Composition and characteristics of particles in the ocean : evidence for present day resuspension

Abstract

This study of particulate matter in the water column and the underlying surface sediments verifies the occurrence of local, present-day resuspension in the deep sea. The location of the major portion of this work was the South Iceland Rise, a region influenced by the flow of Norwegian Sea Overflow Water. Measured current velocities exceeded 20 cm/sec in the axis of the bottom current for the duration of the deployments, approximately two weeks. Particulate matter was sampled with Niskin bottles, to obtain the standing crop of suspended matter and with sediment traps, to obtain the material in flux through the water column. Box cores were taken to obtain surface sediment samples for comparison with the trap samples. Suspended particulate matter (SPM) and light-scattering studies demonstrate that in the Iceland Rise area the correlation of the L-DGO nephelometer to concentration of SPM differs between clear water and the nepheloid layer. Correlations of light scattering to SPM concentration also differ regionally, but for predicting concentration from light scattering, regression lines at two locations are indistinguishable. Particle size distributions have lower variance in the nepheloid layer than those in clear water which have roughly equal volumes of material in logarithmically increasing size grades from 1-20 μm. Apparent density differences between SPM in clear water and the nepheloid layer are not distinguishable in the Iceland Rise study; apparent densities increase in the nepheloid layer in the western North Atlantic. An apparent density of 1.1 g/cm3 adequately separates clear water from nepheloid layer samples in this region. Compositional variations seen between clear water and the nepheloid layer include a decrease in small coccoliths and an increase in clays and mineral matter. These compositional variations are more dramatic in the western North Atlantic region, due to dissolution of carbonate at the seafloor, later resuspended into the nepheloid layer. Sedimentological evidence of resuspension and redistribution of material are: 1) presence of sediment drifts throughout the Iceland Basin; 2) occurrence of coarse, glacial age sediments beneath the axis of the bottom current; and 3) differences in mineralogy, carbonate and organic carbon contents between surface sediments beneath the bottom current and those in a channel. A comparison of the vertical flux of material measured by sediment traps at 500 meters above bottom (mab) with the accumulation rate in cores, shows that the present-day surface input is an order of magnitude smaller than the accumulation rate. This observation suggests transport of material into some sections of the region by bottom currents or by turbidity currents. The horizontal flux of particulate matter into and out of the region by the bottom current is 100 kg/sec. This material may contribute to the formation of Gardar sediment drift downstream. The trends in % CaC03 and % organic carbon through the water column and in the surface sediments suggest that dissolution of carbonate and decomposition and consumption of organic carbon occurs primarily at the seafloor. These data also suggest preferential preservation at channel stations and/or preferential erosion beneath the bottom current. A comparison of sediment-trap samples with box-core surface samples further supports present-day resuspension. Benthic foraminifera, iron-oxidicoated planktonic foraminifera and the glacial, subpolar planktonic foraminifera (Neogloboquadrina pachyderma (sinistral)) in traps at 10,100 and a few specimens at 500 mab, provide conclusive evidence for local resuspension. The coarse fraction (>125 μm) of the sediment trap material collected at 10 mab comprises 21-34% of the samples Calculations indicate that this material is locally derived (few kilometers) resuspended material.