the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Oct 2024
Rising Arctic Seas and Thawing Permafrost: Uncovering the Carbon Cycle Impact in a Thermokarst Lagoon System in the outer Mackenzie Delta, Canada
Abstract. Climate warming in the Arctic is directly connected to rising sea levels and increasing erosion of permafrost coasts, leading to inland migrating coastlines and the transformation of coastal permafrost lakes into thermokarst lagoons. These lagoons represent transitional zones between terrestrial to subsea permafrost environments. So far, the effect of the transition on the carbon cycle is fairly unknown. In this study, we conducted long-term anoxic incubation experiments on surface samples from thermokarst lagoons with varying degrees of sea connectivity. We also included terrestrial permafrost and active layer as endmembers to investigate variations in carbon dioxide (CO2) and methane (CH4) production within lagoon systems and along a land-sea transition transect on Reindeer Island, Northeast Mackenzie Delta, Canada. Results show that CH4 production peaks at 4.6 mg CH4 gC⁻¹ in younger, less connected lagoons with high-quality organic matter, leading to up to 18 times higher GHG production (in CO2 equivalents) compared to open lagoons. CO2 production is higher under marine conditions (3.8 to 5.4 mgCO2 g-1C) than under brackish conditions (1.7 to 4.3 mgCO2 g-1C). Along a land-sea transect, CO2 production increased with increasing marine influence. These findings suggest that the landward migration of the sea, resulting in the inundation of permafrost lowlands and thermokarst lakes, may lead to increased GHG emissions from Arctic coasts in the future.
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RC1: 'Comment on egusphere-2024-2891', Anonymous Referee #1, 25 Nov 2024
General comments:
The authors studied greenhouse production along the land-sea transect with increasing salinity to explore how future sea level rise in the Arctic may alter the carbon cycling of permafrost region. I found this study well-conceived, with interesting and relevant questions. The experiment is also well executed and seems to generate some very robust results. Thus, I do not have any questions regarding the design or results of the study, but I do find the writing is a little hard to understand with convoluted sentences and repetitive descriptions, especially Discussion. The Discussion of this work is unnecessarily long, with large part of it not to interpret the results but repeating the description of the results. I suggest that the authors largely shorten the Discussion (ideally to 2/3 or 3/5 of current length), and cite some recent studies to highlight the most interesting findings of this study, and emphasize the significance of this findings to future climate change. Thanks for the good work and interesting findings.
Specific comments:
- When presenting statistical analysis, it is good to show the F values and sample size, such that the audience can better evaluate the robustness of the results.
- Map of the study sites was (the Reindeer Island and the surrounding lagoon system) was presented twice (first in figure 1c, and again in figure 4 left). It seems unconventional and really unnecessary. I wonder why the authors need to present it twice. Given the limited space for figures in the journal, I suggest to replace one of the study site figures with the PCA results, which is currently enclosed in the supplementary document but seems important to the study.
- I noticed that the Wilcoxon signed-rank test was used for paired samples for statistic comparisons, why not paired Student's t-test? Wilcoxon test requires the strong assumption of symmetric distribution of the data, I am curious if this assumption is met for the analysis?
- All analyses were performed in R, please show the specific R packages associated with each statistical analysis such as to increase the reproducibility of their work.
Technical corrections:
Please pay attention to the format of terminology (e.g. CH4, number subscripted, not CH4), and also make sure the presentation of units or reference of figures/tables is consistent throughout the manuscript (e.g. figure/figures, or fig./figs.).
Citation: https://doi.org/10.5194/egusphere-2024-2891-RC1 -
RC2: 'Comment on egusphere-2024-2891', Anonymous Referee #2, 05 Dec 2024
Jenrich and colleagues have sampled from a set of changing permafrost affected lagoons and small-delta-island lakes in Canada's NW Arctic and conducted a long-term, low-temperature incubation study exploring carbon mineralization (and net methane and carbon dioxide production) and potential controls related to the different land/marine-forms. I admittedly do not know the geomorphological context of the study well at all, but have experience in controls on microbial biogeochemistry in terrestrial wetlands, including permafrost affected sites. Overall, I found the manuscript and study interesting, seemingly very relevant (again with the caveat that I don't know arctic marine coastal/delta island geomorphology), and easy to read. The manuscript at large is long and some parts wordy based on my preference, but generally (with a few small objective formatting errors and typos), there isn't much technically incorrect writing, and I would defer to the authors and journal requirements about the general nature of the written presentation. Some of my specific technical suggestions are:
1) For a more lay reader to the geomorphological context like myself, consider expanding the explanation of the historical and contemporary transition of isolated lakes to thermokarst and increasingly marine-connected lagoons. I generally really like your figures, but I did struggle to see the 1972 coastline as highlighted in the Figure 1 legend.. I suppose I was expecting to see what was a clearly a lake then that is now 52y later a lagoon, but did not. Some additional context about the timing of the coastal erosion and permafrost change in the study sites I think would be useful.
2) There was a prior expectation in the introduction about the role of sulfate reduction (more prominent in S rich marine sediments) influencing methanogenesis that didn't fully play out. There could be a disconnect between a single sulfate measurement in bulk cores at that beginning of the incubation that isn't relevant in incubations running for multiple hundreds of days in closed anoxic systems. Overall, it did seem that the stronger the influence from the marine environment the lower the methane (and increasingly important carbon dioxide as a mineral anaerobic microbial product).
3) In other marine sediments, anaerobic respiration pathways have been shown to be stratified by depth with electron acceptor recharge occurring from the top down because of the oxygenated, mineral-rich water column. SRB might occupy a small horizon, and bacterial fermentations and methanogenesis occur in a slightly deeper horizon (on the scale of millimeters in low turbulent/laminar flowing waters). Are there related laboratory artifacts due to the static/sealed small jars used (v say a column approach with sediment below water and a source of electron acceptor recharge)? I think this warrants some explanation and discussion.
4) Following from 3 above, are there other anaerobic respiration pathways (that would influence proportions of carbon dioxide v methane and methanogenesis overall) beyond sulfate reduction that deserve some attention?
5) In the ecosystems in-situ, and as represented in-vitro in your incubations, is there an expected or potential role for anaerobic methane oxidation? There is a large contrast of the low net methane measured in the marine influenced samples: does this imply low activity of methanogens per se, that produced methane is being consumed, or both? Does it matter?
6) I would like to see some more overall speculation in the Discussion what patterns seen in the lab-based, long-term, static jar incubation approach might mean in the field (both how the lab system does and does not represent field conditions and resources). E.g., considering redox potential layering in situ, the fate of mineral C products in overlying alkaline, oxygenated water, etc.
Citation: https://doi.org/10.5194/egusphere-2024-2891-RC2
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