Sensitivity of pelagic calcification to ocean acidification

Abstract. Ocean acidification might reduce the ability of calcifying plankton to produce and maintain their shells of calcite, or of aragonite, the more soluble form of CaCO3. In addition to possibly large biological impacts, reduced CaCO3 production corresponds to a negative feedback on atmospheric CO2. In order to explore the sensitivity of the ocean carbon cycle to increasing concentrations of atmospheric CO2, we use the new biogeochemical Bern3D/PISCES model. The model reproduces the large scale distributions of biogeochemical tracers. With a range of sensitivity studies, we explore the effect of (i) using different parameterizations of CaCO3 production fitted to available laboratory and field experiments, of (ii) letting calcite and aragonite be produced by auto- and heterotrophic plankton groups, and of (iii) using carbon emissions from the range of the most recent IPCC Representative Concentration Pathways (RCP). Under a high-emission scenario, the CaCO3 production of all the model versions decreases from ~1 Pg C yr−1 to between 0.36 and 0.82 Pg C yr−1 by the year 2100. The changes in CaCO3 production and dissolution resulting from ocean acidification provide only a small feedback on atmospheric CO2 of −1 to −11 ppm by the year 2100, despite the wide range of parameterizations, model versions and scenarios included in our study. A potential upper limit of the CO2-calcification/dissolution feedback of −30 ppm by the year 2100 is computed by setting calcification to zero after 2000 in a high 21st century emission scenario. The similarity of feedback estimates yielded by the model version with calcite produced by nanophytoplankton and the one with calcite, respectively aragonite produced by mesozooplankton suggests that expending biogeochemical models to calcifying zooplankton might not be needed to simulate biogeochemical impacts on the marine carbonate cycle. The changes in saturation state confirm previous studies indicating that future anthropogenic CO2 emissions may lead to irreversible changes in ΩA for several centuries. Furthermore, due to the long-term changes in the deep ocean, the ratio of open water CaCO3 dissolution to production stabilizes by the year 2500 at a value that is 30–50% higher than at pre-industrial times when carbon emissions are set to zero after 2100.


Introduction
Ocean uptake of atmospheric CO 2 leads to a decrease in carbonate ion concentrations, a reduction in pH, a shoaling of the calcium carbonate (CaCO 3 ) saturation horizons, and a subsequent increase in CaCO 3 dissolution. The current rate at which this process, known as ocean acidification, is occurring will likely have large biological consequences for ocean ecosystems within the near future. Past studies reported a decrease in calcification with decreasing saturation state before undersaturation is reached (refer to Fabry et al., 2008 for a synthesis of existing experimental evidence). Calcifying plankton (mainly coccolithophores, foraminifers, and pteropods; Schiebel, 2002;Kleypas et al., 2006) might be especially vulnerable to the decreasing saturation state as these organisms secrete calcite and aragonite, two important forms of CaCO 3 , to form their shells. In addition to potentially large impacts on the marine calcifiers, a decrease in CaCO 3 production causes a higher ocean uptake of CO 2 , i.e. a negative feedback on atmospheric CO 2 .
Published by Copernicus Publications on behalf of the European Geosciences Union. The goal of this study is to document the performance of a new cost-efficient 3-dimensional circulation-marine ecosystem model and to apply this Bern3D/PISCES model to study calcification under ocean acidification and related feedbacks to atmospheric CO 2 . We address uncertainties by applying a wide range of parameterizations for the dependency of calcification on the chemical state of surface water and for a range of 21st century and long-term CO 2 emission scenarios. A novel parameterization for the production of calcite by zooplankton is introduced and its impact on the cycle of CaCO 3 and on atmospheric CO 2 is evaluated. The combination of a cost-efficient circulation model and a complex marine ecosystem model allows us to go a step beyond existing studies in terms of tested parameterizations and scenarios. It is, as far as we are aware of, the first time that calcite production by mesozooplankton is considered in global change simulations.
The following chain of feedbacks is considered in this model study. Anthropogenic emissions cause atmospheric CO 2 to rise, carbon uptake by the ocean to increase, and carbonate ion concentration and the saturation state with respect to CaCO 3 to decrease. In response, production and dissolution of CaCO 3 change and affect the concentration of dissolved inorganic carbon (DIC) and alkalinity, and in turn, the CO 2 partial pressure in surface water and CO 2 uptake by the ocean. Finally, the altered ocean uptake leads to a different evolution of atmospheric CO 2 as compared to a situation where the CaCO 3 cycle would not respond to changes in CO 2 and the saturation state. The production of CaCO 3 removes DIC, but twice as much alkalinity from the water; the combined effect of removing DIC and alkalinity is to increase pCO 2 (Frankignoulle et al., 1994). Thus, a decrease in CaCO 3 production and the related increase in DIC and alkalinity, tends to increase carbon uptake and to lower atmospheric CO 2 . Similarly, shallower dissolution of CaCO 3 tends to increase DIC, alkalinity in surface water and to lower pCO 2 in surface waters and in the atmosphere.
Various model experiments have been performed during the recent years in order to identify future changes in calcification rates and associated feedbacks on atmospheric CO 2 (Heinze et al., 2004;Ridgwell et al., 2007Ridgwell et al., , 2009Gehlen et al., 2007;Hofmann and Schellnhuber, 2009). The corresponding global biogeochemical models deal with carbonate production as a biogeochemical function rather than attempting to reproduce the complexity of biological responses. Until now, parameterizations of the response of calcification to ocean acidification were generally fitted to experiments done on coccolithophores (mainly Emiliana huxleyi) and CaCO 3 was on the form of calcite only. With the exception of the study by Ridgwell et al. (2007), which provided a relatively large CO 2 -CaCO 3 production/dissolution feedback response to atmospheric CO 2 when simulating continued carbon emissions after the year 2100, all model experiments projected a small CO 2 -CaCO 3 feedback on atmospheric CO 2 . Ridgwell et al. (2007) estimated that the response of E. huxleyi to in-creasing atmospheric CO 2 is quite small compared to other coccolithophore species, foraminifera and corals. It is thus possible that biogeochemical models that only let CaCO 3 production be associated to E. huxleyi, may underestimate the CO 2 -CaCO 3 feedback on atmospheric CO 2 . Gangstø et al. (2008) extended the study by Gehlen et al. (2007) to include aragonite and, due to limited data on aragonite producing pteropods, they used a dependency equivalent to Gehlen et al. (2007) to project the response of calcite and aragonite production and dissolution to increasing atmospheric CO 2 concentrations. However, CO 2 -CaCO 3 production/dissolution feedbacks were not calculated for this model version. No model has so far explicitly included calcite production by mesozooplankton (foraminifera) and simulated its response to ocean acidification. The sensitivity of the marine carbonate cycle to CaCO 3 production by heterotrophs, as well as to the choice of the functional relationship between CaCO 3 production and seawater chemistry remains an open question.
We thus address the following questions: how important is the choice of shape of the relationship fitting the response of calcification to changing carbonate chemistry? Would the inclusion of aragonite and calcite produced by zooplankton change the predicted evolution of CaCO 3 production and dissolution, as well as the feedbacks on atmospheric CO 2 ? How fast is the expected reversibility of the ocean chemistry after a CO 2 perturbation? Would this be affected by the future changes in CaCO 3 production and dissolution?
The amount of data published about the response of calcifying organisms to increasing CO 2 has augmented over the last few years. Concerning open ocean calcifying organisms, coccolithophores (and especially E. huxleyi) have got much attention (Riebesell et al., 2000;Zondervan et al., 2001Zondervan et al., , 2002Sciandra et al., 2003;Delille et al., 2005;Langer et al., 2006;Iglesias-Rodriguez et al., 2008;Shi et al., 2009;Casareto et al., 2009;Godoi et al., 2009;Gao et al., 2009;Müller et al., 2010). However, the number of experiments performed on foraminifera and pteropods is also growing (e.g. Wolf-Gladrow et al., 1999;Bijma et al., 1999;Fabry et al., 2008;Comeau et al., 2009). Despite the continuously increasing number of studies published about the response of plankton to decreasing saturation state there are still large uncertainties related to the future evolution of calcification rates and the associated feedbacks. Due to these uncertainties, a number of sensitivity studies with a model including an advanced representation of the marine ecosystem would be of high advantage. However, these kinds of models typically require large amounts of computing time preventing a cost-efficient evaluation of different parameterizations.
The new Bern3D/PISCES model, a combination of the ecosystem model PISCES and the cost-efficient circulation model Bern3D, allows us to explore the above mentioned questions further. More recent experiments on both coccolithophores (Iglesias-Rodriguez et al., 2008 andShi et al., 2009) and pteropods (Comeau et al., 2009;J. Büdenbender, S. Lischka, and U. Riebesell, personal communication, 2009) permit us to improve previous studies on the response of calcification to increasing levels of atmospheric CO 2 . However, due to a still limited quantity of observations available, only "idealized" scenarios may be projected. We have therefore chosen a sensitivity study approach to our work. We split our study into three parts: in the first part we explore the change in calcite production in response to increasing atmospheric CO 2 , using different parameterizations of this response. Here, calcite is produced by nanophytoplankton. In the next section we broaden the study to include aragonite and calcite produced by mesozooplankton, investigating changes in production, dissolution and ocean chemistry in addition to quantifying the associated CO 2 -CaCO 3 production/dissolution feedbacks. Finally we use a range of emission scenarios to study the sensitivity and evaluate the reversibility of the ocean carbonate system to a CO 2 perturbation.
The formal outline is as follows. In Sect. 2, the Bern3D/PISCES model and the new parameterizations for CaCO 3 production are described. Model setup and experiments are described in Sect. 3. Section 4 provides an evaluation of the equilibrium state of the Bern3D/PISCES model with observation-based estimates for circulation and watermass distribution, CaCO 3 production, the distribution of dissolved inorganic carbon and alkalinity and of the saturation state as well as for the global ocean pelagic CaCO 3 budget. Section 5 presents the results for transient simulations and projections of atmospheric CO 2 , CaCO 3 production and dissolution, and of the CO 2 -CaCO 3 feedback. Concluding remarks follow in Sect. 6.

The Bern3D/PISCES model
The Bern3D/PISCES model couples the Bern3D costefficient, dynamic ocean circulation model (Müller et al., 2006) and the biogeochemical model PISCES. PISCES was developed within the circulation model NEMO/OPA (Aumont and Gehlen et al., 2006Gehlen et al., , 2007Gangstø et al., 2008) which has a relatively high resolution in both time and space requiring large computing resources. The low resolution Bern3D/PISCES model facilitates sensitivity analyses, millennial-scale paleoclimatic studies, and multiscenario analyses.

The Bern3D dynamical model
The Bern3D model is a global ocean circulation model (Müller et al., 2006) which is based on the 3-dimensional rigid-lid model of Edwards et al. (1998) and Edwards and Marsh (2005). It has a horizontal resolution of 36 × 36 grid boxes and 32 vertical layers with exponentially increasing thickness towards the ocean bottom. The surface layer has a thickness of 39 m, the bottom layer of 397 m. The at-mosphere is described by one well mixed box and the formulations of air-sea gas exchange are based on the updated OCMIP-2 protocol , and downscaled by 19% as suggested by Müller et al. (2008). The model is forced with seasonal fields of temperature, salinity and wind stress. Sea surface temperatures and salinities are constrained by using restoring and mixed boundary conditions. Restoring boundary conditions force SST and SSS to remain close to modern-day observational values. With mixed boundary conditions we prescribe freshwater fluxes on the ocean surface instead of SSS, allowing circulation-salinity feedbacks to develop upon perturbation of the model state (Bryan, 1986). An additional freshwater flux correction is applied (by removing freshwater from the Atlantic and distributing it in the North Pacific) in order to intensify and deepen the Atlantic meridional overturning circulation (MOC). The model includes an isopycnal diffusion scheme and Gent-McWilliams parameterization for eddy-induced transport (Griffies, 1998). The standard Bern3D model has been tuned towards data-based chlorofluorocarbon and radiocarbon inventories and is able to capture the large-scale ocean circulation, the characteristics of the most important water masses and the large-scale distributions of a range of ventilation timescale tracers (Müller et al., 2006). The Bern3D model has been applied for a wide range of studies (e.g. Müller et al., 2008;Sidall et al., 2007;Ritz et al., 2008;Tschumi et al., 2008;Parekh et al., 2008;Gerber et al., 2009;Gerber and Joos, 2010).
In the current study we use the parameter values for the Bern3D model as given by Müller et al. (2006), with a few exceptions. The diapycnal diffusivity has been increased from 1 × 10 −5 m s −2 to 5 × 10 −5 m s −2 and the Atlantic-to-Pacific freshwater flux correction was set equal to 0.2 Sv in order to strengthen the circulation fields and subsequently improve the oxygen and nutrients fields of the biogeochemical model. The time step used for the Bern3D model is reduced by a factor of 2 compared to the standard version (Müller el al., 2006), with the PISCES component called every ∼2 h in order to assure numerical stability. Future atmospheric CO 2 depends also on the evolution of terrestrial carbon sources and sinks, and this is uncertain (e.g. Denman et al., 2007). Here, we have not included a land biosphere component as the focus is on the ocean carbonate cycle.

The PISCES model
PISCES is a global ocean biogeochemical model (Aumont et al., 2003;Aumont and Bopp, 2006;Gehlen et al., 2006) which simulates the biogeochemical cycle of oxygen, carbon and the main nutrients controlling marine biological productivity: nitrate, ammonium, phosphate, silicate and iron. Biological productivity is limited by the external availability of nutrients and light. The model includes two phytoplankton size classes (nanophytoplankton and diatoms) and two zooplankton size classes (microzooplankton and 436 R. Gangstø et al.: Sensitivity of pelagic calcification to ocean acidification mesozooplankton). The C/N/P ratios are assumed constant for all species. For phytoplankton, the prognostic variables are total biomass, iron, chlorophyll and silicon contents. The internal ratios of Fe/C, Chl/C and Si/C are predicted by the model. For zooplankton, the only prognostic variable is total biomass. The bacterial pool is not modeled explicitly. PISCES comprises three non-living compartments for organic carbon: small particulate organic carbon (POC s ), big particulate organic carbon (POC b ) and semi-labile dissolved organic carbon (DOC). The C/N/P composition of dissolved and particulate matter is also coupled to Redfield stochiometry, whereas the iron, silicon and calcite pools of the particles are fully simulated and their ratios relative to organic carbon are allowed to vary. The particulate detrital pools (POC s , POC b , biogenic silica and calcite) are fuelled by mortality, aggregation from nanophytoplankton and diatoms, fecal pellet production and grazing.
The standard version of the model  considers CaCO 3 in the form of calcite only and assigns calcite production to nanophytoplankton, albeit without dependency of carbonate production on saturation state. The latter was implemented by Gehlen et al. (2007) for the sake of studying the future evolution of the marine carbonate cycle. The version developed for this study distinguishes two mineral phases of CaCO 3 , aragonite and calcite (Gangstø et al., 2008). Changes in the concentration of CaCO 3 in the mineral form l arise from production, Prod, dissolution with the dissolution rate, λ, and sinking of CaCO 3 particles with sinking speed ω: We distinguish between dissolution in the open water and from the ocean bottom. Dissolution in open water occurs only in undersaturated water ( < 1) and the rate λ is dependent on the saturation state, , with respect to the appropriate mineral phase: CaCO 3 particles falling on the ocean floor dissolve immediately in the overlying grid cell independent of . This additional dissolution has only a minor effect on the ocean chemistry. Only the open water dissolution is included in our further calculations and comparisons to observations. Production of CaCO 3 assigned to the plankton of type j and of mineral phase l is taken to be proportional to all the loss terms of this plankton type: Prod j,l =R j,l · {Loss Terms} j .
No production of calcite or aragonite occurs if the water is undersaturated with respect to the appropriate mineral phase. In oversaturated water, the fraction R j,l is not allowed to fall below 0.01 or exceed 0.8 . The model simulates dissolved inorganic carbon and total alkalinity. The carbon chemistry follows the OCMIP protocol (http://www.ipsl.jussieu.fr/OCMIP). A detailed description of the model including model equations and parameters can be found as auxiliary material in Aumont and Bopp (2006). In the present study, the model alkalinity field was initialized with a global mean alkalinity value of 2.410 mol m −3 , which lies between the global average from GLODAP (Key et al., 2004) and the one from Goyet et al. (2000). In the NEMO/PISCES model, half of the fraction of calcite production that is linked to loss of nanophytoplankton by grazing is assumed to be dissolved in the guts of zooplankton; only the remaining half affects alkalinity in open water and is added to the CaCO 3 pool . Here, the percentage of CaCO 3 that interacts with the alkalinity pool was increased from 0.5 to 0.85, which slightly improves the alkalinity fields. Nitrogen fixation and denitrification as well as river input and silicate deposition from dust were all set to 0 in the present study. In order to improve low oxygen values at depth and low surface concentrations of nutrients in the Bern3D/PISCES model, resulting from differences in circulation strength between the two dynamical models, the sinking speed of the largest class of particulate organic carbonate (POC b ) was reduced to a constant value of 60 m d −1 . Total pre-industrial CaCO 3 production is tuned in all setups to about 1.0 Pg C yr −1 by adjusting the scaling factors in Eqs. (4), (9) and (11). The values of the scaling factors are not given as they depend largely on the carbonate chemistry and circulation fields of the model used.

Calcite production by nanophytoplankton
The equations for calcite production by nanophytoplankton in the PISCES model are detailed by Gehlen et al. (2007). The proportion of calcifying phytoplankton is low in oligotrophic regions and calcifiers are often the dominant species when nanophytoplankton blooms. To mimic these conditions, the following parameterization is applied for the fraction of calcite production by nanophytoplankton, R P,C (Aumont and : In this parameterization, f i is a scaling factor used to match a global calcite production by nanophytoplankton of about 1.0 Pg C yr −1 (see Tables 1 and 2). L denotes limitation by the availability of phosphate, nitrate, ammonium, and iron and is taken to be identical to the nutrient limitation factor for nanophytoplankton growth. T is water temperature in • C and P is mass of nanophytoplankton, with the term max(1, P /2) in units of µmol C l −1 . The last factor in above equations denotes the ratio of particulate inorganic (PIC) to organic (POC) carbon for calcifying nanophytoplankton under optimal condition and as a function of the saturation state. Subscript i refers to different parameterizations of PIC C /POC, and subscript C to calcite.   (Gehlen et al., 2007), and PISC-ARAG (Gangstø et al., 2008). Parameterizations for the carbonate cycle are comparable between versions CAL-MIC2 and PISC-CAL and between versions CAL-ARAG and PISC-ARAG. The lower boundary represents the deepest model grid boxes. The flux at the lower boundary is not to be compared with estimates of the burial flux; the latter is not modeled in this study. Units are Pg C yr −1 .   (MIC1), a linear curve that is forced to go through C = 1 (LIN1) and a linear curve that was freely fitted to the data (LIN2), which is also used for calcite production by mesozooplankton (FORAM-C). In addition, the Michaelis-Menten curve that was used in Gehlen et al. (2007) and Gangstø et al. (2008) is included (MIC2). The red curve denotes the parameterization used for aragonite production (ARAG). The respective sensitivities, d(PIC-POC)/d , are shown in (b).
According to experimental results of the coccolithophore E. huxleyi obtained by Riebesell et al. (2000), Zondervan et al. (2001Zondervan et al. ( , 2002 and Delille et al. (2005), the calcification rate may be described by a threshold value of the saturation state below which the calcification decreases rapidly. Gehlen et al. (2007) fitted the PIC C /POC variable to these experimental results. Since then, new experiments with E. huxleyi have been performed (Iglesias-Rodriguez et al., 2008 andShi et al., 2009). When considering the calcification rates, these studies show an increase with decreasing saturation state, which is in contrast to previous studies on E. huxleyi. However, when looking at the PIC-POC values, the data points from these two studies fit well into the group of the other experimental data (Fig. 1a).

Michaelis-Menten type relationship between calcite production and saturation
In previous studies (Gehlen et al., 2007;Gangstø et al., 2008) a Michaelis-Menten approach was adopted for the PIC-POC ratio of calcifying nanophytoplankton, in order to mimic the decreasing calcification with decreasing saturation state (Riebesell et al., 2000;Zondervan et al., 2001Zondervan et al., , 2002Delille et al., 2005). The PIC-POC ratio for calcifying nanophytoplankton as a function of saturation state is expressed as: , (PIC C /POC) max is the maximum ratio reached under optimal growth conditions for calcifying organisms, the max value that PIC C /POC would have on Fig. 1a if the Michaelis-Menten curve was extended to C = ∞. The parameter K max corresponds to a half saturation constant, i.e. the value of ( C − 1) where PIC/POC equals half the value of (PIC C /POC) max . Finally, C is the saturation state with respect to calcite. The values (PIC C /POC) max and K max were previously set equal to 0.8 and 0.4, respectively (Gehlen et al., 2007;Gangstø et al., 2008). New data published (Iglesias-Rodriguez et al., 2008 andShi et al., 2009) encourage us to make a new fit, termed MIC1, to the data. Only data without severe light and nutrient limitation are considered , thus excluding one of the data sets from Zondervan et al. (2002), in addition to another data set which should be discarded due to large uncertainties (Zondervan et al., 2002). These two data sets were included in the original fit MIC2. The new fit MIC1 provides an upper limit of the PIC-POC value for calcifying organisms equal to 1.04, whereas K max becomes 1.11. The PIC C /POC ratio is shifted upwards for high saturation state values for the new fit MIC1 compared to the fit MIC2 (Fig. 1a). We refer to simulations with the new parameterization by the term "CALMIC1", to those with the original fit by "CALMIC2" (Table 1).

Linear relationships between calcite production and saturation
As the quantity of experiments performed on pelagic plankton is not yet massive, we may question the validity of using a Michaelis-Menten type correlation, and wonder how the use of a linear relationship between CaCO 3 production and saturation state would influence projected calcification. In parameterization LIN1, (PIC C /POC) and thus production is R. Gangstø et al.: Sensitivity of pelagic calcification to ocean acidification 439 forced to go through zero for C equal 1: with the slope ppcal equal to 0.19. No CaCO 3 production is allowed in undersaturated water. In parameterization LIN2, data points are fitted with no forcing (the production is still set to 0 when the saturation state becomes smaller than 1): The offset ppcal2 is 0.55 and the slope ppcal3 is with 0.06 more than three times smaller than in LIN1 (Fig. 1a). We expect thus a stronger response in calcite production for LIN1 than for the other parameterizations. We refer to the linear parameterization forced through zero (Eq. 6) by "CALLIN1" and to the unconstrained linear fit (Eq. 7) by "CALLIN2" (Table 1).

Aragonite production by mesozooplankton
The equations for aragonite production and dissolution are specified in Gangstø et al. (2008). One third of pelagic CaCO 3 production is assigned to aragonite producing mesozooplankton as a function of saturation state of seawater with respect to aragonite. This percentage is in the upper range of most estimates. Pelagic aragonite production is reported to contribute ∼10% to the total pelagic CaCO 3 production by Fabry (1990). However, estimates of aragonite production and fluxes in the pelagic ocean are scarce and cover a large range, extending from 10 to 50% of the total global CaCO 3 flux (Berner, 1977;Berger, 1978;Berner and Honjo, 1981;Fabry and Deuser, 1991). Due to the lack of observations available at the time of this study, the modeled dependency of the calcification on saturation state was done in an equivalent way as for calcite following a Michaelis-Menten curve with (PIC A /POC) max and K max equal to 0.8 and 0.4, respectively. Recently, new data has become available allowing a re-assessment of the parameterization. Comeau et al. (2009) studied the calcification response of the pteropod Limacina helicina to decreasing saturation states and report a decrease in aragonite production of 28% with a reduction in saturation state from 1.9 to 1.0 (CO 2 increased from 350 to 760 µatm). The authors did however not provide the corresponding PIC A to POC ratios. Another study on L. helicina was performed in Kongsfjorden, Spitsbergen, Norway. Here, the PIC A to POC ratios varied between 0.078 and 0.106 (J. Büdenbender, S. Lischka, and U. Riebesell, personal communication, 2009). A linear relationship is developed from the two data sets. We let the maximum PIC A -POC value from the second study correspond to conditions with A equal 1.9 and calculate a 28% linear decrease in PIC A -POC down to A = 1 (Comeau et al., 2009). This results in the parameterization: where the offset pparag equals 0.076 and the slope pparag2 is 0.033. The PIC A -POC ratio is multiplied with mesozooplankton biomass, M, and a scaling factor f M,A , which is used to tune the aragonite production to 1/3 of global calcification. The factor R M,A for aragonite production by mesozooplankton becomes: In contrast to the factor R, for nanophytoplankton, R M,A does not depend on temperature and nutrient limitation. Following Eq. (3), R M,A is multiplied with the sum of the loss terms for mesozooplankton to yield the aragonite production. We refer to the simulation including the aragonite parameterization (Eq. 8) and the new Michaelis-Menten parameterization for calcite (Eq. 5) as "CALARAG" (Table 1).

Calcite production by mesozooplankton
Foraminifers are, in addition to coccolithophores, major calcite producers in the pelagic ocean. They may account for between 32 and 80% of the CaCO 3 that accumulates on the sea floor (Schiebel, 2002). They are an important link to the upper trophic levels (e.g. Legendre and Le Févre, 1995;Rowe et al., 2008). Studies of foraminifera show reduced calcification in response to ocean acidification (e.g. Wolf-Gladrow et al., 1999;Bijma et al., 1999;Lombard et al., 2010). Most of these studies include changes in shell weight only. The shell mass was reported to decrease by 4-8% for a doubling of atmospheric CO 2 (Spero et al., 1997;. Moy et al. (2009) suggest that a decrease in shell weight of 30-35% may have taken place since pre-industrial times (thus within less than a doubling of atmospheric CO 2 ). Dissard et al. (2010) confirms the correlation between change in carbonate ions and change in shell weights. Lombard et al. (2009) project a decrease in shell weight of between 20 and 27% within the end of the century. The decrease in shell weight is in some studies reported to be non-linear (Bijma et al., 2002;Kuroyanagi et al., 2009), and in another study linear (Lombard et al., 2009). When it comes to changes in calcification rates of foraminifers, we are only aware of the data by Lombard et al. (2009). They suggest a reduction in calcification rates of between 6 and 13% by the end of this century compared to present day conditions, and these data indicate a linear shape of the response in calcification to increasing atmospheric CO 2 . A linear relationship to simulate the response in calcite production by foraminifera to decreasing saturation state is used here. We have chosen to use the unforced linear dependency on saturation state for the corresponding PIC/POC variable: and to describe the fraction R M,C for calcite production by mesozooplankton: where f M,C is again a scaling factor used to tune the calcite production to 1/3 of global calcification. The parameter ppcal2 is set to 0.55 and ppcal3 to 0.06. We refer to the simulation including the aragonite parameterization (Eq. 8), the new Michaelis-Menten parameterization for calcite (Eq. 5), and the calcite parameterization from Eqs. (10) and (11) as "CAL2ARAG".

Sensitivity of the different parameterizations
To highlight the sensitivity of the parameterizations described in the previous sections, their derivatives d(PIC-POC)/d( ) were calculated (Fig. 1b). The derivative of the Michaelis-Menten equations becomes: For the linear equations the derivatives correspond to the slope constants 0.1928 (LIN1), 0.0573 (LIN2) and 0.0333 (ARAG).

Model setup and simulations
The physical Bern3D model was first integrated over 10 000 yr using restoring boundary conditions. Next, air-sea freshwater fluxes were diagnosed and averaged over the last 1000 yr to provide mixed boundary conditions. The model was then switched to mixed boundary conditions, and an additional freshwater flux from the Atlantic to the Pacific was included. The model was further run for 5000 yr. Finally, the PISCES model was added and the Bern3D/PISCES model was integrated over 3000 yr with atmospheric CO 2 concentrations kept constant at 278 parts per million (ppm). All the variables presented in this study (alkalinity, saturation state, CaCO 3 production etc.) are given as annual means. Nine model versions are run to explore the projected changes in calcification to increasing atmospheric CO 2 concentrations, six with and three, termed NODEPC, NODE-PCA, NODEPC2A, without a dependency of calcification on saturation state (Table 1). Atmospheric CO 2 is prescribed for the period from 1766-2006. From 2007-2100, projected CO 2 emissions of the IPCC scenarios RCP8.5 (referred to as High) and RCP6 (Medium) are used (Van Vuuren et al., 2008;Strassmann et al., 2009) in addition to a scenario with zero carbon emissions (Low). Most of the model versions are run with the High scenario only, whereas multi-scenario runs are performed with the CALARAG and NODEPCA versions (Table 1). From year 2100 to year 2500 the CO 2 emissions are set to 0 for the High and Medium scenarios. An additional set of runs is performed with the High scenario and all model versions where the calcification is set to 0 from the year 2000. All scenario runs are accompanied with corresponding control runs with constant pre-industrial CO 2 concentrations.

Model evaluation/pre-industrial state
In order to validate the new Bern3d/PISCES model, a thorough model-data comparison is presented next. Preindustrial model output is compared to pre-industrial observations and observation-based estimates by first removing anthropogenic perturbations from the data sets whenever these were available, such as for DIC and saturation state. For the other variables, we follow common praxis and compare pre-industrial model output with present-day observations, thereby implicitly assuming that the difference between pre-industrial and present-day observations would be small. Fig. 3. Vertically integrated CaCO 3 production for (a) the version CALMIC1 that represents calcite production by nanophytoplankton only, (b) the difference to version CALARAG that represents calcite and aragonite production and (c) the difference to CAL2ARAG that represents calcite production by nanophyto-and mesozooplankton and aragonite production.

Circulation and water masses
Maximum overturning reaches 23 Sv (1 Sv = 10 6 m 3 s −1 ) at around 45 • N in the North Atlantic, with a southward transport out of the Atlantic between 1000 and 2000 m depth of about 16 Sv (Fig. 2). This is comparable to the estimates of Talley et al. (2003) of maximum overturning and southward transport equal to 18 Sv for most latitudes. The modeled overturning is larger than in earlier model versions (Müller et al., 2006) where a lower diapycnal diffusivity has been applied. North Atlantic Deep Water (NADW) is now propagating a little too deep. A weak modeled overturning cell in the deepest part of the Atlantic corresponds to AABW. The value of the transport from the AABW is lower than estimates, which suggests a transport up to 8.5 Sv (Talley et al., 2003). In the South Pacific, the northward flow of AABW takes place below 2000 m and maximum deep overturning is 14 Sv, comparable to the estimate by Talley et al. (2003) of 13 Sv. Deep equatorial upwelling in the Pacific is with about 9 Sv likely too much and results in nutrient trapping in the thermocline of the equatorial Pacific (not shown). The formation of North Pacific Intermediate Water (NPIW) is rather strong compared to observations (Talley et al., 2003), and propagates too deep. Although most of the Pacific deepwater is supposed to flow southwards again at intermediate depths, too high radiocarbon values at the surface of the North Pacific compared to observations (not shown) may indicate too little upwelling of old water masses here. Deep convection occurs mainly in the North Atlantic south of Greenland and in the Ross and Weddell Sea next to Antarctica. Compared to the standard Bern3D model as used in earlier studies, the Bern3D/PISCES simulates more overturning and younger water masses. As far as the marine biological cycle is concerned, we expect too much biological production and too much nutrient trapping in the Equatorial Pacific due to upwelling.

CaCO 3 production
The spatial pattern of CaCO 3 production modeled with the calcite-only version CALMIC1 (Fig. 3a) shows the major large-scale features of the observations (Iglesias-Rodriguez et al., 2002a;. Values are low in the ocean gyres and high in the North Atlantic, North Pacific, and Southern Ocean, as well as in the near coastal upwelling zones in the eastern Pacific and eastern South Atlantic. However, whereas the estimates from satellite images show little calcification in lower latitudes compared to higher latitudes, a larger part of the total modeled calcification occurs in lowlatitude upwelling areas. At least for the Equatorial Pacific, this may be linked to model deficiencies as mentioned in the previous section. The modeled annual average calcification reaches about 12 g C m −2 yr −1 (∼1 mg C m −3 d −1 ) in the areas with maximum concentrations of nanophytoplankton. In the North Atlantic, the model simulates annual average calcification rates of between 0.2 and 0.5 mg C m −3 d −1 . In regions away from the coast, differences in modeled CaCO 3 production are relatively small between the different model versions (Fig. 3). In other words, CaCO 3 production by nanophytoplankton and by mesozooplankton has a similar spatial pattern, as seen in Fig. 4. This reflects the dependency of mesozooplankton on the availability of nanophytoplankton as a source of food. Mesozooplankton graze on nanophytoplankton, in addition to grazing on diatoms, microzooplankton and POC. In near-coastal areas, the CaCO 3 production is, however, substantially higher in the versions CALARAG and CAL2ARAG where 1/3 and 2/3 of the production are related to mesozooplankton (Fig. 3).  Zonally-averaged production is higher in the Southern Ocean (<50 • S) and for the maximum south of the equator for the version with mesozooplankton calcification (CALARAG, CAL2ARAG) than for the versions with CaCO 3 production by nanophytoplankton only (Fig. 5a). This reflects the relatively higher aragonite and meso-calcite production in these productive regions, caused by the relatively higher abundance of mesozooplankton than nanophytoplankton here (Fig. 4b). The model versions with no dependency of CaCO 3 production on saturation state broadly follow similar zonal patterns.
Observations and estimates of pteropod and foraminifera calcification are scarce. Most of the aragonite production occurs in subpolar and polar areas (Lalli and Gilmer, 1989). Sediment trap studies have shown that in the Southern Ocean, south of the Polar Front, pteropods are the major source of the carbonate flux (>50%) to the depth (Hunt et al., 2008). In the Sub-Antarctic Zone, tests from coccolithophores and foraminifers dominate the flux Honjo, 2004). The model versions with aragonite both provide quite high CaCO 3 production rates in polar and subpolar areas, coherent with literature reporting that aragonite shells of pteropods are especially abundant here (e.g. Collier et al., 2000;Honjo et al., 2000;Urban-Rich et al., 2001;Accornero et al., 2003). However, compared to the high latitudes, the model simulates a higher aragonite production in lower latitudes. While studies report that pteropods are also significant contributors to the CaCO 3 flux in low latitudes, such as in the Indian Ocean (Panchang et al., 2007) and the Equatorial Atlantic (Volbers et al., 2001), the highest biomass of pteropods is expected to be found in cold, high-latitude waters (Lalli and Gilmer, 1989). This may indicate an underestimation of aragonite production in high latitudes or an overestimation in low latitudes by the Bern3D/PISCES model. Since aragonite production is linked to the modeled mesozooplankton distribution, a bias in the latter could explain the latitudinal trend. Figure 5b shows the zonal average modeled distribution of mesozooplankton compared to a data set from the National Marine Fisheries Service (NMFS), compiled by Buitenhuis et al. (2006). The dataset has a global, yet temporally sparsely resolved coverage and includes samples taken over the top 200 m. While the overall pattern of spatial variability displayed by the observations is reproduced, the model underestimates the global mesozooplankton concentrations from the dataset, and especially in the North. The global average value of the model equals 0.46 mmol m −3 , compared to 0.84 mmol m −3 from the NMFS data set. The model shows a better match to observations from the Continuous Plankton Recorder survey (CPR) (Buitenhuis et al., 2006), which are taken from regular surveys in the North Atlantic Ocean only, and are therefore not shown. The average value of the model in this region equals 0.65 mmol m −3 , compared to 0.60 mmol m −3 from the CPR data set. The improved fit might be due to the better data coverage provided by a time-series type of survey as the CPR, compared to a sparse and synoptic data set like NMFS. The mismatch in the North when comparing to the NMFS dataset suggests that the under/overestimation of aragonite production in the high/low latitudes may be linked to a bias in the modeled mesozooplankton distribution. The parameterization of aragonite production does not include a dependency on temperature, making the missing temperature effect another likely explanation of the he latitudinal trend with higher average rates of aragonite production occurring at low latitudes.
CaCO 3 production is simulated down to a depth of ∼500 m with little differences in basin-averaged production profiles between the different model versions (Fig. 6a). Most of the production is simulated above 200 m. While the vertical distribution of coccolithophorids (nanophytoplankton) is function of light and nutrient availability, the vertical migrations of pteropods are not taken into account.
In conclusion, CaCO 3 production appears to be reasonably represented in the Bern3D/PISCES model when compared with available observation-based estimates and their uncertainty. Aragonite production is likely overestimated in low and mid-latitude regions compared to high-latitude regions as production is linked to mesozooplankton abundance without considering that many aragonite producing species are found predominantly in cold, high-latitude waters.

Alkalinity and DIC
The observed spatial patterns of DIC and alkalinity in the surface ocean are well matched by the model (Figs. 5c and  d, 7a and b and 8a-d). The Bern3D/PISCES reproduces the high alkalinity values in the north and south Atlantic and the high DIC concentrations in the Southern Ocean. However, the surface alkalinity and DIC concentrations are on average lower than observed. DIC and alkalinity are slightly higher in the versions including aragonite production and dissolution. This is due to the 50% higher solubility of aragonite compared to calcite, which increases shallow dissolution and thus the alkalinity and DIC concentrations at the surface and in the upper part of the water column. Consequently, including aragonite in the model improves the simulated concentrations of surface alkalinity and DIC ( Fig. 5c and d).
At depth, the model generally reproduces observationbased alkalinity and DIC patterns ( Fig. 7a and b). Existing mismatches can be explained by deficiencies in the simulated ocean circulation. DIC and alkalinity increase along the deep ocean transport path, due to the water column remineralization of organic carbon and CaCO 3 . Modeled DIC concentrations in most of the Atlantic are low compared to observations and for all model versions. This is likely related to the  . Concentrations are shown for the Bern3D/PISCES versions CALMIC1, representing calcite production by nanophytoplankton only (blue), CALARAG, representing calcite and aragonite production (red) and CAL2ARAG, representing calcite production by nanophyto-and mesozooplankton and aragonite production (green). Observation-based estimates are shown by black solid lines (GLODAP; Key et al., 2004) and saturation concentrations for CO 2− 3 with respect to calcite and aragonite are indicated by black, dashed lines. The alkalinity and DIC concentrations at depth are slightly improved in the versions including aragonite, due to the rearranging of DIC and alkalinity concentrations in the water column caused by shallow water aragonite dissolution (Fig. 6b). An exception is the alkalinity in the deep Atlantic and Pacific, which is more over-and underestimated, respectively, in the model versions including aragonite (Fig. 7a). The differences between the model versions with calcite only and the model versions including aragonite appear in the Taylor diagram (Fig. 9), where modeled global alkalinity and DIC are compared to pre-industrial values from the Global Ocean Data Analysis Project (GLODAP) (Key et al., 2004). Due to the mismatches in the deep Atlantic and Pacific, the correlation coefficient r between modeled and data-based alkalinity is ∼0.8 for the versions without aragonite, and ∼0.7 for the versions including aragonite. For DIC, the correlation coefficient is higher than 0.9 for all versions and independent of the form of CaCO 3 , whereas the standard deviation becomes closer to unity when aragonite is included. To conclude, except for some discrepancies in the Atlantic and Pacific mainly related to deficiencies in modeled circulation, the observed alkalinity and DIC concentrations are fairly well represented by the model.

Saturation state
Modeled surface CO 2− 3 concentrations, calcite and aragonite saturation state compare well with observations (Figs. 5e and f, 8e and f). The modeled surface saturation state is on average slightly too high. At depth, the observed values of aragonite saturation state are relatively well represented in the Southern and Indian Ocean (Figs. 7c, 10e-h), with aragonite saturation horizons around the depth of 1000 m. Mismatches between model results and observations in the Atlantic and Pacific deep ocean (Fig. 10a-d) are linked to deficiencies in the modeled DIC and alkalinity fields. Due to these mismatches, a modelobservation comparison of the exact depth of the saturation horizons with respect to calcite and aragonite results in low correlations in the Taylor diagram (Fig. 9). However when comparing the patterns between modeled A and GLODAP observations, the diagram shows correlation coefficients of about 0.95 for all versions and a relative standard deviation just above unity. This indicates an overall good representation of the aragonite saturation state in the Bern3D/PISCES.

Global carbonate budgets
Next, the global pre-industrial CaCO 3 production, export and dissolution fluxes are discussed in the context of observation-based estimates and compared with those from the NEMO/PISCES model (Tables 1 and 2) (Gehlen et al., 2007 andGangstø et al., 2008). Total CaCO 3 production was tuned to ∼1 Pg C yr −1 for all versions in agreement with published estimates (Iglesias-Rodriguez et al., 2002b;Lee, 2001;Jin et al., 2006;Berelson et al., 2007;. The modeled CaCO 3 export at 100 m (∼0.8 Pg C yr −1 ) is smaller than the net production, because CaCO 3 is also produced below 100 m (Fig. 6a). Model export matches well the estimate by Berelson et al. (2007), but is higher than proposed by Sarmiento et al. (2002).
The pelagic CaCO 3 dissolution of all model versions is within the range of estimates of 0.5 ± 0.2 Pg C yr −1 . The total pelagic CaCO 3 dissolution is substantially lower in calcite only versions (CALMIC1 and CALMIC2; ∼0.4 Pg C yr −1 ) than in those with aragonite (CALARAG and CAL2ARAG; ∼0.6 Pg C yr −1 ). This is re- Fig. 9. Taylor diagram comparing modeled global fields of aragonite saturation state, alkalinity, and DIC for different parameterizations of the CaCO 3 production to observation-based estimates (GLODAP; Key et al., 2004). The modeled and estimated saturation horizons with respect to calcite (CSH) and aragonite (ASH) are added.
lated to the higher solubility of aragonite compared to calcite. Including aragonite in the model yields a substantial CaCO 3 dissolution in the upper part of the water column compared to the version without aragonite (Fig. 6b, Table 2). This increased shallow water CaCO 3 dissolution in CALARAG, compared to CALMIC1 and CALMIC2, matches better with observation-based estimates Milliman and Droxler, 1996) and confirms the results found by Gangstø et al. (2008). It also supports the hypothesis that the estimated loss of CaCO 3 or excess of alkalinity in the upper part of the water column may be at least partly attributed to aragonite dissolution (Milliman and Droxler, 1996;Berelson et al., 2007) and upward mixing (Friis et al., 2007).
In summary, the global CaCO 3 production, export and dissolution fluxes in the Bern3D/PISCES model are comparable to observation-based estimates.

Atmospheric CO 2 and saturation state
Next, we will present the projected anthropogenic perturbation in the CaCO 3 cycle and the evolution of underlying drivers for carbon emission commitment scenarios (e.g. Frölicher and Joos, 2010). Carbon emissions are prescribed up to 2100 according to one of the RCP scenarios and then hypothetically (and unrealistically) set to zero to study the long-term impacts of 21st century emissions on the CaCO 3 cycle and CO 2 . We will first present results for the high-emission, no climate-policy intervention scenario, before showing the range of scenarios in Sect. 5.4. Note that global warming is not modeled here and production of organic material remains constant throughout the simulation. The changes in CaCO 3 production and dissolution are thus only forced by changes in the saturation state with respect to CaCO 3 . The evolution of CaCO 3 production is therefore closely linked to the evolution of the saturation state in the surface ocean (Figs. 11b and 12), which itself closely follows atmospheric CO 2 (Fig. 11a). The evolution of open water CaCO 3 dissolution is influenced by the growing extent of undersaturated water (Fig. 13) and the degree of undersaturation.
Differences in projected changes in CO 2 , in surface saturation with respect to calcite ( C ) or aragonite ( A ) and in the volume of undersaturated water are relatively small between the different model versions (Figs. 11b, 12, and 13). Atmospheric CO 2 increases to almost 1000 ppm by 2100 for the High scenario. Afterwards, CO 2 decreases only slowly to about 600 ppm by 2500 AD, even though emissions are zero after 2100. C decreases from about 5.2 to 2.2 and A from about 3.4 to about 1.4 by the year 2100 and for all versions (Fig. 11b). Saturation increases again afterwards at the surface. Regionally, the largest decrease in saturation is found in the tropics, whereas surface waters in highlatitude become undersaturated with respect to aragonite over the course of this century and remain undersaturated by 2500 under the High scenario (Fig. 12). Undersaturation with respect to aragonite is imminent in the Arctic and becomes also widespread in the Southern Ocean. The evolution of the surface ocean aragonite saturation state projected by model versions CALARAG and NODEPCA (Fig. 12) is close to identical up to the year 2100 and differences are small after 2100. In other words, the overall effect from the decreasing calcification on the saturation state is small.
Following Steinacher et al. (2009) and Frölicher and Joos (2010), we distinguish different classes of saturation with respect to aragonite and compute global changes in the volume occupied by undersaturated and oversaturated water masses as an indicator of whole ocean acidification (Fig. 13). Compared to GLODAP observations, our model correctly simulates the volume occupied by the upper three classes, whereas the model underestimates the volume of undersaturated water (crosses in Fig. 13). The volume of water oversaturated with respect to aragonite decreases from about 60% to only 15% of the ocean volume until 2200. Saturation changes long after emissions have been stopped as anthropogenic carbon continues to invade the slowly ventilated deep ocean. Water that is more than three times supersaturated disappears in the next decades and remains absent until the year 2400 for the High case. This illustrates the longlasting impacts of anthropogenic carbon emissions.

CaCO 3 production and dissolution
Calcite-only production by nanophytoplankton: under the High scenario, the CaCO 3 production decreases to between 0.69 and 0.82 Pg C yr −1 by the year 2100, for most of the calcite-only versions (Fig. 11c). This corresponds to a 20-34% reduction relative to pre-industrial values. The exception is version CALLIN1 which experiences a reduction in calcification of 66%. In this version the PIC-POC is forced to zero for C approaching one and the production depends particularly sensitively on saturation over the entire C -range (Fig. 1). After 2100, the CaCO 3 production increases again, following the increase in saturation state and the decrease in CO 2 . By the end of the scenario, at the year 2500, the production in the CALLIN1 version with the most sensitive parameterization has increased to about 0.6 Pg C yr −1 , compared to ∼0.9 Pg C yr −1 for the other 3 calcite-only versions (Fig. 11c). Hence, with the exception of the more extreme linear parameterization, the different calcite parameterizations shown in Fig. 1a, whether linear or functions of a Michaelis-Menten curve, provide similar responses of the calcite production to the simulated perturbation in the saturation state.
The decrease in calcite production projected by the Bern3D/PISCES model of 20-66% confirms results from literature. Gehlen et al. (2007) reported a decrease in production equal to 27% by the end of their scenario, where the atmospheric CO 2 was increased at a rate of 1% per year, which is higher than the rate of increase of the IPCC RCP8.5 scenario selected for this study. It resulted in shorter time duration, and the atmospheric concentration reached a maximum of 1144 ppm after 140 yr (compared to 992 ppm in our study). Although not directly comparable due to the  Fig. 13. Simulated global annual mean changes in the entire ocean volume of supersaturated (light blue to red) and undersaturated (blue) waters with respect to aragonite for the High emission commitment scenario where emissions are set to zero after 2100. Differences are small between the versions with (color, black contours), and without (white contours) CaCO 3 production depending on saturation. The black dashed line indicates the separation between over and undersaturated water with respect to calcite. The crosses denote the respective volumes of pre-industrial saturation state derived from the GLODAP data set. differences in the underlying scenario and its duration, we note the agreement with our result for the same parameterization. Heinze (2004) projects a decrease in calcification of approximately 30% at atmospheric CO 2 concentrations equal to 1000 ppm, when fitting a linear dependency to the CaCO 3 production rates as a function of CO 2 reported by Riebesell et al. (2000). Ridgwell et al. (2007) report a larger Fig. 14. Projected evolution of (a) calcite and (b) aragonite production (g C m −2 yr −1 ) for the CALARAG version. reduction in CaCO 3 export production of about 60% at an atmospheric CO 2 content equal to 1000 ppm. This reduction value is comparable to our estimate obtained for the linear parameterization forced to the intercept (LIN1). While the parameterization LIN1 provides a poor fit to the data and should be viewed as an extreme end-member for the sensitivity study, such a strong response of calcification on saturation state can a priori not be ruled out based on Ridgwell et al. (2007). Based on available evidence, a global decrease in pelagic carbonate production of about 30% in response to ocean acidification under a business-as-usual-scenario (without climate change) seems likely by the year 2100. However, uncertainties in these estimates are large.
Calcite and aragonite production by zooplankton and by nanophytoplankton: the two versions including aragonite, CALARAG and CAL2ARAG, generally yield a larger decrease in CaCO 3 production (Fig. 14) than the versions with calcite only (Fig. 11c). The total CaCO 3 production (calcite and aragonite) decreases by 35 and 31% within the year 2100 for the versions CALARAG and CAL2ARAG, respectively (Table 3). By the year 2500, the production has reached 0.80 and 0.78 Pg C yr −1 , respectively, corresponding to a 19 and 18% decrease compared to pre-industrial times. The larger reduction in CaCO 3 production of the versions including aragonite is caused by the higher solubility of aragonite particles compared to calcite particles. The aragonite saturation state is lower than the calcite saturation state already at pre-industrial times (Fig. 11b) and it decreases to even lower values with increasing atmospheric CO 2 .
The different parameterizations for calcite and aragonite production imply different changes in the amount of CaCO 3 produced per unit change in the saturation state (Fig. 1b). For C > 4, the calcite production parameterization LIN1 is by far the most sensitive to changes in the saturation. MIC1 becomes most sensitive for C below 2.5. The sensitivity of aragonite production to changes in saturation state is comparably low. Pre-industrial zonal average surface values for C were lying within the range of 2.5-6.5. This implies that the model version with parameterization LIN1 will show stronger reductions in CaCO 3 compared to other versions at least as long as C remains above 2.5.
The shape of the relationship between seawater saturation state and pelagic carbonate production, the functional group to which it is assigned, as well as the particular CaCO 3 polymorph produced are at the origin of uncertainties in model projections. Since the sensitivity study discussed above demonstrated an overall low sensitivity of the carbonate cycle to the shape of the parameterization, a linear curve to describe the response of calcifying mesozooplankton to decreasing saturation state appears justified at this time. Future studies will provide insight into the calcification mechanism and its controls, thereby enabling the derivation of improved model parameterizations. By including the more soluble aragonite, as well as calcite producing mesozooplankton, we increase the sensitivity of CaCO 3 production to increasing atmospheric CO 2 .
The Bern3D/PISCES model version CALARAG (calcite produced by nanophytoplankton and aragonite by mesozooplankton) projects a larger reduction in total CaCO 3 production (−35%) by the year 2100 than obtained with the NEMO/PISCES model (−19%). While the latter assumed a Michaelis-Menten (MIC2) type dependency of CaCO 3 production on seawater saturation state for calcite and aragonite (Gangstø et al., 2008), a linear parameterization was used for aragonite production as a function of A in the model version CALARAG along with parameterization MIC1 for calcite. The linear parameterization ARAG has a stronger sensitivity to changes in A compared to MIC2 for A > 5.5 (Fig. 1b). Pre-industrial values of A remained below that threshold in both models, implying an increasing higher sensitivity of aragonite production to changes in A in NEMO/PISCES with progressing ocean acidification. The opposite holds for calcite production with parameterization MIC1 being more sensitive than MIC2. The stronger response of the Bern3D model reflects the combination of (i) differential sensitivities of parameterizations, differences in (ii) the modeled pre-industrial saturation horizon and (iii) the rate of transfer of anthropogenic CO 2 from the surface to the deep ocean (ventilation, deep convection). Differences in the emission scenarios, scenario RCP8.5 yields atmospheric CO 2 levels about 165 ppm higher by the year 2100 than SRESA2 (Gangstø et al., 2008), most likely play a minor role. The substantial differences obtained with the same biogeochemical model, but coupled to different ocean general circulation models highlights the need for a systematic model inter-comparison study.
CaCO 3 dissolution: the absolute magnitude of CaCO 3 dissolution in the open water column (Fig. 11d) depends (i) on the amount of CaCO 3 produced (Fig. 11c), and (ii) on the saturation state with respect to CaCO 3 in the thermocline and deep ocean (Fig. 13). The relative importance of the two factors is quantified with the help of simulations that do not include a dependency of the CaCO 3 production on the saturation state. In these simulations, labeled "NODEP", production remains constant and changes in dissolution are entirely driven by changes in the dissolution rate (Eq. 2).
Global CaCO 3 dissolution in the open water increases to reach a plateau after 2200 in the NODEP cases for the High scenario (Fig. 11d). The evolution in dissolution roughly parallels the increase in the volume of undersaturated water (Fig. 13). The evolution in global open water dissolution is more complex in the versions that include the dependency of production on saturation. Global CaCO 3 dissolution is projected to decrease until 2100 in response to decreased CaCO 3 export. Then, open water dissolution increases and raises well above the pre-industrial level, mainly in response to decreasing deep ocean saturation. The relative dissolution (Fig. 11e), i.e. the ratio between open water dissolution to production, increases first gradually until the year 2100, then rapidly until the year 2200, followed by a slight trend to reach ∼90-95% for all versions. In other words, the fraction of CaCO 3 that is dissolved in the open water increases from pre-industrial ∼40% for the calcite-only versions and ∼60% for the versions including aragonite to almost 100%. Obviously, the expansion of the volume of undersaturated water with respect to aragonite from pre-industrial 40% to around 80% (and with respect to calcite from pre-industrial 20% to 60%) of the total ocean volume causes a corresponding increase in open water CaCO 3 dissolution. Trends in production and dissolution are getting smaller towards 2500, but the system is still away from equilibrium by the end of the simulation as expected from the century to multi-millennial perturbation life time of an anthropogenic CO 2 perturbation.

CO 2 -CaCO 3 production/dissolution feedback on atmospheric CO 2
In the following, we will quantify the CO 2 -CaCO 3 production/dissolution feedback with respect to changes in CaCO 3 production only by taking the difference between a model version that includes a dependency of CaCO 3 production on saturation state and the corresponding version (NODEP) that does not incorporate such a dependency. The feedback responses in atmospheric CO 2 are from −2.5 to −11.4 ppm by 2100 for the High scenario (Table 3). Thus, the feedback is small compared to the total atmospheric CO 2 perturbation of about 710 ppm by the year 2100, despite a decrease in CaCO 3 production of between 20 and 66% in the different versions. The feedback increases after year 2100, although emissions have been set to zero and CaCO 3 production increases again (Fig. 11c). This increase is linked to the slow time scale of surface-todeep transport in the ocean, and the complex interplay between tracer transport, production, and shallow dissolution of CaCO 3 . Both model versions with aragonite, CALARAG and CAL2ARAG, provide a feedback on the atmospheric CO 2 of −5.8 ppm by the year 2100. The further evolution is also similar between the two versions. After the peak around year 2200 follows a reduction in feedback towards the year 2500, which is stronger than in the versions with calcite only. This reflects a faster recovery of the carbonate system when aragonite is considered, due to the higher solubility of aragonite compared to calcite. By the year 2500, most model versions provide a feedback effect due to decreasing calcification of between −5.1 and −10.9 ppm. The more extreme parameterization (LIN1) yields a stronger CO 2 -CaCO 3 production/dissolution feedback corresponding to −24.3 ppm by this time. The feedback effects of the model versions with aragonite, CALARAG and CAL2ARAG, correspond to −6.8 and −6.0 ppm, respectively. Our results fall in a narrow range considering the variety of model parameterizations and emission scenarios addressed in this study. They range from −3 to −11 ppm by the year 2100, in the case of continuous CO 2 emissions up to the year 2100. Our results compare well to previous studies which report negative feedbacks to atmospheric CO 2 extending from −2 to −12 ppm integrated over the duration of the simulations (Heinze, 2004;Gehlen et al., 2007;Ridgwell et al., 2007;Hofmann and Schellnhuber, 2009). Despite the diversity in model systems, scenarios and parameterizations all studies converge to project a rather modest negative CO 2 -CaCO 3 production/dissolution feedback to increasing atmospheric CO 2 .
In order to estimate the potential upper limit of the CO 2 -CaCO 3 feedback, calcification was set to 0 after the year 2000 for all model versions (Fig. 15a). While the previous simulations resulted in a maximum feedback of −11 ppm with the strongest parameterization LIN1 by the year 2100, the equivalent maximum CO 2 -CaCO 3 feedback when the calcification is set to 0 after 2000 corresponds to −30 ppm (Fig. 15b). The corresponding maximum value when 1/3 aragonite is included is −23 ppm.
In all the scenarios presented, the two model versions including aragonite show a very similar response to increasing atmospheric CO 2 despite the very different parameterizations for calcite production. In both versions, two thirds of total CaCO 3 production are in the form of calcite at steady state. However, in the model version CALARAG calcite is produced by nanophytoplankton only, whereas in the model version CAL2ARAG half of the calcite production is by nanophytoplankton and half by mesozooplankton at steady state. The similar response of the two versions in scenarios with increasing CO 2 is most likely explained by the fact that the abundance of zooplankton is coupled to the food source, i.e., phytoplankton, and that the sensitivities of the different parameterizations of calcite production to saturation state changes are relatively similar. This study suggests that at present and considering the still limited experimental evidence on which to draw for improved process parameterizations, the first order biogeochemical response of the open ocean CaCO 3 cycle is adequately reproduced by a model including one calcifying plankton functional type.
PISCES includes only an indirect approach to the ballast effect, by assuming that 50% of the organic matter of the calcifiers is associated with the shell. Since calcite is significantly denser than organic matter, 50% of the biomass of dying calcifiers is routed to the fast sinking POC. A reduction of calcifying nanophytoplankton goes along with a reduction of its contribution to the pool of fast sinking POC. Mesozooplankton contributes to the same pool of fast sinking particles, but as their total biomass is not affected by acidification, there will be no effect of decreasing aragonite production on POC fluxes. The model most likely underestimates the decrease in ballasting of organic C fluxes by CaCO 3 (Armstrong et al., 2001;Klaas and Archer, 2002) as a direct consequence of ocean acidification. A reduction in the ballast effect would decrease the penetration depth of organic C. The resulting shallower remineralization depth of organic C would provide a positive feedback to atmospheric CO 2 , which might well be of similar or larger magnitude as the CO 2 -CaCO 3 production/dissolution feedback (Barker et al., 2002;Heinze et al, 2004;Hofmann and Schellnhuber, 2009;Kwon et al., 2009). Climate change, with higher temperatures is at the origin of an increase in stratification of the ocean waters. Chemical changes related to the temperature effect (decreasing solubility of CO 2 with increasing water temperature), but also a slowing down of the surface to deep transport of water masses, combine to a positive feedback to the atmospheric CO 2 Greenblatt and Sarmiento, 2004;Friedlingstein et al., 2006;Plattner et al., 2008).
Impacts of climate change on saturation are small except in polar waters. The decrease in A in the Arctic is amplified by surface freshening and sea ice retreat which may cause increased uptake of anthropogenic carbon (Steinacher et al., 2009). Going along with the projected increase in stratification may be a reduction in marine net primary productivity and export production (Steinacher et al., 2010). The reorganization of surface ocean ecosystems with a shift from diatoms towards nanophytoplankton  has also been suggested. The ultimate fate of CaCO 3 production will thus depend on the interplay between ocean acidification and direct climate change effects. While this discussion highlights the large uncertainties still linked to projections of changes in the marine carbonate cycle and associated feedbacks to atmospheric CO 2 , it also suggests that the magnitude of published feedback estimates might represent an upper limit on time scales of decades to a few centuries.

Sensitivity to future CO 2 emissions
This section addresses the sensitivity of the marine carbonate cycle to projected atmospheric CO 2 trajectories. We focus on the legacy of historical and future CO 2 emissions in terms of impacts on the production/dissolution of marine carbonates ( Fig. 16c and d), as well as on the surface ocean saturation state with respect to aragonite (Fig. 16b). To this end we compare the business-as-usual IPCC scenario RCP8.5 (referred to as High) to the alternative pathway RCP6 (Medium) and to a Low scenario. For the High scenario, total cumulative CO 2 emissions of 1916.9 Pg C between 2007 and 2100 translate into a maximum of atmospheric CO 2 of 992 ppm in the year 2100 (Fig. 16a). Atmospheric CO 2 peaks at 702 ppm in 2100 for the Medium scenario (with total cumulative CO 2 emissions of 1138.0 Pg C for the period 2007-2100). In contrast to the High and Medium cases for which emissions increase up to the year 2100 and are then set to zero, the emissions are set to zero after the year 2007 for the Low scenario.
Plotting surface ocean saturation state, CaCO 3 production, respectively dissolution as a function of atmospheric CO 2 allows identifying time lags in the system response to the perturbation. The saturation state with respect to aragonite closely follows atmospheric CO 2 (Fig. 16b) suggesting it to be approximately in equilibrium with atmospheric forcing. We observed a small shift towards lower values of the saturation state at identical atmospheric CO 2 values before and after 2100 attributed to the ongoing uptake of CO 2 by the ocean and related changes in carbonate chemistry.
In order to relate changes in surface ocean saturation state with respect to aragonite to habitat suitability for calcifiers, we follow the classification proposed by Kleypas et  al. (1999) for tropical coral ecosystems and applied by others at the scale of the global ocean (Steinacher et al., 2009;Frölicher and Joos, 2010). Following this scheme and in terms of carbonate chemistry, A > 4 stands for optimal, 3 < A < 4, for adequate conditions, while 2 < A < 3 and 1 < A < 2 are indicative of a marginal, respectively inadequate carbonate chemistry for coral growth. Finally, values of A < 1 indicate undersaturated conditions and hence an unsuitable environment for most calcifiers (e.g. pteropods). We stress that uncertainties with ecosystem impacts are large. The high case scenario projects large and, over the dura-tion of the simulation, irreversible changes in surface ocean saturation state. "Suitable habitats" for aragonite producing pelagic organisms such as pteropods (3 < A > 4) are lost and conditions remain at "best marginal" (2 < A < 3) by the year 2500. Our model results further confirm the early onset and persistence of undersaturation of high latitude waters (Orr et al., 2005;Steinacher et al., 2009). The evolution of A is projected to be less severe in the medium case emission scenario. While carbonate chemistry shifts towards "marginal conditions" for the development of pteropods around 2100, the saturation state of low latitude waters increases back to values above 3 by the year 2500. Similarly, the extension and duration of undersaturation in high latitude regions is less severe. Keeping in mind the scarcity of data on consequences of large decreases in saturation state for calcifying organisms and from a safety principle, only the low emission scenario allows for moderate changes in A and hence the prevalence of suitable conditions for pteropods. The evolution of ocean saturation state with respect to aragonite projected by the Bern3D/PISCES model is consistent with simulations with the comprehensive NCAR climate-carbon cycle model and we refer to the literature for a further discussion on changes in the saturation state and the reversibility of the impacts of 21st century emissions on the carbon cycle and climate (Frölicher and Joos, 2010;Steinacher et al., 2009).
As the atmospheric CO 2 increases, the global mean CaCO 3 production decreases at the same rate for all three emission scenarios, until the peak CO 2 values of 992 ppm (High), 702 ppm (Medium) and 342 ppm (Low) are reached. Thereafter, the production increases following nearly the same curves back towards, but without reaching, the original CaCO 3 production values. This is a direct consequences of the still higher pCO 2 levels in 2500 and hence the lower surface ocean saturations. The dissolution decreases steadily with increasing atmospheric CO 2 due to the reduced supply of CaCO 3 particles (Fig. 16c), also increasing again after the peak of atmospheric CO 2 is reached. However, it increases at a larger rate than it decreased and the dissolution rate by the year 2500 becomes much higher than it was initially. While the global mean dissolution rate was around 0.6 Pg C yr −1 at the start of the scenarios, it reaches between 0.65 (Low) and 0.75 (Medium and High) Pg C yr −1 by the end of the scenario. This non-linearity occurs because, contrary to the CaCO 3 production, the CaCO 3 dissolution takes place in the deep ocean. Whereas the surface ocean CO 2 concentration is nearly in equilibrium with the atmospheric CO 2 , the deep ocean chemistry recovers much more slowly from the CO 2 perturbation (Fig. 16e).
The resulting feedbacks on atmospheric CO 2 for the 3 scenarios are given in Table 3. Compared to the High scenario which provides a reduction in atmospheric CO 2 of about 5.8 ppm by the year 2100 and 6.8 ppm by the year 2500 due to decreasing calcification and increasing dissolution, the Medium and Low scenarios produce the respective negative CO 2 -CaCO 3 feedbacks on atmospheric CO 2 of −4.1 and −1.3 ppm by the year 2100, and −5.0 and −0.8 ppm by the year 2500. Figure 17 presents the evolution of surface ocean A (annual mean) with time for the Medium and Low CO 2 emission scenarios and with the CALARAG model version. In the Medium scenario (Fig. 17a), the undersaturation starts in Arctic and the Southern Ocean and is only slightly later than with the High scenario. Although a much smaller area of the higher latitudes become undersaturated, the undersaturation in Arctic is maintained for several centuries in both the High and Medium scenario. The Low scenario provides oversaturation everywhere in the surface areas. With this scenario, the surface waters start to recover already soon after the year 2000 when the saturation state of the surface waters slowly returns towards near-pre-industrial values. However, even with the Low scenario, the pre-industrial values are not reached within the year 2500 in most regions. In line with other studies (e.g. Frölicher and Joos, 2010), the results indicate that future anthropogenic CO 2 emissions may lead to irreversible changes in A for several centuries.

Conclusions
In this study we used the new biogeochemical Bern3D/PISCES model to explore the sensitivity of the ocean carbon cycle to increasing concentrations of atmospheric CO 2 . With a range of sensitivity studies, we explore the effect of (i) using different parameterizations of CaCO 3 production fitted to available laboratory and field experiments, of (ii) letting calcite and aragonite be produced by auto-and heterotrophic plankton groups, and of (iii) using CO 2 emissions from the range of the most recent IPCC Representative Concentration Pathways (RCP). Under a high-emission scenario, the CaCO 3 production of all the model versions decreases from ∼1 Pg C yr −1 to between 0.36 and 0.82 Pg C yr −1 by the year 2100. From the variety of parameterizations and model versions, the changes in CaCO 3 production (and dissolution) resulting from ocean acidification provide only a small overall negative feedback on the atmospheric CO 2 equal to between −1 and −11 ppm by the year 2100. The similar response to increasing atmospheric CO 2 by the model version with calcite produced by nanophytoplankton and the one with calcite produced by mesozooplankton may indicate that either of the two plankton groups may be used when simulating future changes in marine pelagic calcification if changes in calcite production in response to environmental drivers are indeed similar for nano-and mesozooplankton. Moreover, in the light of the uncertainties associated with the response of calcifying organisms and the overall similar feedback strengths yielded by the different model versions, the biogeochemical response of the pelagic CaCO 3 cycle to ocean acidification may at present be adequately capture by a model with a single calcifying plankton type (e.g. nanophytoplankton in the standard model setting). The maximum potential limit of the CO 2 -CaCO 3 production/dissolution feedback by the year 2100, by setting the calcification to 0 after the year 2000, equals −30 ppm.
We quantified changes in the marine CaCO 3 cycle for two emission commitment scenarios in which carbon emissions followed RCP8.5 (High) and RCP6 (Medium) until 2100 and are set to zero after 2100, and one scenario where the emissions are set to zero after the year 2007 (Low). In all simulations, the surface aragonite saturation state decreases in parallel to increasing atmospheric CO 2 . The resulting decreases in CaCO 3 production and increases in dissolution have a negligible effect on surface ocean carbonate chemistry. Ongoing ocean acidification is thus not buffered by changes in CaCO 3 production and dissolution. Under the High scenario, undersaturation of Arctic and Southern Ocean surface waters is projected within few decades, and is maintained over several centuries. In the Arctic, both the Medium and the High scenario give early and long-term undersaturation. Although undersaturation of the surface waters is overall not reached under the Low scenario, we note that the saturation state remains below pre-industrial levels by the year 2500. The results confirm previous studies (Orr et al., 2005;Steinacher et al., 2009;Frölicher et al., 2010) indicating that future anthropogenic CO 2 emissions may lead to irreversible changes in A for several centuries. The projected changes in saturation state and decreasing calcification may have large consequences for marine organisms Guinotte and Fabry, 2008;Kleypas et al., 2006). When interpreting our results in the light of a classification of surface ocean saturation state with respect to aragonite in terms of suitability to marine calcifiers, both scenarios High and Medium suggest large scale habitat loss to occur within a few decades and prevailing up to several centuries. Following the precautionary principle, only scenarios with low carbon emissions seem recommendable when large scale changes in ocean ecosystems and their functioning are to be avoided.
The legacy of ocean acidification and ongoing CO 2 uptake after the unrealistic shut-down of emissions after 2100 is also seen in the deep ocean. The volume of undersaturated water masses remains superior to its pre-industrial value by the year 2500. As a consequence of the slow recovery of the deep ocean after a CO 2 perturbation, the CaCO 3 dissolution of all model versions continues to increase over the scenario. By the end of the High scenario, the CaCO 3 dissolution-production ratio has stabilized at a value that is 30-50% higher than its initial value. Changes in CaCO 3 production will lead to rain ratio changes, which together with a reduction in CaCO 3 sedimentation and burial will modify the marine carbonate cycle for several thousand years. The interaction with marine sediments will ultimately bring the system back to a new equilibrium state (Archer, 2005).