Earth and Planetary Science Letters 234 (2005) 299 – 315 www.elsevier.com/locate/epsl Frontiers The role of the global carbonate cycle in the regulation and evolution of the Earth system Andy Ridgwella,T, Richard E. Zeebeb a Department of Earth and Ocean Sciences, The University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4 b University of Hawaii at Manoa, SOEST, Honolulu, HI, United States Received 8 October 2004; received in revised form 7 March 2005; accepted 17 March 2005 Available online 27 April 2005 Abstract We review one of the most ancient of all the global biogeochemical cycles and one which reflects the profound geochemical and biological changes that have occurred as the Earth system has evolved through time—that of calcium carbonate (CaCO3). In particular, we highlight a Mid-Mesozoic Revolution in the nature and location of carbonate deposition in the marine environment, driven by the ecological success of calcareous plankton. This drove the creation of a responsive deep-sea sedimentary sink of CaCO3. The result is that biologically driven carbonate deposition provides a significant buffering of ocean chemistry and of atmospheric CO2 in the modern system. However, the same calcifying organisms that under-pin the deep-sea carbonate sink are now threatened by the continued atmospheric release of fossil fuel CO2 and increasing acidity of the surface ocean. We are not yet in a position to predict what the impact on CaCO3 production will be, or how the uptake of fossil fuel CO2 by the ocean will be affected. This uncertainty in the future trajectory of atmospheric CO2 that comes from incomplete understanding of the marine carbonate cycle is cause for concern. D 2005 Elsevier B.V. All rights reserved. Keywords: Earth system; carbon cycle; carbonate; calcifiers; ocean chemistry; CO2; fossil fuel 1. Introduction The geochemical or long-term carbon cycle primarily involves the exchange of carbon between the dsurficialT and dgeologicT reservoirs . The former comprise atmosphere, oceans, biosphere, T Corresponding author. Tel.: +1 604 822 2449; fax: +1 604 822 6088. E-mail address: email@example.com (A. Ridgwell). 0012-821X/$ – see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.03.006 soils, and exchangeable sediments in the marine environment (Fig. 1) while the latter include crustal rocks and deeply buried sediments in addition to the underlying mantle. How carbon is partitioned between the various reservoirs of the surficial system and between surficial and geologic reservoirs is what sets the concentration of CO2 in the atmosphere. Life, and the cycle of organic carbon, as well as its geological (and subsequent fossil fuel exhumation) is of particular importance in this regard. The cycle of 300 A. Ridgwell, R.E. Zeebe / Earth and Planetary Science Letters 234 (2005) 299–315 Box 1 Carbonate chemistry d101T and jargon buster The mineral calcium carbonate (CaCO3) has a crystal lattice motif comprising one calcium ion (Ca2+) ionically bound to one carbonate ion (CO32 ), configured in different polymorphic forms; e.g., calcite, a trigonal structure, or aragonite, which is orthorhombic. Precipitation may be described by the following reaction: Ca2+ + 2HCO3 YCaCO3 + CO2(aq) + H2O. Of the reactants required for this, Ca2+ is naturally abundant in sea-water and at one of the highest concentrations of all ionic species in the ocean. Bicarbonate ions (HCO3 ) are also ubiquitous in sea-water and are formed through the dissolution of CO2 gas. Under typical marine conditions, carbon dioxide will largely hydrate to form a proton (H+) and a bicarbonate ion (HCO3); H2O + CO2(aq)YH+ + HCO3 (see Fig. 3), while true carbonic acid (H2CO3) is only present in very small concentrations. A fraction of HCO3 dissociates to form a carbonate ion (CO32 ); HCO3 YH+ + CO32 . The sum total; CO2(aq) (+ H2CO3) + HCO3 + CO32 is collectively termed dissolved inorganic carbon (dDICT). The climatic importance of the CaCO3 precipitation reaction arises because although the sum total of dissolved carbon species (DIC) is reduced, the remaining carbon is re-partitioned in favor of CO2(aq), resulting in a higher partial pressure of CO2 ( pCO2) in the surface ocean. (Another way of thinking about this is in terms of removing CO32 and shifting the aqueous carbonate equilibrium reaction CO2(aq) + CO32 + H2O X 2HCO3 to the left to compensate.) The counter-intuitive and often confusing consequence of all this is that the precipitation of carbonate carbon drives an increase in ocean pCO2, and with it, an increase in atmospheric CO2 concentration. Conversely, dissolution of CaCO3 drives a pCO2 (and atmospheric CO2) decrease. Whether CaCO3 precipitates or dissolves depends on the relative stability of its crystal structure. This can be directly related to the ambient concentrations (strictly, activities) of Ca2+ and CO32 by the saturation state (also known as the solubility ratio) X of the solution, defined; X = [Ca2+] [CO32 ]/K sp, where K sp is a solubility constant . The precipitation of calcium carbonate from sea-water is thermodynamically favorable when X is greater than unity and occurs at a rate taking the form of a proportionality with (X 1)n , where n is a measure of how strongly the precipitation rate responds to a change in CO32 . Conversely, CaCO3 will tend to dissolve at X b 1.0, and at a rate proportional to (1 X)n . As well as the concentrations of Ca2+ and CO32 , depth in the ocean is also important because K sp scales with increasing pressure. Since K sp and X are inversely related, the greater the depth in the ocean the more likely the ambient environment is to be under-saturated (i.e., X b 1.0). The depth at which X = 1.0 occurs is termed the equilibrium calcite saturation horizon (CSH). (Similar terminology can be applied to the aragonite polymorph.) Although calcite becomes thermodynamically unstable just below this, dissolution proceeds only extremely slowly. The (greater) depth at which dissolution impacts become noticeable is termed the calcite lysocline . In practice this is taken as the inflection point in the trend of sedimentary CaCO3 content vs. water depth. For want of a more robust definition, a chemical lysocline is sometimes defined at X = 0.8, a value which marks a distinct increase in dissolution rate . Deeper still, and dissolution becomes sufficiently rapid for the dissolution flux back to the ocean to exactly balance the rain flux of calcite to the sediments. This is known as the calcite (or carbonate) compensation depth (CCD). Because in the real World the boundary in depth between sediments that have carbonate present and those in which it is completely absent is gradual rather than sharp, the CCD is operationally defined, and variously taken as the depth at which the CaCO3 content is reduced to 2 or 10 wt.%. A. Ridgwell, R.E. Zeebe / Earth and Planetary Science Letters 234 (2005) 299–315 301 a terrestrial biota CO2 marine biota 4 1 ls soi ree fs deep -sea sedim ents 3 Lysocline 2 Carbonate compensation depth (‘CCD’) oceanic crust continental crust b 8 CO2 CO2 CO2 7 5 subducting ocean plate 6 Fig. 1. The global biogeochemical cycling of calcium carbonate. (a) Modes of CaCO3 transformation and recycling within the surficial system and loss to the geological reservoir (labeled d1T through d4T). #1 Precipitation of calcite by coccolithophores and foraminifera in the open ocean; Ca2+ + 2HCO3 YCaCO3 + CO2(aq) + H2O. #2 Carbonate reaching deep-sea sediments will dissolve during early diagenesis if the bottom water is under-saturated and/or the organic matter flux to the sediments is sufficiently high. #3 Precipitation of CaCO3 by corals and shelly animals, with a significant fraction as the aragonite polymorph. Because modern surface waters are over-saturated relatively little of this carbonate dissolves in situ, and instead contributes to the formation of reefal structures or is exported to the adjoining continental slopes. #4 Precipitation of CaCO3 results in higher pCO2 at the surface, driving a net transfer of CO2 from the ocean to the atmosphere. (b) Modes of CaCO3 transformation and recycling within the geologic reservoirs and return to the surficial system (labeled d5T through d8T). #5 CaCO3 laid down in shallow seas as platform and reef carbonates and chalks can be uplifted and exposed to erosion through rifting and mountain-building episodes. CaCO3 can then be directly recycled; CO2 + H2O + CaCO3YCa2+ + 2HCO3 . #6 Thermal breakdown of carbonates subducted into the mantle or deeply buried. The decarbonation reaction involved is essentially the reverse of silicate weathering, and results in the creation of calcium silicates and release of CO2; CaCO3 + SiO2YCO2 + CaSiO3. #7 Weathering of silicate rocks; 2CO2 + H2O + CaSiO3YCa2+ + 2HCO3 + SiO2. #8 Emission to the atmosphere of CO2 produced through decarbonation. This closes the carbon cycle on the very longest time-scales. 302 A. Ridgwell, R.E. Zeebe / Earth and Planetary Science Letters 234 (2005) 299–315 carbon in its inorganic, calcium carbonate (CaCO3)1 form also affects atmospheric CO2, but by more subtle means. It also plays a fundamental role in regulating ocean chemistry and pH—a major factor in the viability of calcareous marine organisms. Before exploring some of the roles that the global carbonate cycle plays in the functioning of the Earth system (Section 2), we first discuss the two constituent parts of this cycle; (a) precipitation and burial of CaCO3, and (b) weathering and geologic recycling, illustrated in Fig. 1a and b, respectively. Then, in Sections 3 and 4 we highlight the ways in which carbonate cycling on Earth has evolved through time, and look to the future and the increasing impact that fossil fuel CO2 release will have on the system. We finish with a brief perspective on the implications for future research. Readers are referred to Box 1 for a brief primer on aqueous carbonate chemistry and CaCO3 thermodynamics. 1.1. Carbonate precipitation and burial Today, the surface of the ocean is everywhere more than saturated (dover-saturatedT) with respect to the solid carbonate phase, with a mean value for the saturation state (dXT—see Box 1) of calcite of 4.8. In other words, the minimum thermodynamical requirement for calcite to precipitate is exceeded by a factor of almost 5 (for aragonite, X is 3.2). Despite this, the spontaneous precipitation of CaCO3 from the water column is not observed in the ocean . This is because the initial step of crystal nucleation is kinetically unfavorable, and experimentally, spontaneous (homogeneous) nucleation does not occur in sea water solutions until X calcite N ~20–25 . Although CaCO3 precipitation occurs as cements and coatings in the marine environment, it is primarily associated with the activities of living organisms, particularly corals, benthic shelly animals, plankton species such as coccolithophores and foraminifera, and pteropods, and where it takes place under direct metabolic control. In comparison, carbonate deposition in fresh-water systems is of 1 For the purposes of this review we simply refer to dcalcium carbonateT, but recognize that carbonates exhibit a range of substitutions of Ca2+ by Mg2+ with a generic composition of Mgxd Cad (1 x)d CO3. only minor importance globally, and will not be discussed further here. While not in itself sufficient to drive substantial abiotic precipitation, the saturation state of the modern ocean surface is favorable to the preservation of carbonates deposited in shallow water (neritic) environments. Long-term accumulation of this material can result in the formation of extensive marine topographical features such as barrier reefs and carbonate banks and platforms. A different fate awaits CaCO3 precipitated in the open ocean by plankton such as coccolithophores
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