Sediment export from the Sepik River, Papua New Guinea: evidence for a divergent sediment plume

The Sepik River, with an estimated annual sediment load of ∼85×106 t yr−1, empties directly into a submarine canyon that transverses a narrow continental shelf (<5 km) on the north coast of Papua New Guinea. As such, it serves as a possible analogue for rivers discharging onto margins during low stands of sea level. A substantial portion of the riverine sediment appears to be transported seaward in the canyon, most of the remainder being stored at least temporarily on the proximal shelf and slope. Salinity and suspended-sediment distributions along the axis from the river mouth to the coastal ocean suggest that sediment is dispersed via a plume with both surface and near-bottom components. Rapid settling may occur just seaward of a shallow bar at the head of the canyon, ∼1 km upstream of the river mouth. The rapid settling probably results from a combination of factors, including reduction of turbulent mixing in the presence of strong salinity stratification, convergent bottom flows, and changing particle characteristics. Sediments initially trapped at the bar may continue down the steep slope as a hyperpycnal (negatively buoyant) flow or be deposited temporarily near the bar and then flow down the canyon in episodic turbidity currents. Shallow seismic observations, along with textural and radioisotope measurements of seabed sediments, are not inconsistent with two distinct dispersal pathways. An acoustically transparent, possibly muddy drape extends across the outer shelf and slope west of the river mouth, suggesting deposition from a surface plume. Silt-rich bottom sediments with lower 210Pb activities along the axis of the canyon are consistent with rapid deposition and minimal scavenging of dissolved 210Pb. Intermediate water-depth turbid layers observed along isopycnal surfaces in deeper water suggest a possible terrestrial source for elevated levels of particulate aluminum and iron at depth in the equatorial Pacific. The formation of a divergent sediment plume may be common in other rivers emptying onto a steep slope during flood periods.