The Role of Diagenesis in Shaping the Geochemistry of the Marine Carbonate Record

Abstract

Carbonate sediments and rocks are valuable archives of Earth's past whose geochemical compositions inform our understanding of Earth's surface evolution. Yet carbonates are also reactive minerals and often undergo compositional alteration between the time of deposition and sampling and analysis. These changes may be mineralogical, structural, and/or chemical, and they are broadly referred to as diagenesis. Building on work over the past 40 years, we present an overview of key carbonate diagenesis terminology and a process-based framework for evaluating the geochemical impacts of carbonate diagenesis; we also highlight recent experimental and field observations that suggest metal isotopes as valuable diagenetic indicators. Our primary objectives are to demonstrate the value of coupling quantitative and analytical approaches, specifically with regard to metal isotopes and Mg/Ca, and to focus attention on key avenues for future work, including the role of authigenesis in impacting global geochemical cycles and the isotopic composition of the rock record. Quantitative frameworks utilizing well-understood diagenetic indicators and basic geochemical parameters allow us to assess the extent of diagenetic alteration in carbonate sediments. The reactivity, duration of reaction, and degree of isotopic or elemental/chemical disequilibrium determine the extent to which carbonates may be altered. Metal isotopic ratios (δ44Ca, δ26Mg, 87Sr/86Sr) can be used to constrain the extent and rate of carbonate recrystallization. Diagenetic signals may be globally synchronous, while diagenetic fluxes may impact global geochemical cycles.