Contrasting styles of off-shelf sediment accumulation in New Guinea

Abstract

The accumulation of river sediment (and associated materials) off the continental shelf is widely acknowledged to have been significant during the last glacial maximum, when shelf accommodation space was reduced in many areas. However, the importance of off-shelf sinks during modern sea-level conditions is largely unknown. This study examined off-shelf sediment accumulation for two dispersal systems, the Gulf of Papua and the Sepik margin, using radionuclide ( 210Pb, 234Th), sediment texture and structure data. These study areas were selected because they have a similar source of sediment (the Papuan Fold Belt) with dramatically different margin morphologies. The Gulf of Papua is part of a foreland basin that lies in the northern Coral Sea, and receives over 3×10 8 tons of sediment annually from numerous rivers. The majority of this load accumulates on the shelf, and a small fraction (<5%) is estimated to deposit off the continental shelf in Pandora Trough. Advection of sediment by intermediate and bottom nepheloid layers is hypothesized to be the primary off-shelf transport mechanism. Hemipelagic supply to Pandora Trough is evidenced by modest seabed accumulation rates (<2 mm/yr) for bioturbated cores. The Sepik margin behaves very differently. The Sepik River discharges about 1×10 8 tons of sediment annually, and a submarine canyon extends essentially into the river mouth. Rapid settling of as much as 90% of river material at the plume lift-off point generates very turbid near-bottom waters that are postulated to regularly or episodically produce gravity-driven flows in the canyon. A core collected in the canyon axis at 650 m water depth contains at least 24 cm of excess 234Th, confirming the rapid deposition of recently supplied material. Furthermore, sedimentary structures observed in X-radiographs have characteristics of mud turbidites. Forty-seven discrete turbidites are identified in a core with excess 210Pb at its base, indicating a minimum recurrence interval of 2 years. Based on the present and other research, a spectrum of off-shelf transport behavior is proposed for river-sediment dispersal systems. To evaluate the global potential for modern off-shelf transport, the distance to the 200-m isobath is determined for ∼100 major rivers. Nearly 30% of the rivers have shelves <50 km wide (13 are <20 km in width), and many of these are along collision margins. These data suggest that off-shelf sedimentation, such as the type occurring on the Sepik margin, may be more widespread than previously recognized, and this has important implications (e.g. burial site) for the fate of carbon and other particle-associated materials.