Occurrence and significance of microbialites in the uplifted Tasmaloum reef (SW Espiritu Santo, SW Pacific)

In the SW Pacific Ocean, subduction of the d'Entrecasteaux ridge system has caused rapid uplift of the central New Hebrides Island Are. The maximum up lift rate of 6 mm yr(-1) occurs alone the southwest coast of Espiritu Santo Island, near Tasmaloum. The Tasmaloum uplifted reef sequence internal stru cture, which is strongly Linked to its tectonic context, was investigated t hrough a series of drill-holes to depths up to 42 m. Although a stable trop ical 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 ac cumulated during the last glacial period. During the post-glacial sea-level rise, the vertical succession of reef assemblages reflects environmental a nd bathymetric variations controlled by the interplay of rapid, but variabl e 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 t o 10 ka, The development of microbialite crusts is related to nutrient enri chment of interstitial waters through mixing with meteoric groundwater, Aft er 6 ka, when sea level ceased rising in the region and continuing uplift c aused rapid emergence of the reef microbialites disappear within the subtid al assemblages. Several explanations can be put forward for their disappear ance. In particular, nutrient input changes are a likely cause. A new hydro logic and oceanographic regime was established when sea level ceased rising . This change was accompanied by warming of tropical waters. (C) 1999 Elsev ier Science B.V, All rights reserved.