Coral community changes in the Great Barrier Reef in response to major environmental changes over glacial-interglacial timescales
The Ribbon Reef 5 borehole offers a unique record of reef growth spanning the entire history of the northern Great Barrier Reef (GBR). Previous studies have reported the main stratigraphical, lithological and chronological patterns, as well as basic descriptions of the coralgal assemblages, but no detailed coral community analysis was undertaken. We present a quantitative analysis of the nature and distribution of Pleistocene coral communities and apply several statistical tools to define recurrent coral associations and compare the eight reef-building cycles recognized throughout the evolution of the GBR. The start of significant reef building occurs at 137 m based on a major change in coral community structure and the inception of the reef cycles (Cy1–8). This revision, along with available stratigraphical and chronological data, suggests that barrier reef initiation may have occurred prior to MIS 11, earlier than previous reports. The coral assemblages at 137 m reflect the transition from lower mesophotic (60–100 m) to upper mesophotic (30–60 m) settings, while the eight reef cycles above are characterized by three recurrent shallow-water reef-coral associations: Porites–Montipora-faviids (Po-Mo-Fa), pocilloporids (Poc), and Acropora–Isopora (Acro-Iso). Typically, these cycles begin with the Po-Mo-Fa association and end with the Acro-Iso association, reflecting shallowing and a catch-up growth mode. However, the first two cycles are characterized by a transitional phase dominated by the Poc association. The dominance of pocilloporids during the early stages of the GBR’s history and the long-term shift to an Acropora–Isopora-dominated community may reflect an increase in competitive pressure of acroporids over pocilloporids. Our findings are consistent with the view that reef coral community structure is predictable over 100-kyr time scale. However, variations within reef cycles highlight the importance of environmental changes operating at millennial time scales. Further studies are needed to better refine the reef chronology and clarify the influence of environmental variables (i.e. sea surface temperature, turbidity) on reef coral community structure.
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