This chapter presents a general overview of the major controls of trace metals in seawater, developed from the extensive research on trace metals over the last few decades. The reader should be given a first-order understanding and insight into trace metal biogeochemistry in the oceans, rather than presented with a comprehensive review of the distribution of each trace metal. Each of the trace metals discussed will undoubtedly prove to have unique characteristics and subtle differences from this version, yet the comparison with these characteristics will serve as a good springboard to a more complete understanding.

Controls of Trace Metals in Seawater

The geochemical balance of the rare earth elements and neodymium isotopes in the oceans

Since the early 1970s, marine chemists have gained a first-order understanding of the concentrations, distributions, and chemical behaviors of trace metals in seawater. Important factors initiating this quantum leap in knowledge were major advances in modern analytical chemistry and instrumentation, along with the development and adoption of clean techniques. An instrumental development in the mid-1970s that spurred the early research on trace metals was the availability of the sensitive graphite furnace as the sample introduction system to an atomic absorption spectrometer. More recently, the appearance of inductively coupled plasma (ICP) mass spectrometers has provided an even more sensitive and powerful instrumental capability to the arsenal of marine chemists. In addition to these instruments back in shore-based laboratories, there has been the development of sensitive shipboard methods such as stripping voltammetry and flow injection analysis (FIA) systems with either chemiluminescence or catalytically enhanced spectrophotometric detection. Along with the development of these highly sensitive analytical techniques came a recognition and appreciation of the importance of handling contamination issues by using clean techniques during all phases of sampling and analysis. This is

6.02 – Controls of Trace Metals in Seawater

https://pure.rug.nl/ws/files/3286277/1994MarChemdeBaar.pdf

Cadmium versus phosphate in the world ocean

Iron and manganese in the wake of the Kerguelen Islands (Southern Ocean)

Dissolved trace metal concentrations are reported for a section in the southwestern Indian Ocean, extending from 7 to 27°S around the 56°E meridian. Overall distributions conform to those found in other oceans, with Cd, Cu, Ni and Zn showing recycled, or nutrient-like, behaviour, whereas Mn is enriched in the mixed layer relative to deep water. Deep-water concentrations of the recycled elements are intermediate between those for the North Atlantic and North Pacific Oceans, as would be expected from known patterns of deep-ocean circulation. For Cd and Zn, depletion in the surface layer approaches detection limits while for Cu and Ni significant near-surface concentrations (0.9 and 2.1 nM, respectively) are found. Relationships between the concentrations of metals and nutrients are similar to those described in the literature for the recycled metals for other regions, although significant variations exist from ocean to ocean. Maximum manganese concentrations in the upper water column are about or below 1 nM, as expected for a region remote from continental sources of airborne particulaet material. Minor variations in maximum concentration with latitude are evident as a result of the complex geochemistry of this element. Deep water manganese concentrations are uniformly low, showing no evidence for benthic or hydrothermal sources.

P.J. Statham

Papers

Dissolved trace metals in the southwestern Indian Ocean