Mn, Ni, Cu, Zn and Cd in the Western North Atlantic

Abstract

The concentrations of Mn, Ni, Cu, Zn and Cd have been determined on surface and deep water samples from the western North Atlantic. The results from a single vertical profile are compared to published results from the North Pacific and interpreted with respect to the hydrographie characteristics of both oceans. Cd, Zn and Ni have nutrient-type distributions in both oceans. They are depleted in surface waters, increase rapidly across the thermocline, then increase or decrease only slightly with depth. The North Atlantic deep waters at depths of 1 to 3 km have average concentrations of Cd, Zn and Ni equal to 0.29, 1.5 and 5.7 nmol kg⁻¹, respectively; values substantially lower than their corresponding values in the North Pacific at similar depths of 0.94, 8.2 and 10.4 nmol kg⁻¹. Cu concentrations increase gradually with depth in both oceans, with a North Atlantic deep water (1 to 3 km) average value of 1.7 nmol kg⁻¹ relative to 2.7 nmol kg⁻¹ at similar depths in the North Pacific. Mn concentrations decrease with depth through the thermocline with deep North Atlantic values on the order of 0.6 nmol kg⁻¹.

The Atlantic surface samples comprise a transect from the shelf waters off New England to the Sargasso Sea southeast of Bermuda. Metal concentrations are higher in the continental shelf and slope waters with the Gulf Stream separating these from the lower oceanic values. The shelf water vs. open ocean concentrations are: Mn, 21 vs. 2.4 nmol kg⁻¹; Ni, 5.9 vs. 2.3 nmol kg⁻¹; Cu, 4.0 vs. 1.2 nmol kg⁻¹; Zn, 2.4 vs 0.06 nmol kg⁻¹; and Cd, 200 vs. 2 pmol kg⁻¹. The high shelf water values appear to result from an external, presumably continental source. By comparison, increased levels of Ni, Cu, Zn and Cd in the coastal waters off central California have been interpreted to result primarily from the upwelling of nutrient rich waters. Mn and Cu concentrations are significantly higher in the Sargasso Sea than in surface waters of the North Pacific central gyre: 2.4 compared to 1.0 nmol kg⁻¹ for Mn and 1.2 vs. 0.5 nmol kg⁻¹ for Cu. Nickel concentrations are not significantly different: 2.1 to 2.4 nmol kg⁻¹ in both oceans. Zn concentrations are depleted to approximately 0.06 nmol kg⁻¹in both oceans, while Cd is depleted in both regions to values close to 2 pmol kg⁻¹.