Mathematics – Logic
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...5..581r&link_type=abstract
Treatise on Geochemistry, Volume 5. Editor: James I. Drever. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 60
Mathematics
Logic
2
Scientific paper
Soils play an important role in the carbon cycle as the nutrition of photosynthesized biomass. Nitrogen fixed by microbes from air is a limiting nutrient for ecosystems within the first flush of ecological succession of new ground, and sulfur can limit some components of wetland ecosystems. But over the long term, the limiting soil nutrient is phosphorus extracted by weathering from minerals such as apatite (Vitousek et al., 1997a; Chadwick et al., 1999). Life has an especially voracious appetite for common alkali (Na+ and K+) and alkaline earth (Ca2+ and Mg2+) cations, supplied by hydrolytic weathering, which is in turn amplified by biological acidification (Schwartzmann and Volk, 1991; see Chapter 5.06). These mineral nutrients fuel photosynthetic fixation and chemical reduction of atmospheric CO2 into plants and plantlike microbes, which are at the base of the food chain. Plants and photosynthetic microbes are consumed and oxidized by animals, fungi, and other respiring microbes, which release CO2, methane, and water vapor to the air. These greenhouse gases absorb solar radiation more effectively than atmospheric oxygen and nitrogen, and are important regulators of planetary temperature and albedo (Kasting, 1992). Variations in solar insolation ( Kasting, 1992), mountainous topography ( Raymo and Ruddiman, 1992), and ocean currents ( Ramstein et al., 1997) also play a role in climate, but this review focuses on the carbon cycle. The carbon cycle is discussed in detail in Volume 8 of this Treatise.The greenhouse model for global paleoclimate has proven remarkably robust (Retallack, 2002), despite new challenges ( Veizer et al., 2000). The balance of producers and consumers is one of a number of controls on atmospheric greenhouse gas balance, because CO2 is added to the air from fumaroles, volcanic eruptions, and other forms of mantle degassing (Holland, 1984). Carbon dioxide is also consumed by burial as carbonate and organic matter within limestones and other sedimentary rocks; organic matter burial is an important long-term control on CO2 levels in the atmosphere (Berner and Kothavala, 2001). The magnitudes of carbon pools and fluxes involved provide a perspective on the importance of soils compared with other carbon reservoirs ( Figure 1). (6K)Figure 1. Pools and fluxes of reduced carbon (bold) and oxidized carbon (regular) in Gt in the pre-industrial carbon cycle (sources Schidlowski and Aharon, 1992; Siegenthaler and Sarmiento, 1993; Stallard, 1998). Before industrialization, there was only 600 Gt (1 Gt=1015g) of carbon in CO2 and methane in the atmosphere, which is about the same amount as in all terrestrial biomass, but less than half of the reservoir of soil organic carbon. The ocean contained only ˜3 Gt of biomass carbon. The deep ocean and sediments comprised the largest reservoir of bicarbonate and organic matter, but that carbon has been kept out of circulation from the atmosphere for geologically significant periods of time (Schidlowski and Aharon, 1992). Humans have tapped underground reservoirs of fossil fuels, and our other perturbations of the carbon cycle have also been significant ( Vitousek et al., 1997b; see Chapter 8.10).Atmospheric increase of carbon in CO2 to 750 Gt C by deforestation and fossil fuel burning has driven ongoing global warming, but is not quite balanced by changes in the other carbon reservoirs leading to search for a "missing sink" of some 1.8±1.3 GtC, probably in terrestrial organisms, soils, and sediments of the northern hemisphere (Keeling et al., 1982; Siegenthaler and Sarmiento, 1993; Stallard, 1998). Soil organic matter is a big, rapidly cycling reservoir, likely to include much of this missing sink.During the geological past, the sizes of, and fluxes between, these reservoirs have varied enormously as the world has alternated between greenhouse times of high carbon content of the atmosphere, and icehouse times of low carbon content of the atmosphere. Oscillations in the atmospheric content of greenhouse gases can be measured, estimated, or modeled on all timescales from annual to eonal (Figure 2). The actively cycling surficial carbon reservoirs are biomass, surface oceans, air, and soils, so it is no surprise that the fossil record of life on Earth shows strong linkage to global climate change (Berner, 1997; Algeo and Scheckler, 1998; Retallack, 2000a). There is an additional line of evidence for past climatic and atmospheric history in the form of fossil soils, or paleosols, now known to be abundant throughout the geological record ( Retallack, 1997a, 2001a). This chapter addresses evidence from fossil soils for global climate change in the past, and attempts to assess the role of soils in carbon cycle fluctuations through the long history of our planet. (30K)Figure 2. Variation in atmospheric CO2 composition on a variety of timescales ranging from annual to eonal (a) Keeling et al., 1982; reproduced from Carban Dioxide Review 1982, 377-385 (b) Petit et al., 1999; reproduced by permission of Nature Publishing Group from Nature 1999, 399, 429-436 (c) Retallack, 2001d; reproduced by J. Geol. 2001, 109, 407-426 (d) Berner and Kothavala, 2001; reproduced by permission of American Journal of Science from Am. J. Sci. 2001, 301, 182-204.
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