A 210Pb sediment budget and granulometric record of sediment fluxes in a subarctic deltaic system: The Great Whale River, Canada

To elucidate how modern river discharge conditions of the Great Whale River (GWR) are represented in the marine sedimentary record, eight box and gravity cores were examined in terms of 210Pb and 137Cs radiochemistry, granulometry and physical sedimentary structures. These data were analyzed to provide insights into sedimentary processes and patterns at the study site. Sediment accumulation in the study area appears to be a relatively steady process over time-scales of 50–100 yr, allowing biological activity to overprint the primary depositional fabric. Subtle differences between 137Cs and 210Pb sediment accumulation rates (SARs) suggest an offshore shift in the locus of fine sediment deposition during the past ∼150 yr, which may be a result of ongoing climatic warming leading to decreasing sea-ice coverage and a more energetic marine environment. Under present day conditions 23% (40,000 t/yr) of the discharged sediment appear to accumulate in a 25 km2 area off the river mouth. The remaining 77% (136,000 t/yr) are either deposited further offshore, possibly along the northeastern shore as a result of Hudson Bay's counterclockwise circulation, or dispersed into the Hudson Bay system. Grain diameter frequency analyses suggest that environmental processes controlling sediment transport and deposition vary over decadal time scales. Although, we cannot define an exact cause for this pattern, these shifts may be related to variations in river discharge, wave climate, possibly due to windier conditions or less sea-ice dampening, bioturbation or a combination of all. This suggests that also longer term river discharge signals are preserved in the marine sedimentary record offshore the Great Whale River. In summary, no major change in sediment discharge over the past ∼150 yr was observed. However, the offshore shift in the locus of sediment deposition suggests, that a warming climate will lead to more energetic marine conditions, less sea-ice coverage, and an increased offshore transport of terrestrial matter.