OCCURRENCE AND SIGNIFICANCE OF MICROBIALITES IN THE UPLIFTED TASMALOUM REEF (SW ESPIRITU SANTO, SW PACIFIC)

Abstract In the SW Pacific Ocean, subduction of the d'Entrecasteaux ridge system has caused rapid uplift of the central New Hebrides Island Arc. The maximum uplift rate of 6 mm yr −1 occurs along the southwest coast of Espiritu Santo Island, near Tasmaloum. The Tasmaloum uplifted reef sequence internal structure, which is strongly linked to its tectonic context, was investigated through a series of drill-holes to depths up to 42 m. Although a stable tropical coast would undergo approximately 120 m of post-glacial sea-level rise, the net relative sea-level rise on such a rapidly uplifting coast is only about 20 m. Colonization of the Tasmaloum fringing reef occurred by 24 ka, upon a pre-reef substrate composed of a thick bioclastic sand formation accumulated during the last glacial period. During the post-glacial sea-level rise, the vertical succession of reef assemblages reflects environmental and bathymetric variations controlled by the interplay of rapid, but variable rates of sea-level rise and more or less constant uplift of 5–6 mm yr −1 . Microbialite crusts, composed of high-magnesian calcite laminae, occur in the Tasmaloum reef from 20 to 6 ka and are particularly abundant from 16 to 10 ka. The development of microbialite crusts is related to nutrient enrichment of interstitial waters through mixing with meteoric groundwater. After 6 ka, when sea level ceased rising in the region and continuing uplift caused rapid emergence of the reef, microbialites disappear within the subtidal assemblages. Several explanations can be put forward for their disappearance. In particular, nutrient input changes are a likely cause. A new hydrologic and oceanographic regime was established when sea level ceased rising. This change was accompanied by warming of tropical waters.