Research programme on coral reefs in the Western Indian Ocean
Georg Heiss
gheiss at geomar.de
Fri Dec 15 16:02:19 UTC 1995
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
More information about the Coral-List
mailing list