[Coral-List] Darwin was WRONG about reef formation

[Rupert Ormond]

Thanks David,

This sounds like the interpretation I was trying to re-call after Doug 
et al asked about when I mentioned that decades ago I was teaching 
students that the Maldives atolls were not atolls - at least in the 
Darwinian sense.

I still struggle to understand however how exactly the Maldives manages 
to get mini-atolls within ring reefs within atolls. As a non-geologist I 
could only guess that this was the result of eustatic fluctuations in 
sea-level, with the largest scale atolls being formed when sea level was 
lowest and conditions within unfavourable to coral growth, and smaller 
ring-reefs being able to form inside them once sea level was higher 
again, and so on.

It would be nice to know if the evidence supports this?

Thanks

Rupert

*Prof. Rupert Ormond
*Co-Director, Marine Conservation International
Hon. Professor, Centre for Marine Biodiversity & Biotechnology, 
Heriot-Watt University, Edinburgh




On 27/10/2020 13:29, David Blakeway via Coral-List wrote:
> Doug - Their interpretation for the Maldives is that the carbonate sequence
> was all produced in shallow water, over the subsiding volcanic plateau.
> Deposition rates varied across the plateau, some areas remaining shallow
> and other areas drowning. The areas that remained shallow developed into
> flat-topped sea-level banks during a long period of relatively stable sea
> level just over 3 million years ago. Atoll morphology subsequently
> developed over the flat-topped banks, and is therefore independent of the
> deeply buried volcanic substrate. I think it is a good model, with some
> solid supporting evidence from the Maldives. But the article would be so
> much better if it exercised more restraint and balance; e.g. by conceding
> that there may be multiple paths to atoll morphology, by acknowledging that
> Darwin had already suggested some atolls develop from submerged banks, and
> by citing previous work apparently contradictory to their conclusions.
>
> By the way, Darwin's tentative explanation of the Maldives was to slowly
> submerge a large elongate island surrounded by a barrier reef, like New
> Caledonia. The reef "*...after repeated subsidences, would become during
> its upward growth separated into distinct portions; and these portions
> would tend to assume an atoll-like structure, from the coral growing with
> vigour round their entire circumferences, when freely exposed to an open
> sea." *Referring to these sub-atolls, he later says* "...these again,
> during long periods of subsidence, would sometimes become dissevered into
> smaller atolls.*" I know it is just logic, but sure seems like magic!
>
>
> On Tue, Oct 27, 2020 at 3:22 AM Dennis Hubbard <dennis.hubbard at oberlin.edu>
> wrote:
>
>> Thanks David:
>>
>> Going back to a thread from a couple years back, this is the reason that
>> extensive citing of the literature (and not just the most recent and "hot"
>> articles) is so important.
>>
>> Dennis
>>
>> On Mon, Oct 26, 2020 at 10:57 AM David Blakeway <
>> fathom5marineresearch at gmail.com> wrote:
>>
>>> The Droxler & Jorry article provides a valuable perspective but, as has
>>> been pointed out by others, it sells Darwin short. The article claims that
>>> most modern atolls have developed over flat-topped Pliocene banks, and
>>> therefore that Darwin’s fringing reef to barrier reef to atoll model is
>>> wrong. However, Darwin already knew his model did not apply to all atolls.
>>> For example:
>>>
>>>
>>> “*...if, therefore, corals were to grow up from a bank, with a level
>>> surface some fathoms submerged, having steep sides and being situated in a
>>> deep sea, a reef not to be distinguished from an atoll, might be formed..*.”
>>> (Darwin 1842, chapter 5)
>>>
>>>
>>> Furthermore, the article fails to cite a recent paper that describes a
>>> fringing reef to barrier reef transition at Tahiti, reconstructed from 35
>>> logged and dated cores (Blanchon et al. 2014; open access at
>>> https://www.nature.com/articles/srep04997).  Such selective citation is
>>> especially disappointing in a review article.
>>>
>>>
>>>
>>> An additional problem with the article, from my perspective, is that it
>>> invokes the antecedent karst hypothesis to explain the atoll rims. A more
>>> parsimonious explanation, developed by the first scientists to survey
>>> atolls in the early 17th century, is simply that corals and coralline algae
>>> grow better in the turbulent and well-oxygenated water on the outer edges
>>> of submerged structures.
>>>
>>>
>>> The karst hypothesis, in my opinion, is impeding coral reef science
>>> because it views reefs as passive structures – it denies the reef-building
>>> organisms any agency in creating reef form. We really need some young
>>> ecologists to take reef geomorphology forward!
>>>
>>>
>>>
>>> David Blakeway
>>>
>>
>> --
>> Dennis Hubbard - Emeritus Professor: Dept of Geology-Oberlin College
>> Oberlin OH 44074
>> (440) 935-4014
>>
>> * "When you get on the wrong train.... every stop is the wrong stop"*
>>   Benjamin Stein: "*Ludes, A Ballad of the Drug and the Dream*"
>>
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