A combination of the international GEOTRACES project and developments in multi collector inductively coupled plasma mass spectrometry technology have made the study of the marine environment using isotope geochemistry more widespread, and encouraged the investigation of increasingly ambitious targets. This thesis investigates applications of two isotope systems: cadmium (Cd) and lead (Pb).

Most studies of the Cd isotope composition (IC) of seawater have concluded that significant biological isotope fractionation takes place in the surface ocean. An exception to this is Yang et al. (Yang S.-C., Lee D.-C. and Ho T.-Y., 2012. The isotopic composition of cadmium in the water column of the South China Sea. Geochim. Cosmochim. Acta 98, 66-77) (2012), who suggest this is not the case in the South China Sea. Using a box model, it is shown here that biological Cd isotope fractionation most likely does take place.

Organic compounds released from resins that are used for trace element separations can have a detrimental impact on the quality of isotopic analyses, and this seems particularly problematic for Cd. Results of tests suggest this may be due to anomalous mass bias behaviour, and carrying out a liquid-liquid extraction using heptane removes the organic resin residue and improves data quality. This technique is expected to be applicable to many other resins and isotope systems.

Ferromanganese crusts have been widely used in palaeoceanographic studies. Here, Cd IC records for three crusts are presented. Most samples show ICs that are similar to modern deep water, suggesting diffusion may have smoothed away any signal. A new approach to modelling diffusion in crusts implies isotopic signals are likely to diffuse faster than previously suggested.

Compared to the North Atlantic, the Pb concentration and IC of surface South Atlantic seawater is poorly understood. Here, data for the western South Atlantic (GEOTRACES GA02) shows a rather homogenous Pb IC. Lead concentrations have decreased since the 1990s and there may be some movement towards the natural Pb IC of Patagonian mineral dust, the likely principal natural source of Pb to this region.