Sediment Diagenesis and Benthic Flux
Abstract
Chemical reactions in marine sediments and the resulting fluxes across the sediment-water interface influence the global carbon cycle and the pH of the sea and affect the abundance of CaCO3 and opal-forming plankton in the ocean. On very long timescales these diagenetic reactions control carbon burial in sedimentary rocks and the oxygen content of the atmosphere. Sedimentary deposits that remain after diagenesis are the geochemical artifacts used for interpreting past changes in ocean circulation, biogeochemical cycles, and climate. This chapter is about the processes of diagenesis and burial of the chemical elements that make up the bulk of the particulate matter that reaches the seafloor (organic matter, CaCO3, SiO2, Fe, Mn, and aluminosilicates).Understanding of sediment diagenesis and benthic fluxes has evolved with advances in both experimental methods and modeling. Measurements of chemical concentrations in sediments, their associated pore waters and fluxes at the sediment-water interface have been used to identify the most important reactions. Because transport in pore waters is usually by molecular diffusion, this medium is conducive to interpretation by models of heterogeneous chemical equilibrium and kinetics. Large chemical changes and manageable transport mechanisms have led to elegant models of sediment diagenesis and great advances in understanding of diagenetic processes.We shall see, though, that the environment does not yield totally to simple models of chemical equilibrium and chemical kinetics, and laboratory determined constants often cannot explain the field observations. For example, organic matter degradation rate constants determined from modeling are so variable that there are essentially no constraints on these values from laboratory experiments. In addition, reaction rates of CaCO3 and opal dissolution determined from modeling pore waters usually cannot be reproduced in laboratory experiments of these reactions. The inability to mechanistically understand reaction kinetics calculated from diagenesis models is an important uncertainty in the field today.Processes believed to be most important in controlling the preservation of organic matter have evolved from a focus on the lability of the substrate to the protective mechanisms of mineral-organic matter interactions. The specific electron acceptor is not particularly important during very early diagenesis, but the importance of oxygen to the degradation of organic matter during later stages of diagenesis has been clarified by the study of diagenesis in turbidites deposited on the ocean floor during glacial periods.Evolution of thinking about the importance of reactions between seawater and detrital clay minerals has come full circle since the mid-1960s. "Reverse weathering" reactions were hypothesized in very early chemical equilibrium (Sillen, 1961) and mass-balance ( Mackenzie and Garrels, 1966) models of the oceans. Subsequent observations that marine clay minerals generally resemble those weathered from adjacent land and the discovery of hydrothermal circulation put these ideas on the back burner. Recent studies of silicate and aluminum diagenesis, however, have rekindled awareness of this process, and it is back in the minds of geochemists as a potentially important process for closing the marine mass balance of some elements.
- Publication:
-
Treatise on Geochemistry
- Pub Date:
- December 2003
- DOI:
- 10.1016/B0-08-043751-6/06112-0
- Bibcode:
- 2003TrGeo...6..293E