On the deposition of sediment within glacier-influenced fjords: Oceanographic controls

Abstract

Of the primary pathways for the dispersal of sediment in fjords, only ice-contact processes are unique to fjords under the influence of glaciers. The effects of ice-front melting are found to be overwhelmed by the influences of glaciofluvial discharge and iceberg calving. Ice-front melting is, however, more important for side-entry glaciers compared to fjord-head trunk glaciers. Submarine discharge is almost always in the form of vertically rising buoyant jets. Underflows in marine fjords are considered rare and no direct evidence exists to support their importance or even existence. The release rate of supraglacial sediment due to calving depends on the distribution of supraglacial material and the style of calving. Because iceberg calving and overturning can create solitary shallow-water waves (tsunamis), ice-rafted sediment may be deposited well above the high tide line. Ice-contact processes cause most sediment to be deposited very near the ice terminus. Thus for a stable ice front position, the rate of sediment accumulation decreases rapidly with distance from the glacier. For an unstable position, sediment accumulation depends on the rate of ice terminus retreat or advance. Away from the ice front, sediment deposition is controlled by circulation in the upper water column. The distance a particle is carried out into the fjord will depend on surface plume velocity and particle removal processes. Across-basin effects include the influence of the Coriolis and centrifugal forces acting on the surface plume. The influence of floating ice, i.e., sea ice, icebergs or floating ice shelves, on sediment dispersal within “arctic” glacier-influenced fjords is considered minor although very system dependent. Similarly, the range of deep-water currents in silled fjords is highly variable although most currents arise from the flushing of deep water. Deep-water renewals may import or export sediment from fjords, depending on the season of the flushing event — a process considered minor compared to other depositional processes. In the case of fjords without sills, tidal currents can be important at depth. Redepositional pathways are many and varied, all very system dependent. Much of the sediment deposited at the ice terminus is subsequently remobilized, through the action of iceberg calving, ice-push phenomena, or simply through slope instabilities associated with a rapidly and vertically accreting sediment mass.