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Sedimentary Diagenesis, Depositional Environments, and Benthic Fluxes This article was originally published in Treatise on Geochemistry, Second Edition published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non- commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Aller R.C. (2014) Sedimentary Diagenesis, Depositional Environments, and Benthic Fluxes. In: Holland H.D. and Turekian K.K. (eds.) Treatise on Geochemistry, Second Edition, vol. 8, pp. 293-334. Oxford: Elsevier. © 2014 Elsevier Ltd. All rights reserved. Author's personal copy 8.11 Sedimentary Diagenesis, Depositional Environments, and Benthic Fluxes RC Aller, Stony Brook University, Stony Brook, NY, USA ã 2014 Elsevier Ltd. All rights reserved. 8.11.1 Introduction 293 8.11.1.1 Early Diagenesis and the Global Sedimentary Cycle 293 8.11.1.2 General Classes of Conceptual Diagenetic Models 294 8.11.1.3 Diagenetic Regimes and Depositional Environments 294 8.11.2 Diagenetic Oxidation–Reduction Reactions 298 8.11.2.1 Biogeochemical Redox Reaction Sequence 298 8.11.2.1.1 Coupled stoichiometric relations 299 8.11.2.1.2 Pore water solutes as reaction indicators 300 8.11.2.2 Reaction Rates and Kinetics 300 8.11.2.3 Sedimentary Redox Reaction Patterns 301 8.11.2.3.1 Redox oscillation 303 8.11.2.3.2 Microenvironmental heterogeneity 303 304 8.11.3 Diagenetic Transport Processes 8.11.3.1 Diffusive Transport 304 8.11.3.2 Advective Transport 305 8.11.4 Diagenetic Transport–Reaction Models 305 8.11.4.1 General Mass Balance Relations and Reference Frames 305 8.11.4.2 Simplification of Diagenetic Models 306 8.11.4.2.1 Characteristic scaling and steady state 306 8.11.5 Patterns in Boundary Conditions and Reaction Balances 307 8.11.5.1 Spatial Patterns in Sediment Accumulation and Biogenic Transport 307 8.11.5.2 Spatial Patterns in Reactive Corg Delivery and Magnitudes of Benthic Fluxes 308 8.11.5.3 Global Patterns in the Scaling of Redox Reactions 309 8.11.5.4 Sedimentary Record of Diagenetic Reaction Dominance and Balances 309 8.11.5.5 Temporal Patterns in Boundary Conditions and the Scaling of Reactions 311 8.11.6 Corg Burial and Preservation: Reactants and Diagenetic Regime 313 8.11.6.1 Patterns in Corg Distributions and Particle Associations 313 8.11.6.2 C Preservation: O , Accumulation Rates, and Diagenetic Regimes 315 org 2 8.11.7 Carbonate Mineral Dissolution–Alteration–Preservation 317 8.11.7.1 Coupling of Redox Reactions and Carbonate Diagenesis 317 8.11.7.2 Carbonate Mineral Equilibria and Saturation States 317 8.11.7.3 Kinetics of Biogenic Carbonate Dissolution in Sediments 318 8.11.7.4 Shallow Water Carbonate Dissolution 320 8.11.7.5 Dissolution of Carbonate in Deep-Sea Deposits and Internal Reaction Patterns 321 8.11.7.6 Benthic Alkalinity Fluxes 321 8.11.8 Biogenic Silica and Reverse Weathering 324 8.11.8.1 Patterns in Biogenic SiO2 Distributions 324 8.11.8.2 Diagenetic Fates, Equilibria, and Dissolution Reaction Kinetics of Biogenic SiO2 324 8.11.8.3 Authigenic Silicate Formation and Reverse Weathering 326 8.11.9 Future Directions 328 Acknowledgments 328 References 329 8.11.1 Introduction interactions of these deposits with overlying water and the atmosphere. Seabed geochemical interactions are driven by 8.11.1.1 Early Diagenesis and the Global Sedimentary Cycle components of sedimentary mixtures that are thermodynami- The global sedimentary cycle and attendant biogeochemical cally unstable and undergo a wide range of reactions, including processes are major controls on the composition of the Earth’s hydrolysis, dissolution, oxidation–reduction, precipitation, and surface over a wide spectrum of timescales (Drever et al., 1988; recrystallization. Many such reactions are rapid, biologically Kump et al., 2000;seeChapter 9.15). One part of this grand mediated, and occur in the upper few microns to meters of cycle is the deposition and buildup of lithogenic and biogenic deposits where both dissolved and particulate reactants and debris at the seafloor and the dynamic physical and chemical products can readily exchange with adjacent regions. Treatise on Geochemistry 2nd Edition http://dx.doi.org/10.1016/B978-0-08-095975-7.00611-2 293 Treatise on Geochemistry, Second Edition, (2014), vol. 8, pp. 293-334 Author's personal copy 294 Sedimentary Diagenesis, Depositional Environments, and Benthic Fluxes The chemical and physical changes that accompany these reac- controls on organic matter remineralization and preservation, tions and occur during the initial stages of sediment accretion at and developing predictive models to describe them, are central < low temperature (usually 50 C) define early diagenesis. The goals in diagenetic research. factors and mechanisms internal to deposits that govern diagen- esis are referred to as diagenetic processes, and the associated 8.11.1.2 General Classes of Conceptual Diagenetic Models mass transport of solutes, fluids, and particulate material across the upper surface of deposits are termed benthic or sediment– Five general classes of conceptual models are commonly uti- water fluxes (Burdige, 2006; Schulz, 2006). lized separately or in combination for the investigation of Because sedimentary deposits behave as open systems, early diagenetic processes and as a basis for interpretation and pre- diagenetic reactions and associated mass refluxing can have diction of compositional patterns and mass fluxes (Berner, major impacts on ocean water composition and ecosystem 1980; Boudreau, 1997; Burdige, 2006; Emerson and Hedges, processes. These impacts are particularly obvious in shallow 2003; Middelburg and Soetaert, 2004; Van Cappellen et al., < waters ( 50 m) where benthic release of regenerated nutrients 1993): (1) thermodynamic models to evaluate solution (e.g., N, P, and Si) interacts closely with the biologically pro- speciation, solution saturation states, reaction probability, ductive photic zone and can supply 25–80% of planktonic and mineral composition and stabilities in the context of nutrient requirements (Heip et al., 1995; Jahnke, 2004; known energy relations; (2) stoichiometric models to define Middelburg and Soetaert, 2004). However, more subtle yet or infer reactions and reaction balances and to examine rela- significant effects on a range of elemental cycles (e.g., N, P, tionships between reactant and product abundances; (3) trans- Si, K, Li, F, and Fe) in the ocean and atmosphere can also port models to evaluate mass fluxes, compositional relations, be demonstrated and are still being quantified (Jahnke, 1996; and system time dependence; (4) kinetic models to examine Michalopoulos and Aller, 2004; Middelburg and Soetaert, system time dependence, extent of reactions, and mechanistic 2004; Middelburg et al., 1996; Sayles, 1979; Severmann relationships between reactants and products; and (5) et al., 2010; see also Chapter 10.12). Diagenetic processes ecological models that combine theories of microbial substrate can significantly alter sediment composition and modify initial competition, benthic community structure, and ecological inputs of constituents that would otherwise be directly useful interactions with thermodynamic, transport, and kinetic for paleoenvironmental reconstruction, for example, selective models of sedimentary biogeochemical cycling. As illustrated dissolution of skeletal fossils, but these same processes can in the succeeding text, combinations of these models, for leave an alternative record of environmental conditions in example, transport–kinetic reaction–equilibrium modeling, the form of authigenic mineral suites and derived composi- play major roles in the recognition, quantification, and inter- tional relationships (e.g., S and Fe; Goldhaber, 2003; Lyons pretation of diagenetic processes, the quantification of mass et al., 2009; Raiswell and Canfield, 1998). The recognition and fluxes, and the prediction of diagenetic responses under steady accurate reading of this record depends on a detailed under- and unsteady conditions. Proper application of any of these standing of diagenesis in varied depositional conditions. models requires a thorough understanding of the depositional Biogenic debris in particular is often inherently unstable at environment and ecosystem context in which diagenesis Earth surface conditions and, once separated from sites of occurs. formation and active maintenance by energy flow, undergoes progressive net reactions. Organic matter, carbonates (CaCO3 8.11.1.3 Diagenetic Regimes and Depositional and CaxMgxÀ1CO3), and opaline silica (SiO2) are the domi- Environments nant general classes of biogenic solids involved in early diage- netic reactions, authigenic mineral product formation, and The expression of particular sets of early diagenetic reactions in biogeochemical cycling in the seabed (Emerson and Hedges, deposits and their interactions with overlying water depend 2003; Martin and Sayles, 2003;seeChapters 9.4 and 10.12). strongly on the depositional environment and the correspond- Examples of the overall patterns
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