Zusammenfassung
Interest in global ocean circulation has increased since coupled ocean-atmosphere models have suggested that the current mode of circulation is fragile. Natural changes have occurred in the past, for example, in conjunction with glaciations, but the models suggest that ocean circulation may also be heavily perturbed by anthropogenic greenhouse emissions, with dramatic consequences for global climate. However, to understand how ocean circulation and climate may change in the future, we must first understand how the system operated in the past, without man's influence. New isotopic tracers are now helping to elucidate past ocean circulation patterns, but controversies remain regarding the relative influence of ocean circulation and weathering on the isotopic signals. Analyses of the salinity and temperature of water masses are used by oceanographers to reconstruct the present-day ocean circulation. For example, the salt content of Atlantic seawater differs depending on where the water originated (see the figure below). To determine the distribution of former water masses, paleoceanographers rely on isotopic measurements of marine sediments, which reflect the distinct isotope composition of the water masses from which they formed. For the past 20 years, the stable isotope ratios of carbon have been used as a tracer for labeling present-day water masses (1). They are also preserved in the shells of marine organisms, but the carbon isotope ratios in planktonic foraminifera, for example, are modified from those of the water masses in which they live by temperature (1), the availability of nutrients (1), and carbonate (2). Similarly, Cd/Ca elemental ratios in foraminifera are prone to thermodynamic effects (3). In addition, information from 14C is limited by its short half-life of 5700 years (1). Therefore, there is demand for a supplementary set of tracers.
