Algal encrustation and the interpretation of radiocarbon dating of coral clasts

Abstract

A wide variety of encrusting organisms, ranging from filamentous algae to corals, occur within coral reef lagoons. However, many of these, such as macroalgae, are not readily preserved in the sedimentary record (Noble et al. 1995; Perry 1996). Those organisms which secrete a carbonate skeleton can contribute substantially to the sediment budget. Coralline algae are probably the most ubiquitous encrusting species on many reefs which secrete a hard skeleton, and may not only cover loose debris but also can form an important structural component of the reef framework (e.g. Camion and Montaggioni 1994; Rasser and Riegl 2002). Although other processes, such as diagenetic cementation, may be more important in long-term stabilization of reef sediments (Rasser and Riegl 2002), coralline algae quickly colonize vacant substrates within a lagoon, including both bedrock and unconsolidated gravel. The most common species of coralline algae require light to grow, and encrustation therefore occurs most commonly on surface sediment (Adey and Macintyre 1973; Adey 1986; Rasser and Riegl 2002). The growth rate of coralline algae is, however, not the main determining factor in crust distribution and abundance, with herbivory often being very important, along with water movement and sedimentation (Steneck 1985, 1986). Clasts still, however, require a finite time of surface exposure to be encrusted, although secondary encrustation (e.g. Foraminifera) can occur below the surface, such as in the upper part of reef frameworks. An assessment of the sedimentary history of lagoon infill is often based on radiocarbon dating of larger clasts. The standard technique is to remove the encrustation. This is especially the case for conventional radiocarbon dating techniques which require sample sizes of 5 g (preferably >25 g) or more, and the encrustation is viewed as potential contamination. The question therefore arises as to which event is actually being dated. The radiocarbon age of the coral clast will relate to the death of the coral, whereas the outer layer of algal encrustation may correspond more closely to the last period of exposure. Therefore, the removal of the outer layer may in fact give a relatively older age of deposition and, hence, a lower sedimentation rate. This study compares the age of a series of coral clasts, their associated algal encrustation, and surrounding unconsolidated sediment in order to determine if there is a significant difference in their age. It examines the implications of using these clasts in reconstructing the depositional history of a reefal lagoon. The dates reported form part of a larger study of the Holocene lagoonal evolution of Lord Howe Island, the southernmost coral reef in the world, reported in Kennedy and Woodroffe (2000).