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A Simple Model for the Caco3 Saturation State of the Ocean: the ‘‘Strangelove,’’ the ‘‘Neritan,’’ and the ‘‘Cretan’’ Ocean

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A Simple Model for the Caco3 Saturation State of the Ocean: the ‘‘Strangelove,’’ the ‘‘Neritan,’’ and the ‘‘Cretan’’ Ocean Article Geochemistry 3 Volume 4, Number 12 Geophysics 12 December 2003 1104, doi:10.1029/2003GC000538 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society A simple model for the CaCO3 saturation state of the ocean: The ‘‘Strangelove,’’ the ‘‘Neritan,’’ and the ‘‘Cretan’’ Ocean Richard E. Zeebe Alfred Wegener Institute for Polar and Marine Research, Germany ([email protected]) Peter Westbroek Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 AR Leiden, Netherlands ([email protected]) [1] A simple model of the CaCO3 saturation state of the ocean is presented. It can be solved analytically and is intended to identify the fundamental controls on ocean carbonate ion concentration. It should also attract researchers unfamiliar with complex biogeochemical models. Despite its limitations, the model- calculated CaCO3 saturation state of today’s ocean agrees well with observations. In general, the model reveals three distinctly different modes of operation: The ‘‘Strangelove Ocean’’ of high supersaturation which is dominated by inorganic CaCO3 precipitation, (2) the ‘‘Neritan Ocean’’ of indefinite saturation dominated by biogenic shallow-water CaCO3 precipitation, and (3) the ‘‘Cretan Ocean’’ of low saturation 2À dominated by biogenic pelagic CaCO3 precipitation. In the latter mode, the deep ocean [CO3 ]is 2+ remarkably stable, provided that the biogenic production of CaCO3 exceeds the riverine flux of Ca and 2À CO3 . This explains the overall constancy of the saturation state of the ocean documented over the last 100 Ma. The model is then used to address diverse questions. One important result is that the recovery of the oceanic carbonate chemistry from fossil fuel neutralization in the future will be accelerated due to expected reduced biogenic calcification. Components: 14,037 words, 12 figures, 2 tables. Keywords: CaCO3 saturation; model; biogenic calcification; Strangelove; Neritan; Cretan. Index Terms: 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4267 Oceanography: General: Paleoceanography; 4825 Oceanography: Biological and Chemical: Geochemistry. Received 3 March 2003; Revised 18 July 2003; Accepted 21 August 2003; Published 12 December 2003. Zeebe, R. E., and P. Westbroek, A simple model for the CaCO3 saturation state of the ocean: The ‘‘Strangelove,’’ the ‘‘Neritan,’’ and the ‘‘Cretan’’ Ocean, Geochem. Geophys. Geosyst., 4(12), 1104, doi:10.1029/2003GC000538, 2003. 1. Introduction state of the ocean over geologic time. The prod- uct of the concentrations of calcium and carbon- 2+ 2À [2] One of the most important feedbacks that ate ion ([Ca ] Â [CO3 ]) must have exceeded control the oceanic carbonate system and thus the solubility product of calcite or aragonite, atmospheric pCO2 is the CaCO3 saturation state otherwise the CaCO3 would have dissolved. In of the ocean. Calcium carbonate rich sediments other words, the ocean must have been supersat- cover a huge portion of Earth’s history which urated with respect to at least one of these constitutes a strong constraint on the saturation minerals over much of its history. The CaCO3 Copyright 2003 by the American Geophysical Union 1 of 26 Geochemistry 3 zeebe and westbroek: model for caco saturation state Geophysics 3 10.1029/2003GC000538 Geosystems G saturation state of seawater, , is usually ex- pressed as: ½Ca2þ ½CO2À ½Ca2þ ½CO2À ¼ sw 3 sw ¼ sw 3 sw ; ð1Þ * 2þ 2À Ksp ½Ca sat ½CO3 sat where the subscript ‘sw’ refers to the in situ 2+ 2À concentration of Ca and CO3 in seawater, Ksp* is the solubility product of CaCO3 (aragonite or calcite) at the in situ conditions of temperature, salinity, and pressure, and ‘sat’ refers to the 2+ 2À corresponding Ca and CO3 concentration at saturation. >1 signifies supersaturation, whereas < 1 signifies undersaturation. Figure 1. Calcite and aragonite saturation horizon in [3] In the modern ocean, the saturation state is the ocean [after Broecker and Peng, 1987]. As pressure tightly coupled to the position of the so-called increases with depth, so does the solubility of calcite 2À ‘‘snow line,’’ the depth above which the ocean and aragonite ([CO3 ]sat). The crossover between the in floor is mainly covered with calcite while below it situ carbonate ion concentration (solid curve) and the is largely calcite-free. The ocean floor hence saturation concentration for calcite (dashed curve) and aragonite (dot-dashed curve) determines the saturation resembles a landscape with snow-covered moun- horizon. tains. The transition from calcite-rich to calcite- depleted sediments is not abrupt but rather gradual calcite lysocline depth (an observed quantity) and the depth of rapid increase in the rate of corresponds to the calcite saturation depth (a dissolution as observed in sediments is called the thermodynamic quantity) and calcium carbonate lysocline. The depth at which the sediments are falling from the surface to the deep ocean is mainly virtually free of calcium carbonate is called the preserved in supersaturated waters above the calcium carbonate compensation depth (CCD). saturation horizon, while it starts to dissolve below [4] Today, the depth of the calcite lysocline is the saturation horizon in undersaturated waters primarily controlled by the saturation state of (cf. discussion in section 4.5). bottom water, i.e., by the product of the in situ [5] The feedback between the calcite saturation concentrations of Ca2+ and CO2À. In fact, it is 3 state – i.e., the deep ocean carbonate ion concen- controlled by [CO2À] because the Ca2+ concen- 3 tration – and the lysocline works as follows. tration is spatially virtually constant and much Assume there is an imbalance between the riverine larger than that of CO2À. Equation (1) can then 3 influx of dissolved CaCO derived from weather- be written as: 3 ing on the continents and the CaCO3 burial in the 2À deep sea. If the riverine flux is larger than the ½CO3 sw ¼ 2À : ð2Þ burial, then the carbonate ion concentration will ½CO3 sat increase. This leads to a deepening of the saturation The crossover of the in situ carbonate ion horizon (Figure 1) and thus to an increased burial 2À concentration in seawater ([CO3 ]sw)andthe because a larger fraction of the seafloor is now carbonate ion concentration at calcite saturation bathed in supersaturated water. Finally, the burial 2À ([CO3 ]sat) is called the calcite saturation depth or will again become equal to the riverine flux, and a saturation horizon (SH), see Figure 1. Because new balance is restored. This mechanism is called calcite solubility increases with pressure and thus calcite compensation and its e-folding time scale is with depth, the seafloor is bathed in supersaturated of the order of 10,000 years. In the absence of water above the saturation depth, while it is bathed pelagic production and thus deep sea burial, bio- in undersaturated water below. In today’s ocean the genic shallow-water and inorganic CaCO3 precip- 2of26 Geochemistry 3 zeebe and westbroek: model for caco saturation state Geophysics 3 10.1029/2003GC000538 Geosystems G itation are the major players controlling the satu- influence of the carbonate chemistry on biogenic ration state (see below). calcification during periods of high atmospheric pCO2. [6] In the current paper, we attempt to formulate a simple model of the CaCO3 saturation state of the ocean—the model can be solved analytically. Our 2. The Model desire for simplicity is twofold. First, the model shall be used to identify the essential parameters [8] The model to be presented calculates the deep controlling the saturation state of the ocean, while ocean carbonate ion concentration and thus the details that can be shown to only cloud this picture CaCO3 saturation state of the deep ocean which are ignored. This should help to stimulate and is accomplished by considering the input and 2+ 2À clarify our thinking on the fundamental operation output fluxes of CaCO3,Ca ions, and CO3 ions. The model will be based on boundary con- of the marine CaCO3 cycle. It is therefore antici- pated that the model is also of use for scientists ditions for the modern ocean. unfamiliar with biogeochemical models imple- [9] A single reservoir is considered, representing mented in general ocean circulation- or box mod- the deep ocean (Figure 2). The most shallow part of els. Second, the model shall be employed to the ocean floor is located at a depth of 1 km, while investigate diverse problems on various time scales the deepest part is located at 6.5 km. In between, a which would require long integration times if linear increase of the ocean floor area fraction with sophisticated models are used. depth is assumed. Shallow-water environments [7] In the following, the model is introduced with above 1 km are included in the model through boundary conditions representative for the modern CaCO3 accumulation (see section 2.5.1). Continen- 2+ 2À ocean (section 2). Then, the steady-state solutions tal weathering releases Ca and CO3 ions that of the model are presented which reveal three are transported through rivers into the ocean, gen- different modes of operation to be termed the erating an influx, Fin. Note that in the current ‘‘Strangelove,’’ the ‘‘Neritan,’’ and the ‘‘Cretan’’ context the description of precipitation and disso- 2+ 2À Ocean (section 3). We will demonstrate that today’s lution of CaCO3 via Ca +CO3 Ð CaCO3 and 2+ À ocean operates in the latter mode and that the Ca + 2HCO3 Ð CaCO3 +CO2 +H2O is equiv- alent because total dissolved carbon (ÆCO2) and model-calculated CaCO3 saturation state agrees well with observations. The modern ocean’s satu- total alkalinity (TA) are changed in a ratio of 1:2 in ration state is shown to be a consequence of the both cases. Seawater above the calcite lysocline is assumed to be supersaturated and calcite falling on interplay of the biogenic CaCO3 production at low supersaturation—a biological process—and the that part of the ocean floor is thought to be pre- served (cf.
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