the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Jul 2023
Reviews and syntheses: Sediment-stressed reefs over the past 420 Myr
Abstract. The evolution of reefs over geologic time is diverse and includes a range of different builders. An understanding of the consequences of natural and anthropogenically-driven sediment influx to reef systems is crucial to planning future protection and mitigation strategies. Most reef systems are associated with clear water settings, however, many reef communities have evolved in turbid water environments stressed by high rates of sediment influx. Conventionally, these mixed carbonate-clastic environments have been considered unfavourable to reef organisms. Utilising case-studies of sediment-stressed reefs from the Devonian to Recent, we clearly demonstrate that reef organisms can survive, and even thrive, under the influence of clastic sediment influx. Ten case-studies were selected on the basis of: i) the presence of a mixed carbonate-clastic matrix, and ii) the existence of a coral framework. For each example, the system was characterised in terms of sediment input, organism growth forms (with a focus on corals) and the overall reef morphology. The host sediment from Cenozoic reefs was found to be typically better-described than that within Paleozoic and Mesozoic communities. This may be due to the closer affinity between Cenozoic communities and recent species when compared to more ancient systems. The same reasoning accounts for the paucity of data describing the internal structure of many fossil reefs, a feature also related to outcrop quality. This study clearly demonstrates that, while reefs in sediment-impacted environments are common, there is no general developmental model that can be applied to all reefs. No relationship was identified between the nature of the reef builders, the character of the siliciclastic component and the reef structure. We demonstrate that, in the majority of cases, the clastic matrix within reefs, both ancient and recent, is insufficiently described – this inhibits understanding of mixed carbonate-clastic reef systems and significantly compromising forecasts of future reef development.
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Preprint
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Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1728', Anonymous Referee #1, 10 Aug 2023
This paper concerns ancient reef communities that have evolved in turbid waters inferred to be stressed by high rates of sediment influx. It covers a range of case-studies of coral reefs from the Devonian to Recent. The authors demonstrate that apparent clastic flux does not appear to hinder reef organism growth, but in many cases there is simply no adequate description. This underscores the current lack of a general model for all reef growth, but notwithstanding this, some general conclusions for diagnosing reefs in sediment-stressed environments are drawn.
First, I think this is a very worthwhile endeavour. These reefs certainly need far more attention.
But I feel there is a missed opportunity here to summarise what is known about modern ‘sediment-stressed’ coral reefs. We know that some coral taxa can do really well in these settings (e.g. Porities) - but others hate it. So here there are important distinctions to be made. What exactly are the survival strategies? When did those clades with survival strategies appear and diversify? Are there differences in the very local hydrodynamic regime (i.e. waves and currents) that help even though the incoming clastic flux might be high? Does the presence of other binding agents such as coralline algae help corals survive and form a reef framework?
What are the general metrics (such as size/longevity/overall biodiversity, etc.) of modern clastic-influenced reefs compared to those that grow in pure carbonate systems? The authors note that all of the described siliciclastic-associated reefs ‘form sedimentary bodies that are significantly thinner and with a smaller footprint than contemporaneous reefs that formed in ‘blue water’ settings’, so perhaps other metrics are available. How does clastic flux affect coral morphology? I know that that the authors state that there is no discernible relationship between siliciclastic sediment influx and coral growth morphology, but does this refer to ancient or modern reefs? Even some summary of available metrics in tabular form would help.
The figures are generally rather schematic. In Figure 1 it might be good to add more information on different clades – particularly in important groups of scleractinian corals such as the rise of the more generalist poritids, and acroporoids. The abundance has no scale, so am not really sure what this means. Is it in any way normalised to available tropical shallow marine shelf area?
I fully concur with the call to arms at the end of the paper. We need far more detailed information on these reefs – all aspects including the local hydrodynamic settings and particularly thin sections of the various sediment matrices. This should perhaps be the basis for comparison – % clastic grains, types, and their variability; clastic grain size distribution; etc. But a key issue here is how in fact, does sediment flux, relate to % in the final ancient sediment? Yes – researchers need to demonstrate that the reefs grow concurrent with clastic input, not just that it infilled a framework post-mortem.
What we really want to know is how does % clastic input actually produce different ‘cut-offs/tipping point’ for initial settlement vs. mature coral reef growth, leading to potential differences in overall size, morphology, etc.
Citation: https://doi.org/10.5194/egusphere-2023-1728-RC1 -
AC1: 'Reply on RC1', Tanja Unger, 25 Sep 2023
We thank the anonymous referee for his/her time and feedback and for pointing out the value of this review. Especially for highlighting the need for more detailed information on sediment-stressed reefs of the Earth's history, we concur.
Thanks for pointing out the lack of a scale bar in Figure 1. That was corrected.
Concerning the main criticism (the "missed opportunity"), we somehow wonder, is it truly a reviewer's task to say something like "…. I agree with your paper, but you should have written a totally different paper…?" Please do not misunderstand this comment; it is not meant to be disrespectful. We simply point out that we mainly intended to summarise the state-of-the-art regarding sediment-stressed reefal communities in the geological past (and yes, we add one example of a recent reef system). And this is what we have done. We are sorry that fossil reefal ecosystems are, apparently, not the main field of interest of the reviewer. If we intended to write a paper on Recent sediment-stressed reefs, we would have done so. But then, as pointed out in the second review, there is significant literature on actualistic or geologically young sediment-stressed reefal ecosystems, but, to our knowledge, no paper critically reviews what we have (and what we have not) in the field of ancient reefal systems. This might not be the reviewer's view, but from our perspective, we took the opportunity to (at least in part) close this gap. There is probably no need to emphasise that one cannot simply translate messages from recent scleractinian coral reefs to, say, Mesozoic or Palaeozoic reefal organisms.
Citation: https://doi.org/10.5194/egusphere-2023-1728-AC1
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AC1: 'Reply on RC1', Tanja Unger, 25 Sep 2023
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RC2: 'Comment on egusphere-2023-1728', Michael Risk, 06 Sep 2023
Review of Unger et al.
This MS purports to be about the history of reef and sediments over time. This is a noble venture, to be sure. Perhaps we can use the present to be a key to the past, and understand more about fossil reefs. The MS itself is well-written, and beautifully illustrated.
It cannot be accepted in its present form. It will need to be either rejected, or completely rewritten. Reasons for this seemingly harsh verdict are given below, in no order of importance. It may seem I have relied heavily on my own work, in suggesting holes in this MS, but-there’s a ton of stuff out there, and I am lazy.
First of all, the title. This MS isn’t about “sedimentation”, it’s about siliciclastic sedimentation. True, much sediment stress does come from clastic sources, but that is far from the complete picture. In fact, I have wrestled for years (unsuccessfully) with the problem of teasing out stress effects from carbonate sediments.
Next, au assume that reefs co-existing with, or occurring near, shaley interbeds is de facto evidence of sediment stress. This is a catastrophic mistake. Au need to review the literature on modern sediment stress. They will learn that size matters, that timing matters-and what does NOT matter is, who is interbedded with whom. Given geologic timescales, there could easily be centuries of time between growth of reef critters and the muds lying nearby. I repeat: the fact that reefy critters occur near mud interbeds is NOT proof of sediment stress, and all allusions to this effect must be removed.
What’s frustrating here is that there are things to look for, that would help show the way-but au have blown right by the signposts.
First, try Cortes and Risk, 1985, Bull Mar Sci. I see au have found the Rogers paper-which is of no help here. In C&R, they will find hints: change in shape and size distribution of corals under sediment stress. Incorporation of siliciclastics.
Next, Johnson and Risk, 1987, Sedimentology. Au need to pick through this one. See how we tried to deal with the impacts of sediments on reef growth. Some more pointers there.
Again next, Risk, 2014. Assessing the effects of sediments and nutrients on coral reefs. Current Opinions on Envl Sustainability. Again, some criteria to look for.
The Conclusions aren’t. This is at best a wish list, at worst a research proposal. This needs to be thrown out, and replaced with real, well, Conclusions.
The MS is well-written, and the figures are outstanding. A lot of work has gone into this, and it would be a shame to chuck all that out the window. It’s up to au, but I would suggest removing much of the talk about sediment stress that is guilt by association, specifying au are dealing with siliciclastics, and concluding with a list of Things to Look For to Find Out.
Some less-important issues:
L 18: semicolon, not comma, after “settings”
L 96: there is evidence of symbiosis far down in the Paleozoic. Risk et al, 1987, PALAIOS.
Fig. 1. Abundance, not abundancy. And this sort of diagram was appeared many times already…au would be well advised to look at the Reefs section in Facies models.
Table 2. Please don’t conclude sediment impact from simple proximity to muds.
L417. Hamstrung.
Citation: https://doi.org/10.5194/egusphere-2023-1728-RC2 -
AC2: 'Reply on RC2', Tanja Unger, 25 Sep 2023
Concerning: Review of Dr. M. Risk - Reply by authors
We greatly appreciate the open review by Dr. Risk, the many positive comments, and the time and effort he has spent to provide feedback. We regret that this paper was not written as he would have. The reviewer approaches the paper from the viewpoint of recent, sediment-stressed scleractinian coral reefs; we wrote our paper mainly from the viewpoint of the geological archive, with all limitations inherent to studies of ancient carbonate systems. We regret not citing Dr. Risk's papers; that was not meant to be disrespectful. As pointed out by Dr. Risk, there is quite a bulk of published literature and classically, every reviewer has his/her personal view of which paper must be cited or not.
That said, most of the key points of criticism raised came, at least in our view, as a surprise as we felt that we abundantly used most of the arguments brought forward by the reviewer. Please allow us to explain why we feel so by using the two main points of criticism as examples.
- Next, au assume that reefs co-existing with, or occurring near, shaley interbeds is de facto evidence of sediment stress. This is a catastrophic mistake. Au need to review the literature on modern sediment stress. They will learn that size matters, that timing matters-and what does NOT matter is, who is interbedded with whom.
Our reply: Please allow us to copy-paste text from the abstract of our MS: "…No relationship was identified between the nature of the reef builders, the character of the siliciclastic component and the reef structure. We demonstrate that, in the majority of cases, the clastic matrix within reefs, both ancient and recent, is insufficiently described – this inhibits understanding of mixed carbonate-clastic reef systems and significantly compromises forecasts of future reef development…".
We extensively reviewed the literature on ancient and modern sediment stress of marine reefal communities. That is what most people do when writing a review paper. We are well aware of the fact that grain size (see lines 114, 210 etc.) and timing (see lines 342, 399 etc.) matter, and the simple interbedding of reefal intervals with clastic units cannot be used as criteria for sediment stress (see chapter 4.1 and tables 1 and 2). We discuss these points in our previous study (Unger et al. 2022, Sedimentology) reviewed in this paper and discussed in the present article under review here.
For this review article, we selected papers that provided at least some form of evidence on clastic sedimentation DURING reef growth from the numerous examples in the literature. We know this is often difficult to document. As indicated by the reviewer, hard arguments such as clastic sediment layers encased in the corallum or calcareous sponge skeleton are rarely described by workers focussing on the taxonomy of the reef builders (see extensive discussion in Unger et al. 2022). The issue of grain size, clay versus sand, is discussed repeatedly in tables, in the sections describing the case examples and the discussion. In fact, we feel that the points raised by Dr. Risk were considered.
- What's frustrating here is that there are things to look for, that would help show the way-but au have blown right by the signposts.
Our reply: While we find the literature dealing with modern sediment-stressed reefs very instructive, we would like to point out that most palaeo-ecologists would strongly reject the simple translation of features found in modern scleractinian corals to Palaeozoic coral and stromatoporoid reefs to name one example. In fact, a significant bulk of the literature deals with what we can learn from the morphology of these ancient reef builders concerning turbid waters and sediment stress (see, for example, Zapalski et al. 2021, Rogers, 1990, Mitchell 202a, b, Jones et al., 2015 etc.). We cited many of these papers but, of course, also make use of studies of more recent reefs.
Summing up: We greatly appreciate the feedback but are somewhat uncertain where the reviewer wants us to go. Read (and cite) all papers about recent scleractinian coral reefs growing in a sediment-stressed environment and, ignoring differences in the ecology of ancient reefal systems, apply these lessons to case examples throughout the rock record. If this is Dr. Risk's advice, we fear, with all due respect, we cannot follow it.
Citation: https://doi.org/10.5194/egusphere-2023-1728-AC2
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AC2: 'Reply on RC2', Tanja Unger, 25 Sep 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1728', Anonymous Referee #1, 10 Aug 2023
This paper concerns ancient reef communities that have evolved in turbid waters inferred to be stressed by high rates of sediment influx. It covers a range of case-studies of coral reefs from the Devonian to Recent. The authors demonstrate that apparent clastic flux does not appear to hinder reef organism growth, but in many cases there is simply no adequate description. This underscores the current lack of a general model for all reef growth, but notwithstanding this, some general conclusions for diagnosing reefs in sediment-stressed environments are drawn.
First, I think this is a very worthwhile endeavour. These reefs certainly need far more attention.
But I feel there is a missed opportunity here to summarise what is known about modern ‘sediment-stressed’ coral reefs. We know that some coral taxa can do really well in these settings (e.g. Porities) - but others hate it. So here there are important distinctions to be made. What exactly are the survival strategies? When did those clades with survival strategies appear and diversify? Are there differences in the very local hydrodynamic regime (i.e. waves and currents) that help even though the incoming clastic flux might be high? Does the presence of other binding agents such as coralline algae help corals survive and form a reef framework?
What are the general metrics (such as size/longevity/overall biodiversity, etc.) of modern clastic-influenced reefs compared to those that grow in pure carbonate systems? The authors note that all of the described siliciclastic-associated reefs ‘form sedimentary bodies that are significantly thinner and with a smaller footprint than contemporaneous reefs that formed in ‘blue water’ settings’, so perhaps other metrics are available. How does clastic flux affect coral morphology? I know that that the authors state that there is no discernible relationship between siliciclastic sediment influx and coral growth morphology, but does this refer to ancient or modern reefs? Even some summary of available metrics in tabular form would help.
The figures are generally rather schematic. In Figure 1 it might be good to add more information on different clades – particularly in important groups of scleractinian corals such as the rise of the more generalist poritids, and acroporoids. The abundance has no scale, so am not really sure what this means. Is it in any way normalised to available tropical shallow marine shelf area?
I fully concur with the call to arms at the end of the paper. We need far more detailed information on these reefs – all aspects including the local hydrodynamic settings and particularly thin sections of the various sediment matrices. This should perhaps be the basis for comparison – % clastic grains, types, and their variability; clastic grain size distribution; etc. But a key issue here is how in fact, does sediment flux, relate to % in the final ancient sediment? Yes – researchers need to demonstrate that the reefs grow concurrent with clastic input, not just that it infilled a framework post-mortem.
What we really want to know is how does % clastic input actually produce different ‘cut-offs/tipping point’ for initial settlement vs. mature coral reef growth, leading to potential differences in overall size, morphology, etc.
Citation: https://doi.org/10.5194/egusphere-2023-1728-RC1 -
AC1: 'Reply on RC1', Tanja Unger, 25 Sep 2023
We thank the anonymous referee for his/her time and feedback and for pointing out the value of this review. Especially for highlighting the need for more detailed information on sediment-stressed reefs of the Earth's history, we concur.
Thanks for pointing out the lack of a scale bar in Figure 1. That was corrected.
Concerning the main criticism (the "missed opportunity"), we somehow wonder, is it truly a reviewer's task to say something like "…. I agree with your paper, but you should have written a totally different paper…?" Please do not misunderstand this comment; it is not meant to be disrespectful. We simply point out that we mainly intended to summarise the state-of-the-art regarding sediment-stressed reefal communities in the geological past (and yes, we add one example of a recent reef system). And this is what we have done. We are sorry that fossil reefal ecosystems are, apparently, not the main field of interest of the reviewer. If we intended to write a paper on Recent sediment-stressed reefs, we would have done so. But then, as pointed out in the second review, there is significant literature on actualistic or geologically young sediment-stressed reefal ecosystems, but, to our knowledge, no paper critically reviews what we have (and what we have not) in the field of ancient reefal systems. This might not be the reviewer's view, but from our perspective, we took the opportunity to (at least in part) close this gap. There is probably no need to emphasise that one cannot simply translate messages from recent scleractinian coral reefs to, say, Mesozoic or Palaeozoic reefal organisms.
Citation: https://doi.org/10.5194/egusphere-2023-1728-AC1
-
AC1: 'Reply on RC1', Tanja Unger, 25 Sep 2023
-
RC2: 'Comment on egusphere-2023-1728', Michael Risk, 06 Sep 2023
Review of Unger et al.
This MS purports to be about the history of reef and sediments over time. This is a noble venture, to be sure. Perhaps we can use the present to be a key to the past, and understand more about fossil reefs. The MS itself is well-written, and beautifully illustrated.
It cannot be accepted in its present form. It will need to be either rejected, or completely rewritten. Reasons for this seemingly harsh verdict are given below, in no order of importance. It may seem I have relied heavily on my own work, in suggesting holes in this MS, but-there’s a ton of stuff out there, and I am lazy.
First of all, the title. This MS isn’t about “sedimentation”, it’s about siliciclastic sedimentation. True, much sediment stress does come from clastic sources, but that is far from the complete picture. In fact, I have wrestled for years (unsuccessfully) with the problem of teasing out stress effects from carbonate sediments.
Next, au assume that reefs co-existing with, or occurring near, shaley interbeds is de facto evidence of sediment stress. This is a catastrophic mistake. Au need to review the literature on modern sediment stress. They will learn that size matters, that timing matters-and what does NOT matter is, who is interbedded with whom. Given geologic timescales, there could easily be centuries of time between growth of reef critters and the muds lying nearby. I repeat: the fact that reefy critters occur near mud interbeds is NOT proof of sediment stress, and all allusions to this effect must be removed.
What’s frustrating here is that there are things to look for, that would help show the way-but au have blown right by the signposts.
First, try Cortes and Risk, 1985, Bull Mar Sci. I see au have found the Rogers paper-which is of no help here. In C&R, they will find hints: change in shape and size distribution of corals under sediment stress. Incorporation of siliciclastics.
Next, Johnson and Risk, 1987, Sedimentology. Au need to pick through this one. See how we tried to deal with the impacts of sediments on reef growth. Some more pointers there.
Again next, Risk, 2014. Assessing the effects of sediments and nutrients on coral reefs. Current Opinions on Envl Sustainability. Again, some criteria to look for.
The Conclusions aren’t. This is at best a wish list, at worst a research proposal. This needs to be thrown out, and replaced with real, well, Conclusions.
The MS is well-written, and the figures are outstanding. A lot of work has gone into this, and it would be a shame to chuck all that out the window. It’s up to au, but I would suggest removing much of the talk about sediment stress that is guilt by association, specifying au are dealing with siliciclastics, and concluding with a list of Things to Look For to Find Out.
Some less-important issues:
L 18: semicolon, not comma, after “settings”
L 96: there is evidence of symbiosis far down in the Paleozoic. Risk et al, 1987, PALAIOS.
Fig. 1. Abundance, not abundancy. And this sort of diagram was appeared many times already…au would be well advised to look at the Reefs section in Facies models.
Table 2. Please don’t conclude sediment impact from simple proximity to muds.
L417. Hamstrung.
Citation: https://doi.org/10.5194/egusphere-2023-1728-RC2 -
AC2: 'Reply on RC2', Tanja Unger, 25 Sep 2023
Concerning: Review of Dr. M. Risk - Reply by authors
We greatly appreciate the open review by Dr. Risk, the many positive comments, and the time and effort he has spent to provide feedback. We regret that this paper was not written as he would have. The reviewer approaches the paper from the viewpoint of recent, sediment-stressed scleractinian coral reefs; we wrote our paper mainly from the viewpoint of the geological archive, with all limitations inherent to studies of ancient carbonate systems. We regret not citing Dr. Risk's papers; that was not meant to be disrespectful. As pointed out by Dr. Risk, there is quite a bulk of published literature and classically, every reviewer has his/her personal view of which paper must be cited or not.
That said, most of the key points of criticism raised came, at least in our view, as a surprise as we felt that we abundantly used most of the arguments brought forward by the reviewer. Please allow us to explain why we feel so by using the two main points of criticism as examples.
- Next, au assume that reefs co-existing with, or occurring near, shaley interbeds is de facto evidence of sediment stress. This is a catastrophic mistake. Au need to review the literature on modern sediment stress. They will learn that size matters, that timing matters-and what does NOT matter is, who is interbedded with whom.
Our reply: Please allow us to copy-paste text from the abstract of our MS: "…No relationship was identified between the nature of the reef builders, the character of the siliciclastic component and the reef structure. We demonstrate that, in the majority of cases, the clastic matrix within reefs, both ancient and recent, is insufficiently described – this inhibits understanding of mixed carbonate-clastic reef systems and significantly compromises forecasts of future reef development…".
We extensively reviewed the literature on ancient and modern sediment stress of marine reefal communities. That is what most people do when writing a review paper. We are well aware of the fact that grain size (see lines 114, 210 etc.) and timing (see lines 342, 399 etc.) matter, and the simple interbedding of reefal intervals with clastic units cannot be used as criteria for sediment stress (see chapter 4.1 and tables 1 and 2). We discuss these points in our previous study (Unger et al. 2022, Sedimentology) reviewed in this paper and discussed in the present article under review here.
For this review article, we selected papers that provided at least some form of evidence on clastic sedimentation DURING reef growth from the numerous examples in the literature. We know this is often difficult to document. As indicated by the reviewer, hard arguments such as clastic sediment layers encased in the corallum or calcareous sponge skeleton are rarely described by workers focussing on the taxonomy of the reef builders (see extensive discussion in Unger et al. 2022). The issue of grain size, clay versus sand, is discussed repeatedly in tables, in the sections describing the case examples and the discussion. In fact, we feel that the points raised by Dr. Risk were considered.
- What's frustrating here is that there are things to look for, that would help show the way-but au have blown right by the signposts.
Our reply: While we find the literature dealing with modern sediment-stressed reefs very instructive, we would like to point out that most palaeo-ecologists would strongly reject the simple translation of features found in modern scleractinian corals to Palaeozoic coral and stromatoporoid reefs to name one example. In fact, a significant bulk of the literature deals with what we can learn from the morphology of these ancient reef builders concerning turbid waters and sediment stress (see, for example, Zapalski et al. 2021, Rogers, 1990, Mitchell 202a, b, Jones et al., 2015 etc.). We cited many of these papers but, of course, also make use of studies of more recent reefs.
Summing up: We greatly appreciate the feedback but are somewhat uncertain where the reviewer wants us to go. Read (and cite) all papers about recent scleractinian coral reefs growing in a sediment-stressed environment and, ignoring differences in the ecology of ancient reefal systems, apply these lessons to case examples throughout the rock record. If this is Dr. Risk's advice, we fear, with all due respect, we cannot follow it.
Citation: https://doi.org/10.5194/egusphere-2023-1728-AC2
-
AC2: 'Reply on RC2', Tanja Unger, 25 Sep 2023
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