Crown-of-thorns starfish outbreaks on the Great Barrier Reef: a geological perspective based upon the sediment record

Over the last 30 years, the crown-of-thorns starfish (Acanthaster planci) has caused extensive damage to many reefs in the Great Barrier Reef Province. Surface sediment of two such reefs, John Brewer Reef and Green Island Reef, has high densities of A. planci skeletal elements relative to their abundance in the surface sediment of Heron Island Reef which, during the same 30 years, maintained very low-density starfish populations. Carbon-14 accelerator mass spectrometry (AMS) dating indicates that skeletal elements from the surface sediment of John Brewer and Green Island Reefs are of contemporary age. Core sampling shows that subsurface sediment at John Brewer and Green Island Reefs contains A. planci element densities comparable to those found in the surface sediment at these localities. Physical and biological eworking of elements within the sediment precludes the recognition of individual outbreaks in core stratigraphy. AMS element dates and conventional bulk sediment dates show that subsurface elements are generally prehistoric and conform to an age structure preserved in the sediment pile. The density and distribution of subsurface elements suggest that A. planci outbreaks are not a recent phenomenon, but have been an integral part of the ecosystem for at least 7000 years on John Brewer Reef and 3000 years on Green Island Reef.