stable isotope ratios act as naturally-occurring tracers for organic matter, making possible, under certain conditions, the quantification of coastal-offshore exchanges. In general, organic matter has isotope ratios characteristic of its origin (e. g. plants with different modes of photosynthesis and different growth conditions, anthropogenic compounds). These ratios are maintained as the organic matter moves through the biosphere and geosphere. A mixture of organic matter from two sources has isotope ratios intermediate between those of the two sources, in proportion to the fraction of...