The distribution of colloidal aluminum and organic carbon in coastal and open ocean waters off Nova Scotia

Abstract

Cross-flow filtration has been used to study the size distribution of aluminum and organic carbon in an operationally defined colloidal size range (10,000 MW-0.45 μm; ~1–10 nm-0.45 μm) in coastal and open ocean waters off Nova Scotia. Colloidal aluminum was consistently <5% of the “dissolved” (0.45 μm filtered) fraction by direct measure and 15% of “dissolved” Al by difference (0.45 μm filtered minus 1–10 nm filtered). Concentrations of Al measured in the colloidal fraction were high in the surface waters of the shelf, 0.44–0.55 nM (3–4% of “dissolved”), and decreased to levels of 0.09–0.15 nM (~ 1% of “dissolved”) in deeper waters. The open ocean vertical distribution of colloidal Al was similar to that on the shelf, with elevated concentrations in surface waters, 0.16–0.19 nM (~ 1% of “dissolved”), decreasing to 0.01–0.08 nM (<0.5% of “dissolved”) in deep waters. Colloidal organic carbon was ~ <10–15% of “dissolved” organic carbon; however, we recognize that our organic carbon measurements may be underestimated by our analytical method which employs UV-photooxidation. Laboratory experiments, using 234Th tracer and several surface water samples, showed an increasing percentage of colloidal thorium from open ocean to nearshore waters; 234Th tracer was always slightly greater than Al in the colloidal size fraction. The relationship between the apparent distribution coefficient (K′d) for metals and particle concentration (Cp) for oceanic waters has been examined. Model predictions show that, for most of the global ocean, K′d will be independent of Cp. This result is in agreement with oceanic and laboratory K'd values determined for aluminum.