Mid Pleistocene Age for the Great Barrier Reef

Determining the age of the Australian Great Barrier Reef (GBR) is complicated by the lack of appropriate absolute age dating methods and the technical difficulty of recovering deep fossil reef material. The GBR region is characterized by a highly-productive carbonate factory adjacent to a monsoon-dominated tropical forest with rivers that discharge a substantial volume of terrigenous sediment. Here we leverage this unique setting to indirectly constrain the age of the GBR by reconstructing the geomorphologic evolution of the continental shelf from a non-rimmed to a rimmed margin. Contradictory to the traditional reciprocal sedimentation model for mixed carbonate-siliciclastic systems in other areas, terrigenous sediment accumulation on the upper Northeast Australian continental slope peaks as the shelf is reflooded during glacial termination and sea-level transgression. This is because terrigenous sediment accumulated behind the barrier reef during the preceding glacial (low sea level) instead of being directly deposited to the slope ("transgressive shedding").

Using a spliced, multiproxy record from ODP Hole 1198A and the adjacent GeoB22218-1, each recovered from the Marion Plateau (~350 mbsl), seaward of the shelf break, we show that reworking of terrigenous sediment to the slope during glacial terminations began by at least MIS 17. Simultaneous deposition of shallow-water carbonates strongly suggests a response to sea-level rise, rather than e.g., increased precipitation and river discharge. We therefore interpret that the antecedent patch reefs preceding the formation of the GBR coalesced into an effective barrier by MIS 19. We are also investigating whether the formation of a barrier was coincident with significant environmental change by analyzing micro- and nano-fossil assemblages and planktic foraminifer trace element geochemistry. Interestingly, terrigenous sedimentation processes do not appear to be dominated by sea-level prior to the emplacement of a barrier reef. This may be related the Northern Hemispheric sea-level signal being overprinted by local, Southern Hemisphere insolation-driven monsoon variability. Thus, traditional mixed carbonate-siliclastic sedimentation models are likely inappropriate for this region, regardless of the presence or absence of a barrier reef.