EXPERIMENTAL BIVALVE AND CORAL TAPHONOMY, MADANG, PNG: CONTROLS ON BURIAL/EXHUMATION TRUMP NUTRIENTS AND MICROSTRUCTURE IN REEF ENVIRONMENTS

Fidelity of reefal deposits to their living communities controls our use of this important environmental record. Reef taphonomy is a complex system of intense exposure and burial processes, affecting the widest range of skeletal sizes, forms, and microstructures found in any marine environment. In order to experimentally constrain this variation, bivalve and coral skeletons were deployed for one year across a range of reef environments in Madang Lagoon, Papua New Guinea. Sites included a high energy, low nutrient patch reef (Padoz), a windward moderate energy fringing reef (Gossem), and a estuarine low energy, high nutrient fringing reef (Nagada). Skeletons were deployed both at and 10cm below the sediment-water interface and included 2 species of bivalve (Gafrarium tumidum and Geloina coaxans) and three species of coral (Acropora cf. pulchra, Fungia spp., and Goniastrea retiformis). Across taxa, specimen loss occurred almost exclusively at the high energy site, predominantly of specimens deployed on the sediment surface. Net weight change was positive for exposed bivalves and negative for buried ones, in both cases within 10% of the initial weight. By contrast, weight loss among corals was ubiquitous with the exception of a few Acropora, and often ranged between 10-20%. Both bivalves and corals showed lower surface alteration if originally buried. Exposed specimen surfaces are obscured by encrustors, otherwise they are dull to chalky. Acropora consistently displayed higher surface alteration than other species. Encrustation was pervasive at all sites after 1 year. The degree to which buried shells were exhumed and encrusted was high at Padoz, moderate at Nagada, and low at Gossem (where bivalves and Fungia were rarely exhumed), however exhumed shells had maximum surface coverage in the lower energy sites. Bioeroders were significantly more abundant in exposed rather than exhumed skeletons, and preferentially attacked the higher organic, larger Geloina. NMDS ordinations of epibiont types indicate separation among the sites, however the gradients are not consistent among the experimental taxa. The dominant control on taphonomic condition therefore seems to be the interaction of environmental energy with skeletal form and size (affecting exposure), with a secondary control of skeletal microstructure.