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5.18 Soils and Global Change in the Carbon Cycle Over Geological Time G 5.18 Soils and Global Change in the Carbon Cycle over Geological Time G. J. Retallack University of Oregon, Eugene, OR, USA 5.18.1 INTRODUCTION 1 5.18.2 APPROACHES TO THE STUDY OF PALEOSOLS 3 5.18.2.1 Molecular Weathering Ratios 4 5.18.2.2 Strain and Mass-Transfer Analysis 5 5.18.2.3 Analyses of Stable Isotopes of Carbon and Oxygen 7 5.18.3 RECORD OF PAST SOIL AND GLOBAL CHANGE 7 5.18.3.1 Origins of Soil 8 5.18.3.2 Archean–Paleoproterozoic Greenhouse Paleosols 9 5.18.3.3 Proterozoic Icehouse Paleosols 10 5.18.3.4 Cambro-Ordovician Greenhouse Paleosols 11 5.18.3.5 Terminal Ordovician Icehouse Paleosols 13 5.18.3.6 Siluro-Devonian Greenhouse Paleosols 13 5.18.3.7 Late Devonian to Permian Icehouse Paleosols 13 5.18.3.8 Triassic–Jurassic Greenhouse Paleosols 15 5.18.3.9 Early Cretaceous Icehouse Paleosols 17 5.18.3.10 Cretaceous–Paleogene Greenhouse Paleosols 17 5.18.3.11 Neogene Icehouse Paleosols 17 5.18.3.12 Pleistocene Glacial and Interglacial Paleosols 19 5.18.4 SOILS AND GLOBAL CARBON CYCLE CHANGES 21 ACKNOWLEDGMENTS 23 REFERENCES 23 5.18.1 INTRODUCTION amplified by biological acidification (Schwartz- mann and Volk, 1991; see Chapter 5.06). These Soils play an important role in the carbon mineral nutrients fuel photosynthetic fixation cycle as the nutrition of photosynthesized bio- and chemical reduction of atmospheric CO2 mass. Nitrogen fixed by microbes from air is a into plants and plantlike microbes, which are at limiting nutrient for ecosystems within the first the base of the food chain. Plants and photo- flush of ecological succession of new ground, synthetic microbes are consumed and oxidized and sulfur can limit some components of wet- by animals, fungi, and other respiring land ecosystems. But in the long term, the lim- microbes, which release CO2, methane, and iting soil nutrient is phosphorus extracted water vapor into the air. These greenhouse by weathering from minerals such as apatite gases absorb solar radiation more effectively (Vitousek et al., 1997a; Chadwick et al., 1999). than atmospheric oxygen and nitrogen, and are Life has an especially voracious appetite for important regulators of planetary temperature common alkali (Na þ and K þ ) and alkaline- and albedo (Kasting, 1992). Variations in earth (Ca2 þ and Mg2 þ ) cations, supplied solar insolation (Kasting, 1992), mountainous by hydrolytic weathering, which is in turn topography (Raymo and Ruddiman, 1992), 1 2 Soils and Global Change in the Carbon Cycle over Geological Time and ocean currents (Ramstein et al., 1997) also tapped underground reservoirs of fossil fuels, play a role in climate, but this review focuses on and our other perturbations of the carbon cycle the carbon cycle. The carbon cycle is discussed have also been significant (Vitousek et al., in detail in Volume 8 of this treatise. 1997b; see Chapter 8.10). The greenhouse model for global paleocli- Atmospheric increase of carbon in CO2 to mate has proven remarkably robust (Retallack, 750 Gt C by deforestation and fossil fuel burn- 2002), despite new challenges (Veizer et al., ing has driven ongoing global warming, but is 2000). The balance of producers and consumers not quite balanced by changes in the other car- is one of a number of controls on atmospheric bon reservoirs, leading to a search for a ‘‘mis- greenhouse gas balance, because CO2 is added sing sink’’ of some 1.871.3 Gt C, probably in to the air from fumaroles, volcanic eruptions, terrestrial organisms, soils, and sediments of and other forms of mantle degassing (Holland, the northern hemisphere (Keeling et al., 1982; 1984). Carbon dioxide is also consumed Siegenthaler and Sarmiento, 1993; Stallard, by burial as carbonate and organic matter 1998). Soil organic matter is a big, rapidly within limestones and other sedimentary cycling reservoir, likely to include much of this rocks; organic matter burial is an important missing sink. long-term control on CO2 levels in the atmos- During the geological past, the sizes of, and phere (Berner and Kothavala, 2001). The fluxes between, these reservoirs have varied magnitudes of carbon pools and fluxes involved enormously as the world has alternated provide a perspective on the importance of between greenhouse times of high carbon soils compared with other carbon reservoirs content of the atmosphere, and icehouse (Figure 1). times of low carbon content of the atmosphere. Before industrialization, there was only Oscillations in the atmospheric content of 15 600 Gt (1 Gt ¼ 10 g) of carbon in CO2 and greenhouse gases can be measured, estimated, methane in the atmosphere, which is about the or modeled on all timescales from annual to same amount as in all terrestrial biomass, but eonal (Figure 2). The actively cycling surficial less than half of the reservoir of soil organic carbon reservoirs are biomass, surface oceans, carbon. The ocean contained only B3Gt of air, and soils, so it is no surprise that the biomass carbon. The deep ocean and sediments fossil record of life on Earth shows strong comprised the largest reservoir of bicarbonate linkage to global climate change (Berner, 1997; and organic matter, but that carbon has been Algeo and Scheckler, 1998; Retallack, 2000a). kept out of circulation from the atmosphere There is an additional line of evidence for for geologically significant periods of time past climatic and atmospheric history in the (Schidlowski and Aharon, 1992). Humans have form of fossil soils, or paleosols, now known Atmosphere 600 100 50 50 0.6 74 74 Land biota 610 0.6 50 Rivers Surface ocean 1,000 Soil & detritus 0.8 Biota 3 10 1,560 6 1 DOC 700 4 90 100 Sediments 6 Intermediate and deep waters 4.8 × 107 1.2 × 107 38,000 Carbonate Organic 0.2 Figure 1 Pools and fluxes of reduced carbon (bold) and oxidized carbon (regular) in Gt in the preindustrial carbon cycle. Sources: Schidlowski and Aharon (1992), Siegenthaler and Sarmiento (1993), and Stallard (1998). Approaches to the Study of Paleosols 3 335 330 325 dioxide (ppm) dioxide Atmospheric carbon Atmospheric 320 Dec-68 Dec-69 Dec-70 Dec-71 Dec-72 Dec-73 (a) Calendar year 300 250 200 dioxide (ppm) dioxide Atmospheric carbon Atmospheric 150 300 200 100 0 (b) Thousands of years ago 6,000 4,000 2,000 dioxide (ppm) dioxide Atmospheric carbon Atmospheric 0 300 200 100 0 (c) Millions of years ago 8,000 6,000 4,000 dioxide (ppm) dioxide 2,000 Atmospheric carbon Atmospheric 0 400 200 0 (d) Millions of years ago Figure 2 Variation in atmospheric CO2 composition on a variety of timescales ranging from annual to eonal (a) Reproduced from Keeling et al. (1982). (b) Reproduced by permission of Nature Publishing Group from Petit et al. (1999). (c) Reproduced from Retallack (2001d). (d) Reproduced by permission of American Journal of Science from Berner and Kothavala (2001). to be abundant throughout the geological 5.18.2 APPROACHES TO THE STUDY record (Retallack, 1997a, 2001a). This chapter OF PALEOSOLS addresses evidence from fossil soils for global climate change in the past, and attempts to as- Many approaches to the study of paleosols sess the role of soils in carbon cycle fluctuations are unlike those of soil science, and more like through the long history of our planet. soil geochemistry prior to the earlier part of the 4 Soils and Global Change in the Carbon Cycle over Geological Time twentieth century (Thaer, 1857; Marbut, 1935). tropical regions for geologically significant pe- Such measures of soil fertility as cation- riods of time (Oxisols of Soil Survey Staff, exchange capacity and base saturation that 1999), the molar ratio of alumina over bases are used for characterizing surface soils (Buol can reach 100 or more, indicating that the slow et al., 1997) are inappropriate for the study of progress of hydrolysis has almost gone to paleosols because of profound modification of completion. the cation-exchange complex during burial Application of this approach to a Preca- and lithification of paleosols (Retallack, mbrian (1,000 Ma) paleosol from Scotland 1991). Many paleosols are now lithified and (Figure 3) showed the expected decrease of amenable to study using petrographic thin sec- hydrolytic weathering down from the surface, tions, X-ray diffraction, electron microprobe, and an overall degree of hydrolytic alteration and bulk chemical analysis (Holland, 1984; that is modest compared with deeply weathered Ohmoto, 1996; Retallack, 1997a). modern soils (Figure 4). Effects of hydrolysis of this Precambrian paleosol can also be seen in petrographic thin sections and electron 5.18.2.1 Molecular Weathering Ratios microprobe analyses, which document convers- ion of feldspar into clay (Retallack and Soil formation (see Chapter 5.01) is not only Mindszenty, 1994). a biological and physical alteration of rocks, Other molar weathering ratios can be devised but a slow chemical transformation following to reflect leaching (Ba/Sr), oxidation (FeO/ a few kinds of reactions that seldom reach Fe2O3), calcification (CaO þ MgO/Al2O3), and chemical equilibrium. In many soils, the most salinization (Na2O/K2O). Two of these ratios important of these reactions is hydrolysis: the reflect differential solubility of chemically com- incongruent dissolution of minerals such as parable elements, but the calcification ratio feldspars to yield clays and alkali and alkaline- quantifies the accumulation of pedogenic cal- earth cations in solution. A useful proxy for cite and dolomite, and the ratio of iron of dif- the progress of this reaction in soils and ferent valence gives reactant and product of paleosols is the molar ratio of alumina (repre- iron oxidation reactions. In the Precambrian senting clay) over the sum of lime, magnesia, paleosol illustrated (Figure 4), these molar ra- soda, and potash (representing major cationic tios indicate that the profile was oxidized and nutrients lost into soil solution).
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