the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Feb 2024
Spatiotemporal variation of modern lake, stream, and soil water isotopes in Iceland
Abstract. As global warming progresses, changes in high-latitude precipitation are expected to impart long-lasting impacts on earth systems, including glacier mass balance and ecosystem structure. Reconstructing past changes in high-latitude precipitation and hydroclimate from networks of continuous lake records offers one way to improve forecasts of precipitation and precipitation-evaporation balances, but these reconstructions are currently hindered by the incomplete understanding of controls on lake and soil water isotopes. Here, we study the distribution of modern water isotopes in Icelandic lakes, streams, and surface soils collected in 2002, 2003, 2004, 2014, 2019 and 2020 to understand the geographic, geomorphic, and environmental controls on their regional and interannual variability. We find that lake water isotopes in open-basin (through-flowing) lakes reflect local precipitation with biases toward the cold season, particularly in lakes with sub-annual residence times. Closed-basin lakes have water isotope and deuterium excess values consistent with evaporative enrichment. Interannual and seasonal variability of lake-water isotopes at repeatedly sampled sites are consistent with instrumental records of winter snowfall, summer relative humidity, and atmospheric circulation patterns, such as the North Atlantic Oscillation. In contrast to the cold-season bias in Icelandic lakes, summer surface soil water isotopes reflect summer precipitation overprinted by evaporative enrichment that can occur throughout the year, although the soils sampled were shallower than rooting depths for many plant types. This dataset provides new insight into the functionality of water isotopes in Icelandic environments and offers renewed possibilities for optimized site selection and proxy interpretation in future paleohydrological studies on this North Atlantic outpost.
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RC1: 'Comment on hess-2024-1', Anonymous Referee #1, 08 Mar 2024
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In this manuscript, Harning et al. present a detailed investigation of Icelandic hydrology through a multi-year isotopic (δ2H, δ18O, d-excess) observations of lake, stream, and soil water. The number of isotopic observations here represents a remarkable dataset that will be a valuable contribution to the Arctic isotopic (modern and paleo) community. The authors explore the isotopic observations across a range of spatial and temporal scales to understand the key physical drivers of isotopic variations of these waters. They strive to place these observations in the context of paleoclimate studies that utilize these isotopic ratios in different archives to investigate past climatic changes. The authors offer a range of important recommendations for paleoclimate researchers through their modern water isotopic observations.
I think this rich dataset and set of analyses are certainly worthy of publication. However, there are some comments that I feel should be addressed before being published.
Major comments
1. Use of different datasets.
One area that limits the effectiveness of this investigation is uncertainty in which parts of the dataset are being discussed in any given section. There are some times when the authors discuss the full set of data while the majority of time they focus only on those samples in 2014/19/20. It is unclear why there is not a greater focus on the full dataset. My recommendation would be to either treat all data equally throughout the manuscript, or just focus on the 2014/2019/2020 data (and you could include a subsection on the old data to provide additional context if desired). The language used at times is clearly just in thinking about the 14/19/20 data, but it is not written explicitly that this is what the authors are discussing. Some of my minor comments below might simply be related to me mistaking what subset of lakes you are discussing.As presented, the early years of data are often just an afterthought, where they could actually provide significant value. Especially for the implications to paleo proxies, I do not understand why a closer examination of the earlier data is not completed. Having 6 years of data comparison as compared to 3 years is a big difference for constraining the key drivers of lake water changes. I am guessing you cannot do the repeated comparison of the 7 resampled lakes, but there are plenty of other examinations of the data across all the years that could be completed (see below).
2. Interannual comparisons and key drivers.
I think the exploration of interannual comparisons is particularly valuable for their paleoclimate implications. However, I think the analysis could be improved in several important ways that would allow the analysis to more effectively convey the conclusions the authors draw here. For certain changes, I think there are potential alternative explanations for why a given isotopic change occurred, where some additional analysis might be able to better eliminate some explanations and lead readers to the particular conclusions the authors have made.First, I would recommend including the early lake data (2002, 03, 04) to the interannual variability analysis. Obviously, the climate system has changed rather substantially over this time period, so it would be interesting to see if there are significant differences between this earlier data and the more recent data. Exploring these longer term changes would seemingly help to further evaluate a number of the suggested conclusions of key lake water drivers.
Related to this point, bringing in the earlier data would allow the authors to compare to the precipitation isotopic record from the GNIP station in Reykjavik. The precipitation isotopic composition would vary significantly around Iceland, but having this information of the isotopic inputs to lake water would be particularly useful to connecting to some of the conclusions on seasonal biases. Looking at the interannual variability in the Reykjavik precipitation isotopic record in comparison to the lake water would be helpful in this assessment, rather than just as a broad climatology assessment (e.g., that winter precip is depleted, etc).
A single Local Evaporation Line (LEL) is determined for the whole dataset. I would recommend computing the LEL each year. This is an easy way to summarize the suite of lake data for each year, instead of needing to compare a single lake to itself directly. This would also allow for easy comparison with the 2002-2004 data, even if not the same exact lakes. The system should respond to similar larger-scale variations, so, while not a perfect one, a comparison of these years would be meaningful and provide greater context for the Icelandic hydrologic system. Spatial variations could also be studied in this manner. The dynamics are certainly different in this location, but studies like that by Leng and Anderson (2003) that examines drivers of interannual and spatial LEL variations in West Greenland or Kopec et al. (2018) that examines the LEL changes in response to the NAO could serve as an example of how to examine the full system instead of (or in addition to) a lake-by-lake approach used here.
Lastly, a common practice in these sorts of lake water isotopic studies is to examine the intersection of the LEL with the Local Meteoric Water Line (LMWL). This intersection is thought to be the average input of the lake, and then that water evaporates along the LEL (e.g., Gibson et al., 2016). Does this LEL-LMWL intersection reflect the precipitation values observed in Reykjavik? Is there significant interannual/seasonal variability in this intersection? This would be helpful in evaluating the relative seasonal influences in this system and how they might change year to year. In particular, a key conclusion here is that these lakes are driven by cold-season precipitation – does this intersection with the LMWL reflect that conclusion? If so, it is good evidence supporting this conclusion. If not, there might be other factors that should be considered here.
Minor commentsLine 59. Do not need the second ‘however’ in this paragraph.
Figure 1. Precipitation. Why not just use the modern window for Reykjavik precipitation (1992-2018)? It is more relevant to the sampling effort here.
Figure 2. Near end of caption – you write “form” and seemingly mean ‘from’.
Figure 2 – what about the closed lakes outside the B-D narrower axes range? There are many closed lakes with higher δ2H and/or δ18O values. I assume they help define the LEL.
Line 140. More info and/or citations would be helpful on this statement. Could cite Steen-Larsen et al. 2015 for their study on the isotopic composition of water vapor in Iceland. I also think exploring more fully would be important for the NAO discussion later in the manuscript.
Line 150. Why were these 7 lakes chosen to be revisited? I assume a lot of it is for simple logistics purposes (which is very understandable!), but is there further justification that can be offered for why these ones are particularly good for exploring these interannual changes?
Line 193. How was catchment area measured in Google Earth? There must be numerous assumptions and uncertain decisions that had to be made in this process. Were any groundtruthed?
Line 204. I think it is fine to just pick a value here to use, but it would seem quite possible to better inform this number. If it is a generally drier year, the value would likely be smaller, and if it is a wet one, it would be larger. A simple experiment to explore the interannual variation question later in the manuscript would be to try a few different values of this factor to approximate a drier and wetter year.
Line 230. The lake with a d2H value of -1.35‰ does not fall within the range of meteoric waters? I think you can just delete that part of the sentence.
If you do want to assess how the waters compare with the Reykjavik precip, then it would probably be better to examine the precip during times close to when the lake water was sampled (e.g., precip for the year prior to the lake sample).Line 244-245. The precipitation amount at the ‘closest weather station’ could be rather different than what falls in the lake basin, particularly with elevation differences. There is not necessarily a better way to do this analysis (you could try a reanalysis product, but that is not ideal either), but I would note that there could be significant uncertainty added in this analysis. You can correct for the spatial differences in temperature using the lapse rate as you did here, but this is a harder one to “correct”. I say all this because a weaker correlation to precipitation amount than say temperature might not actually be physically meaningful with the uncertainty here.
Figure 4E. The sustained low RH for Torfdalsvatn (red) in 2019 seems very unlikely. Could there be instrument issues (or RH calculation issues) here?
Figure 7. It would be worth comparing the Reykjavik GNIP precipitation isotopic values to the stream flow. That could be helpful in assessing the strength or lack of an evaporation signal in this water.
Line 328-330. I think this is true for open lakes (as said in the next sentence), but I think there are plenty of lakes that plot significantly different than the LMWL.
Line 384. Consistent with what observation?
Lines 394-395. This does not seem correct to me. My understanding of the NAO is that when it is positive (strong low pressure over Iceland), the North Atlantic storm track centers right on Iceland, and would yield high precipitation and humidity (low evaporation). This setup has strong south to north transport. You can see this in any meteorological or reanalysis dataset (i.e., NAO has strong positive correlations between precipitation, humidity, southerly winds across this region). It would seem quite unlikely for the dominant moisture sources during the positive phase to be from the north. The negative phase removes Iceland from the primary storm track (making it drier), on average. If 2019 is the negative NAO year, then it would seem to me the higher d2H and lower d-excess values are showing an expression of the relative dryness (higher evaporation / lower inputs) while the other two years see higher inputs / lower evaporation, on average.
Line 396. How is the time delineated here? My interpretation in reading this would be that NAO is computed for the calendar year of each of these years (2014, 2019, 2020). To understand the effects on the lake water, you would likely want some time period (e.g., one year) before the sampling took place. Looking quickly at the data, I do not think it would change your NAO values too much, but it would be a more appropriate assessment.
Line 427-428. Say specifically what the time change is associated with the enrichment / depletion.
Line 539-541. In the Interannual section, you showed the NAO in 2019 was negative (which is correct), not positive as written here. That said, I do agree with the conclusion overall as written here (if the NAO value was correct) – i.e., drier conditions driving the observed enrichment and low d-excess - but this is not consistent with the argument presented in 5.1.2. I think this is how you should argue the point in 5.1.2.
References
Gibson, J. J., Birks, S. J., & Yi, Y. (2016). Stable isotope mass balance of lakes: a contemporary perspective. Quaternary Science Reviews, 131, 316-328. https://doi.org/10.1016/j.quascirev.2015.04.013.Kopec, B. G., Feng, X., Posmentier, E. S., Chipman, J. W., & Virginia, R. A. (2018). Use of principal component analysis to extract environmental information from lake water isotopic compositions. Limnology and Oceanography, 63(3), 1340-1354. https://doi.org/10.1002/lno.10776.
Leng, M. J., & Anderson, N. J. (2003). Isotopic variation in modern lake waters from western Greenland. The Holocene, 13(4), 605-611. https://doi.org/10.1191/0959683603hl620.
Steen‐Larsen, H. C., Sveinbjörnsdottir, A. E., Jonsson, T., Ritter, F., Bonne, J. L., Masson‐Delmotte, V., et al. (2015). Moisture sources and synoptic to seasonal variability of North Atlantic water vapor isotopic composition. Journal of Geophysical Research: Atmospheres, 120(12), 5757-5774.https://doi.org/10.1002/2015JD023234.
Citation: https://doi.org/10.5194/hess-2024-1-RC1
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