[Coral-List] Darwin was wrong

[Walter Goldberg]

The paper by Droxler & Jorry  in Annual Reviews in Marine Science (not yet published) argues that Plio-Pleistocene sea level fluctuations and antecedent karst is pre-eminent in determining atoll formation compared with the role of subsidence. This emphasis on more recent history is not new as others have pointed out. An equally old argument concerns the very nature of atolls and whether they are limited to annular formations on volcanic platforms. Long ago, David Stoddard pointed  out that most atolls are elliptical rather than annular (Mar. Geol. 3: 369-383, 1965), but more importantly, he asked how many atolls there are, and that becomes the more interesting question. Depending on the narrowness or the breadth of how the atoll family is embraced, the number can be quite variable. There are atolls on continental shelves, as well as those whose rims are actively growing despite being subtidal, among other variations that Darwin did not take into account (see my review of Atolls of the World https://smithsonian.figshare.com/arb ). But even if an atoll is limited to those with volcanic platforms, that does not mean that Darwin was wrong about subsidence. The main problem with the Annual Review paper is the misleading title that challenges Darwin’s “deeply ingrained theory”, an errand that it does not accomplish. Perhaps it is not too late to fix it.
Walter

Walter M. Goldberg
Professor Emeritus
Dept. of Biological Sciences
Florida International University
Miami, FL 33199





Date: Wed, 21 Oct 2020 10:32:48 -0400
From: Dennis Hubbard <dennis.hubbard at oberlin.edu<mailto:dennis.hubbard at oberlin.edu>>
To: Rupert Ormond <rupert.ormond.mci at gmail.com<mailto:rupert.ormond.mci at gmail.com>>
Cc: Coral Listserver <coral-list at coral.aoml.noaa.gov<mailto:coral-list at coral.aoml.noaa.gov>>
Subject: Re: [Coral-List] Darwin was WRONG about reef formation
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To some extent, this is probably a matter of semantics. The term "atoll" is
a geomorphic one - an emergent annular feature built by the accumulation of
coral and related materials (algae, sediment, etc.). I don't think there is
any resistance to this. However, from a process standpoint, features with
this generic shape can result from one of (at least) two very different
processes. The one we most often acknowledge is a subsiding volcano that is
cooling and sinking as it moves away from its initial hotspot location. As
long as reef accretion offsets subsidence, the feature will remain at or
near to sea level (read Ricky Grigg's paper in an old issue of Coral Reefs
on his "Darwin Point". .Another alternative is related to wave refraction
behind an exposed or nearly exposed high on a larger shallow bank. The
resulting wave pattern will create two sedimentary spits on either side of
(and just downdrift from) the high; this leads to a roughly circular or
oblate feature resembling its more-common cousins with a volcanic origin.

Dennis