the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A comparative isotopic study of the biogeochemical cycle of carbon in modern stratified lakes: the hidden role of DOC
Robin Havas
Christophe Thomazo
Miguel Iniesto
Didier Jézéquel
David Moreira
Rosaluz Tavera
Jeanne Caumartin
Elodie Muller
Purificación López-García
Karim Benzerara
Abstract. The carbon cycle is central to the evolution of biogeochemical processes at the surface of the Earth. In the ocean, which has been redox-stratified through most of the Earth’s history, the dissolved organic carbon (DOC) reservoir holds a critical role in these processes because of its large size and involvement in many biogeochemical reactions. However, it is rarely measured and examined in modern stratified analogs and yet commonly invoked in past C cycle studies. Here, we characterized the C cycles of four redox-stratified alkaline crater lakes from Mexico. For this purpose, we analyzed the concentrations and isotopic compositions of DOC together with dissolved inorganic and particulate organic C (DIC and POC). In parallel we measured physico-chemical parameters of the water columns and surficial bottom sediments. The four lakes have high DOC concentrations (from ~ 15 to 160 times the amount of POC, averaging 2 ± 4 mM; 1SD, n=28) with an important variability between and within the lakes. All lakes exhibit prominent DOC peaks (up to 21 mM), found in the oxic and/or anoxic zones. δ13CDOC signatures also span a broad range of values from -29.3 to -8.7 ‰ (with as much as 12.5 ‰ variation within a single lake), while δ13CPOC and δ13CDIC varied from -29.0 to -23.5 ‰ and -4.1 to +2.0 ‰, respectively. The DOC peaks in the water columns and associated isotopic variability seem mostly related to oxygenic and/or anoxygenic primary productivity through the release of excess fixed C in three of the lakes (Atexcac, La Preciosa and La Alberca de los Espinos). By contrast, the variability of [DOC] and δ13CDOC in Lake Alchichica could be mainly explained by partial degradation and accumulation in anoxic waters. Overall, DOC records metabolic reactions that would not have been clearly detected if only DIC and POC reservoirs had been analyzed. For example, DOC analyses evidence an active DIC-uptake and use of a DIC-concentrating mechanism by part of the photosynthetic plankton. Despite the prominent role of DOC in the C cycle of these lakes, variations of [DOC]/δ13CDOC and associated reactions are not reflected in the sedimentary organic carbon record, hence calling for special care when considering sediments as reliable archives of metabolic activities in stratified water columns. Overall, this study brings to light the need of further investigating the role of DOC in the C cycles of modern stratified analogs.
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Robin Havas et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2022-149', Anonymous Referee #1, 03 Oct 2022
The manuscript by Havas et al. presents an extensive biogeochemical dataset of the water column and surficial sediment from four alkaline stratified Mexican lakes. The authors put great emphasis on measuring carbon speciation in the lake in order to obtain a better understanding of the role of DOC on the overall carbon dynamics. They argue that the lakes used in their study can be used as modern analogues of the stratified freshwater Oceans on early Earth.
The methodology is very well described and shows the great care that the authors took to obtain pristine samples. I would have liked to see a bit more detail on the description of the Ion Chromatography, but this is rather a personal taste. Overall, this section is well written and does not leave much to be desired.
The results part is absolutely excessive and requires a major revision. I can fully understand the authors, after having spent so much time in the field and the lab to collect and analyze these samples, one really wants to do justice to every single data point. However, this is absolutely overwhelming for the readers. It is just not necessary to explain every curve or datapoint in every single dataset. Nobody will be able to memorize just a tiny fraction of it. The figures are very nice, although in the current manuscript the axis labels are too tiny, I really had to zoom in to read them. With such nice figures the authors should instead focus on the take-home messages. What are the most important observations that one can make from these data? All the readers need to know are those facts that will carry them through the discussion. There is much more information in the data, but they only distract from the main story.
That brings me to the discussion, and that is also a complete overkill. Given the huge high-quality dataset that is presented here, the authors fell into the trap in trying to present multiple stories in one manuscript. I’ve read it twice and still failed to see how the different sub-chapters in the discussion connect to each other. Maybe they do, maybe they don’t, I really couldn’t tell. Overall, I would strongly recommend to cut out parts of the discussion and put them into a separate manuscript. This is just too much. If you want to keep everything together, then shorten it considerably and make this story more understandable. I really think that there is a good story (or several stories) in the dataset, but currently it is hidden underneath a thick layer of distracting (albeit correct) information.
It does not make sense to make a detailed correction of this manuscript, it needs a thorough revision first. In closing I recommend major revision. This is great work that should be published, but not in its current shape
Citation: https://doi.org/10.5194/bg-2022-149-RC1 -
AC1: 'Reply on RC1', Robin Havas, 08 Nov 2022
Dear reviewer,
First we would like to thank you for providing your useful comments. Besides, we appreciate that your evaluation supports the quality of our work and data, as well as the fact that it should be published.
Nonetheless, your review does bring up major concerns regarding the result and discussion sections and notably the overwhelming amount of information they contain, distracting the reader from the important conclusions of the study. To solve this issue, you suggest to reduce the length of the document and “cut out part of the discussion and put them into a separate manuscript”.
Indeed, the current manuscript combines a large number of data, trying to connect several natural systems through multiple analytical techniques. Condensed in one document, this does result in probably too many pieces of information and conclusions. Therefore, we agree that the different messages of the paper could be better addressed in distinct manuscripts. More precisely, we think that our paper could be subdivided into DIC/POC interpretations on one side and DOC on the other side.
As written to the editor and the other reviewer, the two manuscripts could be submitted in parallel as “Part 1” – “Part 2” publications. We would take care that their respective messages clearly stand out and avoid repetition about geological settings, methods, etc. Overall, this combination would provide a comprehensive picture of the C cycle of these lakes and an easy comparison between the different datasets.
We believe such a reorganization will improve the stream line of each article and facilitate their understanding. It will also reduce their respective length. Each paper would include less methods to describe, which thus could be more detailed if required (such as ion chromatography for example).
You described the result section as excessively long with a number of superficial information. In the proposed outline, the first paper contains DIC/POC data and physico-chemical parameters only, while the second one would contain DOC and nutrient concentrations data only. Together with the fact that we do not need to describe each parameter in detail, this reorganization could greatly reduce the length of this section in each manuscript allowing to focus on the main results.
The respective discussion sections would also be more concise and consistent because each paper will focus on fewer points. Last, it will be possible to read the conclusions independently but they will be easy to connect between the two papers.
We think this reorganization should allow to retrieve the important messages from our dataset in a clearer and more direct way, and hope the reviewer will agree.
Thank you again,
Robin Havas, on behalf of all co-authors
Citation: https://doi.org/10.5194/bg-2022-149-AC1
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AC1: 'Reply on RC1', Robin Havas, 08 Nov 2022
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RC2: 'Comment on bg-2022-149', Anonymous Referee #2, 08 Oct 2022
In this study, the authors analysed the carbon in lakes that acts as analogue of Precambrian Oceans. The manuscript is well written, the methods are state-of-the art, the figures are clears, and the conclusion are well supported by the results. Overall, the study greatly improves our understanding of how biological reactions are recorded and should be of great interest for the readers of Biogeosciences. However, I do have two major concerns.
First, the introduction suggests that biogeochemical processes are facing natural selection. However, there are no references backing up such a statement. To the best of my knowledge, there are no empirical evidence indicating that biogeochemical processes are facing selection. Either the authors need to provide evidence that biogeochemical processes are selected and develop that point a bit more or change the terminology. I think everyone would agree that the diversity of biological enzymes is higher now than it was in the early earth. However, I conceptually fail to see why geological (e.g., erosion) or chemical (e.g., redox reaction) processes must have evolved. The authors need to provide further evidence that those processes evolve or focus the manuscript on what can be demonstrated empirically.
Second, the structure and length of the manuscript, especially the discussion, is not helping the reader to understand the message of this manuscript. The discussion is about 550 lines long (more lines than all other sections of the manuscript combined) and contains additional figures. I do want to apologize to the authors that it took me so long for giving my review back but part of this is because the discussion is pretty much an additional manuscript. I guess only half of the data in this manuscript are in fact needed to make a good story. I would suggest splitting the current manuscript into two manuscripts, one focussing on the data presented in Figure 2, 3, 5 and another one with the isotope data. The introduction claim that the manuscript will describe “the C cycle of four modern redox-stratified alkaline crater lakes”. Well in that case, I only need two sub section in the results/discussion: (1) source and fate of inorganic carbon and two (2) source and fate of organic carbon. In addition, describing all processes in all lakes, one by one, is not helping the reader to extract useful information, on the contrary. I would rather suggest using the results from all lakes and provide a general conclusion.
Minor comments:
The setting/context section is really appreciated. However, I think there is a need to point to Figure 2 (which is currently poorly located) already in the setting/context section. For instance, after reading that lakes were stratified, I was wondering why the authors talk about pH without mentioning the water depth and without presenting the pH profile.
Line 67: delete data
Line 161: What semi-calibrated means?
Line 165: Why different volumes?
Line 166: Does pre-ashed mean pre-combusted? Add temperature and time if this is the case.
Line 171: Please provide details on the transport from the lake to the laboratory.
Line 175: Was the water filtered, stored in a container and then, put in the Exetainer? Or the filter went directly into the exetainer?
Line 196: Was the acid added in the field? Also, we need to know the type, concentration and volume of acid added. This is important because we need to know by how much the samples were diluted after the addition of acid.
Line 197: The name of the laboratory should be written in either French or English throughout the manuscript, and the city in which the laboratory is located should be mentionned. Currently we have a mix of French and English.
Line 206: So, the autosampler was loaded with one lake sample, then six samples of DI water, then another lake sample. The first DI water sample (after the lake sample) was not considered, correct? Not sure I get how the samples were processed and which sample was discarded.
Line 265: Place into brackets and move “between 34.5 and 35 mM” after “throughout the water column”.
Citation: https://doi.org/10.5194/bg-2022-149-RC2 -
AC2: 'Reply on RC2', Robin Havas, 08 Nov 2022
Dear reviewer,
We first thank you for reading our manuscript and giving your constructive comments and we appreciate that they are supportive of our work.
Some minor comments were given and will be incorporated in the final publication(s) after clarification of the communication’s structure. Furthermore, two major concerns were brought up and are answered in detail here below.
First, the reviewer noticed a mistake in the introduction which suggests “that biogeochemical processes are facing natural selection”. As noticed by the reviewer, this is not backed up by empirical evidence but rather a problem of clarity and terminology. In the first sentence, it is the “C cycle and biogeochemical conditions” rather than processes which are suggested to have evolved throughout the Earth’s history. We can make this clearer to avoid confusion. Then, we assess that “processes affecting the C cycle were likely different from those occurring in most modern, well oxygenated environments” because of the widespread stratification of past oceans. While this sentence mostly refers to biological processes – which indeed face natural selection – it also suggests that different processes (e.g. chemical) were in action but did not necessarily “evolve”. For example, redox reactions did not fundamentally evolve themselves but their relative occurrences have changed with respect to changes in the oxidation state of Earth’s surficial environments. Accordingly, we could rephrase this part as follows: “while biological processes evolved through time, dominant chemical processes occurring in anoxic conditions were likely not the same than in fully oxygenated oceans”
The second main point of the reviewer is about the length of the manuscript, and notably its discussion, which is indeed long and dense. In line with the comment of the first reviewer, it is suggested to drastically reduce the size of our manuscript and it is explicitly advised to split the current document “into two manuscripts”.
We realize the size and density of the discussion may be tough to read and retain all at once. Besides, we agree that the dataset described here – although composed of interconnected data – could be presented as several independent ones. Therefore, we agree that the study and its conclusions would benefit from splitting the manuscript in two.
In the current manuscript, we describe the effects of specific physico-chemical and biological parameters on the C cycle of four stratified lakes along a gradient of water chemistries using a unique methodology. Firstly, we aim at assessing the biogeochemical reactions occurring in the water column and how they are recorded in the surficial sediments, through “traditional” DIC and POC results. Then, we want to present the interests of additional DOC results for the understanding of these C cycles.
Your comment suggests to split the study into (i) a manuscript about concentration data (“Figures 2, 3 and 5”) and (ii) “another one with isotope data”. Additionally, you suggest to organize the result/discussion sub-sections as sources and fates of inorganic and organic C.
We think that presenting the sources and fates of the different C reservoirs is indeed a good way to describe the C cycles of these lakes and thus should be included (as we did for DOC – discussion part 5.3.1). However, we believe it is difficult to interpret isotopic signatures of the different C reservoirs without also discussing their concentration, since the first directly relates to the second. Thus, we suggest an alternative outline for the two papers which, compared to the initial manuscript, (i) puts the sources and fates of the C reservoirs more at the front and (ii) better reflects and connects the important messages of our work together.
In brief, we suggest to split the current manuscript in two manuscripts submitted in parallel as “Part 1” – “Part 2” publications with DIC/POC results on one side and DOC on the other. To illustrate the advantages and feasibility of this restructuring we present its potential outline in more detail below.
The first paper would aim at characterizing the sources and fates of DIC and POC using their elemental and isotopic signatures and putting the emphasis on (i) how the different water chemistries influence these C reservoirs and (ii) how this is recorded in their surficial sediments. This first publication will thus set the ground for understanding the C cycle of closely related stratified lakes under different physico-chemical constraints. Fostered by the integrated comparison of several environments, it will show how local effects and the alkalinity gradient (via distinct C sources) strongly influence the DIC record, whereas POC mainly reflects the variable ecosystems and redox structure of the lakes. Moreover, regarding the C sinks, it will show that upper sediment organic C captures different parts of the water columns, while carbonates are in isotopic equilibrium with oxycline DIC. Nonetheless, this conventional DIC/POC dataset leaves equivocal conclusions for the identification of some metabolisms (such as anoxygenic photosynthesis) and shows an incomplete carbon mass balance.
In the potential second paper, we would focus on this missing C reservoir, namely the DOC. Through the description of DOC elemental and isotopic signatures, this second manuscript would aim at highlighting how this innovative tracer complements and deepens our understanding of the C cycle in stratified environments, traditionally based on DIC and POC data. To achieve this, we would first present DOC sources and fates, then describe the inferences drawn for planktonic C uptake and release, and finally the implications for the geological record and “big DOC” hypotheses. As conclusions, DOC analyses (i) detail and unveil the location of photosynthetic organisms in the water columns, (ii) unravel planktonic cellular processes (C-uptake, -release) characterized using d13CDOC but invisible via POC results and (iii) provide boundary conditions to the “big DOC” hypotheses.
We think that this reorganization will clarify the structure and thus the respective messages of each article in addition to considerably reducing their length.
We would like to thank the reviewer again for providing a thorough review up to proposing an alternative structure of the article. We hope that the details of the alternative suggested outline will also convince the reviewer.
Yours sincerely,
Robin Havas, on behalf of all co-authors
Citation: https://doi.org/10.5194/bg-2022-149-AC2
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AC2: 'Reply on RC2', Robin Havas, 08 Nov 2022
Robin Havas et al.
Robin Havas et al.
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