Kristin Orians

Chemical Oceanography: Trace metals in seawater

The study of biogeochemical cycles often requires using inverse methods. Distributions of various elements, their isotopes, and their chemical speciation in the natural environment are investigated, providing clues as to the mechanisms that produce these patterns. The more chemical probes which are available in the study of the environment, the better constrained our understanding of the biogeochemical cycles becomes. The rapidly increasing pressure on the Earth`s ecosystem by the actions of humans makes an understanding of the feedback mechanisms and controls on this system critical.

Trace elements are valuable as tracers of many processes in the marine environment, and may be directly involved in biological processes, either as limiting nutrients or potential toxins. We are looking at the distributions of various "particle-reactive" constituents in the oceans. These elements, such as Al, Ga, In, Ti, Zr, Hf, Mn and Fe, are removed rapidly from seawater to the underlying sediments via chemical "scavenging" onto sinking particles. Since they are not in the oceans long enough to become thoroughly mixed, these minor components of seawater have concentrations which vary tremendously with time and space. This is in marked contrast to the major ions, which make seawater "salty", and remain nearly constant from place-to-place in the oceans. The large dynamic range makes these elements particularly useful as tracers of oceanic processes - inputs, internal cycles and removal processes. We are currently looking at the distribution of various trace metals in the northeast Pacific and the western Arctic. This study aims to evaluate 1) the sources of metals to the ocean from rivers, shelf and slope sediments, and from ice melt, and 2) the removal of metals from the surface waters by particle scavenging and phytoplankton blooms.

A second area of interest is the interaction of trace metals with organic ligands naturally present in the environment. This is important as the bioavailability and/or toxicity of a metal is not a function of its total concentration, but of the concentration of certain forms (or chemical species) of the metal. Research in this area involves developing methods for looking at various fractions of the metal, which are present in picomolar concentrations - either by looking directly at the complexes or, more commonly, by looking at the availability (lability) of the metal of interest, which is operationally defined by the method being used. We are using electrochemical methods to look at metal lability, and a combination of chromatography methods and mass spectrometry to look at metal organic complexes in the marine environment and in metal-stressed phytoplankton cultures.