Evidence of very rapid reef accretion and reef growth under high turbidity and terrigenous sedimentation

Abstract

Global-scale deteriorations in coral reef health are projected to lead to a progressive decline in reef-building potential and ultimately to states of net reef erosion. These transitions may be driven by various human disturbances and by climate change; however, increased terrestrial sediment and nutrient yields from anthropogenically modified coastal catchments are widely recognized as a major threat. As water quality deteriorates, reduced coral cover and species diversity are commonly inferred, and lower reef accretion rates and impaired reef development are assumed consequences. Here we present a detailed chronostratigraphic growth history, constrained by 40 accelerator mass spectrometry radiocarbon dates for Middle Reef, an inshore turbid-zone reef on Australia’s Great Barrier Reef, that challenges the assumption that high terrestrial sediment inputs inherently restrict reef accretion rates and inhibit reef development. We establish that Middle Reef has vertically accreted very rapidly for more than 700 yr, at an average rate of 8.3 mm yr−1. Accretion rates varied across the reef at different times, but it is significant that the periods of most rapid accretion (averaging 13.0 mm yr−1) coincide with phases of reef development dominated by fine-grained terrigenoclastic sediment accumulation. We suggest that this is in large part a function of a high rate of terrigenous sediment accumulation aiding the postmortem preservation of coral skeletal material. Both maximum and site-averaged accretion rates match or exceed those documented for most clear-water, mid- and outer-shelf reefs in the region over the past 9000 yr, and those determined for many reefs throughout the Indian and Pacific Oceans over the same period. While examples of inshore coral reefs that have been degraded in the short term by excessive terrestrial sedimentation clearly exist, others clearly tolerate high sedimentation and turbidity, and our data confirm that sustained and long-term rapid reef growth is possible in these environments.