Controls of Trace Metals in Seawater

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 necessary due to low concentrations of trace metals in seawater relative to the ubiquitousness of metals on a ship or in a laboratory (e.g., dust, steel hydrowire, rust, paint with copper and zinc antifouling agents, brass fittings, galvanized material, sacrificial zinc anodes, etc.). As a result, seawater concentrations of most trace metals have now been accurately determined in at least some parts of the oceans, and their oceanic distributions have been found to be consistent with oceanographic processes.The concentrations and distributions of trace metals in seawater are controlled by a combination of processes. These processes include external sources of trace metals delivered by rivers along ocean boundaries, by wind-blown dust from arid and semi-arid regions of the continents, and by hydrothermal circulation at mid-ocean ridges. Processes removing trace metals from seawater include active biological uptake or passive scavenging onto either living or nonliving particulate material. Much of this particulate material (along with its associated trace metals) is internally recycled either in the water column or in surficial sediments. The ultimate sink of trace metals is generally marine sediments. These various sources and sinks are superimposed on the general circulation and mixing of the oceans, resulting in the characteristic distributions of each trace metal. One of the first examples of the emergence of oceanographically consistent vertical profiles was for the trace-metal cadmium (Boyle et al., 1976; Martin et al., 1976; Bruland et al., 1978a). These studies demonstrated that the distribution of dissolved cadmium in the sea follows a pattern similar to that of the nutrients phosphate and nitrate. Sparked by these surprising results, several investigators during the following two decades were able to obtain excellent data sets on a wide variety of trace metals. This chapter will attempt to provide a basic overview of what is known about the controls of the concentrations and distributions of trace metals in the open ocean. Subtleties in their distributions will not be presented. The distributions of trace metals in coastal regions are more dynamic and complicated and will not be discussed in this chapter.The bulk of the data for vertical profiles of trace metals in seawater are from papers published in the 1980s and 1990s and most of the profiles are from either the North Pacific or North Atlantic. There is a paucity of vertical profiles from the South Atlanticand South Pacific. It has recently been argued that a new "GEOSECS"-type trace-metal program needs to be in place in order to provide appropriate global coverage of trace metals. Much of the impetus for such a program comes from the recognition of iron as an important micronutrient influencing global biogeochemical cycles in the oceans (Moore et al., 2002) and the potential role of other trace metals such as zinc. In particular, there is a pressing need for an expansion of the global database of dissolved iron distributions in the oceans. These measurements are needed to both initiate and verify models and to identify processes not contained in existing models.There have been a number of reviews of trace elements in seawater that form a foundation for this chapter. Among them are: Bruland (1983) on oceanographically consistent data sets; Burton and Statham (1990) on trace metals in seawater; and Donat and Bruland (1995) on trace elements in oceans. There are two reviews that deal with more of the biological role of trace metals: Bruland et al. (1991) on interactive influence of bioactive trace metals on biological production in ocean waters; and Hunter et al. (1997) on biological roles of trace metals in natural waters. A highly complementary chapter in this Treatise that deals with the influence of essential trace metals on biological processes has been written by Morel et al. (Chapter 6.05). Turning to "on-line" sources of information, Nozaki has done an excellent job perusing the available literature and compiling vertical profiles from the North Pacific for each element in a periodic table that makes an excellent figure (http://www.agu.org/eos_elec/97025e.html). Ken Johnson, a marine chemist at the Monterey Bay Aquarium Research Institute (MBARI), has a web site with a periodic table of the elements containing a brief review of information on each element (http://www.mbari.org/chemsensor/pteo.htm).