Hydrography and sedimentation under the deep western boundary current on Björn and Gardar Drifts, Iceland Basin

Abstract

Abstract The interaction of Northern Component Waters overflowing the Iceland–Scotland Ridge with sediments further south in the Iceland Basin has been responsible for highly variable sedimentation patterns since at least the late Miocene (∼5 million years ago), and has formed both Bjorn and Gardar Drifts. Presently this process is maintained by a flux of approximately 3 Sv (measured along the south Icelandic slope) of Iceland–Scotland Overflow Water (ISOW) with a mean regional flow direction to the SW, which affects sediment transport on Bjorn Drift below ∼1350 m depth and over most of the Gardar Drift. Coriolis deflection of ISOW flow causes it to move as a Deep Western Boundary Current (DWBC) in the Iceland Basin, which redistributes sediments under the influence of seabed topography. Here, hydrographic and sedimentological data are used to illustrate the effects and extent of the vigorous DWBC activity. The study presents a baseline against which inferred changes in the past can be assessed. North of ∼58°N bottom nepheloid layers with heavy suspended sediment loads are often encountered, particularly under the denser and faster-flowing core of ISOW on the eastern slope of Gardar Drift. In this area the presence of migrating, regular sedimentary waves is seen on 3.5 kHz profiles. This is consistent with both the near-bottom flow velocity probably in excess of ∼10 cm s −1 estimated by the geostrophic method and the presence of well-sorted foraminiferal sand on the seabed. South of ∼58°N dilute bottom nepheloid layers, associated with more tranquil conditions near the seabed (inferred from photographic and sedimentological evidence), show that currents on Gardar Drift are weak. Although most of the south Iceland Basin is mantled by both regular and irregular, non-migratory mudwaves, the absence of migrating structures (with the exception of current intensification associated with discrete basement highs) indicates that more sluggish currents characterise this region. Throughout the region highly variable mean Holocene sediment accumulation rates are inferred (2–40 cm ka −1 ) as a result of the DWBC interacting with the topography.