Heterogeneity, uncertainty and process identification in early diagenesis : new model developments with applications to biological mixing

Abstract

Within the last decades, there have been spectacular developments in experimental and analytical techniques that allow geochemists and biologists to acquire ever more detailed data sets on aquatic sediments. These data sets often combine high-resolution chemical distributions with rate determinations, and information on resident biological communities and their activities. This wealth of data, in tum, creates a need for new diagnostic models that account for the complex interactions documented by field and experimental studies. Models of early diagenesis must therefore integrate knowledge from a wide variety of scientific fields, from transport theory and chemistry, to molecular biology and benthic ecology. Only by incorporating meaningful representations of the dominant processes, are these models able to scale reactive transport interactions from the local to the regional and, ultimately, global scale. This thesis focuses on the quantitative description of (1) biologically-induced transport processes, and (2) the coupling of reaction and transport processes. It presents three innovative approaches to quantify pore water transport other than molecular diffusion. Two of the approaches compute site-specific depth distributions of solute mixing intensities, but they differ fundamentally in the type of input data. One approach is based on chemical concentration and rate measurements, the other uses ecological data on the infaunal community. Despite their differences, both approaches yield comparable bioirrigation intensities. In the third approach, measured benthic oxygen uptake fluxes across a wide variety of oceanic environments are used to derive global relationships for enhanced solute transport rates in sediments. This last approach bridges the gap between site-specific studies of pore water irrigation and regional to global assessments of the role of benthic-pelagic coupling in ocean biogeochemistry. The estimates of enhanced solute transport intensities clearly demonstrate that bioirrigation has a major global impact on solute exchanges between the water column and sediments.