Spatio-temporal patterns in the postglacial flooding of the Great Barrier Reef shelf, Australia

Abstract

The shelf of the Great Barrier Reef (GBR) was progressively marine flooded from the last glaciation maximum (LGM) (ca 20 ka BP) until the last sea-level highstand (ca 6 ka BP), affecting the depositional evolution of the GBR margin and associated deposits. However, the physiographic variables related to this process have not been fully characterized, especially in relation to the sedimentary processes at the shelf margin. For this study, we used a bathymetric model of the entire shelf and a shelf margin sub-set, divided into 33 latitudinal zones. Postglacial marine flooding was simulated and flooded area (km2), flooding magnitude (km2 per sea-level increment), flooding rate (km2 ky−1) and coastline length (km) were estimated for each zone, from 130 m to 0 m below present sea level, representing the period from 20 ka to 6 ka BP. Our results show that the postglacial marine flooding did not occur uniformly and that some sub-regions (e.g. the southern-central GBR) had early and rapid flooding. Coastal complexity increased in the mid-postglacial, reaching maximum values at around 9 ka BP. This reflects a coastal landscape evolving from a linear, laterally connected coast to a complex coast dominated by estuaries and lagoons, partly returning to its initial linearity during highstand. Flooding trends and geological evidence make two depositional relationships apparent. Firstly, the timing and magnitude of the off-shelf sediment flux appears linked to the presence and orientation of a shelf-edge rim, and to the extension and morphology of the evolving drainage network. Secondly, the periods of shelf-edge reef development and demise seem to respond to the remobilisation, trapping or redirection of fine sediments. We propose a sedimentation model for the shelf margin and the slope driven by the interplay of sea-level rise and shelf physiography, and we highlight two fundamental processes: (1) the cross-shelf sediment transport related to coastline retreat under rising sea levels, and (2) the effectiveness of transient embayments in redirecting or trapping sediments. The quantifications provided in this study have implications in the estimation of Pleistocene carbonate budgets and the atmospheric carbon cycle, as well as for past human migrations.