Atlantic surface particulates: composition, settling rates and dissolution in the deep sea

Abstract

The concentrations of Al, Fe, Mn, Ni, Cu and particulate organic carbon (estimated from weight loss on ignition) in suspended phases collected from several tons of surface Atlantic waters are reported. The salient features of the results are: (1) Organic matter is the major component of surface particulates. It constitutes ~50% by weight of the total suspended matter. (2) The mean concentration of Al in the particulates is low, ~0.2% by weight (range 0.03-0.9), which suggests that the average abundance of crustal materials in surface particles is <5%. (3) The residence time of aluminosilicate particles in seawater is estimated to be about 4 years, 1-2 orders of magnitude less than that expected from Stokes' settling velocity for 2-μm particles, typical of pelagic clays. It appears from these data that the aluminosilicates are transported to sediment as large particles probably through fecal matter. (4) Significant correlation between particulate organic carbon and the concentrations of Fe, Mn and Ni has been observed. Multivariant analysis indicates that on average about 30% Fe and 50% Mn are associated with particulate organic carbon. However, the direct removal of Fe, Mn and Ni from seawater to sediments by organic matter is only a small part of their total precipitation rate on the ocean floor. (5) The average Cu/Al and Ni/Al ratios in the surface particulates are at least an order of magnitude higher than those in Atlantic pelagic clays. This suggests that Cu and Ni are fractionated from Al prior to the deposition of surface-water particles on the ocean floor.