SEDIMENTATION, RESUSPENSION AND CHEMISTRY OF PARTICLES IN THE NORTHWEST ATLANTIC
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Sediment traps were deployed to sample vertical fluxes of particulate matter both immediately above and within the nepheloid layer on the continental slope and rise of the western North Atlantic. The data from the traps are used to estimate the settling flux of particles and to attempt to distinguish between the primary flux (settling material from the upper water column) and the resuspended flux (settling material that has been resuspended from some region of the sea floor). From these data and from measurements of the net nepheloid standing crop of particles, estimates of the mean residence time for particles resuspended in the nepheloid layer are on the order of weeks to months from 15 to 500 m above bottom. Horizontal advection and diffusion are more important than vertical mixing in maintaining the upper portion of the nepheloid layer. The estimated flux of Al to the North Atlantic as atmospheric dust is one to two orders of magnitude less than the primary flux of Al measured just above the nepheloid layer, yet the range of accumulation rates of Al measured on the sea floor with radioisotopes is close to the values we define as the primary flux. Horizontal transport of Al-rich particles (mostly aluminosilicates) must therefore be significant even above the nepheloid layer. The size of 80-90% of the particles collected in the traps both above and within the nepheloid layer was less than 63 m. The mean size of particles trapped in the nepheloid layer was about 22 m, and above the nepheloid layer, 11 m. The particles were probably in large aggregates when they encountered the traps, but identifiable fecal pellets are only a small fraction of the material collected in the traps. Between 6 and 15% of the primary production of particulate organic carbon in the euphotic zone was collected in floating sediment traps at the 100 m depth, indicating that most particulate organic carbon is recycled in the surface water. Only 2-3% of primary production of organic carbon was collected as primary flux 500 m above the sea floor. From an intercomparison of the composition of particles falling rapidly (collected in traps), falling slowly or not at all (collected in water bottles), and resting on the sea floor (from core tops), it was found that elements associated with biogenic matter, such as Ca, Sr, Cu and I, were carried preferentially by the rapidly falling particles. Once these particles reached the sea floor the concentration of those elements was decreased through decomposition, respiration, or dissolution. Dissolution appears to be rapid near or on the sea floor, because radiolarians, diatoms, and juvenile foraminifera were rare in core samples even though abundant in all traps. The dynamic nature of the nepheloid layer makes it possible for particles to be resuspended many times before they are finally buried, enabling sediment to be carried long distances from its origin. The recycling of particles near the sea floor may also increase dissolution of siliceous and carbonate matter. 1985.