Barbin, V.

Debenay, J. P.

Decrouez, D.

Friedman, Gerald M.

Ramseyer, K.

Roux, M.

Schein, E.

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Geol. Mag. 130 (2), 1993, pp. 269-270. Copyright © 1993 Cambridge University PressDISCUSSIONS269Discussion of cathodoluminescence of Recent biogenic carbonates: an environmental andontogenetic fingerprintG. M. Friedman comments: Barbin et al. (1991) state 'mostauthors consider recent primary biogenic carbonates to benon-luminescent.' They quote several authors who havemade such negative statements, but they do not referencestudies that have demonstrated the usefulness of cathodo-luminescence in the study of modern biogenic carbonatesthat make up reefs. Barbin et al. (1991) are not alone inquestioning the uselessness of cathodoluminescence inmodern biogenic carbonates as expressed in the work oftheir predecessors. Most recently Major (1991) likewisecommented' cathodoluminescence in carbonates is primarilya result of diagenesis', especially 'postdepositional pre-cipitation or recrystallization'.The purpose of this discussion is to explain the usefulnessof cathodoluminescence in the study of modern biogeniccarbonates in reefs, as developed by Hill in 1976 andFriedman (1985). The reason why these studies have notbeen quoted are (a) Hill's (1976) study has remained anunpublished Master's of Science thesis (C. M. Hill, unpub.Master's thesis, Renneselaer Polytechnic Institute) and (b)Friedman's (1985) paper was published under a title whichdoes not include the word cathodoluminescence and wasconcerned with problems of carbonate classification ratherthan using luminescence as a tool.Blasting open a coral reef or drilling through such a reefrarely displays framework-building organisms in growthpositions. Biting and boring and rasping or mechanicalbreakdown convert solid colonies of calcium carbonateskeletons secreted by the reef organisms into an ever-increasing supply of skeletal particles that accumulate withinthe reef or in the vicinity of the reef. Boreholes throughmodern reefs reveal mostly debris; framework builders in aposition of growth are sporadic or even absent. Bothskeletal particles and interparticle cement in reefs consist ofhigh-magnesium calcite and aragonite. High-magnesiumcalcite commonly luminesces, whereas aragonite does notluminesce and appears black under an electron beam. Threekinds of submarine cryptocrystalline cement occur in reefs.These are: (1) cements that luminesce brightly, (2) cementsthat luminesce dimly or faintly, and (3) cements that do notluminesce at all. Under plane-polarized light or crossednicols these three kinds of cement appear identical. Inplaces, a cement 'stratigraphy' can be demonstrated wheretwo kinds of cement appear in sharp lateral stratigraphiccontact, one luminescent and the other non-luminescent(Friedman, 1985, figs 1E, F). These differences in behaviourunder an electron beam relate to differences in theconcentration of activator (Mn2+) ions. Because the geo-chemical characteristics of manganese are similar to those ofmagnesium, manganese replaces magnesium in the high-magnesium calcite lattice in sufficient concentration toproduce luminescence. The sharp boundaries between thethree kinds of cement indicate variations in concentration ofMn2+ ions and different times and/or rates of precipitation.The same principle of bright luminescence to non-lu-minescence applies to skeletal particles, especially those ofcoralline algae.In many reef samples the bulk of the skeletal particles arecomposed of aragonite which does not luminescence. Bycontrast the high-magnesium calcite cement shows strongluminescence. In such samples a particle-cement ratio canbe measured under an electron beam.Under cathodoluminescence modern Red Sea reefsluminesce more than Pleistocene reefs. Pleistocene reefsamples are weakly or non-luminescent. The reason fordecreased luminescence is the diagenetic change fromluminescent high-magnesium calcite to less luminescent low-magnesium calcite. The presence of Fe2+ ions in low-magnesium calcite restricts luminescence. This observationis at variance with the experience of others who claim thatdiagenetic processes result in cathodoluminescence (Major,1991).The colour plates of Barbin et al. (1991) showingcathodoluminescence and transmitted light photomicro-graphs are most instructive. They should be compared withthose previously published (see Friedman, 1985).V. Barbin, K. Ramseyer, J. P. Debenay, E. Schein, M. Roux& D. Decrouez reply: The discussion by Friedman and thequoted paper (Friedman, 1985) stresses the cathodo-luminescence (CL) characteristics of cryptocrystallineeogenetic high-magnesium calcite and aragonite cement andonly briefly mentioned CL of unaltered biogenic skeletalparticles, which was the aim of our paper.Our studies on biogenic carbonates (Barbin et al. 1991;Barbin, 1992) revealed clearly that the mode of carbonateprecipitation, i.e. biomineralization versus crystallizationfrom a pore fluid, are two different processes with regard totrace element incorporation into carbonate minerals. State-ments true for one process may therefore not necessarilybe identical for the other mode of crystallization.A good example for this difference in Friedman's claimthat eogenetic high-magnesium calcites commonly lumi-nesce. In recent crinoids and echinoids we generally observedonly a faint blue-green luminescence (if any at all). Also theassertion that aragonite does not luminesce may be true foreogenetic cements, but is not necessarily true for biogenicparticles. As early as 1972, Sommer described yellow-greenluminescing aragonitic shells and we also show thisphenomenon in our paper (see for example Fig. 2, parts 1 b,2b and 3b in Barbin et al. 1991). Barbin (1992) also showedluminescent growth bands in different species of Nautilus. Inaddition, the aragonitic skeletons of corals generally exhibita weak blue luminescence which can be difficult to detectbeside strongly luminescing particles.We thank Friedman for his comments, but do not feelthat they in any way alter the main conclusions, which wereto show (1) that recent biogenic carbonate skeletons oftenluminesce, (2) that luminescence in bioclasts is not always afingerprint for diagenetic alterations, and (3) that CL ofhttps:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800009924Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 21:12:56, subject to the Cambridge Core terms of use, available at270 DISCUSSIONSmodern animal skeletons may be used to unravel en-vironmental conditions during the life of the animal.ReferencesBARBIN, V. 1992. Fluctuation in shell composition in Nautilus(Cephalopoda, Mollusca): evidence from cathodo-luminescence. Lethaia 25, 391-400.BARBIN, V., RAMSEYER, K., DEBENAY, J. P., SCHEIN, E.,Roux, M. & DECROUEZ, D. 1991. Cathodoluminescenceof Recent biogenic carbonates: an environmental andontogenic fingerprint. Geological Magazine 128, 19-26.FRIEDMAN, G. M. 1985. The problem of submarine cementin classifying reefrock: an experience in frustration. InCarbonate Cements (eds N. S. Schneidermann andP. M. Harris), pp. 117-21. Tulsa: SEPM, special pub-lication no. 36.MAJOR, R. P. 1991. Cathodoluminescence in Post-MioceneCarbonates: Luminescence Microscopy and Spectro-scopy. In Qualitative and Quantitative Applications (edsC. E. Barker, H. Y. ColburnandC. Kopp),pp. 149-53.SEPM Short Course no. 25, 198 pp.SOMMER, S. E. 1972. Cathodoluminescence of carbonates, 2.Geological applications. Chemical Geology 9, 275-84.GERALD M. FRIEDMAN, Department of Geology, Brooklyn College and Graduate School of the CityUniversity of New York, Brooklyn, NY 11210, U.S.A. and The Northeastern ScienceFoundation, Rensselaer Center of Applied Geology, 15 Third Street, P.O. Box 746, Troy, NY12181-0746, U.S.A.v. BARBIN, E. SCHEIN & M. ROUX, Laboratoire des Sciences de la Terre, Universite de Reims, B.P. 347,F-51062 Reims Cedex, France.K. RAMSEYER, Geologisches Institut, Universitat Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland.j . p. DEBENAY, Laboratoire de Geologie, Universite d'Angers, Boulevard Lavoisier, Belle-Beille,F-49045 Angers Cedex, France.D. DECROUEZ, Museum d'Histoire Naturelle, 1 route de Malagnou, C.P. 434, CH-1211 Geneve 6,Switzerland.https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800009924Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 21:12:56, subject to the Cambridge Core terms of use, available at

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