Showing papers in "Treatise on Geochemistry in 2014"

Composition of the Continental Crust

Abstract: This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.

Cosmochemical Estimates of Mantle Composition

Abstract: The composition of the primitive mantle derived here shows that Earth was assembled from material that shows many of the same chemical fractionation processes as chondritic meteorites. These processes occurred at the initial stage of the solar system formation, under conditions thought to be present in the solar nebula. But the stable isotope record excludes chondritic meteorites as the ‘building blocks’ of Earth. Meteorites formed in local environments separated from that part of the inner solar system where much of the material forming the terrestrial planets was sourced.

Trace Elements in River Waters

Abstract: In this chapter, we have tried to review the recent literature on trace elements in rivers, in particular by incorporating the results derived from recent ICP-MS measurements. We have favored a “field approach” by focusing on studies of natural hydrosystems. The basic questions which we want to address are the following: What are the trace element levels in river waters? What controls their abundance in rivers and fractionation in the weathering + transport system? Are trace elements, like major elements in rivers, essentially controlled by source-rock abundances? What do we know about the chemical speciation of trace elements in water? To what extent do colloids and interaction with solids regulate processes of trace elements in river waters? Can we relate the geochemistry of trace elements in aquatic systems to the periodic table? And finally, are we able to satisfactorily model and predict the behavior of most of the trace elements in hydrosystems?

The geochemistry of acid mine drainage

Abstract: Mining and mineral processing generates large volumes of waste, including waste rock, mill tailings, and mineral refinery wastes. The oxidation of sulfide minerals in the materials can result in the release of acidic water containing high concentrations of dissolved metals. Recent studies have determined the mechanisms of abiotic sulfide-mineral oxidation. Within mine wastes, the oxidation of sulfide minerals is catalyzed by microorganisms. Molecular tools have been developed and applied to determine the activity and role of these organisms in sulfide-mineral-bearing systems. Novel tools have been developed for assessing the toxicity of mine-waste effluent. Dissolved constituents released by sulfide oxidation may be attenuated through the precipitation of secondary minerals, including metal sulfate, oxyhydroxide, and basic sulfate minerals. Geochemical models have been developed to provide improved predictions of the magnitude and duration of environmental concerns. Novel techniques have been developed to prevent and remediate environmental problems associated with these materials.

Geochemistry of Mercury in the Environment

Abstract: This chapter focuses principally on the low-temperature environmental biogeochemistry of mercury. The current understanding of mercury cycling at Earth's surface (soils, sediments, natural waters, and the atmosphere) is presented. The coverage, as appropriate, includes the anthropogenic interferences (i.e., mercury pollution) and biological mediation, which affect significantly the speciation, behavior, and fate of mercury in the environment. Some of the gains made in the understanding of the environmental biogeochemistry of mercury since Goldschmidt's groundbreaking work are summarized. Much of this advancement has come since the early 1970s, and the development in mercury research continues at breakneck pace.

Devolatilization during subduction

Abstract: Subducted crust refertilizes the subarc mantle wedge as well as the deep convecting mantle. In the subarc region, mass transfer occurs mainly through fluids, melts, or supercritical liquids. Experiments show that devolatilization reactions may occur at almost any depth in the chemically complex subducting lithosphere. Extreme temperature gradients across the slab lithosphere, the subduction channel, and the mantle hanging wall promote diachronous fluid production. Fluids are dominated by H2O, and the temperature of melting is controlled by the availability of H2O and CO2. The exact nature of the transfer agent and its composition depends on the source lithologies and the temperature regime, which vary significantly between different subduction zones. Nevertheless, P–T paths calculated from mechanical subduction zone models suggest that, in most subduction zones, the subducted crust remains at subsolidus conditions in the subarc regime. In a few, the pelitic solidus is overstepped, and fluid-saturated melting may be induced by flushing of H2O derived from more deep-seated metamorphic reactions. A steady volatile flow into the subarc wedge does not imply a steady supply of most trace elements, because low temperatures prevent diffusive equilibration, and, thus, the reactive volumes, in combination with the residual minerals formed through the devolatilization reactions, determine the efficiency of trace element transfer to the mantle wedge. Three main lithologies contribute to the devolatilization signal: peridotites, the mafic oceanic crust, and pelitic sediments + eroded continental crust, all three of which may contain carbonates that were added through hydrothermal or sedimentary processes. Most of the subducted CO2 survives the subarc regimen, while most of the H2O is lost to the subarc mantle, making CO2 the dominant volatile that is transported into the deeper mantle. At > 200–250 km depth, a fairly high temperature stability of hydrous phases relative to a typical subduction P–T path, in combination with relatively low melting temperatures for carbonated lithologies, leads to the possible formation of carbonatites within the subducting crust. Finally, examination of volcanic arcs worldwide reveals that their position is controlled not by devolatilization from the slab, but mainly by convection in the mantle wedge (which generally does not reach steady state) and to a minor extent by the structure of the overriding plate.

Arsenic and selenium

Abstract: This chapter outlines the main effects of arsenic and selenium on human and animal health, their abundance and distribution in the environment, sampling and analysis, and the main factors controlling their speciation and cycling. Such information should help identify aquifers, water resources and soils at risk from high concentrations of arsenic and selenium, and areas of selenium deficiency. Human activity has had, and is likely to continue to have, a major role in releasing arsenic and selenium from the geosphere and in perturbing the natural distribution of these and other elements over the Earth's surface.

Salinization and Saline Environments

Abstract: This chapter provides an overview of global salinization phenomena and investigates the different mechanisms and geochemical processes that are associated with salinization. The overview includes salinization of rivers, lakes, and groundwater from different parts of the world. Special emphasis is given to the distinction between natural processes and anthropogenic forcing that generates salinity, such as wastewater contamination and agricultural runoff. As such, two anthropogenic salinization cycles are introduced – the agricultural and the domestic. The role of the unsaturated zone in shaping the chemical composition of dryland salinization is also discussed. An overview of the effects of salinity on the occurrence of health-related contaminants such as fluoride, oxyanions (arsenic, selenium, boron), radionuclides, trihalomethanes, and fish-kill algae is presented. Some useful geochemical and isotopic fingerprinting tracers are introduced for elucidating the salinity sources. Finally, the chemical and isotopic compositions of man-made ‘new water’ that is produced from desalination are analyzed with implications for predicting the chemical and isotopic compositions of future water resources in the Anthropocene Era.

Experimental Constraints on Core Composition

Abstract: The composition of the Earth's core is a fundamental issue in geochemistry that bears on our understanding of the origin, evolution, and dynamics of the planet. This article reviews experimental constraints on core composition. Following an introduction of general approaches, the authors summarize the evidence for iron being the dominant component of the core and experimental results on the phase diagram and physical properties of iron at high pressures and high temperatures. The review focuses on experimental results on iron-rich systems containing one or more candidate light elements sulfur, silicon, oxygen, carbon, and hydrogen, including the phase relations in single phase, binary, or ternary systems, the densities and sound velocities of iron-rich alloys, and the partitioning of light elements between iron-rich alloys and silicate/oxide phases. Also provided is a brief review of experimental constraints on the abundance and behavior of minor and trace elements in the core, including the heat-producing potassium, the nominally lithophile element niobium, and the highly siderophile elements platinum, rhenium, and osmium. The article concludes with a summary of the latest constraints on core composition and an outlook of obtainable goals for experimental studies in the near future.

Iron Formations: Their Origins and Implications for Ancient Seawater Chemistry

Abstract: Iron formations are economically significant, iron- and silica-rich sedimentary rocks that are restricted to Precambrian successions. There are no known modern or Phanerozoic analogues for these deposits that are comparable in terms of areal extent and thickness. Although many aspects of iron formation origin remain debatable, it is generally accepted that secular changes in the style of deposition are genetically linked to plate tectonic processes, mantle plume events, and evolution of Earth's surface environments. Two types of Precambrian iron formations have been recognized based on depositional and tectonic settings. Iron formations formed proximal to volcanic centers are interlayered with or laterally linked to submarine volcanic rocks and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger sedimentary rock-hosted iron formations are developed in passive-margin settings and typically lack a direct association with volcanic rocks. A full gradation between these two end-members exists in the rock record. Texturally, iron formations are divided into two groups. Banded iron formation (BIF) is predominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is more common in middle to late Paleoproterozoic successions, having been deposited in shallow-marine settings after the rise of atmospheric oxygen at ~2.4 Ga. Secular changes in the style of iron formation deposition have been linked to a diverse array of environmental changes. Geochronologic studies emphasize the periodicity in deposition of giant iron formations, which are coeval with large igneous provinces (LIPs). Giant sedimentary rock-hosted iron formations first appeared ~ 2.6 Ga, possibly when the construction of large continents changed the heat flux across the core–mantle boundary. From ~ 2.6 to ~ 2.4 Ga, global mafic-to-ultramafic magmatism culminated in the deposition of giant sedimentary rock-hosted iron formations in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited immediately before a shift from reducing to oxidizing conditions in the ocean–atmosphere system. Counterintuitively, enhanced magmatism at 2.50–2.45 Ga, which likely delivered large amounts of reductants to shallow-marine environments, may have triggered atmospheric oxidation. After the rise of atmospheric oxygen ~ 2.4 Ga, GIF became more abundant in the rock record than BIF. Iron formations largely disappeared ~ 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global-scale glaciations, during the so-called snowball Earth events. In the Phanerozoic, deeper-water iron formation deposition became restricted to local areas of closed to semi-closed basins, where volcanic and hydrothermal activity was extensive, such as in back-arc basins. In contrast, episodically deposited, basin-scale Phanerozoic oolitic and pisolitic ironstones are linked to periods of intense magmatic activity and ocean anoxia. Late Paleoproterozoic iron formations and at least some Paleozoic ironstones were deposited at the redoxcline, where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to the production of the conditions necessary for deposition of iron formation. Mantle plume events that led to the emplacement of LIPs also enhanced spreading rates of mid-ocean ridges and resulted in higher growth rates of oceanic plateaus; both processes thus contributed to a higher hydrothermal iron flux to the oceans. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the ocean oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the ocean redox state, iron was deposited mainly proximally, generally as oxides or sulfides; manganese was more mobile. It is concluded that occurrences of BIF, GIF, Phanerozoic ironstones, and hydrothermal sedimentary rocks of exhalative origin (exhalites) surrounding VMS systems are a record of a complex interplay among mantle plume events, plate tectonics, and ocean redox conditions throughout Earth's history, in which mantle heat unidirectionally decreased and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter-term fluctuations.

Continental Basaltic Rocks

Abstract: The primary goal of this chapter is to illustrate how geochemical data can be used both to assess the origin of continental basaltic rocks and to study the evolution of the continental lithosphere. The scope of the chapter is limited to a discussion of a select group of ultramafic to mafic ‘intraplate’ igneous rocks, consisting primarily of kimberlites, potassic and sodic alkali basalts, and continental flood basalts. Although basaltic magmatism has occurred throughout Earth's history, the majority of the examples presented here are from Mesozoic and Cenozoic volcanic fields due to the more complete preservation of younger continental mafic igneous rocks. While considerable effort has been expended in studying the chemical differentiation of mafic magmas, the present discussion concentrates on the least differentiated basaltic rocks in a given location. Such rocks generally provide the best estimate the compositions of ‘primary’ magmas generated beneath a given volcanic field and primary magmas provide the most direct insights into the nature of the magma source regions.

Natural Weathering Rates of Silicate Minerals

Abstract: This chapter presents (1) the development of rates that quantitatively describe silicate mineral and rock weathering, (2) a summary of the available literature rate data for the weathering of several common silicate minerals, and (3) a discussion of the chemical, physical, and hydrologic processes that control silicate mineral weathering at the Earth's surface. Quantitative rates of weathering are important in understanding reaction mechanisms and in addressing a number of economic and environmental issues. Mass change, defined in terms of elements, isotopes, or mineral abundances, is determined from either solid-state (soil, regolith, and rock) or solute (pore water, groundwater, and surface water) compositions. Solid-state mass differences reflect weathering over geologic timescales while solute compositions reflect the residence time of the water. These mass losses or gains are normalized to surface area defined on a geographic, volumetric, or mineral-specific basis. The advantages of this approach are that such rates are related to reaction mechanisms and can be used as predictive tools in estimating how weathering will behave under various environmental conditions.

Global Methane Biogeochemistry

Abstract: Methane is the most abundant hydrocarbon in the atmosphere. It plays important roles in atmospheric chemistry and the radiative balance of the Earth. Methane has been studied as an atmospheric constituent for over 200 years. A 1776 letter from Alessandro Volta to Father Campi described the first experiments on flammable ‘air’ released by shallow sediments in Lake Maggiore. This chapter attempts to summarize our current knowledge of the global methane budget, our understanding of physical and microbiological controls on CH 4 sources and sinks, and how well models are representing these processes.

The Biological Pump in the Past

Abstract: The ocean's ‘biological pump’ refers to the coupled biological, chemical, and physical processes that work to concentrate carbon and other biologically active elements in the voluminous ocean interior, sequestering them from the surface ocean and the atmosphere. Current research seeks to understand the relationship of the ocean's biological pump to the Earth's environmental, chemical, and climatic history. Changes in the efficiency of the biological pump are central to most current hypotheses for the cause of the coherent variations of atmospheric CO2 over the ice age climate cycles (i.e., glacial vs. interglacial stages). Here, we review the concepts, tools, and observations relating to this topic. While the biological pump is driven by biological activity in the sunlit surface ocean, its global efficiency is shown to be affected by the ocean's physical circulation, and its net effect on atmospheric CO2 is shown to work through the ocean's acid–base chemistry. We integrate these findings into a proposed recipe for the major dynamics driving CO2 change over the past 800 000 years.

The Biological Pump

Abstract: The biological pump is the set of processes by which inorganic carbon (e.g., carbon dioxide) is fixed into organic matter via photosynthesis and then sequestered away from the atmosphere generally by transport into the deep ocean. This may be accomplished by the passive sinking of particulate organic matter, through the vertical migration of zooplankton, or the downwelling of surface waters rich in dissolved organic matter. In addition to concentrating carbon in the deep sea, the biological pump also significantly affects the distribution of a number of different chemical constituents of ocean water. There is keen interest in being able to predict both the overall capacity and the efficiency of the biological pump in different places and at different times (including in the future). The physical environment, the type of phytoplankton present, the activities of zooplankton, the presence of biominerals and clay minerals, and the structure of the food web all play important roles in determining both the capacity and efficiency of the biological pump on local and regional scales, complicating efforts to portray the biological pump in models.

Fluid Flow in the Deep Crust

Abstract: Fluids are central to a wide array of fundamental lithospheric processes and properties, including mass transfer, heat transfer, reaction progress, the rheological behavior of rocks and seismicity, crustal anatexis, arc magmatism, ore formation, the release of greenhouse gases, such as CO 2 , and the impact of such gases on the long-term carbon cycle. Pelitic sediments and hydrothermally altered oceanic crust will typically lose 2–4 wt% H 2 O from low to high metamorphic grades. Losses from serpentinized ultramafic rocks can exceed 10 wt%. Devolatilization of metacarbonate rocks can liberate 10–20 wt% CO 2 during collisional orogenesis. Degassing intrusions and the mantle are additional fluid sources. The fluids flow pervasively between grains or are channelized into structures, such as fractures, fold hinges, and faults. The flow can be relatively continuous or more pulsed, as exemplified by porosity waves. The transport of dissolved constituents in the fluids occurs by advection (flow), diffusion, and mechanical dispersion, all of which are important during regional flow. Available models and field studies suggest that flow is dominantly upward in deep systems. Time-integrated fluid fluxes can vary by orders of magnitude. The smallest regional fluxes ( 3 m 3 m −2 ) are realized in dominantly pervasive flow systems where flow is distributed over a wide area and/or the availability of fluids is limited. Typical devolatilization of metasediments in orogenic belts is likely to produce larger fluxes ~10 3 –10 4 m 3 m −2 averaged over the regional scale. Still larger fluxes of ~10 4 –10 6 m 3 m −2 require channelization and focusing of flow into veins, shear zones, regional fracture systems, or other conduits. Fluxes of ~10 3 m 3 m −2 or greater can produce substantial mass transfer and metasomatism by, for example, flow up or down temperature gradients or across lithologic contacts. Even when fluxes are smaller, the available flow and diffusion can drive mass transfer at more local scales. Despite several field examples, it remains unclear if large-scale heat transfer by fluids through the deep crust is a widespread phenomenon. Timescales of flow range from 3 years to the duration of orogeny. The shortest timescales correspond to flow associated with fracturing, orogenic thermal pulses, or other transient phenomena. The largest timescales reflect more progressive, long-term devolatilization of orogenic belts. A growing body of evidence indicates that significant flow can occur on geologically short timescales of less than ~10 6 years.

The Contemporary Carbon Cycle

Abstract: Since 1850, the amount of carbon in the atmosphere has increased by ~ 40%, largely as a result of fossil fuel combustion but also from use of land (e.g., deforestation for croplands). Early in this period, the emissions of carbon were largely from land use; now the emissions are largely (~ 85%) from fossil fuels. The annual rates of fossil fuel use and of atmospheric CO 2 growth are accelerating, and averaged 7.9 and 4.1 Pg C per year, respectively, over the decade 2000–10. About half the total emissions each year remains in the atmosphere, and half is taken up by the oceans and terrestrial ecosystems. Despite large annual variations due to terrestrial metabolism, those fractions have remained remarkably stable over the last 50 years. The nearly constant fraction indicates that the annual uptake of carbon by land and oceans has been increasing in proportion to emissions. There is no clear signal, globally, of a saturation of sink strength. Increased emissions regionally from droughts, fires, and thawing of permafrost have apparently been offset by increased uptake of carbon in other regions. The future of these sources and sinks under a changing climate is crucial for predicting the rate of climatic change.

Alkenone Paleotemperature Determinations

Abstract: The alkenone paleotemperature (‘U k 37 ’) method, which relies on determining the unsaturation index of biomarker compounds produced by one group of closely related haptophyte algae, is now widely used to reconstruct near-surface ocean temperatures on timescales ranging from a few centuries to millions of years. In comparison with other methods of paleotemperature estimation, the alkenone method is rapid and extremely precise and appears to obey a global relationship to sea surface temperature (SST). This does not mean that the ecological and physiological complexities of the alkenone-producing organisms may not intrude in some cases on the application of U k 37 to derive past SST or that the results of alkenone paleotemperature reconstructions agree to the stated uncertainties with other paleotemperature proxies. Nevertheless, the alkenone method has proven remarkably successful at generating regionally consistent, physically plausible pictures of past SST change at all timescales and appears to have a reliable place in the toolkit of paleoclimatologists.

Condensation and Evaporation of Solar System Materials

Abstract: The volatile element depletion patterns of planetary size objects and the chemical and isotopic composition of numerous smaller objects such as chondrules and calcium- and aluminum-rich inclusions (CAIs) provide the motivation to consider evaporation and condensation processes in the early solar system. Equilibrium thermodynamic calculations of condensation of phases in the solar nebula provide an important reference with which natural solar system materials can be compared. The theoretical background for fractionation of elemental and isotopic compositions during kinetically controlled evaporation and condensation is explored in some detail. Some parameters can be calculated from first principles, but others must be measured in the laboratory, and a number of evaporation experiments are described and interpreted. Elemental and isotopic fractionations of solar system materials are then intepreted in the context of the theoretical and experimental considerations. A key point is that the processes that led to chondrules and planets appear to have occurred under conditions very close to equilibrium, whereas the processes that led to CAIs involved significant departures from equilibrium.

Formation and Diagenesis of Carbonate Sediments

Abstract: 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).

Manganiferous Sediments, Rocks, and Ores

Abstract: Manganese is of great interest in geochemistry because its minerals are both tracers of redox processes and accumulators of other elements of geochemical significance. The solubility of manganese compared to iron under reducing and mildly oxidizing conditions leads to its export from low-oxygen environments, be they basalt-hydrothermal systems or euxinic sedimentary basins, and its accumulation in oxidizing environments of the shallow ocean or in low-productivity areas of the deep sea. Therefore tracking Mn/Fe ratios provides us a means of reconstructing the oxidation structure of ocean basins, soils, or groundwater systems.

Formation and Geochemistry of Precambrian Cherts

Abstract: Cherts, specifically Precambrian cherts, are fine-grained rocks consisting dominantly or entirely of SiO 2 in the form of microcrystalline quartz, subordinate megaquartz, and (rarely) chalcedonic quartz. Cherts may be chemical precipitates, or they may be replacements of preexisting carbonates, volcanoclastic sediments, or igneous and metamorphic rocks.

Noble Gases as Mantle Tracers

Abstract: The study of the noble gases has been associated with some of the most illustrious names in experimental science, and some of the most profound discoveries. Fundamental advances in nuclear chemistry and physics – including the discovery of isotopes – have resulted from their study, earning Nobel Prizes for a number of early practitioners as well as for their discoverers. In this chapter, we give a brief overview of the discovery of the noble gases and their continuing utility in the Earth Sciences, prior to setting into perspective the present contribution, which focuses on noble gases in the Earth's mantle.

Tropospheric Ozone and Photochemical Smog

Abstract: Ozone and particulates are two of the most important atmospheric pollutants in terms of human health Elevated ozone was first associated with smog events in cities with heavy automobile traffic, especially Los Angeles Elevated ozone and particulates also occur in multiday events over regions 2000 km or more in extent, including both urban and rural areas These occur in densely populated industrialized regions of the United States, Europe, the Mediterranean, and (more recently) China Ozone and particulates both have serious effects on human health, cause damage to agricultural crops, and affect climate Formation of ozone and particulates involves chemical production from organics and nitrogen oxides in the presence of sunlight and warm temperatures The process is significantly impacted by biogenic emission of organics, primarily from trees In recent years, pollution controls have been partially successful in reducing ozone and particulate levels in the United States and Europe This chapter describes the formation process for ozone and particulates, their relation to organics, nitrogen oxides, biogenic emissions, atmospheric dynamics and global cycles, impacts on health, agriculture, and climate

The Geologic History of the Carbon Cycle

Abstract: Geologists, like other scientists, tend to view the global carbon cycle through the lens of their particular training and experience This chapter describes the behavior of the carbon cycle prior to human influence It describes events and processes that extend back through geologic time and include the exchange of carbon between the Earth's surface and the long-term reservoirs in the lithosphere This chapter begins with an overview of the carbon exchanges and processes that control the variations observed in the geologic record of the carbon cycle Then examples of past carboncycle change are described, beginning with the most recent variations seen in cores drilled from glaciers and the sea floor, and concluding with the distant transformations inferred from the rock record of the Precambrian

Green Clay Minerals

Abstract: Color poses a problem for scientific study The first problem is the vocabulary one uses to describe color Mint green, bottle green, and kelly green are nice names but not of great utility in that people's physical perception of color is not always the same A second problem is that color in a spectral identification is the result of several absorption emissions, with overlapping signals, forming a complicated spectrum Interpretation depends greatly on the spectrum of the light source and the conditions of transmission–reflection of the sample In this chapter, reference is made concerning color, to thin section microscopic perception The discussion is based largely upon old data, those which combine iron oxidation states and XRD information It seems clear that green clay mineral names can reflect either compositional differences, mineral structure differences, or differences in geological occurrence

Tracers of Past Ocean Circulation

Abstract: This chapter reviews the major geochemical methods used to reconstruct past ocean circulation. The carbon isotopic composition and trace metal concentrations in the calcite tests of benthic foraminifera are used to reconstruct the nutrient content of deep water masses. These water mass tracers reflect both biological cycling and physical circulation and mixing in the oceans. Conservative water mass tracers which reflect circulation and mixing alone are also discussed. These tracers include both the isotopic and chemical composition of the benthic foraminifera as well as information from sediment pore waters. Methods for inferring the rates of past ocean circulation are also examined, including radiocarbon, the accumulation of the decay products of uranium in sediments, and geostrophic shear estimates. Finally, the ocean circulation inferred for the Last Glacial Maximum based on these techniques is discussed.

The Global Sulfur Cycle

Abstract: Sulfur plays major biogeochemical roles but is also a pollutant most evident in acid rain. Living organisms require sulfur, but the cycle shows some remarkable differences from the important cycle of nitrogen. Its oxidized state forms low solubility sulfates, such as gypsum, and sulfide minerals are common (e.g., iron sulfides), so it is more likely to be stored in sediments than nitrogen. Sulfur can form S–S bonds in a range of polysulfides and polythionates. Volcanoes emit an important source of sulfur compounds, particularly sulfur dioxide and hydrogen sulfide. There are a wide range of volatile sulfides, carbonyl sulfide and carbon disulfide, and a range of organosulfides from the oceans because seawater has high sulfate concentrations. Dimethyl sulfide represents a major flux of sulfur to the atmosphere. Carbonyl sulfide is unreactive in the troposphere and is transferred to the stratosphere along with sulfur dioxide from volcanic eruptions. These sources dominate the sulfur chemistry of the stratosphere. A number of planets and moons of the solar system also have an elaborate sulfur cycles.

13.3 – Stable Isotope Geochemistry of Mineral Deposits

Abstract: In this chapter, the intent is to summarize the results of traditional stable isotope studies (mainly H, B, O, C, and S) that have greatly contributed to the understanding of ore-forming processes over the last 60 years and to provide an up-to-date assessment of the application of new nontraditional isotope systems (Fe, Cu, Zn, Se, Mo, Hg, and Tl).

Organic Nitrogen: Sources, Fates, and Chemistry

Abstract: Nitrogen is an essential element in living systems and plays key roles in biochemical and physiological processes in the cell. It is dispensable for the structure and function of proteins, nucleic acids, and many other organic molecules produced by organisms. This chapter not only aims to provide an overview of previous studies on processes related to organic nitrogen in the natural environment but also tries to bridge between nitrogen biochemistry and geochemistry mainly through isotopic signature to provide useful introduction for the future studies. It describes the biogeochemical aspects of the degradation of nitrogenous compounds, both in the water column and sediments. Such information is useful for interpreting the various types of sedimentary nitrogen represented in the isotopic record. It focuses mainly on nitrogen-related biogeochemical processes in the context of molecular isotopic signatures, with an emphasis on the aquatic realm.