the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Mar 2024
Methane dynamics in the Baltic Sea: investigating concentration, flux and isotopic composition patterns using the coupled physical-biogeochemical model BALTSEM-CH4 v1.0
Abstract. Methane (CH4) cycling in the Baltic Sea is studied through model simulations that incorporate the stable isotopes of CH4 (12C-CH4 and 13C-CH4) in a physical-biogeochemical model. A preliminary CH4 budget identifies benthic release as the dominant CH4 source, which is largely balanced by oxidation in the water column and to a smaller degree by outgassing. The contributions from land loads and net export to the North Sea are of marginal importance. Simulated total CH4 emissions from the Baltic Sea correspond to an average 0.04 g CH4 m−2 y−1, which can be compared to the calibrated sediment source of 0.3 g CH4 m−2 y−1. A major uncertainty is that spatial and temporal variations of the sediment source are not well known. Further, the coarse spatial resolution prevents the model to resolve shallow-water near-shore areas for which measurements indicate occurrences of considerably higher CH4 concentrations and emissions compared to the open Baltic Sea. Modeling of stable CH4 isotopes can help to constrain process rates; to our knowledge this is the first time that CH4 isotopes have been included in a physical-biogeochemical model. A large-scale approach is used in this study, but the parametrizations and parameters presented here could also be implemented in models of near-shore areas where CH4 concentrations and fluxes are typically substantially larger and more variable. Currently, it is not known how important local shallow-water CH4 hotspots are compared to the open water outgassing in the Baltic Sea.
- Preprint
(2028 KB) - Metadata XML
-
Supplement
(2340 KB) - BibTeX
- EndNote
Status: open (until 21 May 2024)
-
RC1: 'Comment on gmd-2023-211', Anonymous Referee #1, 26 Apr 2024
reply
Review: “Methane dynamics in the Baltic Sea: investigating concentration, flux, and isotopic composition patterns using the coupled physical, biogeochemical model BALTSEM-CH4 v1.0” by E. Gustafsson, B. G. Gustafsson, M. Hermans, C. Humborg, and C. Stranne, under review for Geoscientific Model Development
As a very short summary, reactions and tracers were added to an ecosystem model that includes the horizontally integrated depth profiles of model tracers in 13 sub-basins of the Baltic Sea to simulate methane concentrations and isotopes and resolve the associated process rates. The baseline simulation shows high CH4 accumulation in the deeper anoxic parts of the Baltic Sea. Redox zonation is strongly affected over time by inflows from the North Sea and stagnation periods. CH4 concentrations above the halocline are generally much lower due to the presence of oxygen. There, seasonal thermal stratification plays an important role due to its effect on O2 availability and CH4 and O2 solubility. This also strongly affects the d13C-CH4 profiles. Qualitative agreement exists between simulated 1-D profiles, representing larger 3-D volumes of the sub-basins, and measured profiles (e.g., Jakobs et al., 2014). The authors also present a preliminary methane budget, which is, however, highly uncertain.
The introduction does a good job of describing the main research questions for studying methane dynamics in the Baltic Sea. It introduces a problem that the 1-D model cannot resolve, viz., lateral CH4 concentrations from the coastal to open waters. Similarly, there is a significant discussion in the manuscript about benthic methane release from shallower parts that cannot be well resolved. The authors could have chosen to add CH4 isotope tracers and reactions to existing 3-D ecosystem models instead of BALTSEM. This would have allowed simulating lateral gradients and also direct comparison of vertical simulated profiles from deeper parts to measured profiles at specific locations. The authors mention that BALTSEM was chosen since it has been calibrated in the past. However, other 3-D ecosystem models have also been calibrated for the Baltic Sea. Given the link between primary production and sedimentary methane production and the importance of shallow point sources of methane, it may seem that a budget including methane emission may only be confidently constrained with a 3-D model. The authors could elaborate on this choice between 1-D and 3-D models.
In the manuscript, there is great uncertainty regarding the sources of methane (acknowledged in the discussion). The paper assumes a constant flux from the sediment to the water column, independent of time or water depth. There are a couple of issues, including some that are not discussed.
First of all, the text does not elaborate on sources of methane in the water column, which could be particularly important for methane emissions. Studies have shown that methane can be produced in oxic water. Weber et al. (2019) argue that these pathways are needed to explain the general oversaturation of methane in ocean surface waters, and they mention a strong correlation between methane production and net primary production. Could methane production in the water column play an important role in the Baltic Sea? Could the degradation of methylphosphonate form an alternative explanation for higher CH4 emissions (instead of lower CH4 solubility and increased benthic methanogenesis) in exceptionally warm summers, wherein PO4 is more likely a strong limiting factor for primary production? Coccolithophores and zooplankton can also release methane in the water column. Anoxic microzones in sinking particles could harbor methanogenesis both in surface waters and deeper waters. The paper mentions that the model does not reproduce a measured local minimum d13C-CH4 at 30 m depth in the water column (line 431). Local production of methane could potentially explain this.
Secondly, fluxes from sediment to the water are likely not constant. Clearly, the authors are well aware of seasonal variations in CH4 effluxes (lines 458-464) and also that it strongly depends on the oxygen levels in bottom waters (Reed et al., 2011). This makes the simulated temporal patterns less reliable. The discussion and sensitivity analyses sufficiently address this issue and, indeed, show a high sensitivity toward the parameter used for benthic CH4 release.
Thirdly, the introduction mentions gas ebullition versus diffusive sources. This part should be expanded. Other workers have shown that hotspots, such as cold seeps, vents, and mud volcanoes, are significant for methane emissions on the scale of the global ocean (e.g., Hornafius et al., 1999; Weber et al., 2019). There are plenty of studies about cold seeps in the Baltic Sea. To what extent do the authors expect these to dominate emissions to the atmosphere? Methane from these point sources may be laterally transported and affect the CH4 concentrations in surface waters in large parts of the Baltic Sea. There are also other hotspots, such as inundated peat lands. It would be beneficial for readers to know more about the prevalence of cold seeps and methane-rich sediments in both shallow and deeper parts of the Baltic Sea to gain a sense of their importance in the overall budget. It could be very well that methane emissions to the atmosphere are greatly underestimated by the model, as benthic methane release is primarily fitted to CH4 concentrations in the deeper basins.
More than 90% of the citations appear to be works from scientists who have studied the Baltic Sea. Literature from other parts of the world is largely ignored. Occasionally, the wrong articles are cited. For instance, Broman et al. (2020) did not discover methanotrophy (lines 48-50). Not the latest articles, but the articles that made the initial discoveries should be cited. In Table 1, it would be interesting to compare the values of rate constants to the literature. In the discussion, the reaction kinetics are discussed and compared to lake studies. However, there is also a vast body of literature about methane oxidation in ocean waters (e.g., Chan et al., 2019b, 2019b and Pack et al. 2015 show up in a first search attempt).
Overall, I think the strength of the paper is the simulated vertical structure and temporal variability of CH4 concentrations, which is representative of locations with greater water depth. It identifies the interesting role of thermal stratification in surface water, which can affect methane emissions, and the dynamics related to inflows from the North Sea. The authors discuss uncertainties in the overall budget, which could further be improved by considering methane production in the water column and by elaborating on the role of cold seeps and other point sources. Beyond simulating some interesting dynamics, I am currently not convinced that the model will be able to constrain a methane budget for the entire Baltic Sea in the future, as it will always be difficult to represent methane dynamics in coastal areas, and model output cannot be directly compared to measured profiles from particular locations. Maybe the authors can elaborate on whether switching to a 3-D model will be necessary.
Specific comments:
Line 15: “land loads”
I do not understand what land loads could mean, since CH4 is not a solid but a gas. Based on the text, I think river runoff is meant. However, sometimes river runoff and land loads are mentioned in a single sentence as separate sources (e.g., line 594). In the text river runoff, river load, and land loads may denote the same source.Lines 20-21: “to our knowledge this is the first time that CH4 isotopes have been included in a physical-biogeochemical model”
What about Nihous and Masutani (2006)? The full reference is listed below.Line 53-55: “This notion... Humborg et al. 2019)”
Could there not be an alternative explanation, such as increased methane production in the water column?Line 354: “calibrated”
This word is used several times in the text. However, due to the scarcity of data, I think it cannot be called a calibration. Also, it is annoying that the data is not shown. This shows the disadvantage of not using a 3-D resolved model.Lines 430-431: “Furthermore, a local... model run.”
This could indicate a local source of methane in the water column.Lines 482-486: “The rate constant... lakes cited above.”
Rate constants could be compared to kinetic studies of methane oxidation in ocean water instead of lakes. It should be noted that microbes that oxidize methane may have more than one trick on their sleeve (Rogener et al., 2018), allowing them to survive on other energy sources.Line 599: “References”
The reference list is incomplete. At least, Weber et al. (2019) and Roth et al. (2022) are missing.Minor comments:
Line 52-53: “In shallow... Borges et al. (2016)”
The sentence would improve by replacing “emissions” with “seafloor ebullition” and removing the last part.Line 354: “intension”
Line 388-389: “The δ13C-CH4 in water... temperature stratification.”
The transition from the previous sentences is not smooth. It would be good to point the readers to the right figure here.Figure 6: Abbreviations should be explained, as figures should be understandable without reading the text. “ASE” is also not defined in the text.
The term “redox zone” appears to be wrongly used at various locations in the text, where the authors intent to mean “redoxcline”.
References:
Chan, E. W., Shiller, A. M., Joung, D. J., Arrington, E. C., Valentine, D. L., Redmond, M. C., ... & Kessler, J. D. (2019a). Investigations of aerobic methane oxidation in two marine seep environments: Part 1—Chemical kinetics. Journal of Geophysical Research: Oceans, 124(12), 8852-8868.
Chan, E. W., Shiller, A. M., Joung, D. J., Arrington, E. C., Valentine, D. L., Redmond, M. C., ... & Kessler, J. D. (2019b). Investigations of aerobic methane oxidation in two marine seep environments: part 2—isotopic kinetics. Journal of Geophysical Research: Oceans, 124(11), 8392-8399.
Hornafius, J. S., Quigley, D., & Luyendyk, B. P. (1999). The world's most spectacular marine hydrocarbon seeps (Coal Oil Point, Santa Barbara Channel, California): Quantification of emissions. Journal of Geophysical Research: Oceans, 104(C9), 20703-20711.
Nihous, G. C., & Masutani, S. M. (2006). A model of methane concentration profiles in the open ocean. Journal of Marine Research, 64(4), 629-650.
Pack, M. A., Heintz, M. B., Reeburgh, W. S., Trumbore, S. E., Valentine, D. L., Xu, X., & Druffel, E. R. (2015). Methane oxidation in the eastern tropical North Pacific Ocean water column. Journal of Geophysical Research: Biogeosciences, 120(6), 1078-1092.
Rogener, M. K., Bracco, A., Hunter, K. S., Saxton, M. A., & Joye, S. B. (2018). Long-term impact of the Deepwater Horizon oil well blowout on methane oxidation dynamics in the northern Gulf of Mexico. Elem Sci Anth, 6, 73.
Weber, T., Wiseman, N. A., & Kock, A. (2019). Global ocean methane emissions dominated by shallow coastal waters. Nature communications, 10(1), 4584.
Citation: https://doi.org/10.5194/gmd-2023-211-RC1 -
RC2: 'Comment on gmd-2023-211', Anonymous Referee #2, 27 Apr 2024
reply
Gustafsson et al. developed the model BALTSEM-CH4 v1.0, where methane isotopes are included to provide more information about the methane dynamics in the Baltic Sea. They reported their study focusing on three objectives: 1. identify and roughly quantify key CH4 fluxes, 2. set up a preliminary CH4 budget on a Baltic Sea scale, and 3. perform sensitivity experiments on CH4 concentration and isotopic composition depending on transport and transformation processes.
While I would agree that the authors have by and large achieved their objectives, I think the manuscript in its current form won’t meet the curiosity of interested readers, particularly for those who are technical detail oriented and who are likely a significant fraction of gmd’s readership. In the least, I think the authors should provide a thorough technique note that details the model structure with all governing equations and their supporting assumptions, as well as instructions on how initial and boundary conditions are set, and how the numerical solution is obtained.
For example, the current paper leaves me with many questions like:
- How is the reactive-transport problem being formulated?
- Does the model explicitly represent diagenesis?
- Is the sediment represented with explicit biogeochemistry?
- How diagenesis and biogeochemistry are coupled with temperature dynamics and vertical mixing?
- In what way is river load applied?
- And how the lateral and vertical transition of water depth and sediment thickness are handled?
- How is the lateral exchange formulated from the shallow water zone to deep water zone?
- From some part of the paper, it seems the sediment is not explicitly represented. Then How should this be justified if the model is used for long-term projection, where active accumulation/degradation of sediment organic matter will be significant?
Besides, although the authors mentioned calibration in the paper, the results do not show any comparison with observations. (They did say the model more or less agree with some measurement in the text, but I think this is insufficient.) Since there is no differential equation of the reactive-transport system described, I cannot judge how well the model is performing, even I may trust the authors are making confident statement.
In all, I expect the authors do a major revision to present a more convincing manuscript to the readers.
Citation: https://doi.org/10.5194/gmd-2023-211-RC2
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 211 | 42 | 22 | 275 | 23 | 16 | 14 |
- HTML: 211
- PDF: 42
- XML: 22
- Total: 275
- Supplement: 23
- BibTeX: 16
- EndNote: 14
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1