Sedimentation on the Canadian Beaufort Shelf

Abstract

Modern sediment supply to the Beaufort shelf is dominated by the Mackenzie River which annually provides of the order of 1.25 × 10 8 tonnes a −1 of mainly fine grained sediment. Minor contributions of sediment are derived from other rivers, principally along the Yukon coast (1.5 × 10 6tonnes a −1) and from erosion of coastal bluffs (5.62 × 10 6tonnes a −1). Calculations of the sediment mass that has accumulated in the Mackenzie Delta and on the Beaufort Shelf during the Holocene suggests that sediment supply from the Mackenzie must have been higher during the earlier Holocene than it is today. Sea-ice covers the Beaufort Shelf from October to June each year. Ice scouring may be active during the winter, particularly below pressure ridges, but net transport by ice-keels is probably unimportant on the eastern shelf where sedimentation rates are relatively high. Ice scour transport and ice-rafting may be more important on the western shelf where sediment supply is much lower. Discharge and sediment supply from the Mackenzie River are both low during the winter months but rise rapidly in late April and May prior to break-up. A significant proportion of the sediment supply occurs before and during break-up, when open water is minimal. During the open water summer months, a surface plume of brackish water with relatively high concentrations of suspended sediment extends over much of the shelf, east of the delta. The position and dynamics of the plume is alternately controlled by the prevailing northwesterly or easterly winds, but net transport is to the east. The distribution of Holocene mud on the shelf reflects this transport and is closely matched to the maximum extent of the plume. Over the middle and outer shelf (20 m water depth), wave motions influence bottom sediment transport less than 1% of the time. Near-bottom currents, weakly coupled to surface wind forcing, increase during storms and there is some evidence for resuspension of middle and outer shelf sediment during these events. In water depths less than 10 m, the influence of wave motion becomes important. Orbital wave motion combines with strong coastal currents during northwesterly storms to cause massive resuspensions of inner shelf sediments. Net transport during these events is generally parallel to the coast.