution to Arctic trace element biogeochemistry. The seasonal contrast in dissolved REE is convincingly demonstrated, and the dataset is robust. The revisions suggested above are primarily clarifications and structural improvements intended to enhance methodological transparency, mechanistic depth, and reproducibility. No additional sampling, new experiments, or major reanalysis are required. I therefore recommend acceptance following minor revision.

Jaxon Dii Horne

References

Johannesson, K.H. and Zhou, X., 1999. Origin of middle rare earth element enrichments in acid waters of a Canadian High Arctic lake. Geochimica et Cosmochimica Acta, 63(1), pp.153-165.

Johannesson, K.H. and Lyons, W.B., 1995. Rare-earth element geochemistry of Colour Lake, an acidic freshwater lake on Axel Heiberg Island, Northwest Territories, Canada. Chemical Geology, 119(1-4), pp.209-223.

MacMillan, G.A., Chételat, J., Heath, J.P., Mickpegak, R. and Amyot, M., 2017. Rare earth elements in freshwater, marine, and terrestrial ecosystems in the eastern Canadian Arctic. Environmental Science: Processes & Impacts, 19(10), pp.1336-1345.