Carbonates in Continental Settings

Geochemistry, Diagenesis and Applications

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• Publisher: Elsevier
• Year: 2009
• Print ISBN: 9780444535269
• eBook ISBN: 9780444535276
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

This book provides an up-to-date compilation of the latest research on the petrography, facies, paleonvironmental significance and economic aspects of continental carbonates. The overall organization of the book first emphasizes the descriptive aspects and processes operating on carbonate deposits in greatly varied settings, and then considers applications for basin analysis, as well as economic and historical aspects. This volume will be a valuable tool for graduate and postgraduate students as well as for experienced researchers. The first part (volume 61 in this series) will deal with the facies, environments, and processes of carbonates in continental settings.- Covering the greatly varied aspects of carbonate deposits from continental settings deposits- Clear and easy to follow organization of the book- Graduate to postgraduate level- Up to date information, so readers can find references from the classic literature to the most recent research

Chapter 1 The Geochemistry of Continental Carbonates

Daniel M. Deocampo^* (email: [email protected])

Department of Geosciences, Georgia State University, P.O. Box 4105, Atlanta, GA 30302, USA

^*Corresponding author.

Abstract

Carbonate sediments hold a critical component of the continental sedimentary record on earth. Carbonates precipitate under all climatic conditions on earth, from arctic to humid tropical, and are found actively precipitating on all the continents. An understanding of the thermodynamic controls on carbonate precipitation is important for modeling the conditions under which ancient carbonates precipitated. Biotic photosynthesis, respiration, and other aspects of cellular metabolism often alter the thermodynamic status of microenvironments, enhancing precipitation of carbonates. Source waters and evaporative concentration are both important in the development of Mg-bearing carbonates, whether the waters in which precipitation is occurring are surface, pedogenic, or groundwaters. To the degree that isotopic equilibrium is achieved between water and precipitating minerals, stable isotopes of oxygen, carbon, and strontium can be used to reconstruct the isotopic composition of paleowaters. These can be used to infer various aspects of paleoenvironments, including climate, hydrology, and temperature. Such approaches have been tremendously important in studies of carbonates with lacustrine, pedogenic, karstic, or biotic origin. New isotopic techniques continue to be developed, such as the “isotope clumping” paleothermometer. Whether carbonate mineral precipitation is purely physico-chemical, biologically mediated, or truly biosynthesized, modern environmental and experimental studies continue to provide critical perspectives on elemental and isotopic chemistries.

Keywords: carbonate geochemistry; stable isotopes; authigenic carbonate; pedogenic carbonate; ostracods; tufa

1. Introduction

Non-clastic carbonates form an important class of sediment, having precipitated from solution either through inorganic precipitation or through a biologically mediated process. Throughout the sedimentary record of the earth, they comprise an important component of continental sediments, providing insights into paleoenvironmental, paleoecological, and paleoclimatic conditions. Analyses of elemental and isotopic geochemistry are now fundamental to any study of carbonate deposits, subject to the many complexities introduced by the sensitivity of carbonate minerals to early, middle, and late diagenetic alteration.

The precipitation of carbonate minerals from continental water is fundamentally controlled by the thermodynamics of the carbonate mineral systems. Even given the dramatic importance of biota in mediating mineral precipitation reactions, such organisms must somehow achieve mineral supersaturation in order for biomineralization to occur, either within or near cellular tissue (e.g., the proton pump of Lian et al., 2006). Understanding the geochemistry of carbonate minerals, therefore, is fundamentally a question of understanding ionic activities in solution at the time of precipitation.

Despite the simplicity of the thermodynamics, however, carbonate mineral precipitation in near-surface continental environments is tremendously complicated by the geochemical impacts of large and small organisms, and reaction kinetics in an environment with non-ideal ion interactions, variable substrates, fluctuating pH, organic molecules, and gas phase interactions. With these complications in mind, it is nevertheless helpful to consider that, when we observe sedimentary carbonates, the fundamental reason that solid calcite, for example, exists is because Ca_(aq)²⁺ and CO_3(aq)²⁻ were present in sufficient concentrations and activities for calcite to be supersaturated. Hence, a lack of calcareous microfossils in a lacustrine deposit may be directly due to limnological processes inhibiting the bioavailability of Ca_(aq)²⁺, rather than to other ecological factors such as pH or salinity. Understanding the genesis of carbonate minerals, therefore, is a question of understanding what processes promote supersaturation and precipitation (Table 1), be they biotic or abiotic, and what kinetic effects inhibit precipitation (perhaps favoring certain phases). Moreover, understanding how these processes control the incorporation and partitioning of major and trace elements, and stable isotopes (especially of C, O, and Sr) provides a powerful tool for paleoenvironmental reconstruction.

Table 1 Names, chemical formulas, and K values for major carbonate minerals

Sources: (1) Plummer and Busenberg (1982); (2) Kralj and Brecevic (1995); (3) Ball and Nordstrom (1991); (4) Clarkson et al. (1992) (5) Plummer et al. (1988) (6) Busenberg et al. (1984).
Name	Chemical formulas	logK25 (disordered, if available)	References
Calcite	CaCO3	−8.48	(1)
Aragonite	CaCO3	−8.34	(1)
Vaterite	CaCO3	−7.91	(1)
Monohydrocalcite	CaCO3·H2O	−7.15	(2)
Dolomite	CaMg(CO3)2	−17.09 (−16.54)	(3)
Ikaite	CaCO3·6H2O	−6.62	(4)
Huntite	CaMg3(CO3)4	−29.97	(3)
Magnesite	MgCO3	−8.03	(3)
Hydromagnesite	4(MgCO3)·Mg(OH)2·4H2O	−8.76	(3)
Nesquehonite	MgCO3·3H2O	−5.62	(3)
Trona	Na2CO3·NaHCO3·2H2O	−0.80	(3)
Thermonatrite	Na2CO3·H2O	0.13	(3)
Nahcolite	NaHCO3	−0.55	(3)
Natron	Na2CO3·10H2O	−1.31	(3)
Pirssonite	CaCO3·Na2CO3·2H2O	−9.23	(5)
Gaylussite	CaCO3·Na2CO3·5H2O	−9.42	(5)
Kalicinite	KHCO3	−10.06	(5)
Siderite	FeCO3	−10.89	(3)
Malachite	Cu2(OH)2CO3	5.15	(3)
Azurite	Cu3(OH)2(CO3)2	3.75	(3)
Smithsonite	ZnCO3·H2O	−10.0	(3)
Rhodacrosite	MnCO3	−11.43 (−10.39)	(3)
Cerussite	PbCO3	−13.13	(3)
Strontianite	SrCO3	−9.27	(6)
Witherite	BaCO3	−8.56	(3)
Burkeite	Na2CO3·2Na2SO4	−0.772	(3)

The purpose of this chapter is to provide a broad review of the geochemistry of continental carbonates, with emphasis on the development of the sedimentary record. This is a tremendous field, represented by an enormous body of literature. In some ways, the continental sedimentologists of today who work with carbonates face many of the challenges confronting marine geologists of past decades, where fundamental concepts such as reaction kinetics and biogeochemical effects remain problematic and difficult to quantify in the field. Indeed, some problems, such as the “dolomite problem,” are common to both marine and continental sediments. Nevertheless, this chapter attempts to summarize the key aspects of geochemistry relevant to continental sediments. The chapter is organized into first a discussion of the major element geochemistry of waters from which inorganic carbonates are commonly precipitated, and the resulting elemental geochemistry observed in carbonates, especially in lakes. Then the controls on isotopic composition of carbonates are discussed, with examples from lacustrine, pedogenic, tufa, and karst carbonates. The chapter then discusses saline carbonates common to evaporative environments and, finally, ostracodes, perhaps the most common carbonate lacustrine microfossil used in paleolimnology.

2. Precipitation of Inorganic Carbonates

The dissolved constituents that contribute to carbonate mine...

Table of contents

• Cover Image
• Table of Contents
• Elsevier
• Preface
• Chapter 1 The Geochemistry of Continental Carbonates
• Chapter 2 Diagenesis of Carbonates in Continental Settings
• Chapter 3 Silicification of Continental Carbonates
• Chapter 4 Continental Carbonates as Indicators of Paleoclimate
• Chapter 5 Continental Sequence Stratigraphy and Continental Carbonates
• Chapter 6 Economic Aspects of Continental Carbonates and Carbonates Transformed under Continental Conditions
• Chapter 7 Continental Carbonates – Preservation of Natural and Historic Heritage Sites