Antarctic bottom water fluctuations in the Vema Channel: Effects of velocity changes on particle alignment and size

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The anisotropy of magnetic susceptibility (AMS) and mean particle size in the silt fraction have been measured in surface sediments and down selected cores from the Vema Channel. Results show that a function (Fs) which represents magnetic grain long axis alignment is highly correlated with variation in mean size of the silt fraction. An increase in alignment is caused by an increase in bottom water velocity which corresponds to an increase in mean particle size. Best alignments and coarsest particle sizes are found in the axis of the Vema Channel. In contrast, poor particle alignment and fine silt mean characteristics the shallower region east of the channel axis, consistent with lower current velocities. Core CH 115-61 was collected near the Level of Least Motion at the transition between northward-flowing Antarctic Bottom Water (AABW) and southward-flowing North Atlantic Deep Water. A positive relationship exists between silt size and AMS in the core. Fluctuations in particle alignment and mean silt size indicate three or four periods of increased velocity during the past 150,000 years. Geographic orientation of the maximum AMS axes in cores CH 115-60 and -61 was accomplished using the declination of remanent magnetism and an axial dipole as the reference azimuth. The data show that the larger magnetic grains in these sediments are aligned normal to the bathymetric trend of the channel which suggests that these grains are aligned by traction transport. The result indicates that AABW in the Vema Channel has flowed along an azimuth of approximately N30°E for the past 150,000 years. © 1977.

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Earth and Planetary Science Letters

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