Spatial and temporal changes in bottom-water velocity and direction from analysis of particle size and alignment in deep-sea sediment

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We have demonstrated that the size and efficiency-of-alignment of particles in deep-sea sediment can be used to delineate relative velocity and direction of high-velocity bottom water. Our methods have been applied to core-top samples on the east flank of the Vema Channel which is the principal passage for northward flowing Antarctic Bottom Water (AABW) through the Rio Grande Rise in the southwest Atlantic. The size and alignment analyses delineated a zone of high-velocity bottom water below 4150 m which corresponds to the location of AABW as derived from physical oceanographic data. The eastern edge of the AABW was identified as a zone of high sedimentation rate due to the preferential deposition of fine-grained material marginal to the high-velocity flow. Three cores in that zone were used to examine the temporal changes in inferred bottom-water direction and relative velocity using a detailed stratigraphy based on magnetostratigraphy, biostratigraphy, fluctuations in percentage carbonate and oxygen isotopes. The cores were reoriented to the present geographic coordinate system using the stable remanent magnetism. The long-axis alignment of magnetic grains was used to infer bottom-water flow direction. The direction of flow was shown to be consistently northward due to the bathymetric control on the flow and corresponds to the direction determined by a current-meter near the site. The relative velocity of the AABW has fluctuated within the channel but no simple correlation was found with the paleoclimate. The size and alignment analyses were performed on core-top samples from the Amirante Passage in the western Indian Ocean. A zone of high-velocity bottom water was inferred below 4100 m and may correspond to a deep westward boundary current (DWBC) determined from physical oceanographic data. Regions of large sediment waves in the passage correspond to zones of low-velocity flow as inferred by our techniques. The sediment waves may have formed in a shear zone between the high-velocity DWBC and the lower-velocity overlying water mass. Our methods may be routinely applied to the study of deep-sea sediment. The initial results suggest that our geologic parameters correspond to the present benthic physical oceanography. Analysis of older sediments reveals that relative velocity of bottom currents has changed appreciably while the inferred current direction has remained stable due to the bathymetric control at the sites chosen for study. Deep-sea sediment, therefore, may act as a long-term fossil bottom-current meter. © 1980, All rights reserved.

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Marine Geology

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