the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Reviews and syntheses: Sediment-stressed reefs over the past 420 Myr
Tanja Unger
Stephen W. Lokier
Markus Aretz
Mathhieu Saillol
Adrian Immenhauser
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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Tanja Unger et al.
Status: final response (author comments only)
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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 -
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
Tanja Unger et al.
Tanja Unger et al.
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