Evidence for a sedimentary fingerprint of an asymmetric flow field surrounding a short seamount

Physical oceanographic modeling and field studies have shown that kilometer-scale seafloor elevations of comparable breadth and width (abyssal hills, knolls, seamounts) are surrounded by complex flow fields. Asymmetric flow fields, reversed flow and closed streamlines around the topographic feature (Taylor caps), and resonantly amplified tidal currents around the seamount rim potentially control near-bottom particle dynamics, particle deposition at the seafloor and, consequently, the formation of the sedimentary record. We combine numerical modeling and field data to study how such topographically controlled flow-field features are reflected in the sedimentary record. Sediment deposition on a topographically isolated abyssal knoll (height: 900 m) on the Porcupine Abyssal Plain in the Northeast Atlantic (water depth above the abyssal plain: 4850 m) was studied, (1) by comparing the spatial distribution of Pb-210 fluxes, calculated from inventories of sedimentary excess Pb-210, with Pb-210 input from the water column as recorded by sediment traps; and (2) by comparing sedimentary grain-size distributions and Zr/Al ratios (an indicator for contents of the heavy mineral zircon) at slope, summit and far-field sites. Given Rossby numbers greater than or equal to 0.23, a fractional seamount height of similar to 0.2, and the absence of diurnal tides it is concluded that an asymmetric flow field without Taylor cap and without amplified tidal currents around the seamount rim is the principal flow-field feature at this knoll. The results and conclusions are as follows: (1) Geochemical and grain-size patterns in the sedimentary record largely agree with the predicted pattern of flow intensity around the topographic elevation: with increasing current strength (erosiveness) there is evidence for a growing discrepancy between water column-derived and sediment-derived Pb-210-fluxes, and for increasing contents of larger and heavier particles. The topographically controlled flow field distorts a homogeneous particle-flux input signal from the ocean interior and results in kilometer-scale differences of the amount and composition of the deposited material. (2) The fact that, at the summit, the sediment-derived Pb-210 flux is lower than the water-column-derived Pb-210 flux indicates that the passing water is partly advected around and partly advected over the knoll. (3) The orientation of the sedimentary pattern indicates that at least during the past 100 years ( similar to 5 Pb-210 half lives) northward currents prevailed within the lowest similar to 1000 in of the water column on the Porcupine Abyssal Plain. The fact that the modelled spatial current-velocity distribution shows a better match with sedimentary velocity (erosiveness) proxies at higher than at lower inflow velocities suggests that mean far-field current velocities might have been higher in at least the past 100 years as compared to today. More comprehensive studies of this kind could provide information on paleo-changes of the orientation and current velocity of flow fields in the deep ocean. (C) 2004 Elsevier B.V. All rights reserved.