Research programme on coral reefs in the Western Indian Ocean

[Georg Heiss]

Dear colleagues, 
With this message we would like to spread information about a current 
research programme on recent and fossil coral reefs in the Western Indian 
Ocean. 
If you would like to have more detailed information, please contact any of 
the members of the group at the addresses listed at the end of this 
message. 
At the moment we are constructing a WWW-page with more information about 
the activities of the group. As soon as we are online, you will be informed 
via the coral-list. 

Georg Heiss 

_________________________________________________ 
CORAL AND REEF GROWTH IN THE WESTERN INDIAN OCEAN: 
PALEOCLIMATIC IMPLICATIONS 


INTRODUCTION 

Since 1994 a group of European reef geologists including workers from 
France, Germany, Italy and the U.K. has been investigating Western Indian 
Ocean reefs, studying paleoclimatology and sea-level changes. These studies 
comprise localities over a latitudinal range between 4 degrees and 23 
degrees south, spanning in the north the Seychelles Bank, Aldabra-Atoll, 
and Mayotte (Comoro 
Islands), and in the south,Tulear (Madagascar), Reunion and Mauritius. 

The approach is on three time scales using: 

1. Living corals to document coral growth and climatic change  over the 
last 1000 years. 
2. Drill cores in Holocene reefs to cover the last 10,000 years. 
3. Outcrops of Pleistocene limestones to investigate the past 130,000 years 
and including: 

a)  Sampling of raised Eemian limestones; 
b)  Studies of drowned reef terraces (isotope stage 3 and LGM, 20-18 ka) by 
submersible. 

CORAL CORES 

        Cores collected from massive coral colonies (Porites) allow the 
determination of growth bands and of an exact chronology, together with the 
vertical accretion rate of the colony and variations in stable isotopes of 
O and C, and of Sr with high temporal resolution. 

The delta 18O-composition of the skeletal aragonite is used as a proxy for both 
paleotemperature and salinity.  In general, a variation of 0.22 permil is 
equivalent to 1 degree C of temperature change (Epstein et al. 1953) but is 
also linked to evaporation and precipitation (Swart and Coleman 1980). The 
delta 13C-composition serves as a proxy for both the biological 
productivity of the coral and the CO2 content of the contemporary 
atmosphere (Nozaki et al. 1978). 

Tropical sea surface palaeotemperatures can also be determined with high 
precision by means of Sr/Ca-TIMS (thermal ionization mass spectrometry), 
producing a seasonal resolution over at least the past 10,000 years (Beck 
et al. 1992). This method promises to provide an additional independent 
means of determining  temperature which is probably less influenced by 
fluctuations in the volume of ice-caps and oxygen isotope variations or by 
evaporation or freshwater influx (Beck et al. 1992; De Villiers et al. 
1994). 

The isotopic compositions of the skeleton forming the youngest parts of the 
coral cores can be calibrated with temperature readings from automatic 
temperature loggers. These have been installed for two years at some of the 
sampled colonies and provide measurements over this time span in steps of 2 
hours. 

Sampling of coral slabs with computerized drill equipment (sampling in 
steps of 1/100 mm) theoretically allows the determination of up to 100 
samples per year.  For these studies we sample in steps of 1 mm, resulting 
in a resolution of ca. 8-16 samples per year of coral growth, depending on 
the relative growth rates of the corals. 

Samples available from recent Porites-colonies are summarized in Table 1. 
________________________________________________________ 
TABLE 1: RECENT CORAL CORE SAMPLES 

LOCATION__NO. OF CORES__LENGTH OF LONGEST CORE__WATER DEPTH RANGE__DATE of 
FIELD WORK 

Mayotte, Comoros___10___238cm__1 to 7m__March 1994 
Seychelles__10__203cm__2 to 9m__October 1994, November 1995 
Reunion__6__195cm__6 to 13m__October 1994, November 1995 
Tulear, Madagascar__16__404cm__0.5 to 7m__October 1995 
________________________________________________________ 

HOLOCENE DRILL CORES 

Methods 

The Holocene reefs have all been drilled close to their outer edges in 
order to obtain the maximum thickness of the carbonate deposited during the 
Holocene.  Corals and associated organisms (e.g. algae, molluscs) occurring 
within the cores will be identified to species level in order to evaluate 
the palaeobathymetry.  Age determinations based on U/Th-TIMS may then 
provide a dataset for establishing a sea level curve.  Information can also 
be gained on the relative rate of reef growth and its response to sea level 
change, either  'keep up' or 'catch up' styles. 

Core material will also be examined petrographically to determine both the 
degree and style of diagenetic alteration.  Larger coral colonies recovered 
will be investigated using the same methods applied to the recent coral 
cores.  This will provide short sclerochronological datasets which will 
include paleotemperature variability on a sub-annual resolution for 
different time intervals during the Holocene.  Both the general 
environmental parameters and the specific patterns of change which these 
reveal will be compared with those of the recent corals. 

Locations 

The group has recovered cores from Reunion, Mauritius and Mayotte in recent 
years, and drilling operations have been completed at Tulear and on Mahe in 
the Seychelles this year.  On average, Holocene carbonates are more than 
20m thick in the Western Indian Ocean. The oldest Holocene corals collected 
so far are from Mayotte and have been dated at  9,800 yrs B.P. (Colonna 
1994). The underlying substratum varies with the origin and age of the 
islands.  In Reunion, the basement is probably formed by Pleistocene 
basalts, while in Mayotte and Mauritius the Holocene reef sequences overlie 
Pleistocene limestones.  On Mahe they rest on what are probably terrestrial 
silts and clays which themselves overlie Precambrian granite. 

EEMIAN RAISED LIMESTONES 

Methods 

Large EemianPorites-heads have been sampled and are being analyzed using 
methods identical to those applied to the recent samples (sclerochronology, 
stable isotopes).  Petrographic studies are being carried out on material 
from the same outcrops to assess the degree of alteration.  The ages of 
these carbonates will be determined using U/Th dating. 

Locations 

Outcrops of Eemian limestone terraces are exposed on the Seychelles 
Islands, principally in small outcrops beneath granite boulders (Mahe) but 
including thick (metres) in situ algal and coral crusts on Praslin and La 
Digue.  More extensive outcrops are found on Aldabra and other so-called 
'high' limestone islands to the west.  Detailed stratigraphical data are 
available for Aldabra, Kenya, Madagascar, Mauritius and Reunion from 
earlier studies by the group. 

Additional data 

The group also has a sample set collected during a German-French mission to 
Mayotte using the research submersible "Jago".  During this mission several 
coral samples and reef rock samples were recovered.   It has been  shown 
that reefs flourished during isotope stage 3 (50 - 27 ka B.P.) forming 
extended terraces presently drowned to around 80 m deep.  Last glacial 
maximum (LGM) corals were also collected, giving hints of a maximum sea 
level fall of 150 m for the volcanic island of Mayotte.  Such extreme 
values seem to be typical for volcanic islands having no gravity 
differential with the ocean crust and therefore providing a regional 
hydroeustacy (Peltier 1991; Lambeck and Nakada 1992). 

FUNDING 

The studies are financed by a variety of funding agencies. The European 
Union Programme "TESTREEF" (TEmporal and Spatial Variability in Western 
Indian Ocean REEFs) is funding the research in Tulear and Seychelles. 
Investigations on Reunion and Mayotte is funded by PNRCO (Programme 
National Recifs Coralliens- C.N.R.S.), and by the Deutsche 
Forschungsgemeinschaft (DFG Du 129/6 and Du 129/9). 


LITERATURE 
Beck, J.W., Edwards, R.L., Ito, E., Taylor, F.W., Recy, J., Rougerie, F., 
Joannot, P. and Henin, C. (1992): Sea-surface temperature from coral 
skeletal Strontium/Calcium ratios.- Science, 257: 644-647. 

Colonna, M. (1994): Chronologie des variation du niveau marins au cours du 
dernier cycle climatique (0-140 000 ans) dans la partie sud occidental de 
l'Ocean Indien. Implications paleoclimatiques et paleoceanographiques. 
Ph.D. thesis, Aix-Marseille: 1-303. 

De Villiers, S., Shen, G.T. and Nelson, B.K. (1994): The Sr/Ca-temperature 
relationship in coralline aragonite: Influence of variability in 
(Sr/Ca)seawater and skeletal growth parameters.- Geochimica et Cosmochimica 
 Acta, 58: 197-208. 

Epstein, S., Buchsbaum, R., Lowenstam, H.A. and Urey, H.C. (1953): Revised 
carbonate-water isotopic temperature scale.- Geological Society of America 
Bulletin, 64: 1315-1326. 

Lambeck, K. and Nakada, M. (1992): Constraints on the age and duration of 
the age and duration of the last interglacial period and on sea-level 
variations.- Nature, 357: 125-128. 

Nozaki, Y., Rye, D.M., Turekian, K.K. and Dodge, R.E. (1978): A 200 year 
record of Carbon-13 and Carbon-14 variations in a Bermuda coral.- 
Geophysical Research Letters, 5/10: 825-827. 

Peltier, W.R. (1991): The ICE-3G model of Late Pleistocene deglaciation: 
construction, verification and applications.- In: Sabadini, R. (ed.) 
Glacial Isostacy, Sea Level and Mantle Rheology: 95-119 . 

Swart, P.K. and Coleman, M.L. (1980): Isotopic data for scleractinian 
corals explain their paleotemperature uncertainties.- Nature, 283: 557-559. 

_______________ 
Adresses of the group members: 

Colin Braithwaite 
Lilybank Gardens 
Glasgow G1 2 8QQ 
Tel. +44.141.339.8855. Ext. 5449. 
Fax. +44.141.330.4817 
cjrb at geology.gla.ac.uk 

Gilbert F. Camoin 
URA 1208 CNRS 
Universite de Provence 
Centre de Sedimentologie et Paleontologie 
3 Place V. Hugo 
F-13331 Marseille cedex 3 
Tel. +33.91106723 
Fax. +33.91649964 
gcamoin at cerege.fr (by January 96) 

Wolf-Christian Dullo 
GEOMAR 
Research Center for Marine Geosciences 
Wischhofstr. 1-3, Geb. 4 
D-24148 Kiel 
Germany 
Tel     +49-431-7202200 
Fax     +49-431-725391 
cdullo at geomar.de 

Georg A. Heiss 
GEOMAR 
Research Center for Marine Geosciences 
Wischhofstr. 1-3, Geb. 4 
D-24148 Kiel 
Germany 
Tel     +49-431-7202209 
Fax     +49-431-725391 
gheiss at geomar.de 

Lucien Montaggioni 
URA 1208 CNRS 
Universite de Provence 
Centre de Sedimentologie et Paleontologie 
3 Place V. Hugo 
F-13331 Marseille cedex 3 
Tel. +33.91106324 
Fax. +33.91649964 

Marco Taviani 
Istituto di Geologia Marina 
Via Zamboni, 65 
I-40127 Bologna 
Italia 
Tel.    +39-51-244044 
Fax     +39-51-243117 
taviani at boigm2.igm.bo.cnr.it 

Bernard A. Thomassin 
Centre D'Oceanologie de Marseille (O.S.U.) 
Unite associee au CNRS (UA 41) 
Rue Batterie des Lions 
F-13007 Marseille 
Tel.    +33-91041617 
Fax     +33-91041635 
thomassi at com.univ-mrs.fr