the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Aug 2024
Exploring the mechanisms of Devonian oceanic anoxia: impact of ocean dynamics, palaeogeography and orbital forcing
Abstract. The Devonian is a warmer-than-present geological period spanning from 419 to 359 million years ago (Ma) characterized by multiple identified ocean anoxic/hypoxic events. Despite decades of extensive investigation, no consensus has been reached regarding the drivers of these anoxic events. While growing geological evidence has demonstrated a temporal correlation between astronomical forcing and anoxia during this period, underlying physical mechanisms remain unknown, hence questioning causality. Here, we perform multiple sensitivity experiments, using an Earth system model of intermediate complexity (cGENIE), to isolate the influences of specific Devonian climate and palaeogeography components on ocean oxygen levels, contributing to the better understanding of the intricate interplay of factors preconditioning the ocean to anoxia. We quantify the impact of continental configuration, ocean-atmosphere biogeochemistry (global mean oceanic PO4 concentration and atmospheric pO2), climatic forcing (pCO2) and astronomical forcing on background oceanic circulation and oxygenation during the Devonian. Our results indicate that continental configuration is crucial for Devonian ocean anoxia, significantly influencing ocean circulation and oxygen levels while consistently modulating the effects of other Devonian climate components such as oceanic PO4 concentration, atmospheric pO2 and pCO2, and orbital forcing. The evolution of continental configuration provides a plausible explanation for the increased frequency of ocean anoxic events identified during the Middle and Late Devonian periods, as it contributed to the expansion of oxygen-depleted zones. Our simulations also show that both the decreased atmospheric pO2 and increased oceanic PO2 concentration exacerbate ocean anoxia, consistent with established knowledge. The variation of pCO2 reveals a wide range of ocean dynamics patterns, including stable oscillations, multiple convection cells, multistability and hysteresis; all leading to significant variations of the ocean oxygen levels, therefore strongly impacting the preconditioning of the ocean to anoxia. Furthermore, multistability and important hysteresis (particularly slow ocean time response) offer different mechanisms to account for the prolonged duration of some ocean anoxic events. Finally, we found that astronomical forcing substantially impacts ocean anoxia by altering ocean circulation and oxygen solubility, with obliquity consistently emerging as the primary orbital parameter driving ocean oxygen variations.
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RC1: 'Comment on egusphere-2024-1983', Anonymous Referee #1, 27 Aug 2024
Review of "Exploring the mechanisms of Devonian oceanic anoxia: impact of ocean dynamics, palaeogeography and orbital forcing" by Justin Gérard et al.
This manuscript investigates Devonian OAEs by performing and analyzing systematic sensitivity experiments using the cGENIE Earth system model. The multiple sensitivity experiments are used to quantify the impact of continental configuration, ocean-atmosphere biogeochemistry, climate forcing and astronomical forcing on background oceanic circulation and oxygenation during the Devonian. I find that results of these simulation are interesting, providing new insights into the ocean anoxic events, the analyses of the results are basically sound, and the manuscript is well-organized, although I had some questions regarding the simulations and interpretation of results after reading through the paper.
Specific comments are listed below that may help further improve this manuscript:
1. Section 2.3: It would be of much help if a table summarizing the key set-ups of the three sets of simulations could be provided in the manuscript.
2. Lines 208-210: The authors argue that for both reconstruction types, the Middle and Late Devonian exhibit a higher susceptibility to anoxia due to the broader OMZ extent associated with these periods. However, the OMZ extent reaches minimum after 405 Ma, according to Figure 4. Could you explain why “Middle and Late Devonian” is used? Otherwise, please revise the corresponding discussion.
3. Figure 5a: The continental configuration for Vera393M is not consistent with that in Figure 2. For example, there is a passage at around (50S, 50W) in Figure 2, but there is continent at the same grids in Figure 5a.
4. Figure 6: Could you explain why 4 times pCO2 is used here, instead of 2 times or 1 time as control? Also, 0.4 times pO2 and 1.5 times PO4 are used in Figures 6, 7, 8, but why 0.7 times pO2 and 1.25 and 1.75 times PO4 are chosen for Figures 9a and 9b? It would be helpful if the authors could add some clarification on how the values are chosen for different experiments.
5. Lines 251-252: I am a little curious whether it’s robust that the northern overturning circulation is always strengthened with each doubling of pCO2. It would be more convincing if related figures could be provided in the supplementary materials.
6. Figure 8 caption: Should it be anomaly between 4 and 2 times pCO2 for (c), and between 2 and 1 times pCO2 for (f)?
7. Lines 264-270: Could you explain why the active circulation cell does not efficiently ventilate the anoxic areas left in the benthic ocean when the nutrient availability is relatively low?8. Lines 288-291: Could you show the results of Scot390M 4 times pCO2 in Figures 9a and 9b, to better explain the trend observed in Figure 7a?
Citation: https://doi.org/10.5194/egusphere-2024-1983-RC1 -
CC1: 'Reply on RC1', Justin Gérard, 02 Sep 2024
We thank the reviewer for their valuable input and constructive comments on our manuscript. We will provide a detailed response to each of these comments as we move forward in the review process. We are pleased that you had a positive overall impression of the paper.
Citation: https://doi.org/10.5194/egusphere-2024-1983-CC1 -
AC1: 'Reply on RC1', Justin Gérard, 01 Oct 2024
Again, we thank the reviewer for their valuable feedback. Below is a summary of our planned actions in response to each comment.
1. Section 2.3: We agree to include a table summarizing the key simulation setups to improve clarity.
2. Lines 208-210: We refer to the "Middle (about 394-379 Ma) and Late Devonian (about 379-360Ma)" because both continental configurations (Scotese and Vérard) show, on average, a broader OMZ extent compared to the Early Devonian (about 419-394Ma) (Becker et al., 2020). Consequently, these periods exhibit a higher susceptibility to anoxic events, as the system is, on average, already less oxygenated. The minimum OMZ extent observed at 405 Ma (Early Devonian) further supports this assertion, indicating a system with more available oxygen, making it more difficult to reach an anoxic event. We will adapt the manuscript to clarify that we are referring to the average OMZ extent of Early, Middle and Late Devonian and to the susceptibility of reaching an anoxic event.
3. Figure 5a: The mismatch between the continental configurations in Figures 2 and 5a is expected. Figure 2 shows surface bathymetry, while Figure 5a depicts oxygen concentrations at 1.5 km depth. As a result, regions where the ocean depth is less than 1.5 km are displayed as continental cells in Figure 5a (such as the passage at around 50S, 50W). We will add this clarification to the Figure 5 caption to prevent any confusion.
4. In Figure 6, we deliberately avoided using 1 and 2x pCO2 to prevent interference from any limit cycles, which only occur at these values. In Figures 9a and 9b, we aimed to illustrate how important the impact of the limit cycles on dissolved oxygen can be. To achieve this, we selected specific conditions using different pO2 and [PO4] values for the various limit cycles in Scot390M and Scot420M. We will revise the manuscript to clarify how the values of pCO2, PO4 and pO2 are chosen in those Figures.
5. Lines 251-252: This result is robust across all our cGENIE simulations. We will provide the reviewer with an additional figure showing the evolution of MOC strength as a function of pCO2, to illustrate this behaviour.
6. Figure 8 caption: This is indeed a mistake. The correct anomalies should be between 4 and 2x pCO2 for (c), and between 2 and 1x pCO2 for (f). The text will be updated accordingly.
7. Lines 264-270: In our simulations, MOC strengthening is not spatially uniform, with a gradually more pronounced effect in the northern hemisphere. As [PO4] decreases, the benthic anoxic boundary is shifted southward, sometimes reaching areas less affected by MOC strengthening. Consequently, in certain cases (e.g., Vera370M and Vera420M), when [PO4] becomes small, the effect of increased ventilation on the percentage of benthic anoxia can be significantly reduced. We will revise the manuscript to better justify this.
8. Lines 288-291: We will provide the reviewer with an updated version of Figures 9a and 9b, displaying the results for Scot390M at 4x pCO2. However, since no limit cycle is observed at 4x pCO2, we believe this may not offer additional insights to the document.
Citation: https://doi.org/10.5194/egusphere-2024-1983-AC1
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CC1: 'Reply on RC1', Justin Gérard, 02 Sep 2024
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RC2: 'Comment on egusphere-2024-1983', Anonymous Referee #2, 07 Sep 2024
Summary:
Gerard et al. use the Earth system model of intermediate complexity with active biogeochemical cycling, cGENIE, to better understand the mechanisms underlying enhanced ocean anoxia during the Devonian. The sensitivity experiments with cGENIE explore the relative importance of paleogeography, orbital configuration, global mean oceanic PO4 concentration and atmospheric pO2, and atmospheric pCO2 in preconditioning Devonian ocean anoxia. Of these factors considered, Devonian paleogeography plays a leading role in promoting ocean anoxia by directly influencing ocean circulation, which causes an expansion in oxygen depleted zones. Paleogeography also modulates the impacts of orbital configuration, oceanic PO4 concentration, atmospheric pO2, and atmospheric pCO2. An increase in oceanic PO4 and decrease in atmospheric pO2 exacerbate ocean anoxia. Changes in pCO2 lead to large reorganizations of ocean dynamics, and potentially provide an explanation for the prolonged duration of some ocean anoxic events. Also, changes in orbital configuration, particularly obliquity, influence ocean oxygen variability by altering ocean circulation and oxygen solubility. Gerard et al. have produced a novel set of simulations that contribute to our understanding of the drivers of Devonian ocean anoxic events, the results are well organized/communicated, and the model limitations are clearly stated. I only have minor comments and questions that are explained in more detail below.
Main comments:
Section 3.1 (Lines 169-175): How does the number and extent of deepwater formation zones differ between the Scotese and Wright (2018) and Verard (2019a) paleogeographies? Decreased deepwater formation in Verard could also lead to increased global mean ventilation age. I am wondering if the number and extent of deepwater formation zones is similar between the Scotese and Wright (2018) and Verard (2019a) paleogeographies and therefore it is clear that seafloor depth is the most important factor for global mean ventilation age?
Section 3.2.2: Can you explain how this seesaw phenomenon relates to a potentially different surface temperature and salinity response in each hemisphere? How are these temperature and salinity changes impacted by the melting sea ice, which then impact ventilation?
Section 3.2.2 (Lines 276-277): Should there be a hatched area for melted sea ice in Figure 8c?
Minor comments:
Line 56: Should “sensibility” be replaced with “sensitivity”?
Line 32: Break to a new paragraph at “Extensive”
Line 135-136: I would cite Fig. 1 here because from Fig. 1 caption alone, it is not clear why the surface albedo is hemispherically symmetric.
Line 157-159: What justification did you use to choose these ranges in pCO2, PO4, and pO2?
Line 226: Replace period with a space
Figure 5 caption: Please indicate what lines labeled A and H represent. An outline of anoxic and hypoxic conditions?
Citation: https://doi.org/10.5194/egusphere-2024-1983-RC2 -
CC2: 'Reply on RC2', Justin Gérard, 09 Sep 2024
We thank the reviewer for their positive feedback and constructive comments on our work. We appreciate the reviewer’s thoughtful evaluation of the paper. As we continue through the review process, we will carefully address each of the comments and questions.
Citation: https://doi.org/10.5194/egusphere-2024-1983-CC2 -
AC2: 'Reply on RC2', Justin Gérard, 01 Oct 2024
We are grateful for the reviewer's constructive comments and have outlined our actions in response to each of them.
Main comments:- Section 3.1: Throughout our simulations, cGENIE displays, at equilibrium, a very similar number and extent of deepwater formation zones for Scotese and Wright (2018) and Vérard (2019a) paleogeographies. Using Verard’s palaeogeography and raising the ocean floor to match the average depth of Scotese’s configuration led to much more similar average ventilation ages (3 times closer). This further demonstrates that, in our experimental design, ocean depth is the primary factor influencing the ventilation age of the benthic ocean. The manuscript will be adapted to better justify this.
- Section 3.2.2 It is challenging to fully address these points as our simulations represent an equilibrium state, and we prefer to avoid speculative conclusions. As shown in past work (e.g. Gérard and Crucifix 2024), even a detailed analysis of the geostrophic components of equilibrium simulations does not allow for proper identification of the causes of circulation changes. Therefore, separating the changes that are attributable to the seesaw effect from those of a different effect is non-straightforward with the material at hand. We suspect that sea ice does not play a significant role, as its presence is minimal and vanishes completely from 4x pCO2 and higher scenarios. The seesaw phenomenon might instead be more closely related to the bathymetry, which helps structure the ocean circulation. Furthermore, continental configuration likely plays a central role in shaping the differential temperature and salinity responses between hemispheres. While the question of the reviewer is interesting, the explanation we offer here remains speculative and providing a definitive answer might be a bit out of the scope of the present study because our experimental sign is not suited to tackle such a fuller investigation.
- Section 3.2.2: A hatched area will be added to Fig. 8c to match the conventions used in Fig. 8f.
Minor comments:
- Line 56: “Sensibility” will be replaced with “sensitivity”.
- Line 32: We will follow the reviewer's advice and break into a new paragraph at “Extensive”.
- Line 135-136: Fig. 1 will be cited at the end of the following sentence: “The albedo profile depends on the latitude only (see Fig. 1)”.
- Line 157-159: We will add the justification for the chosen range of parameters at the end of section 2.3.
- Line 226: The period will be replaced by a space.
- Figure 5 caption: Indeed, A and H labels correspond to the outline of anoxic and hypoxic conditions. An explicit description of these outlines will be provided in the caption of Fig. 5.
Citation: https://doi.org/10.5194/egusphere-2024-1983-AC2
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CC2: 'Reply on RC2', Justin Gérard, 09 Sep 2024
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