Sediment accumulation determined with 210Pb geochronology and geochemical tracers for Strickland River flood plains, Papua New Guinea

The Strickland River is the primary sediment source for the Fly River system, a large tropical river that ranks in the global top 20 for both water and sediment discharge. As part of a "Source to Sink" NSF Margins program, the patterns and rates of floodplain sedimentation are being investigated. Previous research on the Middle Fly has documented an exponential decrease in sedimentation rates with distance from channel bank and a large influence of distributary floodplain channels in directing sediment to the floodplain environment. In the Strickland, a mine has discharged waste into the river since 1992, and though the total load increase is small for the lowland Strickland, elevated Ag and Pb levels occur in the river sediment, providing a clear environmental tracer across the floodplain. Work on other flood plain environments has demonstrated that 210Pb can be used to map the spatial and temporal patterns of sedimentation. Here we present geochronological results from an intensive floodplain coring campaign conducted in 2003 on the lower Strickland, which employed both 210Pb geochronology and Ag and Pb penetration depths to quantify sedimentation rates. We will first outline our procedure for dating Strickland sediment with 210Pb geochronology and summarize some early results from 36 cores. Flood plain accumulation rates appear to be highest upstream near the gravel-sand transition, low in the middle portion of the river, and higher again in the lower reaches of the Strickland near to its confluence with the Fly River. Overall patterns of sedimentation from 210Pb geochronology seem to be spatially consistent, for series of cores collected along single flood plain transects. We will next compare these results to accumulation rates determined from duplicate cores that were measured for the concentration of heavy metals from the upstream mine. These two techniques are independent and cover different temporal and spatial (in the vertical dimension) scales, so we will outline how they agree and key scientific questions that are posed by any differences. We will conclude with a summary of the rates and spatial patterns of sediment accumulation across Strickland River flood plains.