In the course of the last glacial maximum, around 20 000 years B.P., sea levels reached their lowest point, at 120 to 130 metres below the present level. The subsequent ice-cap melting induced a gradual rise of the oceans up to current levels. In the tropical regions, these large-amplitude fluctuations have contributed to the formation and growth of coral reefs.
IRD researchers at Noumea, in conjunction with scientists from three American universities (1), have just reconstructed the history of the oldest post-glacial reef ever studied in the Pacific which has grown under the influence of sea-level oscillations. This reef is situated at Urelapa, off the island of Espiritu Santo in the Vanuatu group in the South-West Pacific. It shows the longest recorded continuous growth, at 17 000 years, between 23 000 and 6 000 years B.P.. (2). The scientists thus have at their disposal environmental data (sea-levels, quantity of nutrients, temperatures, and so on) covering the entire period of deglaciation (3).
Core samples from five boreholes made on the island were analysed. Data was taken on the sedimentology, palaeontology, radiochronology (dating of the corals) and palaeoecology (study of communities of fossil organisms like corals, algae and molluscs). Character and morphological analysis of the corals (whether tabular, branched, massive or foliaceous) and the calcareous algae species which are associated with them gave clues which put together produced a reconstruction of the different stages of the reef's development (4). These corals reveal a strong ability to adapt. The reef developed according to two different and successive growth processes, in response to variations in environmental conditions and, in particular, to sea-level changes.
From 23 000 to about 11 300 years B.P., the reef developed continuously, keeping to the shallow marine habitats. The corals thus followed the rise in the oceans very closely. The associations of ramified corals, including species of the Acropora genus, with red encrusting algae, as coralgal assemblages, forming the basis of the reef frameworks, found at horizons dating from this period, are specific to shallow well-lit, high wave-energy environments (at depths less than 6 m), ideal conditions for reef growth.
However, from 11 300 to 6 000 years B.P. the coral colony morphology, consisting mainly of massive Porites species, and changes observed in the associations of organisms show a deeper, calm water (i.e. low wave-energy) habitat (10 to 20 m), where light penetration is poorer. Coral reef growth slowed during this period and did not keep up with the rising sea level. In fact, an acceleration of deglaciation and of the rise in sea level are known to have occurred 11 300 B.P. They indeed led to changes in the environmental conditions. These induced in the reef a change-over from a keep-up mode of growth geared to keeping pace with sea-level rise to a catch-up mode. The reef could no longer keep up to the surface. It was temporarily completely submerged, before it could succeed in catching up with the water level. This reef-sequence study showed that the reef nevertheless managed to grow throughout the deglaciation period by modifying its structure and morphology. It did this by favouring species capable of adapting to new environmental conditions.
Coral reef development depends on external factors (sea water temperatures, salinity, nutrient content and input, tectonic activity, the nature of the rock substrate, and so on). The respective roles of these parameters in determining which of the two types of growth process prevails are still not really known. Further drilling investigations in different environments of the Pacific, such as some which have been conducted in New-Caledonia, Tahiti and Vanuatu, should reveal more about the impact of each factor and allow assessment of their variations for all parts of that ocean. The aim of this research is to gain better understanding of the influence of climatic changes on these marine complex ecosystems. Fundamental information that can be obtained on the installation, formation and development of coral reefs in the tropical zone should make it possible in the long run to elaborate models of their growth.
(1) Department of geology and geophysics of the University of Minnesota, Department of physics of the University of Arizona (Tucson) and Institute of Geophysics of the University of Texas (Austin).
(2) The longest continuous growth -14 000 years- known up to now had been found on the barrier reef of Tahiti in 1999. See scientific bulletin n° 106 (January 2000)
(3) They had hitherto, for the tropical zone, only fragmented reef growth data which was spread too widely over time. Reconstitution enabled them only to gather information for the past 22 000 years, with no guarantee of a real continuity.
(4) As the community structure of fossil fauna and flora species is closely bound to the water depth, degree of light penetration and to calm or rough conditions, it is a valuable indicator of the type of reef growth prevailing at a given time.