Chapter 9 - Early diagenesis of deep-sea sediments

Abstract

This review covers the early diagenesis of the various types of deep-sea sediments. It uses pore-water profiles as a sensitive tool to gain insight in diagenetic processes, to depict the mineral reactions associated with the changes in the pore-water chemistry and to group deep-sea sediments into a limited number of diagenetic classes. In doing so, a sequence is followed from the simplest diagenetic systems encountered in pelagic sediments on the ocean crust with pore-water profiles lacking chemical gradients to the most highly reactive organic-matter rich sediments on the continental margins at the oxygen-minimum zone. Pore-water profiles are characteristic of specific sediment types only if they are reaction-controlled. In advection- and/or diffusion-dominated profiles, fluid-chemistry related to the nature of the host sediment is overprinted by ions derived from external sources. Advection and diffusion-dominated profiles are treated at the beginning of the chapter.The simplest type of pelagic sediment in terms of diagenesis and the first to be covered is the brown abyssal clay on the deep ocean floor below the calcite compensation level. The review of the early diagenesis of biogenic siliceous oozes which follows covers a wide range of topics from pore-water chemistry to burial-diagenetic stages, characteristics of the solid silica phases, nature of the conversion mechanisms and their interpretation in terms of Ostwald processes, crystallographic structural changes of opal-CT and quartz during progressive burial, rate-controlling factors of the diagenetic reactions, the formation of bedded chert and the physical diagenesis of biogenic siliceous deep-sea sediments. The next topic is the early diagenesis of biogenic pelagic carbonates which is characterized by recrystallization involving (pressure) solution and reprecipitation leading to the formation of chalk. It is accompanied by a marked redistribution of strontium isotopes in the pore waters. Rhythmic limestone/marlstone alternations are generally attributed to Milankovitch palaeoclimate cycles but may also be accentuated or even originate during burial diagenesis.For hemipelagic sediments the oxidation of organic matter becomes the driving force of diagenetic reactions. During early diagenesis the organic-matter of the sediments is decomposed mostly by bacteria. Microbial organic-matter oxidation is accomplished by five different oxidant-specific populations of bacteria (aerobic bacteria, nitrogen reducers, sulphate reducers, carbonate reducers or methanogens, and fermenters) that follow one another during burial in the order of decreasing energy efficiency of their metabolic reactions and give rise to the vertical sequence of organic-matter decomposition zones comprising (1) the oxidation zone, (2) the nitrate-reduction zone, (3) the sulphate-reduction zone, (4) the carbonate-reduction zone, and (5) the fermentation zone. The latter is followed by (6) the thermocatalytic decarboxylation zone. The oxidation zone corresponds to what is called 'oxic diagenesis', the nitrate-reduction zone to 'suboxic diagenesis', and the top of the sulphate-reduction zone to the top of 'anoxic diagenesis'. The brown abyssal clay which is almost free of organic matter essentially remains in the oxidation zone during burial.A typical representative of suboxic diagenesis are manganese nodules and crusts, as are glauconitic sediments. Phosphorites are diagenetic in origin and frequently occur together with glauconite. Anoxic diagenesis in organic-matter rich hemipelagic sediments leads to the formation of gas-hydrates, if the water depth (i.e. pressure) is sufficient. Euxinic sediments are hemipelagic sediments undergoing anoxic diagenesis in stagnant basins under oxygen-free bottom water. Evaporite dissolution in the subsurface gives rise to a specific pore-water type, as does hydrothermal activity and the intrusion of igneous dykes and sills above sediment-covered mid-ocean ridges. Active margins affected by advective lateral fluid flow show typical step-curves of their pore-water profiles. Highly reactive volcanogenic sediments produce their own pore-water anomalies.A major section is devoted to early diagenetic mineralization reactions in anoxic deep-water sediments covering sulphide precipitation, authigenic carbonates (calcite, siderite, organogenic dolomite, complex authigenic carbonates) and barite. The final section deals with early diagenetic clay mineral formation. © 2011 Elsevier B.V.