Carbonate geochemistry in primary, diagenetic and biological systems

The carbonate minerals calcite, aragonite and dolomite (and their rock-counterparts) precipitate directly from fluids. The mineral-yielding fluids must contain the necessary chemical constituents calcium, magnesium, carbon and oxygen. As the carbonates precipitate they inherit and incorporate chemical signatures that are ultimately governed by the nature of formation fluids. Therefore, carbonate rocks and minerals can be treated as geologic reservoirs for information concerning past fluid chemistries and very powerful geochemical databases.;The following dissertation describes five projects that exploit this carbonate mineral geochemical reservoir across a range of geologic settings. The carbonates presented in the following chapters can be categorized based on their relative time of formation compared to the deposition of the associated geologic unit. These categories are (1) primary, or syndepositional carbonates; and (2) diagenetic, or carbonates precipitated post depositionally and prior to metamorphic temperature and pressure ranges.;In the following studies isotopic, trace elemental, trace compound, petrographic, stratigraphic and textural data are combined in order to determine the formation environments of carbonates and the characteristics of carbonate-yielding fluids. Given the major constituents of carbonate minerals listed above, isotopic analyses of carbon and oxygen are critical measurements and can provide a great deal of evidence regarding the sources and cycling of theses two elements. Oxygen isotopes in carbonates are influenced by a multitude of processes (discussed in detail in the following chapters) most of which are directly reflective of fluid temperatures and their isotopic compositions, both of which can help distinguish between primary or diagenetic environments. Trace element concentrations of iron, manganese and strontium are exploited in chapter 2 in order to further constrain the influence of diagenesis on primary chemical signatures.;Carbon is a major constituent of not only carbonate minerals but also organic compounds, making is isotopic characterization in carbonate minerals a valuable tracer of the sources of carbon in sedimentary environments (Claypool and Kaplan, 1974). As with carbon, sulfur is strongly influenced by biologic processes. Carbonates incorporate trace amounts of sulfate upon precipitation (Burdett et al., 1986) and this sulfate has been shown to substitute directly for the carbonate ion within the crystal lattice (Pingitore et al., 1995). This sulfate is referred to as carbonate-associated sulfate (CAS) and can be extracted and analyzed for not only its bulk concentration but also its isotopic composition, both of which are dictated by the nature of carbonate-yielding fluids (Burdett et al., 1986). Carbonate formation environment can be further characterized when analysis of CAS (the oxidized sulfur species) is combined with isotopic and abundance analyses of pyrite (the dominant reduced sulfur phase in geologic settings). Therefore carbonate minerals act as geologic reservoirs for sulfur systematics as well.;Chapters 1 and 2 deal with Neoproterozoic to middle Cambrian units from northwestern Mexico and eastern California. These deposits are interpreted here as largely primary, however the influence of diagenesis is identified and discussed in each. This time interval is characterized by perhaps the most extreme evolutionary radiation experienced in Earth history, the so-called "Cambrian Explosion". It has long been proposed that certain chemical conditions must have existed in order to support such a drastic radiation, in particular increased marine oxygen concentrations. The data presented here suggests that this time interval was characterized by oceans with low sulfate concentrations---sulfate is a redox sensitive compound and is expected to increase in tandem with oxygen.;Chapters 3 and 4 focus on carbonate concretions from the Miocene Monterey Formation and the late Cretaceous Holz Shale. Concretions have long been known to form within sediments and their distinction as diagenetic is not largely debated. In these chapters, I show that concretionary carbonates retain signatures consistent with particular microbial processes, and directly identify that sulfur cycling plays a large role in past marine sedimentary regimes. In most cases, the identified microbial processes are likely directly responsible for carbonate mineralization. Concretions of the Monterey Formation exhibit chemical characteristics consistent with formation in sediments experiencing organic matter degradation via oxidation by nitrate and/or metal oxides and sulfate. Methanogenesis and sulfide oxidation were also active in zones of concretion precipitation in sediments of the Monterey Formation. Calcitic concretions of the Holz Shale possibly formed in zones experiencing extensive sulfide oxidation. Sulfide oxidation has, until now, not been identified as a reaction associated with carbonate authigenesis.;Chapter 5 highlights the possibility of forming potentially primary-like sedimentary textures through diagenetic processes in the Beck Spring Dolomite of eastern California. In this deposit, an extensive, laminated texture is most reasonably interpreted as diagenetic when geochemical, textural and petrographic data are considered together. This study demonstrates that the distinction between primary and diagenetic must not be made solely on field-based criteria. (Abstract shortened by UMI.)...