Rolf S. Arvidson

The dissolution kinetics of major sedimentary carbonate minerals

Abstract Among the most important set of chemical reactions occurring under near Earth surface conditions are those involved in the dissolution of sedimentary carbonate minerals. These minerals comprise about 20% of Phanerozoic sedimentary rocks. Calcite and, to a significantly lesser extent, dolomite are the major carbonate minerals in sedimentary rocks. In modern sediments, aragonite and high-magnesian calcites dominate in shallow water environments. However, calcite is by far the most abundant carbonate mineral in deep sea sediments. An understanding of the factors that control their dissolution rates is important for modeling of geochemical cycles and the impact of fossil fuel CO 2 on climate, diagenesis of sediments and sedimentary rocks. It also has practical application for areas such as the behavior of carbonates in petroleum and natural gas reservoirs, and the preservation of buildings and monuments constructed from limestone and marble. In this paper, we summarize important findings from the hundreds of papers constituting the large literature on this topic that has steadily evolved over the last half century. Our primary focus is the chemical kinetics controlling the rates of reaction between sedimentary carbonate minerals and solutions. We will not attempt to address the many applications of these results to such topics as mass transport of carbonate components in the subsurface or the accumulation of calcium carbonate in deep sea sediments. Such complex topics are clearly worthy of review papers on their own merits. Calcite has been by far the most studied mineral over a wide range of conditions and solution compositions. In recent years, there has been a substantial shift in emphasis from measuring changes in solution composition, to determine “batch” reaction rates, to the direct observation of processes occurring on mineral surfaces using techniques such as atomic force microscopy (AFM). However, there remain major challenges in integrating these two very different approaches. A general theory of surface dissolution mechanisms, currently lacking (although see Lasaga and Luttge [Science 291 (2001) 2400]), is required to satisfactorily relate observations of mineral surfaces and the concentration of dissolved components. Studies of aragonite, high-magnesian calcites, magnesite, and dolomite dissolution kinetics are much more limited in number and scope than those for calcite, and provide, at best, a rather rudimentary understanding of how these minerals are likely to behave in natural systems. Although the influences of a limited number of reaction inhibitors have been studied, probably the greatest weakness in application of experimental results to natural systems is understanding the often profound influences of “foreign” ions and organic matter on the near-equilibrium dissolution kinetics of carbonate minerals.

Variation in calcite dissolution rates:: A fundamental problem?

A comparison of published calcite dissolution rates measured far from equilibrium at a pH of ∼ 6 and above shows well over an order of magnitude in variation. Recently published AFM step velocities extend this range further still. In an effort to understand the source of this variation, and to provide additional constraint from a new analytical approach, we have measured dissolution rates by vertical scanning interferometry. In areas of the calcite cleavage surface dominated by etch pits, our measured dissolution rate is 10−10.95 mol/cm2/s (PCO2 10−3.41 atm, pH 8.82), 5 to ∼100 times slower than published rates derived from bulk powder experiments, although similar to rates derived from AFM step velocities. On cleavage surfaces free of local etch pit development, dissolution is limited by a slow, “global” rate (10−11.68 mol/cm2/s). Although these differences confirm the importance of etch pit (defect) distribution as a controlling mechanism in calcite dissolution, they also suggest that “bulk” calcite dissolution rates observed in powder experiments may derive substantial enhancement from grain boundaries having high step and kink density. We also observed significant rate inhibition by introduction of dissolved manganese. At 2.0 μM Mn, the rate diminished to 10−12.4 mol/cm2/s, and the well formed rhombic etch pits that characterized dissolution in pure solution were absent. These results are in good agreement with the pattern of manganese inhibition in published AFM step velocities, assuming a step density on smooth terraces of ∼9 μm−1.

Modeling the Mutualistic Interactions between Tubeworms and Microbial Consortia

The deep-sea vestimentiferan tubeworm Lamellibrachia luymesi forms large aggregations at hydrocarbon seeps in the Gulf of Mexico that may persist for over 250 y. Here, we present the results of a diagenetic model in which tubeworm aggregation persistence is achieved through augmentation of the supply of sulfate to hydrocarbon seep sediments. In the model, L. luymesi releases the sulfate generated by its internal, chemoautotrophic, sulfide-oxidizing symbionts through posterior root-like extensions of its body. The sulfate fuels sulfate reduction, commonly coupled to anaerobic methane oxidation and hydrocarbon degradation by bacterial–archaeal consortia. If sulfate is released by the tubeworms, sulfide generation mainly by hydrocarbon degradation is sufficient to support moderate-sized aggregations of L. luymesi for hundreds of years. The results of this model expand our concept of the potential benefits derived from complex interspecific relationships, in this case involving members of all three domains of life.

Plasmonic Nature of the Terahertz Conductivity Peak in Single-Wall Carbon Nanotubes

Plasmon resonance is expected to occur in metallic and doped semiconducting carbon nanotubes in the terahertz frequency range, but its convincing identification has so far been elusive. The origin of the terahertz conductivity peak commonly observed for carbon nanotube ensembles remains controversial. Here we present results of optical, terahertz, and direct current (DC) transport measurements on highly enriched metallic and semiconducting nanotube films. A broad and strong terahertz conductivity peak appears in both types of films, whose behaviors are consistent with the plasmon resonance explanation, firmly ruling out other alternative explanations such as absorption due to curvature-induced gaps.

Formation and Diagenesis of Carbonate Sediments

The first edition of this chapter provided a brief review of the geochemistry of carbonate minerals, summarized general aspects of their thermodynamics and the kinetics of their precipitation and dissolution, and then used the natural division between deep-sea marine carbonates (chalks and pelagic muds) versus shoal-water carbonates (skeletal and nonskeletal materials) to organize details of the formation, distribution, and subsequent diagenetic alteration of these phases. A substantial focus in this treatment was thus the description of the behavior and fate of carbonate minerals in natural settings. In this chapter, this latter material appears largely unchanged. The aim in this revision is not simply to trudge through calcite, dolomite, and magnesite rate equations, detailed reviews of which are available elsewhere; instead, the intent is to provide background for the selective discussion of recent work on carbonate mineral surfaces (mostly calcite).

Crystal chemistry, and thermodynamic and kinetic properties of calcite, dolomite, apatite, and biogenic silica: applications to petrologic problems

Abstract Sedimentary minerals are generally metastable phases that, given time and changing environmental conditions, recrystallize to more stable phases. The actual pathway of stabilization is governed by a host of kinetic factors. Unfortunately, much of the theoretical and experimental work on thermodynamic and kinetic behavior of sedimentary minerals either has not reached field practitioners in sedimentary petrology, or has been conducted under conditions that are difficult to extrapolate to natural sedimentary environments. In this paper we review and present new data on the basic crystal chemistry, thermodynamic and kinetic properties of calcite, dolomite, apatite, and biogenic silica, and discuss the relevance of these data to the solution of geological and geochemical problems. The crystal chemistry and structure of a given magnesian calcite exert a fundamental control on its solubility and solid solution behavior, and this control can be seen most clearly through comparison of synthetic and biogenic phases. Thus, variations in crystal chemistry and structure, through solubility control during diagenesis, yield a range of possible stabilization pathways, whose documentation is the domain of much field-based study. Experimental work involving dolomite has focused on delineation of phase relations in dry and aqueous systems at moderate to high temperatures, determination of reaction pathways followed during dolomitization of calcium carbonate, and measurement of reaction rate. Uncertainties reside in the relevance of these data to the classic problem of low-temperature dolomite formation. We suggest that the effort must now focus on designing experimental systems that effectively mimic natural environments, and yield reaction rate data as a function of temperature and solution composition. Such an example is presented. A primary goal in experimental work involving carbonate fluorapatite has been an understanding of the mechanism of formation of this mineral. We review the state of this knowledge, and also present the results of ongoing dissolution rate experiments. The importance of this work is that it bears directly on the understanding of the role carbonate fluorapatite plays in the biogeochemical cycle of P in the oceans. Many factors influence the solubilities of opaline silica and the silica polymorphs, and solubility plays an important role in controlling silica diagenesis. A model is presented that relates changes in sediment properties including density and acoustic velocity to the stages of silica diagenesis. The model is applied to sediments of Deep Sea Drilling Project Leg 63. The discussions of the sedimentary phases calcite, dolomite, apatite, and biogenic silica in this paper point to several directions for future experimental research on sedimentary mineral-solution reactions. These include emphasis on: (1) experimental studies of synthetic sedimentary mineral-solution reactions to form a foundation for an understanding of natural mineral-solution reactions; (2) experimental investigations of mineral reactions in aqueous solutions under conditions of composition, temperature, and pressure similar to the natural conditions of mineral formation; and (3) studies of the surface (as opposed to bulk) properties of sedimentary minerals in aqueous solutions and their role in reactions governing precipitation and dissolution of sedimentary phases.

Tentative kinetic model for dolomite precipitation rate and its application to dolomite distribution

The ‘dolomite problem’ has a long history and remains one of the most intensely studied and debated topics in geology. Major amounts of dolomite are not directly forming today from seawater. This observation has led many investigators to develop geochemical/hydrologic models for dolomite formation in diagenetic environments. A fundamental limitation of the current models for the growth of sedimentary dolomite is the dearth of kinetic information for this phase, in contrast to that available for calcite and aragonite.We present a simple kinetic model describing dolomite growth as a function of supersaturation using data from published high temperature synthesis experiments and our own experimental results. This model is similar in form to empirical models used to describe precipitation and dissolution rates of other carbonate minerals. Despite the considerable uncertainties and assumptions implicit in this approach, the model satisfies a basic expectation of classical precipitation theory, i.e., that the distance from equilibrium is a basic driving force for reaction rate. The calculated reaction order is high (~ 3), and the combined effect of high order and large activation energy produces a very strong dependence of the rate on temperature and the degree of supersaturation of aqueous solutions with respect to this phase.Using the calculated parameters, we applied the model to well-documented case studies of sabkha dolomite at Abu Dhabi (Persian Gulf), and organogenic dolomite from the Gulf of California. Growth rates calculated from the model agree with independent estimates of the age of these dolomites to well within an order of magnitude. A comparison of precipitation rates in seawater also shows the rate of dolomite precipitation to converge strongly with that of calcite with increasing temperature. If correct, this result implies that dolomite may respond to relatively modest warming of surface environments by substantial increases in accumulation rate, and suggests that the distribution of sedimentary dolomite in the rock record may be to some extent a temperature signal.

Single-crystal plagioclase feldspar dissolution rates measured by vertical scanning interferometry

Abstract Here we introduce a technique for simultaneous measurement of surface normal retreat rates of specific cleavage faces by vertical scanning interferometry and the bulk dissolution rate of a mineral powder. A hydrothermal reactor is used to contain both a well-characterized powder and oriented single crystals with a masked reference surface at elevated temperatures. We show examples using both anorthite and albite reacted at temperatures between 150 and 200 °C. In the case of albite, dissolution rates of fine-grained powders are substantially enhanced compared to those prevailing on large single-crystal cleavage surfaces. Rates developed on the (010) albite cleavage surface are also substantially faster than those on the (001) face, where etch-pit development was relatively modest and surface normal retreat was not detectable within the time frame of the experiment. The reasons for this difference are not immediately clear, but may be related to anisotropy in the distribution of Al-O-Si vs. Si-O-Si bonds in the albite structure, (010) twinning expressed on the (001) surface, and possible disruption of kink propagation across the twin plane.

Temperature Dependence of Mineral Precipitation Rates Along the CaCO3-MgCO3 Join

The large variation in precipitation rate and abundance of mineralscomprising the CaCO3–MgCO3 binary join can be understood in terms of their large differences in activation energy. Following the treatment of Lippmann (1973), activation energy isextrapolated along the join as a linear function of mole percentmagnesium. For the dolomite-type carbonates, the predicted activationenergy is compatible with recent measurements of calcian protodolomitekinetics; cation ordering in ideal dolomite can thus be seen as anadditional contribution to activation energy. Although no activationenergies are available for magnesian calcites, treatment of rate datafor these phases using the formalism of stoichiometric saturationsuggests a possible change in mechanism or rate-limiting step astemperature is decreased from 25 to 5 °C.

Chemostatic modes of the ocean-atmosphere-sediment system through Phanerozoic time

Abstract The essential state of the Phanerozoic ocean-atmosphere system with respect to major lithophile and organic components can be bounded by sedimentary observational data and relatively few model assumptions. The model assumptions are in turn sufficient to constrain and compute the remaining fluxes that result in a comprehensive model describing atmospheric and oceanic evolutionary history over the past 500 m.y. that is in accord with the sedimentary observational data. Two central themes emerge. First, there is a strong coupling of the state of various reservoirs throughout the entire system imposed mainly by negative physical, chemical and biological feedbacks. Second, there is a significant overprint of ‘physical’ processes, such as weathering, by biologically-mediated processes and ecosystem evolution. Ultimately, the Phanerozoic is characterized by two modes of sea water major-ion chemistry, pH and carbonate saturation state, and atmospheric CO2. Importantly, the transition between these two modes may result from the previous state of the system whose impacts lag by tens of millions of years. Thus, the instantaneous state of the system at any given point in time may reflect in part the ‘memory’ of a previous period when fluxes and processes were not in balance. The modern-day problem of ocean acidification mainly reflects the fact that human activities of fossil fuel burning and land use changes are resulting in geologically rapid releases of CO2 to the atmosphere and its absorption by the surface ocean and does not reflect the longer term processes and feedbacks that led to the acidic oceans of the past.