Characterization and Timing of Siliciclastic Sediment Fluxes to Continental Slopes of the Coral Sea During the Late Quaternary

The continental margins of southern Papua New Guinea and northeastern Australia together form the world's largest extant tropical mixed siliciclastic-carbonate depositional system where both rivers and shallow marine organisms supply large amounts of sediment to the shelf. The flux and composition of sediment shed from these margins to surrounding slopes and basins changes dramatically over the late Quaternary. This is to be expected given mixed sediment sources and large amplitude variations in sea-level and climate during this time. Importantly, though, the observed accumulation of the siliciclastic material on slopes deviates significantly from generic sequence stratigraphic models. Recent studies on the northeast Australian margin clearly show greatly increased fluxes coincident with late transgression ca. 12-7 ka rather than lowstand ca. 25-18 ka (Dunbar and Dickens, Sed. Geology, in press; Page et al., Geology, in press). In this study we examine the mineralogy and grain size of the siliciclastic fraction down a series of well-dated cores along the slope of the northeastern Australian margin to further characterize this phenomenon. Sediment samples were taken at ~10 cm intervals down drill cores (collected on ODP Leg 133) and piston cores. Each sample was reacted with weak acetic acid to digest the carbonate component. The remaining siliciclastic component was analyzed for mineralogy using an XRD and for grain size using a Laser Particle Size Analyzer. Preliminary results from ODP Site 820 show that the siliciclastic component deposited during early transgression is dominated by silt but that, from ca. 10 ka until present, the sand percent of this component increased. These results may support either of two end-member models to explain elevated siliciclastic accumulation during late transgression. In the first model, margin physiography controlled siliciclastic accumulation, whereby sediment was stored behind an exposed barrier reef during lowstand, and released to the slope ca. 12-7 ka when flooding of the shelf enabled remobilization. In this case, winnowing of the stored siliciclastic sediment from ca. 10 ka to present may explain the reduced off-shelf flux and greater sand fraction. Alternatively, significant variations in climate may have controlled fluvial discharge to the slope. An arid climate during the Last Glacial, resulting in low river discharge, was followed by intensified precipitation ca. 12-7 ka, vastly increasing river fluxes and siliciclastic accumulation on the slope. In either case, these results could have profound implications for interpretations of mixed siliciclastic-carbonate sequences in the geological record, which are often made on the basis of sediment properties without detailed age constraints. Results from other cores along the northeast Australian margin will be presented at the meeting, and cores from the southern Papua New Guinea margin applicable for this research will be collected in the spring of 2004.