From reef locations at Mayotte (Comoros, 12°50´S, 45°10´E), Mahé (Seychelles, 4°40´S, 55°30´E) and La Réunion (21°5´S, 55°10´E) large coral heads of the species Porites were sampled using an underwater drill, driven by compressed air. All colonies grow at a water depth between 2m and 7m.
Stable isotope records were obtained from the upper part of the cores that comprise between 3 and 6 years of coral growth. Sampling distance for stable isotope measurements was 1mm, thus a resolution of ca 10 samples per year was possible. For the core from Mayotte some additional annual samples were taken. Their record reaches back to the year 1925.
Vertical growth rates of the large colonies are 8.68 (±2.27) mm/yr at La Réunion, 11.2 (±1.53) mm/yr (Mahé), and 11.85 (±4.10) mm/yr (Mayotte). From the southernmost location off La Réunion the most accentuated seasonal oxygen isotopic signal was found with deposition of high density (HD) bands at times of high water temperatures (light oxygen isotopes, d18O: -5.13 to -4.18” PDB).
The Mayotte samples showed a clear seasonal signal (d18O: -5.34 to -4.55 ” PDB), where deposition of HD-bands is linked to rising seawater temperatures.
From Mahé the oxygen isotopic signal is less clear than at the other sites concerning seasonality, probably due to low
latitude and low temperature variations over the year (d18O -values: -5.37 to -4.22 ” PDB). The pattern of HD-deposition is opposite to La Réunion with HD-deposition in the cool season.
Inter-annual variation of oxygen isotopes at Mayotte from 1920 to present show a trend to lighter oxygen isotopes by
ca. 0.25” which is in the same range like in previously studied corals from Aqaba, Red Sea (Heiss, 1994). This might be a sign of global warming effects.
The long-term carbon isotope signal for this core shows a trend to values which are by about 1” lighter than 60 years ago. This trend has been observed in other long-term coral records (Heiss 1994) and may be a response to increased CO2 - values in the atmosphere, caused by burning of fossil fuel.
Heiss, G.A., GEOMAR Report 32, 1-141 (1994).