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The Importance of Terrestrial Weathering Changes in Multimillennial Recovery of the Global Carbon Cycle: a Two-Dimensional Perspective

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The Importance of Terrestrial Weathering Changes in Multimillennial Recovery of the Global Carbon Cycle: a Two-Dimensional Perspective Earth Syst. Dynam., 8, 455–475, 2017 https://doi.org/10.5194/esd-8-455-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. The importance of terrestrial weathering changes in multimillennial recovery of the global carbon cycle: a two-dimensional perspective Marc-Olivier Brault1, H. Damon Matthews2, and Lawrence A. Mysak3 1Department of Geography, McGill University, Montréal, Canada 2Department of Geography, Planning and Environment, Concordia University, Montréal, Canada 3Department of Atmospheric and Oceanic Sciences, McGill University, Montréal, Canada Correspondence to: Marc-Olivier Brault ([email protected]) Received: 31 October 2016 – Discussion started: 2 January 2017 Revised: 8 April 2017 – Accepted: 11 April 2017 – Published: 30 June 2017 Abstract. In this paper, we describe the development and application of a new spatially explicit weathering scheme within the University of Victoria Earth System Climate Model (UVic ESCM). We integrated a dataset of modern-day lithology with a number of previously devised parameterizations for weathering dependency on tem- perature, primary productivity, and runoff. We tested the model with simulations of future carbon cycle perturba- tions, comparing a number of emission scenarios and model versions with each other and with zero-dimensional equivalents of each experiment. Overall, we found that our two-dimensional weathering model versions were more efficient in restoring the carbon cycle to its pre-industrial state following the pulse emissions than their zero-dimensional counterparts; however, in either case the effect of this weathering negative feedback on the global carbon cycle was small on timescales of less than 1000 years. According to model results, the largest con- tribution to future changes in weathering rates came from the expansion of tropical and mid-latitude vegetation in grid cells dominated by weathering-vulnerable rock types, whereas changes in temperature and river runoff had a more modest direct effect. Our results also confirmed that silicate weathering is the only mechanism that can lead to a full recovery of the carbon cycle to pre-industrial levels on multimillennial timescales. 1 Introduction ever, its relevance over time frames of 104 years or shorter has of yet been left mostly unexplored. Here, we introduce a new model of rock weathering devel- 1.1 Rationale oped for use within the University of Victoria Earth System Climate Model (UVic ESCM); this model incorporates a spa- The weathering of carbonate and silicate rocks on land is a tially explicit interactive computation of weathering rates to key process in the global carbon cycle and, through its cou- close the global carbon cycle on multimillennial timescales. pling with calcium carbonate deposition in the ocean, is the The model is based on a lithology-dependent calculation of primary sink of carbon on geologic timescales (Urey, 1952; steady-state weathering fluxes, which are modulated by tran- Walker et al., 1981). The rate at which these processes re- sient changes in environmental conditions akin to the 0-D move carbon from the Earth system is sensitive to changes carbon cycle models already present in the literature (e.g., in the environment, notably temperature (Berner, 1991), bio- Meissner et al., 2012). We tested the model with simulations logical productivity (Lenton and Britton, 2006), and, perhaps of future climate changes following anthropogenic carbon more indirectly, river runoff (Walker and Kasting, 1992). emissions, comparing the output to that of earlier weathering This gives rise to a negative feedback mechanism which reg- models, both 2-D (Colbourn et al., 2013) and 0-D (Lenton ulates the global climate on multimillennial timescales. How- and Britton, 2006). Published by Copernicus Publications on behalf of the European Geosciences Union. 456 M.-O. Brault et al.: The importance of terrestrial weathering changes 1.2 The rock weathering cycle Weathering rates due to silicate hydrolysis tend to be consid- erably slower than from the dissolution of carbonate min- The chemical weathering of rocks is characterized by the erals – it removes on average 0.28 to 0.30 Pg C per year cleavage of bonds of the mineral lattice by water, often in (Amiotte Suchet and Probst, 1995) – hence the effect of at- the presence of a secondary weathering agent – hydronium mospheric CO consumption by silicate weathering only be- or OH ions, low-molecular-weight organic chelators, or car- 2 comes a significant sink of carbon on geologic timescales bonic acid (H CO ; a product of carbon dioxide dissolution 2 3 (105–106C years). For the remainder of this article, dissolu- in rainwater). Rock weathering products, including calcium tion of carbonates (on land) and hydrolysis of silicates will and bicarbonate ions (respectively the most abundant cation be treated separately and referred to as carbonate and silicate and anion in most river waters), can be carried away with weathering, respectively. runoff to rivers and into the ocean. For example, calcium car- bonate dissolution by carbonic acid is given by (Archer et al., 1997) 1.3 A brief history of weathering in global carbon cycle models C C ! 2C C − CaCO3 CO2 H2O Ca 2HCO3 : (1) Variations in rock weathering rates have long been believed The influx of dissolved inorganic carbon (henceforth DIC) to hold a major role in regulating the Earth’s long-term cli- and alkalinity to the ocean surface layer is balanced by mate, and early (non-spatially explicit) carbon cycle models the precipitation and burial of biogenic calcium carbonate were built to investigate the importance of the weathering (CaCO ) in the marine sediments, and ocean alkalinity is a 3 feedback mechanism on various events in Earth’s geologi- key factor in determining the carbonate compensation depth cal history. Walker et al. (1981) developed expressions relat- (CCD), the depth below which the dissolution rate of calcium ing silicate weathering rates to atmospheric pCO (indirectly carbonate exceeds its precipitation rate. In the long term this 2 through vegetation productivity) and temperature (including allows the ocean to maintain a remarkably stable alkalinity, a weak dependency on runoff) and used them to offer a so- as any increases in ocean acidity (such as can be caused by a lution to the faint young Sun paradox by providing a con- CO invasion from the atmosphere) can be neutralized by el- 2 venient mechanism for a slow and steady decrease in atmo- evating the CCD, which dissolves carbonate sediments and spheric greenhouse gas concentrations. Berner et al. (1983) releases carbonate ions (CO2−) back into the ocean. This 3 also linked the rate of atmospheric CO consumption by sili- oceanic buffer factor, along with carbonate dissolution on 2 cate weathering to changes in surface air temperature, atmo- land (due to weathering), is the primary means through which spheric partial CO pressure, and river runoff and offered this ocean alkalinity is restored, and it is responsible for maintain- 2 dependency as a possible explanation for the general decreas- ing both atmospheric and oceanic pCO close to equilibrium. 2 ing trend of atmospheric CO levels on geologic timescales. In short, the weathering of calcium carbonate can accelerate 2 Although these models used rudimentary parameterizations the transfer of CO between the atmosphere and ocean, but it 2 derived from early general circulation models (GCMs) and does not contribute to a permanent return of carbon to the ge- experimental data, they built a foundation for future long- ologic reservoir (Ridgwell and Zeebe, 2005; Sarmiento and term carbon cycle model studies. Gruber, 2006). Following Berner et al. (1983), Berner (1991) built a geo- A certain fraction of rock weathering reactions involve a chemical cycle model, now called GEOCARB, in which the weakening of chemical bonds in the mineral lattice on con- long-term evolution of atmospheric carbon content would be tact with water whereby hydrogen ions replace positively driven by imbalances between CO outgassing by volcanic charged cations (mostly Ca2C and Mg2C) which are bounded 2 activity and the burial of carbonate sediments following the to negatively charged ions, most particularly SiO (silicate) 4 weathering of silicate rocks. The latter was given a depen- structures. One of the most common examples is given by dency on air temperature and atmospheric CO , and it was calcium silicate hydrolysis, as described by the following 2 used to solve a series of mass balance equations in order to schematic reaction (Ebelmen, 1845; Urey, 1952): determine the inward and outward fluxes for the atmosphere– C C ! 2C C C − CaSiO3 2CO2 3H2O Ca H4SiO4 2HCO3 : (2) ocean, land, and mineralogical carbon reservoirs. Subsequent versions were called GEOCARB II (Berner, 1994) and GEO- This equation represents the weathering of any silicate min- CARB III (Berner and Kothavala, 2001), and these further eral into silicic acid (which often precipitates as amorphous improved the weathering parameterizations based on the lat- silica SiO ), and it consumes one more molecule of CO 2 2 est observational data and GCM output. They were later cou- than carbonate dissolution while sending the same amounts pled with a model of atmospheric O and ocean nutrients of calcium and bicarbonate ions to the ocean. The combi- 2 to create COPSE (Bergman et al., 2004), a multi-element nation of Eq. (2) with calcium carbonate precipitation (the geochemical cycling model which introduces
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