The systematics and significance of coralline red algae in the rhodolith sequence of the Amédée 4 drill core (Southwest New Caledonia)

Abstract

The mode of early settlement and development of the New Caledonian barrier reefs is interpreted based on the study of a thick rhodolith sequence found at a depth of 126.5 m near the base of the Amédée 4 drill core. This sequence lies immediately above bedrock and consists of successive reef sequences which belong to the last interglacial stages of the Quaternary. The rhodolith sequence relates to the pioneer stage of the barrier reef development. Based largely on the Corallinaceae taxonomy, growth forms and the composition of floristic and faunistic associations, in combination with sedimentological data, a thick 12.3-m rhodolith sequence (from 116.2 to 128.5 m depth) can be subdivided into six distinct units. The algal assemblage includes ten taxa which are found in present-day coral reef environments (i.e. 8 non-geniculated species: Lithophyllum kotschyanum, L. pustulatum, L. tamiense, Hydrolithon reinboldii, Lithoporella melobesoides, Mesophyllum funafutiense, M. mesomorphum, an unidentified Melobesoideae, one geniculated species Amphiroa fragillissima and the green alga Halimeda). By reference to the biozonation, depth range and water energy setting of their modern counterparts, the different fossil assemblages can be used to reconstruct the conditions of deposition in the early reef stages. The occurrence and shapes of concentrated rhodoliths indicate a development related to active growing phases (fairweather periods) and as well as to erosion and reworking events caused by storms at water depths less than 40 m, thus forming a stabilised substrate upon blocks and debris of substrate. As a result, a large carbonate platform formed. A similar stratal sequence can be observed in the Australian Great Barrier Reef. It strongly supports our hypothesis that calcareous algae have played a significant role in reefal substrate stabilisation and building of barrier reefs.