The Rapid Response of Reef Sediments to Changes in Community Composition: Implications for Time Averaging and Sediment Accumulation

Abstract

ABSTRACT Following Hurricane Allen in August 1980, a variety of natural and anthropogenic stresses combined to significantly alter structure and community composition of reefs along the north Jamaican coast. Comparison of sediment data collected in 1982 (Boss and Liddell 1987) with data collected in 1994 has enabled an assessment of the response of reef sediments to the subsequent changes in carbonate production and productivity. At most sites abundances of the main sediment contributors (coral, Halimeda, Homotrema, coralline algae, and molluscs) have changed significantly, and correlate closely with changes to the reef community. The potential for an individual reef site to preserve evidence of changes in sediment supply depends, however, on the combination of taphonomic processes at t at site. Back-reef sites typically have high rates of biogenic reworking, slow rates of cementation, and limited lateral sediment movement. Short-term changes in the composition of back-reef sediments may result from the short residence time of grains in this environment due to extensive chemical and biochemical dissolution. Such short-term trends in sedimentation are, however, unlikely to be preserved, because of high rates of surface sediment destruction. By contrast, fore-reef framework sands undergo limited physical and biogenic reworking, rapid cementation, and thus shorter residence time in the taphonomically active one. As a result, preservation potential of grains is good, and the potential to record changing patterns of sediment production over time is considerable. Sediments wi hin adjacent sand channels however, show no significant changes in composition and are characterized by rapid lateral and to a lesser extent vertical sediment mixing, slow cementation, generally long residence times, and episodic mass sediment transport. As a result they represent time-averaged assemblages that have little potential to record changing patterns of reef sedimentation. The potential for sediment time-averaging therefore varies enormously among reef sites, and has implications both for assessing periods of community change within Quaternary reef sequences and for interpreting rates of taphonomic destruction within reefs.