Introduction

The cycle of carbon is essential to the maintenance of life, to climate, and to the composition of the atmosphere and oceans. What is normally thought of as the “carbon cycle” is the transfer of carbon between the atmosphere, the oceans, and life. This is not the subject of interest of this book. To understand this apparently confusing statement, it is necessary to separate the carbon cycle into two cycles: the short-term cycle and the long-term cycle. The “carbon cycle,” as most people understand it, is represented in figure 1.1. Carbon dioxide is taken up via photosynthesis by green plants on the continents or phytoplankton in the ocean. On land carbon is transferred to soils by the dropping of leaves, root growth, and respiration, the death of plants, and the development of soil biota. Land herbivores eat the plants, and carnivores eat the herbivores. In the oceans the phytoplankton are eaten by zooplankton that are in turn eaten by larger and larger organisms. The plants, plankton, and animals respire CO2. Upon death the plants and animals are decomposed by microorganisms with the ultimate production of CO2. Carbon dioxide is exchanged between the oceans and atmosphere, and dissolved organic matter is carried in solution by rivers from soils to the sea. This all constitutes the shortterm carbon cycle. The word “short-term” is used because the characteristic times for transferring carbon between reservoirs range from days to tens of thousands of years. Because the earth is more than four billion years old, this is short on a geological time scale. As the short-term cycle proceeds, concentrations of the two principal atmospheric gases, CO2 and CH4, can change as a result of perturbations of the cycle. Because these two are both greenhouse gases—in other words, they adsorb outgoing infrared radiation from the earth surface—changes in their concentrations can involve global warming and cooling over centuries and many millennia. Such changes have accompanied global climate change over the Quaternary period (past 2 million years), although other factors, such as variations in the receipt of solar radiation due to changes in characteristics of the earth’s orbit, have also contributed to climate change.

Processes of the Long-Term Carbon Cycle: Organic Matter and Carbonate Burial and Weathering

The organic subcycle of the long-term carbon cycle, where organic matter burial and weathering are involved, constitutes the major control on the evolution of atmospheric oxygen. It is also important as a secondary factor affecting atmospheric CO2. Thus, it is important to better understand the processes whereby organic matter is buried in sediments and oxidized upon subsequent exposure to weathering during uplift onto the continents. This is especially true of the Paleozoic rise of land plants, which had a large effect on atmospheric CO2 because of increased global organic burial due to the addition of plant debris to sediments. The burial of organic matter in marine sediments is impacted strongly by the availability of the nutrient elements, phosphorus and nitrogen, so a complete discussion of the cycling of organic carbon should involve some discussion of the cycles of these elements. Carbonate burial is the ultimate sink for CO2 derived from the atmosphere via the weathering of Ca and Mg silicates. The location of this burial, shallow water shelves versus the deep sea floor, is important because it affects the probability that the carbonate will be eventually thermally recycled and the carbon returned to the atmosphere. Carbonate weathering is the dominant process affecting river water composition and is a key component of the cycling of carbon. Its importance to the long-term carbon cycle is that, in order to calculate the removal of CO2 from the atmosphere via Ca and Mg silicate weathering, it is necessary to correct total carbonate burial for that derived from carbonate weathering. At present, sedimentary organic matter burial occurs in swamps, lakes, reservoirs, estuaries, and in the open marine environment. The ultimate sources of the organics are land vegetation and marine phytoplankton. Also, soil organic matter, which is intimately associated with clay minerals, is eroded and transported to the sea by rivers (Hedges et al., 1994). A major question is how much of the total global burial is of marine or nonmarine origin. Recent work has shown that organic burial on land is much higher than previously recognized, especially as a result of human activities (Dean and Gorham, 1998; Stallard, 1998).

Processes of the Long-Term Carbon Cycle: Chemical Weathering of Silicates

Carbon dioxide is removed from the atmosphere during the weathering of both silicates and carbonates, but, over multimillion year time scales, as pointed out in chapter 1, only Ca and Mg silicate weathering has a direct effect on CO2. Carbon is transferred from CO2 to dissolved HCO3– and then to Ca and Mg carbonate minerals that are buried in sediments (reaction 1.4). In this chapter the factors that affect the rate of silicate weathering and how they could have changed over Phanerozoic time are discussed. Following classical studies (e.g., Jenny, 1941), the factors discussed include relief, climate (rainfall and temperature), vegetation, and lithology. However, over geological time scales, additional factors come into consideration that are necessarily ignored in studying modern weathering. These include the evolution of the sun and continental drift. The aim of this book is to consider all factors, whether occurring at present or manifested only over very long times, that affect weathering as it relates to the Phanerozoic carbon cycle. Within the past decade much attention has been paid to the effect of mountain uplift on chemical weathering and its effect on the uptake of atmospheric CO2, an idea originally espoused by T.C. Chamberlin (1899). The uplift of the Himalaya Mountains and resulting increased weathering has been cited as a principal cause of late Cenozoic cooling due to a drop in CO2 (Raymo, 1991). Orogenic uplift generally results in the development of high relief. High relief results in steep slopes and enhanced erosion, and enhanced erosion results in the constant uncovering of primary minerals and their exposure to the atmosphere. In the absence of steep slopes, a thick mantle of clay weathering product can accumulate and serve to protect the underlying primary minerals against further weathering. An excellent example of this situation is the thick clay-rich soils of the Amazon lowlands where little silicate weathering occurs (Stallard and Edmond, 1983). In addition, the development of mountain chains often leads to increased orographic rainfall and, at higher elevations, increased erosion by glaciers. All these factors should lead to more rapid silicate weathering and faster uptake of atmospheric CO2.

Atmospheric Carbon Dioxide over Phanerozoic Time

In this chapter the methods and results of modeling the long-term carbon cycle are presented in terms of predictions of past levels of atmospheric CO2. The modeling results are then compared with independent determinations of paleo-CO2 by means of a variety of different methods. Results indicate that there is reasonable agreement between methods as to the general trend of CO2 over Phanerozoic time. Values of fluxes in the long-term carbon cycle can be calculated from the fundamental equations for total carbon and 13C mass balance that are stated in the introduction and are repeated here: . . . dMc/dt = Fwc + Fwg + Fmc + Fmg – Fbc – Fbg (1.10) . . . . . . d(δcMc)/dt = δwcFwc + δwgFwg + δmcFmc + δmgFmg – δbcFbc – δbgFbg (1.11) . . . where Mc = mass of carbon in the surficial system consisting of the atmosphere, oceans, biosphere, and soils Fwc = flux from weathering of Ca and Mg carbonates Fwg = flux from weathering of sedimentary organic matter Fmc = degassing flux for carbonates from volcanism, metamorphism, and diagenesis Fmg = degassing flux for organic matter from volcanism, metamorphism, and diagenesis Fbc = burial flux of carbonates in sediments Fbg = burial flux of organic matter in sediments δ = [(13C/12C)/(13C/12C)stnd – 1]1000. Variants of equations (1.10) and (1.11) have been treated in terms of non–steady-state modeling (e.g., Berner et al., 1983; Wallmann, 2001; Hansen and Wallmann, 2003; Mackenzie et al., 2003; Bergman et al., 2003), where the evolution of both oceanic and atmospheric composition, including Ca, Mg, and other elements in seawater, is tracked over time. However, since the purpose of this book is to discuss the carbon cycle with respect to CO2 and O2, and so as not to overburden the reader with too many mathematical expressions, I discuss only those aspects of the non–steady-state models that directly impact carbon. These are combined with results from steady-state strictly carbon-cycle modeling (Garrels and Lerman, 1984; Berner, 1991, 1994; Kump and Arthur, 1997; Francois and Godderis, 1998; Tajika, 1998; Berner and Kothavala, 2001; Kashiwagi and Shikazono, 2002).

Processes of the Long-Term Carbon Cycle: Degassing of Carbon Dioxide and Methane

Degassing of CO2 and CH4 to the atmosphere and oceans is the process whereby carbon is restored to the surficial system after being buried in rocks. Carbon dioxide is released by a variety of processes. This includes volcanic emissions from the mantle and metamorphic and diagenetic decarbonation of limestones and organic matter. Volcanic degassing can occur over subduction zones, at mid-ocean rises, on the continents, and in the interior of oceanic plates. Degassing can be sudden and violent, as during volcanic eruptions, or slow and semi-continuous in the form of fumaroles, springs, gas vents, and continually degassing volcanic vents. An outstanding example of the latter is Mt. Etna, which contributes about 10% to total global degassing (Caldeira and Rampino, 1992). Metamorphic degassing is concentrated in zones of seafloor subduction (Barnes et al., 1978), crustal convergence (Kerrick and Caldeira, 1998), and crustal extension (Kerrick et al., 1995). Most methane degassing on a geologic time scale occurs from organic matter diagenesis slowly from coal, oil, and kerogen maturation and suddenly from methane hydrate breakdown. A smaller amount of CH4 emanates from mid-ocean hydrothermal vents. Estimates of present-day global volcanic degassing rates are under constant revision (e.g., see Gerlach, 1991; Brantley and Koepenick, 1995; Sano and Williams, 1996; Marty and Tolstikhin, 1998; Kerrick, 2001). A compilation of recent estimated rates of most degassing processes is shown in table 4.1. A constraint on estimates is that none can exceed total global degassing. The latter can be determined from the steady-state assumption that CO2 release by global degassing must be balanced by global uptake by Ca and Mg silicate weathering (Berner, 1990; Berner and Caldeira, 1997). (This assumes essential balance of the organic C subcycle.) Global Ca and Mg silicate weathering, based on river fluxes of these elements to the sea, has been estimated to be about 6 ± 3 × 1018 mol/my (Berner, 1990). Gaillardet et al. (1999) estimate a minimum value for Ca and Mg silicate weathering of 3.6 × 1018 mol/my.

Atmospheric O2 over Phanerozoic Time

The chemical reactions that affect atmospheric O2 on a multimillion-year time scale involve the most abundant elements in the earth’s crust that undergo oxidation and reduction. This includes carbon, sulfur, and iron. (Other redox elements, such as manganese, are not abundant enough to have an appreciable effect on O2.) Iron is the most abundant of the three, but it plays only a minor role in O2 control (Holland, 1978). This is because during oxidation the change between Fe+2 and Fe+3 involves the uptake of only one-quarter of an O2 molecule, whereas the oxidation of sulfide to sulfate involves two O2 molecules, and the oxidation of reduced carbon, including organic matter and methane, involves between one and two O2 molecules. The same stoichiometry applies to reduction of the three elements. Because iron is not sufficiently abundant enough to counterbalance its low relative O2 consumption/release, the iron cycle is omitted in most discussions of controls on atmospheric oxygen. In contrast, the sulfur cycle, although subsidiary to the carbon cycle as to its effect on atmospheric O2, is nevertheless non-negligible and must be included in any discussion of the evolution of atmospheric O2. In this chapter the methods and results of modeling the long-term carbon and sulfur cycles are presented in terms of calculations of past levels of atmospheric oxygen. The modeling results are then compared with independent, indirect evidence of changes in O2 based on paleobiological observations and experimental studies that simulate the response of forest fires to changes in the levels of O2. Because the sulfur cycle is not discussed anywhere else in this book, it is briefly presented first. The long-term sulfur cycle is depicted as a panorama in figure 6.1. Sulfate is added to the oceans, via rivers, originating from the oxidative weathering of pyrite (FeS2) and the dissolution of calcium sulfate minerals (gypsum and anhydrite) on the continents. Volcanic, metamorphic/hydrothermal, and diagenetic reactions add reduced sulfur to the oceans and atmosphere where it is oxidized to sulfate. Sulfur is removed from the oceans mainly via formation of sedimentary pyrite and calcium sulfate.