Fine-grained sediment lofting from meltwater-generated turbidity currents during Heinrich events

Abstract

Turbidity currents generated from sediment-carrying freshwater discharges into the sea contain a fluid that is less dense than ambient seawater. From experiments it is known that such currents will eventually lift from their substrate either in part or as a whole through buoyancy reversal. This ascent will happen when their density is lowered below that of seawater through settling of suspended sediment from the top or deposition from the bottom of the flows. Evidence for large-scale lofting of suspended sediment from the top of giant sand-and gravel-carrying turbidity currents in the Labrador Sea comes from two independent lines of observations: (1) The first is a distinct sedimentary facies consisting of stacked, centimeter-thick graded mud layers that contain grains of ice-rafted debris (IRD) supported by the mud. Deposition of these unusual layers requires a gradedlayer-forming process that is slow enough to allow the incorporation of IRD; this is not possible with normal mud-carrying turbidity currents. (2) The second observation is the presence of a huge abyssal sand and gravel plain in the central Labrador Sea that received its sediment from bed-load-rich meltwater discharges from the Hudson Strait outlet of the Pleistocene Laurentide Ice Sheet. These discharges turned into turbidity currents that released rising columns of freshwater that carried fine-grained suspended sediment and spread out at a water level where their density equaled that of ambient seawater. Deposition from these slow turbid interflows would allow the incorporation of IRD in the accumulating graded mud deposits. The IRD-spiked graded mud facies is restricted to Heinrich layers within 300 km radius of the Hudson Strait ice-stream terminus, tying the sand-carrying turbidity currents via fine-grained sediment lofting to Heinrich events. Estimated total discharge volumes of individual currents are on the order of 10 3 km 3 , supporting the notion that Heinrich ice-rafting events were times of maximum meltwater generation.

A meltwater origin for Antarctic shelf bedforms with special attention to megalineations

“…However, these deposits were considered equivalent to deposits on the Laurentian Fan (Anderson et al, 1986) where much of the sedimentation was from subglacial meltwater sources . These beds on the Weddell Fan approach the scale of outburst-generated turbidity currents in the northwest Atlantic (Hesse et al, 2001;Shaw and Lesemann, 2003;Hesse et al, 2004;Piper et al, 2007). Dowdeswell et al (2006) expressed puzzlement on the source of the meltwater that eroded the gullies.…”

“…Hesse et al (2001) made the surprising discovery of a 700-km-long braid plain on the floor of the Labrador Sea. It was constructed by powerful, hyperpycnal flows generated by meltwater outburst floods (Hesse et al, 2004;Hesse and Khodabakhsh, 2006). Curved megalineations, some in excess of 20 km long, are found on the braid plain (Hesse et al, 2001;Shaw and Lesemann, 2003).…”

Laminated sediments preceding Heinrich event 1 in the Northern North Sea and Southern Norwegian Sea: Origin, processes and regional linkage

“…The lack of any sorting of the fines indicates clearly that the material is hemipelagic in origin and not the product of bottom currents or turbidites. The mean grain size of the laminations is also significantly lower then the grain size of laminations generated by what is thought to be suspended material lofted off meltwater generated turbidites (Hesse et al, 2004).…”

“…In the Labrador Sea, laminations were considered to be the result of deposition through nepheloid layers and the presence of dropstones incorporated in these laminated sediments was used as a criterion for hemipelagic settling (Rashid et al, 2003). The same sediments in the Labrador Sea are now thought to be the product of lofting of meltwater generated turbidites (Hesse et al, 2004), which could explain the irregular and graded laminated records and the channels in the Labrador Sea area.…”

The role of the Iceland Ice Sheet in the North Atlantic during the late Quaternary: a review and evidence from Denmark Strait

“…2) would be associated with meltwater plumes, which would carry fine-grained sediments away from the slope (Hesse and Khodabakhsh, 1998;Hesse et al, 2004). Sediment-rich basal meltwaters that exceed the density of seawater would flow downslope as turbidites, often for hundreds to thousands of kilometres (Chough and Hesse, 1976;Hesse et al, 1987).…”