One-dimensional views of three-dimensional sediments

Recent measurements of trace metals in sediment pore waters at high spatial resolution have revealed significant horizontal and vertical heterogeneity on a submillimeter scale. These measurements are consistent with remobiliz ation occurring from a three-dimensional (3D) stochastic distribution of sm all "microniche sources". However, early diagenetic processes are conventio nally described in 1D terms. Application of 1D reaction-transport models to 3D systems will result in biased estimates of process rates. For the same intrinsic rates of supply and removal, maxima in concentration-depth profil es in 3D systems are likely to be lower, and concentration profile gradient s higher, than in 1D systems. The simple examples considered suggest that p rocess rate estimates may be in error by a factor of 5 when a 1D model is u sed. A simple 3D numerical model of trace metal remobilization in pore wate rs was used to demonstrate how the structure of high-resolution trace metal profiles can be reproduced using a stochastic distribution of microniche s ources. Heterogeneity depends on the scale considered and is more marked wh en measurements are made at high resolution. Heterogeneity is increased by slow transport, fast sinks, and widely separated sources. As the degree of heterogeneity between and within concentration-depth profiles increases, th e estimates of process rates obtained from 1D models become less accurate.