Rapid accretion of inshore reef slopes from the central Great Barrier Reef during the late Holocene

Coral reefs from the inshore Great Barrier Reef (GBR) initiated in the early Holocene, and have undergone a period of quiescence in recent millennia after reaching sea level. However, the capacity for accretion in adjacent reef slopes that are unrestricted by sea-level constraints is largely unknown. To explore this potential, we recovered 38 sediment cores (2-5 m length) from the reef slope (5 m depth) from two inshore fringing reefs (Pandora and Havannah Reefs) from the central GBR. We obtained 115 high-precision U-series ages from the core record to reconstruct a detailed late Holocene accretion record from 1000 yr ago to the present. Computed axial tomography scans of intact cores revealed a coral matrix with voids infilled with fine-grained carbonate-siliciclastic sediment. Accretion within cores was highly constrained through time (R-2 > 0.9) with no evidence of age reversals, indicating continuous and rapid (average 8.8 +/- 1.2 mm/yr) accretion throughout the late Holocene (i.e., 1000 yr ago to the present). Our results indicate rapid late Holocene accretion on reef slopes adjacent to senescent reef flats. Comparisons of these results with published reef accretion rates from Holocene reef flats on the inshore GBR indicate that where accommodation space is available, reef slopes continue to accrete at rates equal to and exceeding that occurring during the mid-Holocene climatic optimum