Ice-proximal sediment accumulation rates in a temperate glacial fjord, southeastern Alaska

Abstract

Sediment traps and echo sounder profiles collected between 1984 and 1987 were used to determine the rate and style of sediment infilling of a fjord basin with a temperate tidewater glacier at its head. McBride Glacier retreated rapidly (0.25 km/yr, 1984-1986; 0.05 km/yr, 1986-1987) enlarging a 90-to 100-m-deep basin adjacent to the terminus. Highly turbid meltwater is discharged subglacially at the base of the water column and rises to the surface forming turbid overflow plumes. Sedimentation rates from suspension settling decrease exponentially with distance from the subglacial stream; from 24 g/cm 2 /day at the terminus to less than 1 g/cm 2 /day, 1.3 km away.The total annual volume of sediment deposition within the ice-proximal basin of McBride Inlet was 3.6 x 10 6 m 3 between 1984 and 1985,3.0 x 10 6 m 3 between 1985 and 1986, and 1.7 x io 6 m 3 between 1986 and 1987. This sediment is contributed by suspension deposition from meltwater plumes, bedload transported by the subglacial stream, direct meltout of glacial debris, ice rafting, and side-entry sources. Rapid glacier flow and large amounts of meltwater and precipitation, a result of the temperate climate, enhance the production and delivery of sediment to the fjord.In McBride Inlet, meltwater discharge is the main ice-proximal sediment source. During 1984-1986, 67 percent of the total volume was deposited by suspension settling alone. Sediment infills the irregular bathymetry of the predepositional basin, producing a smooth, flat basin floor. Within this basin, sediment accumulation rates are as high as 13 m/yr, 300 m from the glacier.Major sediment sources are episodic, including sediment gravity flows, and daily and seasonal variations occur in meltwater discharge. Within temperate Ijords a thick sequence of glacial marine sediment rapidly accumulates, filling proximal basins with interlaminated sand/silt and mud.

Observations and modeling of fjord sedimentation during the 30 year retreat of Columbia Glacier, AK

“…Hallet and others, 1996); so for model simplicity we approximate that the volumes of sediment delivered by proximal and distal accumulation in a fjord of relatively uniform width are the same and set _ S o1 δ 1 ¼ _ S o2 δ 2 . Sedimentation fall-off distances most consistent with published in situ sediment-trap measurements (Cowan and Powell, 1991) are of the order of 10 2 and 10 4 m for δ 1 and δ 2 , hence we use these values.…”

“…The filling of about three quarters of the outer basin long after the glacier had retreated across this area, challenges the common assumption that sediment derived from a glacier only fills the most proximal basin (e.g. Cowan and Powell, 1991;Cowan and others, 2010), and suggests that sediment is transported efficiently throughout the fjord, including passing over significant sills (exceeding tens of meters in height). Fig.…”

“…Our range of sedimentation fall-off distances for δ 1 and δ 2 is constrained by field observations (Cowan and Powell, 1991), thus we vary these values on the order of 10 2 and 10 4 m, respectively. Figure 6 illustrates how the diffusivity and long fall-off distance affect the shape of the sediment deposits and internal stratigraphic architecture; they depend most strongly on the sediment diffusivity, κ.…”

“…Glacially produced sediment transported to the ice/ocean boundary affects the water depth, and thus glacier mass balance and stability, by forming shoals that buttress the glacier, reducing buoyancy at the ice front and decreasing the surface area available for submarine melting. In fjords adjacent to the termini of temperate glaciers, rates of sediment accumulation are among the highest recorded; they can exceed 10 m a −1 at the ice front and ∼1 m a −1 kilometers down the fjord (Cowan and Powell, 1991;Jaeger and Nittrouer, 1999). The importance of the interaction between glaciers and the sediment they erode has long been recognized, as sediment shoals enable tidewater glaciers to advance into deep water (e.g.…”

“…Our model is guided by field studies of sedimentation in fjords (e.g. Cowan and Powell, 1991), and it builds on the 1-D model of Koppes and Hallet (2002), in which the sediment accumulation rate, _ Sðx; tÞ, decreases exponentially with distance down fjord from the terminus, x:…”