The history of Earth in the Solar System has been unraveled using natural radioactivity. The sources of this radioactivity are the original creation of the elements and the subsequent bombardment of objects, including Earth, in the Solar System by cosmic rays. Both radioactive and radiogenic nuclides are harnessed to arrive at ages of various events and processes on Earth.
This collection of chapters from the Treatise on Geochemistry displays the range of radioactive geochronometric studies that have been addressed by researchers in various fields of Earth science. These range from the age of Earth and the Solar System to the dating of the history of Earth that assists us in defining the major events in Earth history. In addition, the use of radioactive geochronometry in describing rates of Earth surface processes, including the climate history recorded in ocean sediments and the patterns of circulation of the fluid Earth, has extended the range of utility of radioactive isotopes as chronometric and tracer tools.
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- Affordably priced sampling of ,content from the full Treatise on Geochemistry
The history of Earth in the Solar System has been unraveled using natural radioactivity. The sources of this radioactivity are the original creation of the elements and the subsequent bombardment of objects, including Earth, in the Solar System by cosmic rays. Both radioactive and radiogenic nuclides are harnessed to arrive at ages of various events and processes on Earth. This collection of chapters from the Treatise on Geochemistry displays the range of radioactive geochronometric studies that have been addressed by researchers in various fields of Earth science. These range from the age of Earth and the Solar System to the dating of the history of Earth that assists us in defining the major events in Earth history. In addition, the use of radioactive geochronometry in describing rates of Earth surface processes, including the climate history recorded in ocean sediments and the patterns of circulation of the fluid Earth, has extended the range of utility of radioactive isotopes as chronometric and tracer tools. Comprehensive, interdisciplinary and authoritative content selected by leading subject experts Robust illustrations, figures and tables Affordably priced sampling of content from the full Treatise on Geochemistry
Cover Page 1
Radioactive Geochronometry 2
Copyright Page 3
Contents 4
Introduction 6
References 7
Contributors 8
Chapter 1: Cosmic-Ray Exposure Ages of Meteorites 10
1.1. Introduction 11
1.2. Calculation of Exposure Ages 12
1.2.1. Basic Equations 12
1.2.2. Factors Influencing Production Rates 12
1.2.3. Measurement Units and Quantities 13
1.2.4. Calibration of Production Rates 14
1.2.4.1. 26Al Versus 21Ne Calibration 14
1.2.4.2. 83Kr/81Kr Calibration 14
1.2.5. Equations for Calculating One-Stage Cre Ages 15
1.2.5.1. 21Ne-22Ne/21Ne Ages 15
1.2.5.1.1. Measurements Needed 15
1.2.5.1.2. Range of Applicability 15
1.2.5.1.3. Limitations 15
1.2.5.2. 38Ar-22Ne/21Ne Ages 16
1.2.5.2.1. Measurements Needed 16
1.2.5.2.2. Range of Applicability 16
1.2.5.2.3. Limitations 16
1.2.5.3. 3He Ages 16
1.2.5.3.1. Range of Applicability 16
1.2.5.3.2. Measurements Needed 16
1.2.5.3.3. Limitations 16
1.2.5.4. 36Cl/36Ar Ages 17
1.2.5.4.1. Measurements Needed 17
1.2.5.4.2. Range of Applicability 17
1.2.5.4.3. Limitations 17
1.2.5.5. 81Kr/Kr Ages 17
1.2.5.5.1. Measurements Needed 17
1.2.5.5.2. Range of Applicability 17
1.2.5.5.3. Limitations 17
1.2.5.6. 40K/K Ages 17
1.2.5.6.1. Measurements Needed 19
1.2.5.6.2. Range of Applicability 19
1.2.5.6.3. Limitations 19
1.2.6. The Importance of Half-Lives 19
1.3. Carbonaceous Chondrites 19
1.3.1. Ci, Cm, Co, Cv, and Ck Chondrites 19
1.3.2. The Cr Clan 20
1.4. H-Chondrites 21
1.5. L-Chondrites 21
1.6. Ll Chondrites 22
1.7. E-Chondrites 22
1.8. R-Chondrites 23
1.9. Lodranites and Acapulcoites 23
1.10. Lunar Meteorites 24
1.10.1. Overview 24
1.10.2. Construction of Cre Histories 24
1.10.3. Production Rate of Lunar Meteorites 27
1.11. Howardite-Eucrite-Diogenite Meteorites 27
1.11.1. Eucrites 27
1.11.2. Diogenites 28
1.11.3. Howardites 29
1.11.4. Kapoeta 29
1.12. Angrites 30
1.13. Ureilites 30
1.14. Aubrites (Enstatite Achondrites) 30
1.15. Brachinites 31
1.16. Martian Meteorites 31
1.17. Mesosiderites 33
1.18. Pallasites 34
1.19. Irons 34
1.20. The Smallest Particles: Micrometeorites, Interplanetary Dust Particles, and Interstellar Grains 35
1.20.1. Background 35
1.20.2. Micrometeorites and Idps 35
1.20.3. Interstellar Grains 36
1.21. Conclusions 37
Acknowledgments 38
References 38
Chapter 2: Early Solar System Chronology 44
2.1. Introduction 45
2.1.1. Chondritic Meteorites as Probes of Early Solar System Evolution 45
2.1.2. Short-Lived Radioactivity At the Origin of the Solar System 45
2.1.3. A Brief History and the Scope of the Present Review 46
2.2. Dating With Ancient Radioactivity 47
2.3. "Absolute" and "Relative" Timescales 48
2.3.1. An Absolute Timescale for Solar System Formation 48
2.3.2. An Absolute Timescale for Chondrule Formation 50
2.3.3. An Absolute Timescale for Early Differentiation of Planetesimals 51
2.4. The Record of Short-Lived Radionuclides in Early Solar System Materials 51
2.4.1. Beryllium-7 52
2.4.2. Calcium-41 52
2.4.3. Chlorine-36 53
2.4.4. Aluminum-26 53
2.4.5. Iron-60 58
2.4.6. Beryllium-10 60
2.4.7. Manganese-53 61
2.4.8. Palladium-107 64
2.4.9. Hafnium-182 64
2.4.10. Iodine-129 64
2.4.11. Lead-205 65
2.4.12. Niobium-92 65
2.4.13. Plutonium-244 and Samarium-146 65
2.5. Origins of the Short-Lived Nuclides in the Early Solar System 65
2.6. Implications for Chronology 68
2.6.1. Formation Timescales of Nebular Materials 68
2.6.2. Timescales of Planetesimal Accretion and Early Chemical Differentiation 70
2.7. Conclusions 71
2.7.1. Implications for Solar Nebula Origin and Evolution 71
2.7.2. Future Directions 72
Acknowledgment 73
References 73
Chapter 3: The Origin and Earliest History Of the Earth 80
3.1. Introduction 81
3.2. Observational Evidence and Theoretical Constraints Pertaining to the Nebular Environment From Which Earth Originated... 81
3.2.1. Introduction 81
3.2.2. Nebular Gases and Earth-like Versus Jupiter-like Planets 82
3.2.3. Depletion in Moderately Volatile Elements 82
3.2.4. Solar Mass Stars and Heating of the Inner Disk 83
3.2.5. The "Hot Nebula" Model 85
3.2.6. The "Hot Nebula" Model and Heterogeneous Accretion 86
3.3. The Dynamics of Accretion of the Earth 87
3.3.1. Introduction 87
3.3.2. Starting Accretion: Settling and Sticking of Dust at 1Au 87
3.3.3. Starting Accretion: Migration 88
3.3.4. Starting Accretion: Gravitational Instabilities 88
3.3.5. Runaway Growth 89
3.3.6. Larger Collisions 89
3.4. Constraints From Lead and Tungsten Isotopes on the Overall Timing, Rates, and Mechanisms of Terrestrial Accretion 90
3.4.1. Introduction: Uses and Abuses of Isotopic Models 90
3.4.2. Lead Isotopes 92
3.4.3. Tungsten Isotopes 93
3.5. Chemical and Isotopic Constraints on the Nature of the Components That Accreted to Form the Earth 98
3.5.1. Chondrites and the Composition of the Disk From Which Earth Accreted 98
3.5.2. Chondritic Component Models 99
3.5.3. Simple Theoretical Components 100
3.5.4. The Nonchondritic Mg/Si of the Earth's Primitive Upper Mantle 100
3.5.5. Oxygen Isotopic Models and Volatile Losses 101
3.6. Earth's Earliest Atmospheres and Hydrospheres 101
3.6.1. Introduction 101
3.6.2. Did the Earth have a Nebular Protoatmosphere? 101
3.6.3. Earth's Degassed Protoatmosphere 103
3.6.4. Loss of Earth's Earliest Atmosphere(s) 104
3.7. Magma Oceans and Core Formation 105
3.8. The Formation of the Moon 106
3.9. Mass Loss and Compositional Changes During Accretion 111
3.10. Evidence for Late Accretion, Core Formation, and Changes in Volatiles After The Giant Impact 113
3.11. The Hadean 114
3.11.1. Early Mantle Depletion 114
3.11.1.1. Introduction 114
3.11.1.2. 92Nb-92Zr 114
3.11.1.3. 146Sm-142Nd 114
3.11.1.4. 176Lu-176Hf 115
3.11.2. Hadean Continents 115
3.11.3. The Hadean Atmosphere, Hydrosphere, and Biosphere 116
3.12. Concluding Remarks-The Prognosis 117
Acknowledgment 118
References 118
Chapter 4: Long-Lived Chronometers 128
4.1. Introduction 128
4.1.1. Basic Principles 128
4.1.2. Application to Meteorites and Planetary Materials: a Historical Perspective 129
4.2. Chondrites and Their Components 130
4.2.1. Formation Ages of Chondritic Components 130
4.2.1.1. Calcium-, aluminum-rich Inclusions 130
4.2.1.2. Chondrules 132
4.2.2. Ages of Secondary Events Recorded In Chondrites 132
4.2.2.1. Aqueous Alteration 132
4.2.2.2. Thermal Metamorphism 133
4.2.2.3. Shock Metamorphism 134
4.3. Differentiated Meteorites 135
4.3.1. Primitive Achondrites: Timing of Incipient Differentiation on Planetesimals 135
4.3.2. Basaltic and Other Achondrites: Timing of Asteroidal Differentiation and Cataclysm 135
4.3.2.1. Crust-formation Timescales From Chronology of Achondrites and Their Components 135
4.3.2.2. Global Differentiation Timescales Based on whole-rock Isochrons and Initial 87Sr/86Sr 137
4.3.2.3. Inner Solar System Bombardment History Based on Reset Ages 140
4.3.3. Iron Meteorites and Pallasites: Timescales of Core Crystallization on Planetesimals 140
4.4. Planetary Materials 142
4.4.1. Timing of Lunar Differentiation and Cataclysm From Chronology of Lunar Samples 143
4.4.1.1. Lunar Differentiation History 143
4.4.1.2. Lunar Bombardment History 144
4.4.2. Timescales for the Evolution of Mars From Chronology of Martian Meteorites 144
4.5. Conclusions 146
4.5.1. A Timeline for Solar System Events 146
4.5.2. Outlook and Future Prospects 146
References 147
Chapter 5: The Geochemistry and Cosmochemistry of Impacts 152
5.1. Introduction: the Use of Geochemistry in Impact Studies 153
5.2. Background on Impact Craters and Processes 153
5.3. Methods 158
5.3.1. General Geochemistry: Major and Trace Elements 158
5.3.2. Rb-Sr and Sm-Nd Isotopes 159
5.3.3. Siderophile Element Studies 160
5.3.4. Osmium Isotopes 162
5.3.5. Chromium Isotopes 164
5.3.6. Tungsten Isotopes 165
5.3.7. Stable Isotopes 166
5.3.8. Other Methods 167
5.3.8.1. Helium-3 167
5.3.8.2. Beryllium-10 167
5.3.8.3. Remote Sensing 168
5.4. Examples 168
5.4.1. Meteorite Craters: Source Rocks And Impactites 168
5.4.2. Extraterrestrial Components in Impactites 171
5.4.2.1. Vredefort Impact Structure (South Africa) 175
5.4.2.2. Morokweng Impact Structure (South Africa) 176
5.4.2.3. Bosumtwi Crater, Ghana 178
5.4.3. Tektites 179
5.4.4. Libyan Desert Glass 181
5.4.5. Cretaceous-Tertiary (K-T) Boundary 181
5.4.5.1. Enrichments and Patterns of the Pges 182
5.4.5.2. Osmium and Chromium Isotopic Data 183
5.4.5.3. A Fragment of the K-T Bolide? 183
5.4.5.4. Presence of Evidence for Impact-induced Wildfires 183
5.4.5.5. Shock Metamorphism 183
5.4.5.6. Distal Impact Glasses 183
5.4.5.7. Impact-derived Diamonds 183
5.4.5.8. Spinel 184
5.4.5.9 Summary 184
5.4.6. Permian-Triassic (P-Tr) Boundary 185
5.4.7. Precambrian Spherule Layers 186
5.4.8. The Earth's Earliest Impact History 189
5.5 Summary 191
Acknowledgment 192
References 192
Chapter 6: Geochronology and Thermochronology in Orogenic Systems 202
Nomenclature 202
6.1. Introduction 203
6.2. Basic Concepts of Geochronology 203
6.2.1. Effects of Branched, Sequential, And Multiparent Decay 204
6.2.2. Fission-track Geochronology 205
6.2.3. Chemical Pb Dating 205
6.3. Analytical Methods 205
6.3.1. Microanalytical Techniques 206
6.4. The Interpretation of Dates as Crystallization Ages 207
6.5. Open-System Behavior: The Role of Diffusion 207
6.5.1. Modes of Diffusion 208
6.5.2. Experimental Constraints on Daughter-isotope Diffusion for Useful Minerals 209
6.6. Closure Temperature Theory 210
6.6.1. Quantitative Estimates of Closure Temperatures 212
6.6.2. The Influence of Input Parameters on Closure Temperature Calculations 213
6.6.3. Qualitative Estimates of Closure Temperatures 214
6.6.4. Fission-track Closure Temperatures and the Partial Annealing Zone 215
6.7. Applications 215
6.7.1. Determining Timescales of Granitic Magmatism 215
6.7.2. Constraining the Cooling Histories of Igneous Rocks 216
6.7.3. Calibrating Metamorphic Histories 216
6.7.4. Calibrating Deformational Histories 218
6.7.5. Estimating Unroofing Rates 219
6.7.6. Monitoring the Evolution of Topography 220
6.7.7. Reconstructing Regional Patterns of Deformation and Erosion 222
6.8. Directions for Future Research 222
Acknowledgment 224
References 224
Chapter 7: Ages and Growth of the Continental Crust from Radiogenic Isotopes 232
7.1. Scope of Available Methods and Data 233
7.2. Determination of Ages of Igneous Events 233
7.2.1. Early Developments in U-Th-Pb Geochronology 233
7.2.2. U-Th-Pb Dating by TIMS-The Isotope Dilution Method 234
7.2.3. Zircon Evaporation Method 234
7.2.4. U-Th-Pb Dating by Ion Microprobe 235
7.2.5. U-Th-Pb Dating by ICP-MS 236
7.2.6. U-Th-Pb Dating of Monazite Using Only Uranium, Thorium, and Lead Concentrations 237
7.3. Determination of Ages of Metamorphism 238
7.3.1. 40Ar/39Ar Thermochronology 238
7.3.2. Rb-Sr Dating 239
7.3.3. Sm-Nd and Lu-Hf Dating 240
7.4. Determination of Ages of Uplift or Exhumation 240
7.4.1. 40Ar/39Ar Dating of Potassium Feldspar 240
7.4.2. FT Dating of Apatite 241
7.4.3. (U-Th)/He Dating of Apatite 241
7.5. Neodymium Isotopes and Chemical Age of Crust 242
7.5.1. Sm-Nd Methodology 242
7.5.2. Juvenile Crust Production Versus Intracrustal Recycling 243
7.5.3. Juvenile Crust Production at 1.9-1.7Ga 244
7.5.4. Juvenile Crust Production in the Canadian Cordillera 245
7.5.5. Juvenile Crust Production in the Altaid Collage of Central Asia 247
7.6. Isotopes and Pre-3 Ga Continental Crust 248
7.6.1. Existence of Ancient Continental Crust 248
7.6.2. Crustal Growth Events and Recycling into the Mantle 249
7.6.3. Acasta Gneisses, Northwest Territories, Canada 249
7.6.4. Narryer Terrane, Western Australia 250
Acknowledgment 251
References 251
Chapter 8: Radiocarbon 260
8.1. Introduction 260
8.2. Production and Distribution of 14C 260
8.3. Measurements of Radiocarbon 261
8.4. Timescale Calibration 261
8.4.1. Calibration Based on Tree Rings 262
8.4.2. Calibration Based on Corals 263
8.4.3. Other Calibration Schemes 264
8.4.4. Cause of the Long-Term 14C Decline 265
8.4.5. Change in Ocean Operation 266
8.5. Radiocarbon and Solar Irradiance 268
8.6. The "Bomb" 14C Transient 270
8.6.1. Radiocarbon as a Tracer for Ocean Uptake of Fossil Fuel Co2 270
8.6.2. Ocean Uptake of 14Co2 and Co2 271
8.6.3. Terrestrial Uptake of 14Co2 and Co2 272
8.7. Future Applications 274
References 274
Chapter 9: Natural Radionuclides in the Atmosphere 278
9.1. Introduction 278
9.2. Radon and Its Daughters 280
9.2.1. Flux of Radon From Soils to the Atmosphere 280
9.2.2. Flux of Radon From the Oceans 282
9.2.3. Distribution of Radon in the Atmosphere 282
9.2.4. Short-lived Daughters of 222Rn in the Atmosphere 282
9.2.5. 210Pb and Its Progeny 283
9.2.5.1. Distribution of 210Pb in the Atmosphere 283
9.2.5.2. Flux of 210Pb to Earth's Surface 284
9.2.5.3. Residence Time of 210Pb and Associated Species in the Atmosphere 285
9.2.5.4. Use of 210Pb as Surrogate for Other Atmospheric Component 288
9.3. Cosmogenic Nuclides 289
9.3.1. Atmospheric Production of Cosmogenic Nuclides 289
9.3.2. 7Be and 10Be 290
9.3.3. 35S and the Kinetics of So2 Oxidation and Deposition 290
9.3.4. Phosphorus Isotopes 291
9.4. Coupled Lead-210 and Beryllium-7 291
9.4.1. Temporal and Spatial Variation 291
9.4.2. Application of the Coupled 7Be-210Pb System to Sources of Atmospheric Species 293
References 295
Chapter 10: Groundwater Dating and Residence-time Measurements 298
10.1. Introduction 299
10.2. Nature of Groundwater Flow Systems 300
10.2.1. Driving Forces 300
10.2.2. Topographic Control on Flow 300
10.2.3. Hydraulic Conductivity and Its Variability 300
10.2.4. Scales of Flow Systems 301
10.2.4.1. Vadose-zone Scale 301
10.2.4.2. Local Scale 302
10.2.4.3. Aquifer Scale 302
10.2.4.4. Regional Scale 302
10.2.5. Sources of Solutes at Various Scales 302
10.2.5.1. Meteoric (recharge) 302
10.2.5.2. Weathering 303
10.2.5.3. Diagenetic 303
10.2.5.4. Connate 303
10.2.5.5. "Basement Waters" 303
10.3. Solute Transport in Subsurface Water 303
10.3.1. Fundamental Transport Processes 304
10.3.1.1. Advection 304
10.3.1.2. Diffusion 304
10.3.1.3. Dispersion 304
10.3.2. Advection-Dispersion Equation 304
10.3.3. Interaction between Hydrogeological Heterogeneity and Transport 305
10.3.3.1. Small-scale Transport and Effective Dispersion 305
10.3.3.2. Large-scale Transport and Mixing 305
10.3.4. Groundwater Dating and the Concept of "Groundwater Age" 306
10.4Summary of Groundwater Age Tracers 307
10.4.1. Introduction 307
10.4.2. Radionuclides for Age Tracing of Subsurface Water 307
10.4.2.1. Argon-37 307
10.4.2.2. Sulfur-35 307
10.4.2.3. Krypton-85 307
10.4.2.4. Tritium 309
10.4.2.5. Silicon-32 309
10.4.2.6. Argon-39 310
10.4.2.7. Carbon-14 310
10.4.2.8. Krypton-81 311
10.4.2.9. Chlorine-36 311
10.4.2.10. Iodine-129 312
10.4.3. Stable, Transient Tracers 312
10.4.3.1. Chlorofluorocarbons and Sulfur Hexafluoride 312
10.4.3.2. Atmospheric Noble Gases and Stable Isotopes 313
10.4.3.3. Nonatmospheric Noble Gases 313
10.5. Lessons from Applying Geochemical Age Tracers to Subsurface Flow and Transport 315
10.5.1. Introduction 315
10.5.2. Approaches 316
10.5.2.1. Direct Groundwater Age Estimation 316
10.5.2.2. Modeling Techniques: Analytical Versus Numerical 317
10.5.2.3. Identification of Sources and Sinks of a Particular Tracer 317
10.5.2.4. Defining Boundary Conditions (modeling Approach) 318
10.6. Tracers At the Regional Scale 318
10.6.1. Introduction 318
10.6.2. Examples of Applications 318
10.6.2.1. Noble-gas Isotopes-Paris Basin 318
10.6.2.2. Great Artesian Basin-noble-gas Isotopes, 36Cl, and 81Kr 320
10.6.2.3. Implications At the Regional Scale 323
10.7. Tracers At the Aquifer Scale 324
10.7.1. Introduction 324
10.7.2. Carrizo Aquifer 325
10.7.3. Implications at the Aquifer Scale 327
10.8. Tracers at the Local Scale 329
10.8.1. Introduction 329
10.8.2. 3H/3He, Cfc-11, Cfc-12, 85Kr-Delmarva Peninsula 330
10.8.3. Implications at the Local Scale 330
10.9. Tracers in Vadose Zones 330
10.10. Conclusions 335
References 336
Chapter 11: Cosmogenic Nuclides in Weathering and Erosion 344
11.1. Introduction 345
11.1.1. Definitions and Nomenclature 345
11.1.2. Quantifying Weathering and Erosion 345
11.1.2.1. Quantifying Physical Erosion Rates 346
11.1.2.2. Quantifying Chemical Weathering Rates 347
11.1.2.3. The Advent of Cosmogenic Nuclide Methods 347
11.2. Cosmogenic Nuclide Systematics at Earth's Surface 347
11.2.1. Cosmic Rays 347
11.2.2. Cosmogenic Nuclide Production 348
11.2.3. Cosmogenic Nuclide Production Profiles 349
11.3. Using Cosmogenic Nuclides to Determine Rates of Surface Lowering and Denudation 351
11.3.1. No Erosion (Surface Exposure Dating) 351
11.3.1.1. Theory 351
11.3.1.1.1. Complications 351
11.3.1.2. Examples 351
11.3.1.2.1. Dating Strath Terraces 351
11.3.1.2.2. Dating Glacial Moraines 352
11.3.2. Rock Erosion 353
11.3.2.1. Theory 353
11.3.2.1.1. Steady Erosion 353
11.3.2.1.2. Complications Due to Unsteady Denudation 353
11.3.2.2. Examples 354
11.3.2.2.1. Denudation of Bedrock Surfaces 354
11.3.2.2.2. Rock Erosion Under Soil Cover 355
11.3.3. Cosmogenic Nuclides in Vertically Mixed Soils 357
11.3.3.1. Theory 357
11.3.3.1.1. Exposure Dating of Vertically Mixed Sediment 357
11.3.3.1.2. Denudation Rates in Vertically Mixed Soils 358
11.3.3.1.3. Complications Due to Selective Dissolution and Quartz Enrichment 359
11.3.3.2. Examples 360
11.3.3.2.1. Exposure Dating Marine Terraces 360
11.3.4. Spatially Averaged Denudation Rates 361
11.3.4.1. Theory 361
11.3.4.1.1. Complications 362
11.3.4.2. Examples 362
11.3.4.2.1. Test of the Method: Comparing Cosmogenic Rates to Absolute Erosion Rates 362
11.3.4.2.2. Transient Versus Dynamic Equilibrium Landscapes 363
11.4. Chemical Weathering Inferred From Cosmogenic Nuclides 365
11.4.1. Chemical Weathering in Noneroding Soils 366
11.4.1.1. Theory 366
11.4.1.1.1. Complications 367
11.4.1.2. Example 368
11.4.1.2.1. Weathering Rates of Glacial Moraines 368
11.4.2. Chemical Weathering Rates of Soil Along an Eroding Slope 369
11.4.2.1. Theory 369
11.4.2.1.1. Complications 369
11.4.2.2. Example 370
11.4.2.2.1. Weathering Rates on a Hillslope in Australia 370
11.4.3. Chemical Weathering Rates of Soils and Landscapes 370
11.4.3.1. Theory 371
11.4.3.1.1. Complications 372
11.4.3.2. Examples 373
11.4.3.2.1. Test of the Mass-balance Approach 373
11.4.3.2.2. Chemical Weathering as a Function of Altitude 374
11.4.3.2.3. Quantifying How Chemical Weathering and Physical Erosion Interrelate 375
11.4.3.2.4. Climatic Effects on Chemical Weathering 376
11.5Summary 378
Glossary 380
References 381
Chapter 12: Geochronometry of Marine Deposits 386
12.1. Introduction 387
12.2. Principles 387
12.2.1. Radioactive Geochronometry 387
12.2.2. Secondary Stratigraphic Procedures 387
12.3. Radioactive Systems Used in Marine Geochronometry 387
12.3.1. Radiocarbon 388
12.3.2. Uranium and Thorium Decay Chain Nuclides 388
12.3.3. Cosmogenic Nuclides 388
12.3.4. Potassium-Argon 389
12.4. Coastal Deposits 389
12.4.1. Applicable Methods and Requirements 389
12.4.2. Unbioturbated Deposits 389
12.4.3. Bioturbated Deposits 390
12.5. Deep-sea Sediments 391
12.5.1. Radiocarbon 391
12.5.2. 230Th and 231Pa 392
12.5.2.1. The Basic Theory 392
12.5.2.2. The Underlying Assumptions 393
12.5.2.3. Applications 393
12.5.2.4. Problems of Erosion and Focusing 394
12.5.3. 10Be 395
12.5.4. 3He 395
12.5.5. Volcanic Layers 396
12.5.6. Extension of Dating Techniques 397
12.5.6.1. The Milankovitch Cycles and Chronology 397
12.5.6.2. Oxygen and Carbon Isotopes in Carbonate Tests 397
12.5.6.3. Magnetic-reversal Stratigraphy 398
12.5.6.4. Element Accumulation: Titanium and Cobalt 398
12.6. Ferromanganese Deposits 398
12.6.1. Applicable Methods 398
12.6.2. The Underlying Assumptions 399
12.6.3. Applications 399
12.7. Corals 400
12.8. Methods Not Depending on Radioactive Decay 402
12.8.1. Amino Acid Racemization 402
12.8.2. Thermoluminescence 402
Acknowledgments 403
References 403
Chapter 13: Chronometry of Sediments and Sedimentary Rocks 408
13.1. Introduction 408
13.2. Chronometry Based on the Fossil Record-First Steps 408
13.3. Refinements in Chronometry Using Fossils 413
13.4. Oil Recovery in California Using Fossil-based Chronometry 416
13.5. Principles of Chorology: the Science of the Distribution of Organisms 418
13.6. Constraints on Chronometry Imposed By Chorology 422
13.7. Radiochronometry 424
13.8. Magnetic Field Polarity and Chronometry 427
13.9. Orbital Chronometry 428
13.9.1. Aurichorology-The Golden Spikes and Global Statotype Section and Points 428
13.10. Terminologies 430
13.11. Summary 430
References 431
Chapter 14: The Early History of Life 434
14.1. Introduction 435
14.1.1. Strangeness and Familiarity-The Youth of the Earth 435
14.1.2. Evidence in Rocks, Moon, Planets, and Meteorites-The Sources of Information 435
14.1.3. Reading the Palimpsests-Using Evidence from the Modern Earth and Biology to Reconstruct the Ancestors and their Home. 436
14.1.4. Modeling-The Problem of Taking Fragments of Evidence and Rebuilding the Childhood of the Planet 436
14.1.5. What Does a Planet Need to be Habitable? 437
14.1.6. The Power of Biology: the Infinite Improbability Drive 437
14.2. The Hadean (~4.56-4.0 Ga AGO) 438
14.2.1. Definition of Hadean 438
14.2.2. Building a Habitable Planet 438
14.2.3. The Hadean Record 440
14.2.4. When and Where Did Life Start? 440
14.3. The Archean (~4-2.5 Ga AGO) 441
14.3.1. Definition of Archean 441
14.3.2. The Archean Record 441
14.3.2.1. Greenland 441
14.3.2.2. Barberton 442
14.3.2.3. Western Australia 442
14.3.2.4. Steep Rock, Ontario, and Pongola, South Africa 443
14.3.2.5. Belingwe 443
14.4. The Functioning of the Earth System in the Archean 444
14.4.1. The Physical State of the Archean Planet 444
14.4.2. The Surface Environment 445
14.5. Life: Early Setting and Impact on the Environment 447
14.5.1. Origin of Life 447
14.5.2. Rna World 448
14.5.3. The Last Common Ancestor 449
14.5.4. A Hyperthermophile Heritage? 452
14.5.5. Metabolic Strategies 452
14.6. The Early Biomes 454
14.6.1. Location of Early Biomes 454
14.6.2. Methanogenesis: Impact on the Environment 454
14.6.3. Prephotosynthetic Ecology 456
14.6.4. Geological Settings of The Early Biomes 456
14.7. The Evolution of Photosynthesis 457
14.7.1. The Chain of Photosynthesis 457
14.7.2. The Rubisco Fingerprint 458
14.7.3. The Evolutionary Chain 458
14.7.4. Anoxygenic Photosynthesis 459
14.7.5. Oxygenic Photosynthesis 461
14.7.6. Archean Oxygen 462
14.8. Mud-stirrers: Origin and Impact of the Eucarya 462
14.8.1. The Ancestry of the Eucarya 462
14.8.2. Possible Settings for the Eukaryote Endosymbiotic Event 464
14.8.3. Water and Mud Stirring-Consequences 464
14.9. The Breath of Life: the Impact of Life on the Ocean/Atmosphere System 465
14.9.1. The Breath of Life 465
14.9.2. Oxygen and Carbon Dioxide 465
14.9.3. Nitrogen and Fixed Nitrogen 465
14.9.4. Methane 466
14.9.5. Sulfur 466
14.10. Feedback from the Biosphere to the Physical State of the Planet 467
Acknowledgment 467
References 467
Chapter 15: Heavy Metals in the Environment-Historical Trends 472
15.1. Introduction 473
15.1.1. Metals: Pb, Zn, Cd, Cr, Cu, Ni 473
15.1.2. Sources of Metals 473
15.1.2.1. Natural 473
15.1.2.2. Anthropogenic 473
15.1.3. Source and Pathways 476
15.2. Occurrence, Speciation, And Phase Associations 476
15.2.1. Geochemical Properties and Major Solute Species 476
15.2.1.1. Lead 476
15.2.1.2. Zinc 477
15.2.1.3. Cadmium 477
15.2.1.4. Chromium 478
15.2.1.5. Copper 478
15.2.1.6. Nickel 479
15.2.2. Occurrence in Rocks, Soils, Sediments, Anthropogenic Materials 480
15.2.3. Geochemical Phase Associations in Soils and Sediments 481
15.3. Atmospheric Emissions of Metals and Geochemical Cycles 484
15.3.1. Historical Heavy Metal Fluxes to the Atmosphere 485
15.3.2. Perturbed Heavy Metal Cycles 486
15.3.3. Global Emissions of Heavy Metals 487
15.3.4. Us Emissions of Heavy Metals 488
15.3.4.1. Lead 488
15.3.4.2. Zinc 489
15.3.4.3. Cadmium 491
15.4. Historical Metal Trends Reconstructed From Sediment Cores 491
15.4.1. Paleolimnological Approach 491
15.4.2. Age Dating 494
15.4.3. Selected Reconstructed Metal Trends 495
15.4.3.1. Lead and Leaded Gasoline: Consequence of the Clean Air Act 495
15.4.3.2. Zinc From Rubber Tire Wear 495
15.4.3.3. Metal Processing and Metal Trends in sediment Cores 497
15.4.3.4. Reduction in Power Plant Emissions of Heavy Metals: Clean Air Act Amendments and the Use of low Sulfur Coal... 500
15.4.3.5. European Lacustrine Records of Heavy Metal Pollution 501
References 504
Appendix 1 510
Appendix 2 511
Appendix 3 515
Appendix 4 516
Index 518
Early Solar System Chronology
K.D. McKeegan University of California, Los Angeles, CA, USA
A.M. Davis University of Chicago, Chicago, IL, USA
2.1.1 Chondritic Meteorites as Probes of Early Solar System Evolution 36
2.1.2 Short-Lived Radioactivity at the Origin of the Solar System 36
2.1.3 A Brief History and the Scope of the Present Review 37
2.2 DATING WITH ANCIENT RADIOACTIVITY 38
2.3 “ABSOLUTE” AND “RELATIVE” TIMESCALES 39
2.3.1 An Absolute Timescale for Solar System Formation 39
2.3.2 An Absolute Timescale for Chondrule Formation 41
2.3.3 An Absolute Timescale for Early Differentiation of Planetesimals 42
2.4 THE RECORD OF SHORT-LIVED RADIONUCLIDES IN EARLY SOLAR SYSTEM MATERIALS 42
2.4.13 Plutonium-244 and Samarium-146 56
2.5 ORIGINS OF THE SHORT-LIVED NUCLIDES IN THE EARLY SOLAR SYSTEM 56
2.6 IMPLICATIONS FOR CHRONOLOGY 59
2.6.1 Formation Timescales of Nebular Materials 59
2.6.2 Timescales of Planetesimal Accretion and Early Chemical Differentiation 61
2.7.1 Implications for Solar Nebula Origin and Evolution 62
REFERENCES 64
2.1 INTRODUCTION
2.1.1 Chondritic Meteorites as Probes of Early Solar System Evolution
The evolutionary sequence involved in the formation of relatively low-mass stars, such as the Sun, has been delineated in recent years through impressive advances in astronomical observations at a variety of wavelengths, combined with improved numerical and theoretical models of the physical processes thought to occur during each stage. From the models and the observational statistics, it is possible to infer in a general way how our solar system ought to have evolved through the various stages from gravitational collapse of a fragment of a molecular cloud to the accretion of planetary-sized bodies (e.g., Cameron, 1995; Shu et al., 1987; André et al., 2000; Alexander et al., 2001; see Chapter 3). However, the details of these processes remain obscured, literally, from an astronomical perspective, and the dependence of such models on various parameters requires data to constrain the specific case of our solar system’s origin.
Fortunately, the chondritic meteorites sample aspects of this evolution. The term “chondrite” (or chondritic) was originally applied to meteorites-bearing chondrules, which are approximately millimeter-sized solidified melt droplets consisting largely of mafic silicate minerals and glass commonly with included metal or sulfide. However, the meaning of chondritic has been expanded to encompass all extraterrestrial materials that are “primitive,” that is, are undifferentiated samples having nearly solar elemental composition. Thus, the chondrites represent a type of cosmic sediment, and to a first approximation can be thought of as “hand samples” of the condensable portion of the solar nebula. The latter is a general term referring to the phase(s) of solar system evolution intermediate between molecular cloud collapse and planet formation. During the nebular phase, the still-forming Sun was an embedded young-stellar object (YSO) enshrouded by gas and dust, which was distributed first in an extended envelope that later evolved into an accretion disk that ultimately defined the ecliptic plane. The chon-drites agglomerated within this accretion disk, most likely close to the position of the present asteroid belt from whence meteorites are currently derived. In addition to chondrules, an important component of some chondrites are inclusions containing refractory oxide and silicate minerals, so-called calcium-and aluminum-rich inclusions (CAIs) that also formed as free-floating objects within the solar nebula. These constituents are bound together by a “matrix” of chondrule fragments and fine-grained dust (which includes a tiny fraction of dust grains that predate the solar nebula). It is important to realize that, although these materials accreted together at a specific time in some planetesimal, the individual components of a given chondrite can, and probably do, sample different places and/or times during the nebular phase of solar system formation. Thus, each grain in one of these cosmic sedimentary rocks potentially has a story to tell regarding aspects of the early evolution of the solar system.
Time is a crucial parameter in constructing any story. Understanding of relative ages allows placing events in their proper sequence, and measures of the duration of events are critical to developing an understanding of the process. If disparate observations can be related temporally, then structure (at any one time) and evolution of the solar system can be better modeled; or, if a rapid succession of events can be inferred, it can dictate a cause and effect relationship. This chapter is concerned with understanding the timing of different physical and chemical processes that occurred in the solar nebula and possibly on early accreted planetesimals that existed during the nebula stage. These events are “remembered” by the components of chondrites and recorded in the chemical, and especially, isotopic compositions of the host mineral assemblages; the goal is to decide which events were witnessed by these ancient messengers and to decipher those memories recorded long ago.
2.1.2 Short-Lived Radioactivity at the Origin of the Solar System
The elements of the chondritic meteorites, and hence of the terrestrial planets, were formed in previous generations of stars. Their relative abundances represent the result of the general chemical evolution of the galaxy, possibly enhanced by recent local additions from one or more specific sources just prior to collapse of the solar nebula ~4.56Ga. A volumetricallyminor, but nevertheless highly significant part of this chemical inventory, is comprised of radioactive elements, from which this age estimate is derived. The familiar long-lived radionuclides, such as 238U, 235U, 232Th, 87Rb, 40 K, and others, provide the basis for geochronology and the study of large-scale differentiation amongst geochemical reservoirs over time (see Chapter 4). They also provide a major heat source to drive chemical differentiation on a planetary scale (e.g., terrestrial plate tectonics).
A number of short-lived radionuclides also existed at the time that the Sun and the rocky bits of the solar system were forming (Table 1). These nuclides are sufficiently long-lived that they could exist in appreciable quantities in the earliest solar system rocks, but their mean lives are short enough that they are now completely decayed from their primordial abundances. In this sense they are referred to as extinct nuclides. Although less familiar than the still-extant radionuclides, these short-lived isotopes potentially play similar roles: their relative abundances can, in principle, form the basis of various chronometers that constrain the timing of early chemical fractionations, and the more abundant radioisotopes can possibly provide sufficient heat to drive differentiation (i.e., melting) of early accreted planetesimals. The very rapid rate of decay of the short-lived isotopes, however, means that inferred isotopic differences translate into...
| Erscheint lt. Verlag | 30.9.2010 |
|---|---|
| Sprache | englisch |
| Themenwelt | Sachbuch/Ratgeber |
| Naturwissenschaften ► Chemie ► Technische Chemie | |
| Naturwissenschaften ► Geowissenschaften ► Geologie | |
| Naturwissenschaften ► Geowissenschaften ► Mineralogie / Paläontologie | |
| Technik ► Umwelttechnik / Biotechnologie | |
| ISBN-10 | 0-08-096709-4 / 0080967094 |
| ISBN-13 | 978-0-08-096709-7 / 9780080967097 |
| Haben Sie eine Frage zum Produkt? |
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