Nature Vol. 278 15 March 1979 281 matters arising

Holocene growth available figures for growth rates at 30m 1. Lighty, R. G., Macintyre, I. G. & Stuckenrath, R. Nature 276, 59~0 ( 1978). on the edge depth there is certainly subjective evi­ 2. Macintyre, I. G. & Glynn, P. W. Proc. 7th Geo!. dence that they are slow. There are com­ Con{. Guadeloupe, 14 (I 97 4 ). of the Florida Shelf monly <2m of 'frame' projecting above 3. Adey, W. H. Res. Bull. 187, 1-67 (1975). 4. Easton, W. H. & Olson, E. A. Bull. geol. Soc. Am. 87, the sand surface. In effect, relief from 711-719 (1976). 5. Goreau, T. F. & Gorcau, N. I. Bull. mar. Sci. 23, 399-464 LIGHTY ET AL. I have raised several present-day surfaces in these depths is no (1973). important lines of speculation concerning gre.ater, and may perhaps be less, than on 6. Goreau, T. F. & Land, L. S. Soc. econ. Paeleont. Miner. the growth of reefs in the Florida and the 'inactive' structures to the north. Spec. Pub/. 18,77-89 (1974). 7. Newell, N.D. & Rigby, J. K. Soc. econ. Paeleonr. Miner. Bahamas areas and elsewhere in the Thus, it seems reasonable to assume Spec. Pub/. 5, 15-79 (1957). Caribbean. The principal contribution of first that surfaces which could have 8. Shinn, E. A. J. Sed. Petrol. 33,291-303 (1963). 9. Ginsburg, R. N. & James, N. P. Geotimes 18, 23-24 these authors has been to show an supported active growth at a time of (1973). effective growth of a shallow-water coral­ rising sea level did not and, second, that 10. Neumann, A. C. & Land, L. S. J. Sed. Perm/. 45,763-786 frame assemblage dominated by the structures which we now see at 30m (1975). 11. Moore, C. H., Graham, E. A. & Land, L. S. J. Sed. Petrol. palmata in a period between 9,440 ± 85 are in effect the surfaces of frames which 46, 174-187 (1976). and 7,145±80yrBP. This extends the were first formed 7,000-10,000 yr ago. 12. Purdy, E. G. Soc. econ. Paeleont. Miner. Spec. Pub/. 18, 9-76 (1974). evidence previously presented by Macin­ There is an obvious incentive to test them, 13. Macintyre, I. G. Atoll. Res. Bull. 185,21-26 (1975). 2 3 tyre and Glynn , Adey and Easton and using the methods described by Macin­ 14. Macintyre, I. G. Bull. Am. Ass. Petrol. Geol. 56,720-738 Olson4 for progressive growth over the tyre13, bearing in mind the suggestion of (1972). 4 past 6,000 yr. Macintyre I that structures on terraces at LIGHTY ET AL. REPLY-The points The structures described lie within a 30-80 m depth in the eastern Caribbean raised by Braithwaite have been discussed depth range (15-30 m) which, only 40 km do indeed represent an older phase of in several studies of Caribbean Holocene 1 to the south, is accessible to Recent coral accretion. reefs. As mentioned in our letter , for growth. One might speculate that, where We may now rephrase the question example, core data from a relict reef conditions of growth are more favourable, posed by Lighty eta/. I What event, taking (depth 18m) off southeastern St Croix reefs of the same age as those described by place -7,000 yr ago, inhibited all shallow­ indicated that a flourishing, shallow-water Lighty et a/. I have been buried beneath water reef growth in the Caribbean? This Acropora palmata (Lamarck) barrier reef growth covering the period from growth was never able to resume in their was killed about 8,000 to 9,000 yr BP, 2 -7,000 yr BP to the present. northern area, presumably for the reasons when the shelf was flooded . Increased Observations by Goreau and Goreau5 which they accept pertaining to cold sur­ water turbidity related to the erosion of and Goreau and Land6 in Jamaica and by face waters. In southern areas, however, soil cover was thought to be responsible Newall and Rigb/ and Shinn8 and myself growth began on new sites along the edge for the demise of this early Holocene reef. in the Florida-Bahamas province show a of the inner break in slope, forming the By the time water conditions returned to remarkable similarity of offshore profiles. present shelf edge patches. normal, the depth of water was too great The first element is a gentle slope to -10- Lighty et a/. I provide figures to show for an A. palmata reef to be re­ 15 m depth (visible as a distinctive terrace that an Acropora palmata reef is capable established, Subsequently, the substrate 1 on Fig. 1 of Lighty et a/. ). It is from this of keeping pace with rising sea level. was colonised by a slowly accreting, that the major reefs in Florida (for exam­ There seems little reason to doubt this deeper water community consisting of ple, Carysfort, Dry Rocks, Alligator Reef, capability, but did it in fact do so? I believe coral heads and octocorals, 2 3 4 and so on) rise and, at least on Andros and that it did not and that, far from creeping That study and others · have proposed Eleuthera, so do the major reef patches. upwards along the surface of a submerging that turbidity, related to shelf erosion Seawards, the surface slopes more steeply slope, the principal locus of growth was accompanying nsmg sea level, has to about 30m before plunging almost established on a new and much shallower generally prevented the continuous vertically in "the wall" described by site. We are left with the conclusion that accumulation of Holocene reefs on wide 9 6 Ginsburg and James , Goreau and Land although sea level rose throughout the shelf platforms of the Caribbean that are and others. The illustrations of Goreau Caribbean there was a relatively abrupt deeper than 10 m below present sea level. and Goreau5 seem to apply throughout change at -7,000 yr BP. The broad Although impressive reef structures are the Caribbean area and suggest only two outlines of reef morphology may have present on platforms <10m below shelf-edge growth sites. Figure 1 of Lighty been defined at some earlier date but it present sea level, these structures cannot et al.I suggests a third, from 15m. was this event which determined the areas be older than 4,000-7,000 yr BP, which is We need to exercise some care in in which would grow. Sadly, in the the period of initial flooding of these plat­ interpreting terrace levels as much of the area described by Lighty eta/. I growth was forms, In south Florida, for example, the mobile sand is derived from the excess no longer possible. present reef tract has developed over the production of the shallow-water zone (see Field-work in Florida and the Bahamas past 7,000 yr (ref. 5) on shallow areas of 11 Neumann and Landio and Moore et a/. ). was supported by the Carnegie Trust for the shelf, behind the relict, shelf-edge reef 6 However, the widespread occurrence the Universities of Scotland whose help is of early Holocene age • within these offshore platforms of exposed gratefully acknowledged. Some deeper shelf areas isolated from Pleistocene limestones indicates that we the effects of shelf erosion might have are not dealing with accretion levels but C. J. R. BRAITHWAITE continuous Holocene reef sections dating with surfaces prepared by erosion during Department of Geology, back to 10,000 yr BP, but no such section 2 glacial periods (compare Purdyi ). The University, has yet been found. Now, although there are no readily Dundee, UK Note, however, that in protected areas,

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