A Tropical View of Quaternary Sequence Stratigraphy and a Dynamic View of the Sink Along the Northeast Australian Margin: The Accumulation of Riverine Material on Slopes Since the Last Glacial Maximum

The northeast Australian continental margin, extending from central Queensland to southern Papua New-Guinea (PNG) and including the Great Barrier Reef, is the outstanding modern example of a tropical mixed siliciclastic/carbonate system, where rivers discharge significant amounts of terrigenous material onto a shelf and marine organisms produce enormous amounts of carbonate material. The long-term sink for this sediment lies in adjacent slopes and basins. Generic models of continental margin evolution predict that siliciclastic fluxes to this sink should be maximal and minimal during major sea-level lowstands and transgressions, respectively. Here we unequivocally show that the opposite occurs. Cores from slopes of this northeast portion of this margin contain a m-scale siliciclastic-rich interval in the upper few meters, with the thickness and depth related to sedimentation rate and proximity to the shelf. Planktonic foraminiferal AMS radiocarbon dates and oxygen isotope records in all cores examined to date consistently show that this interval was deposited ca. 12 to 7 ka, and represents a 4 to 6 fold increase in siliciclastic fluxes. This massive terrigenous discharge to slopes occurred when sea level rose above the shelf/slope break and can be documented along at least 450 km of the margin (essentially where quality cores are available for study), irrespective of modern variations in bathymetry or climate. Either tropical Australia (and presumably PNG) had a significantly different early Holocene climate with greatly enhanced sediment discharge or rivers aggraded on the shelf during lowstand with terrigenous material released to slopes only when waves and tidal currents could remobilise sediment. In this region, quantified sediment discharge from present-day rivers and/or quantified sediment accumulation on the present-day shelf cannot be extrapolated over moderate time frames. This complexity provides an intriguing challenge for source-to-sink studies.