Turnewitsch¹, Robert, Niko Lahajnar², John Thomson¹ and Ian W. Croudace¹

¹Southampton Oceanography Center, European Way, Southampton SO14 3ZH, UK, Tel: ++44 (0) 2380 59 4683, Fax: ++44 (0) 2380 59 6554, E-mail: rxt@soc.soton.ac.uk and ²University of Hamburg – Institute of Biogeochemistry and Marine Chemistry, Grabenstrasse 27, D-20357 Hamburg, Germany

Are asymmetric flow fields around kilometer-scale topographic seafloor elevations reflected in the sediment?

Abyssal hills, knolls and seamounts are ubiquitous and abundant features of the oceanic seafloor. Physical oceanographic studies have indicated that there are asymmetric flow fields enfolding such seafloor elevations: On the northern hemisphere the lateral asymmetry in the flow exhibits an accelerated flow to the left of the elevation, looking downstream, and decelerated flow to the right. We expect this asymmetry to be reflected in the intensity and quality of sediment deposition.

To address the proposed effect on intensity of sediment deposition we assessed the spatial distribution of sedimentary excess ²¹⁰Pb (²¹⁰Pb_xs) inventories and compared these inventories to the expected ²¹⁰Pb input from the water column.

On the summit and on the western slope ²¹⁰Pb_xs inventories are smaller than expected. There is also some evidence for ²¹⁰Pb_xs inventories on the northern and eastern slopes to be somewhat higher than expected. This spatial ²¹⁰Pb_xs pattern coincides with the predicted pattern of flow intensity around the topographic elevation (high on the summit and in the west, lower in the northeast).

Hydrodynamically controlled differential particle deposition across the near-bottom water column is suggested to communicate the hydrodynamic asymmetry of the water column to the underlying sediment as reflected in different grain size distribution, contents of major and minor elements, and prevailing particle types. Given the ubiquity of kilometer-scale topographic seafloor elevations, the possible importance of the described asymmetries for general pattern formation in the deep ocean is stressed.