The behaviour of dissolved Cd, Co, Zn, and Pb in North Atlantic near-surface waters (30°N/60°W–60°N/2°W)

Abstract

In September 1993 (M26) and June/July 1996 (M36), a total of 239 surface samples (7 m depth) were collected on two transects across the open Atlantic Ocean (224 samples) and northwest European shelf edge area. We present an overview of the horizontal variability of dissolved Cd, Co, Zn, and Pb in between the northwest and northeast Atlantic Ocean in relation to salinity and the nutrients. Our data show a preferential incorporation of Cd relative to P in the particulate material of the surface ocean when related to previously published parallel measurements on suspended particulate matter from the same cruise. There is a good agreement with results recently estimated from a model by Elderfield and Rickaby (Nature 405 (2000) 305), who predict for the North Atlantic Ocean a best fit for α_Cd/P=[Cd/P]POM/[Cd/P]_SW of 2.5, whereas the approach of our transect shows a α_Cd/P value of 2.6. The Co concentrations of our transects varied from <5 to 131 pmol kg⁻¹, with the lowest values in the subtropical gyre. There were pronounced elevations in the low-salinity ranges of the northwest Atlantic and towards the European shelf. The Co data are decoupled from the Mn distribution and support the hypothesis of marginal inputs as the dominant source. Zinc varied from a minimum of <0.07 nmol kg⁻¹ to a maximum of 1.2 and 4.8 nmol kg⁻¹ in regions influenced by Labrador shelf or European coastal waters, respectively. In subtropical and northeast Atlantic waters, the average Zn concentration was 0.16 nmol kg⁻¹. Zinc concentrations at nearly three quarters of the stations between 40°N and 60°N were <0.1 nmol kg⁻¹. This suggests that biological factors control Zn concentrations in large areas of the North Atlantic surface waters. The Pb data indicated that significant differences in concentration between the northwest and northeast Atlantic surface waters presently (1996) do not exist for this metal. The transects in 1993 and 1996 exhibited Pb concentrations in the northeast Atlantic surface waters of 30 to 40 pmol kg⁻¹, about a fifth to a quarter of the concentrations observed in 1981. This decline is supported by our particle flux measurements in deep waters of the same region.

Introduction

Most of the information about the behaviour of reactive trace elements in the oceans has been gained from single station profiles, which have allowed a classification into distinct distribution types (Bruland, 1983; Whitfield and Turner, 1987; Donat and Bruland, 1995). Of the trace metals discussed in this paper, Cd and Zn belong to the recycled elements that are biologically mediated and share similar vertical distributions with phosphate, nitrate or silicate. In contrast, Pb and Co (mostly) show higher values in surface waters indicating regulation by external sources. In addition, their concentrations in the deep layers generally decrease along the deep water flow from the North Atlantic to the North Pacific with continuing particle scavenging.

While we are gaining a familiarity with the typical distributions of trace metals, the processes that determine those distributions are still being elucidated. This is especially true for the surface water distributions, which can be affected by a number of physical and biogeochemical processes, including vertical mixing by upwelling or convection, atmospheric deposition, biogenic uptake, and continental input from rivers or shelf sediments. However, studies that improve our understanding of trace metal cycling in surface waters are still scarce. They should ideally focus on high resolution sampling in combination with determinations of nutrients and suspended particles in order to resolve the horizontal mesoscales and smaller scales. Long-range surface water transects for Cd, Cu, and Ni were first made by Boyle et al. (1981) in selected North Atlantic and North Pacific waters, followed by Measures et al. (1984) with measurements of Al, Be, Bi, and Se in the western North Atlantic and the Caribbean, and by Kremling (1985) with data on the distributions of Cd, Cu, Ni, and Mn in the open Atlantic and Brazilian and European shelf areas. More recently, Pohl et al. (1993) presented data from Cd, Cu, Pb, and Zn transects through the Arctic and eastern North Atlantic oceans, and Helmers and Rutgers van der Loeff (1993) and Rutgers van der Loeff et al. (1997) reported the results from continuous profiles of Al, Cd, Cu, and Pb across the Atlantic Ocean, from ∼50°N to 50°S.

Here we present Cd, Co, Zn, and Pb data from a long-distance transect covering the major oceanographic regimes between 30°N/60°W and 60°N/2°W (M36; Fig. 1). The principal goals of this survey were to determine the horizontal distributions and spatial variabilities of the dissolved metals between the northwest and northeast Atlantic Ocean, and to examine and quantify their relationship to the prevailing hydrography and biogeochemical processes. In addition, we use the particulate trace metal data from the same cruise (Kuss and Kremling, 1999a) to examine the partitioning of these elements and the nutritional requirements of bioactive metals (Cd, Co, Zn). Furthermore, we present the data from an earlier (1993) latitudinal profile (M26; ∼30–60°N; Fig. 1) on dissolved Cd, Co, and Pb to provide further insight into the temporal and spatial variabilities of these trace metals.