Influence of geochemical processes on stable isotope distribution in marine sediments

Abstract

Stable isotope geochemistry has become an essential part of marine geochemistry and has contributed considerably to the understanding of the ocean's changing environment and the processes therein. In some fields, such as biogeochemistry and paleoceanography, the application of stable isotopes is still growing due to new microanalytical techniques, permitting a relatively precise analysis of very small samples or single compounds, which allows the investigation of a new generation of problems. Stable isotopes are useful tracers for reconstructing past temperatures, salinities, productivity, pCO 2, nutrients, etc. However, limitations exist on the application of these tracers because of diagenetic processes which may considerably alter the primary signals, due to preferential preservation, decomposition or relocation of particular tracers. On the other hand, stable isotope geochemistry offers the chance to iden-tify and better understand such diagenetic processes, which is essential for the interpreta-tion of isotope variability in marine sediments of the past. Within this chapter, we focus on four elements (C, O, N, S), which participate in most marine geochemical reactions and which are important elements in the biological system. Some recent developments in the use of B isotopes are added to this chapter. We summarize the influence of geochemical processes on the stable isotope distribution of those elements in ocean water and marine sediments. After a short review on the fundamentals of stable isotope fractionation and mass spectrometry, the most important fractionation mechanisms for each element within the marine environment are described, followed by the identification of geochemical processes responsible for syn- or postsedimentary changes in isotope signals.