Aragonite is calcite’s best friend at the seaoor

In the open ocean, calcium carbonates are mainly found in two mineral forms. Calcite, the 10 least soluble, is widespread at the seafloor, while aragonite, the more soluble, is rarely preserved in marine sediments. Despite its greater solubility, research has shown that aragonite, which could contribute between 10 and 90% to pelagic calcium carbonate production, is able to reach the deep-ocean. If large quantities of aragonite settle and dissolve at the seafloor, this represents a large source of alkalinity that buffers the deep ocean and favours the preservation of less soluble calcite, 15 acting as a deep-sea, carbonate version of galvanization. Here, we investigate the role of aragonite dissolution on the early diagenesis of calcite-rich sediments using a novel 3D, micrometric-scale reactive-transport model combined with 3D, X-ray tomography structures of natural aragonite and calcite shells. Results highlight the important role of diffusive transport in benthic calcium carbonate dissolution, in agreement with recent work. We show that, locally, aragonite fluxes to 20 the seafloor could be sufficient to suppress calcite dissolution in the top layer of the seabed, possibly causing calcite recrystallization. As aragonite producers are particularly vulnerable to ocean acidification, the proposed galvanizing effect of aragonite could be weakened in the future, indirectly boosting calcite dissolution further.


Introduction
More than a quarter of the Earth's surface is covered by marine sediments rich in calcium carbonate (CaCO3), 1,2 whose dissolution represents the ultimate natural sink for anthropogenic carbon dioxide (CO2). 3 In the open ocean, most CaCO3 originates from the near surface 4,5 , where it is secreted by organisms as building blocks of their shells and skeletons in diverse crystalline 30 structures. Calcite is the most stable CaCO3 mineral under Earth surface conditions 6 , and it is believed that calcite accounts for the majority of the oceanic CaCO3 reservoir 7 . Thus, in the majority of existing biogeochemical models used to predict and reconstruct Earth climates [8][9][10][11] , all CaCO3 is treated as the mineral calcite. There is, however, growing evidence that aragonite, another CaCO3 mineral more soluble than pure calcite 6 , could account for a large part of, and even 35 dominate CaCO3 production and cycling 12-15 . In the open ocean, aragonite production is dominated by shelled pteropods and heteropods, abundant free swimming sea snails 12,14 , and to a lesser extent, by some foraminifera 16 and coldwater coral species 17 . Upon the organisms death, aragonite shells settle through the water column, 40 where they start to dissolve 18 due to internal organic matter degradation 19 , to their increasing solubility with increasing hydrostatic pressure 20 and to the buildup of metabolic CO2 in deep waters 21 . The remaining aragonite deposits at the seafloor. Below the aragonite saturation depth, the depth at which seawater undersaturation with respect to aragonite first occurs and below which aragonite should dissolve, aragonite grains are rarely preserved in sediments 22 . This largely 45 contrasts with calcite, which is commonly found in marine sediments up to several kilometers below the calcite saturation depth 2,23 . That aragonite disappears shallower than calcite in sediments is coherent with aragonite's greater solubility, but why is aragonite not preserved in sediments below its saturation horizon whilst calcite ordinarily is? Potential reasons include the presence of calcite dissolution inhibitors in sediments, or fast aragonite dissolution kinetics, but both are still 50 uncertain or unsupported by recent laboratory experiments 24,25 .
Although rarely preserved in sediments, there is clear evidence that aragonite reaches the seafloor even deep below its saturation depth. Sediment traps have recorded high concentrations of pteropod genetic material 26 and suspended aragonite 24 far below the aragonite saturation depth. 55 Thus, a large proportion of settling aragonite grains in the ocean could dissolve at or near the sediment-water interface. Let us now consider a sedimentary system in which calcite and aragonite are both present in seawater undersaturated with respect to both minerals, i.e., surrogate for a deepsea sediment. From a thermodynamic perspective, aragonite and calcite should both dissolve, releasing alkalinity and raising CaCO3 saturation states (Ω). Since aragonite is more soluble than 60 calcite, if aragonite dissolution is fast enough, then as long as aragonite is present and dissolving, seawater could remain oversaturated with respect to calcite. As there is nothing to keep seawater saturated with respect to aragonite, since it is the most soluble mineral present, aragonite would eventually fully dissolve. In this picture, the interaction between calcite and aragonite is unidirectional, and the preferential preservation of calcite in sediments is caused by the dissolution 65 of deposited aragonite at the seafloor. This represents a deep-sea, carbonate version of galvanization, in which aragonite sacrifies itself to protect the underlying calcite.
Observing aragonite dissolution at the seafloor in situ is difficult because of the limited spatial and temporal resolution of instruments able to reach the deep ocean. Using existing 70 sediment-porewater models is also an imperfect approach, because these models mathematically express grains (e.g., shells) as a spatial continuum of solid 27-29 rather than three-dimensional entities with microstructures and heterogeneities. Thus, existing models are unable to resolve chemical gradients within a single pore, or across the surface of a single grain. Here, we use a novel three-dimensional model, to simulate dissolution reactions at the micrometer scale for a 75 variety of natural CaCO3 grains virtually placed in seawater (Fig. S1), within which chemical reactions, their rates, and transport processes were resolved. The model equations, assumptions, initial conditions and boundary conditions for each simulation are described in the Materials and Methods. We demonstrate that molecular diffusion generates large disparities in dissolution rates across mineral surfaces within a single CaCO3 shell, which may account for part of the 80 disagreement among published empirical CaCO3 dissolution rate laws. Then, we simulate the dissolution of an aragonite pteropod shell sitting on top of a calcite sediment bed in a typical deepsea setting, and show that aragonite dissolution indeed exerts a galvanizing action by favouring the preservation of surrounding calcite particles.

Heterogeneous dissolution of CaCO3 shells
Most experimental assessments of CaCO3 dissolution rates in seawater to date have measured bulk dissolution rates, by computing dissolution rates from a mass or water-chemistry change over a given amount of time [30][31][32][33] . This approach yields the overall dissolution rate, 90 including transport processes, rather than the rate of true dissolution at the mineral surface 34,35 . In particular, molecular diffusion could lead to a buildup of dissolution products next to the mineral surface, which could locally buffer seawater and raise CaCO3 saturation states 36 .
In volumes of undersaturated seawater, we virtually dissolve a set of foraminifera and 95 pteropod e-specimens obtained from X-ray tomography scans (see Materials and Methods) and present micrometer-scale resolution visualizations of CaCO3 saturation states and dissolution rates (Fig. 1). In each simulation, after only one minute, water inside the dissolving shells is at or near equilibrium with respect to the dissolving CaCO3 phase ( Fig. 1a-d). At this point, dissolution essentially only occurs on the external faces of the shells (Fig. 1e-h). The distributions of calcite 100 dissolution rates across the foraminifera shell surfaces appear bimodal ( Fig. S2): internal faces display dissolution rates approaching zero, while external faces dissolve with rates ranging between 1.5 and 4 ×10 -7 mol m -2 s -1 . Aragonite dissolution patterns are similar. External faces of the pteropod shell dissolve at rates between 4 and 5.5 mol m -2 s -1 while internal faces do not, as they are in contact with seawater at or close to equilibrium with respect to aragonite. This very 105 wide range of values is in line with the range of calcite and aragonite dissolution rates measured in the laboratory, in seawater with a similar bulk chemical composition (Fig. S3). Thus, part of the reason why the variability in measured dissolution rates is so large across experiments could be because solute transport, the rate-limiting step in overall dissolution, is specific to each sample and experimental design. When expressing the overall dissolution rate of a CaCO3 grain, its mass, 110 rather than its surface area, may be a better property of normalization.
The specific surface areas of the foraminifera and pteropod e-specimens used in these simulations, i.e., their surface area per mass unit, are one to two orders of magnitude smaller than specific surface areas measured from the same species using the Kr-BET method 24,31,37,38 . The 115 spatial resolution of our e-specimens is possibly not high enough to capture submicroscale features such as surface roughness and shell microporosity. Since our model underestimates mineral surfaces available for reactions, it also likely underestimates how quickly equilibrium can be reached within dissolving shells and minimizes local transport limitations. In our simulation conditions and in the absence of water advection, only the external faces of CaCO3 shells should 120 dissolve, as the inner parts will be at or close to equilibrium. In the following, we therefore replace calcite foraminifera shells by calcite spheres (Table S2) of similar diameter for simplicity. Pteropod shell dissolution at the seafloor 130 Upon death, pteropods settle rapidly and therefore spend only a few hours or days in the water column 39 . Once at the seafloor, due to very slow accumulation rates of typical deep-sea sediments (a few centimeters per thousand years), pteropods should spend a much greater time at or just below the sediment-water interface (a few decades or centuries, unnacounting for bioturbation and dissolution) and, thus, play a role in early diagenesis of surrounding particles. We 135 simulate the dissolution of an empty pteropod shell placed on a calcite sediment bed overlain by seawater undersaturated with respect to both calcite and aragonite (Ωcalcite ~ 0.64, Ωaragonite ~ 0.46, Table S1). Each calcite particle in this sediment is a sphere with a 150 μm-radius, surrogate for a typical foraminifera. The sediment bed is overlain by a 1.5-mm thick diffusive boundary layer ( Fig. S1), within which solutes are transport via molecular diffusion. These conditions are typical 140 of deep-sea benthic environments 23 . The dissolution simulations were run for 5 minutes, until a steady state was reached.
In a pure-calcite sediment bed, porewaters reach equilibrium with respect to calcite a few hundred μm below the sediment-water interface (Fig. 2) and most of the Ωcalcite gradient is within 145 the diffusive boundary layer rather than the sediment (Fig. 2). This is in agreement with results from previous modeling 40,41 and laboratory 35 works on calcite-rich sediments depleted of organicmatter and aragonite. In this classical setting, the chemical gradients should be laterally homogeneous, and lead to an efflux of dissolution products from the sediment toward the bottom waters. The top layer of calcite grains should dissolve until another layer settles in, and the fraction 150 of the calcite grains that escaped dissolution is buried, eventually, and preserved in the sediment record.
Using the same framework but replacing four calcite spheres at the sediment-water interface by an aragonitic pteropod (Fig. S1), chemical gradients appear much different (Fig. 2). 155 In this simulation, only ~6% of the sediment-water (horizontal) interface surface area (3.15 mm × 3.15 mm ≈ 10 mm 2 , see Materials and Methods) is aragonite, the rest is calcite (32%) and water (62%). In the depth transect across the dissolving pteropod, water Ωcalcite increases from ~0.64 at the top of the diffusive boundary layer to ~1.3 at about 200 μm below the sediment-water interface, before decreasing again deeper in the porewaters and converging toward equilibrium (Fig. 2). 160 Horizontally averaging Ωcalcite over the entire sediment mesh, we find that porewaters are saturated with respect to calcite all the way up to the sediment-water interface due to the presence of the dissolving pteropod shell (Fig. S4). In this setting, dissolution products diffuse from the pteropod shell upward to the bottom waters, but also downward and sideways, and a halo of calcite supersaturation develops in the porewaters beneath the dissolving aragonite (Fig. 2). This causes 165 the calcite grains surrounding the pteropod to be partially in contact with supersaturated water, thermodynamically preventing their dissolution, despite the bottom waters overlaying this sediment being strongly undersaturated with respect to calcite. Over the entire resolved domain, calcite grains sitting at the sediment-water interface only dissolve on their upper half ( Fig. 3) with dissolution rates always lower than those from single-foraminifera simulations (Fig. 1,3). 170 The predicted seawater calcite supersaturation that surrounds dissolving aragonite particles at the seafloor could account for some of the calcite recrystallization occasionally observed on the surface of preserved foraminifera 42,43 . This mechanism could thus require to reconsider the contribution of authigenic CaCO3 formation to the total CaCO3 burial rate in sediments, thought 175 to be ~10% 44 and mainly due to deeper diagenetic processes such as bacterial sulfate reduction 44,45 .  represents a saturation state with respect to calcite of unity, i.e., a transition from undersaturation to supersaturation with respect to calcite that is caused by the dissolving aragonite pteropod.

Implications for CaCO3 cycling
Aragonite can only play a meaningful role in calcite benthic dynamics via its galvanizing action if the aragonite flux to and the residence time at the seafloor are high enough. Unfortunately, 195 little is known about the sources and sinks of aragonite in the ocean. For instance, published estimates of the contribution of pteropod aragonite to global CaCO3 production in the modern ocean span a very wide range from ~10% to ~90% 13,14,46 ; this does not account for aragonite produced by heteropods and benthics organisms. In addition, the spatial distribution of aragonite production and export is unknown, although it is thought that pteropods are most abundant in high-200 latitude systems 47 . At the seafloor, the world-averaged CaCO3 deposition rate is estimated to range between 0.08 and 0.14 mol m -2 a -1 , 7 and we cannot exclude the possibility that a substantial fraction of that amount is deposited in the form of aragonite.
It would take about twice as long for a pteropod to dissolve at the seafloor than when falling through the water column (Fig. 4). The preferential preservation of pteropods at the sediment-205 water interface is due to the strong transport limitation dictated by molecular diffusion within sediment porewaters and the diffusive boundary layer above the bed, and to the presence of dissolving calcite spheres beneath it. It takes ~200 days to fully dissolve an ~800 μg aragonite sphere (Table S2) at the seafloor (Fig. 4). A ~60 μg pteropod shell (Table S2) should, by extrapolation, fully dissolve in ~15 days. Given that one pteropod shell is able to maintain ~10 210 mm 2 of calcite seafloor (super)saturated with respect to calcite all the way up to the sedimentwater interface (Fig. S4), at least one new pteropod shell needs to be delivered every 15 days, every 10 mm 2 of seafloor in order to sustain galvanization by aragonite. Using 60 μg as a typical pteropod shell weight, this translates into an aragonite deposition rate of ~0.14 mol m -2 a -1 , on the higher end of the world-averaged total CaCO3 deposition rate to the seafloor. Thus, it is likely that, 215 locally, calcite particles are preferentially preserved due to aragonite dissolution at the seafloor. because of the increasing difficulty to produce a mesh for dissolving particles as they become smaller. The dotted 220 line represents dissolution for an aragonite sphere sinking through water, the dash/dot line stands for dissolution of a particle in suspension in water, and the solid line is the case for an aragonite sphere sitting above a calcite-spheres sediment.
Diagenetic processes excluded from the present abiotic model could affect the results 225 presented here in various ways. One the one hand, microbial degradation of organic matter that releases CO2 and drives additional CaCO3 dissolution 19,48,49 should reduce the residence time of pteropod shells and other aragonite grains at the seafloor, thus hindering their galvanizing action. On the other hand, biological mixing caused by bioturbating organisms should transport aragonite grains from the sediment-water interface to depth, favouring aragonite preservation and 230 disseminating aragonite "buffering pills" within the sediment. More broadly, our results highlight the need for future model-, field-and laboratory-based studies about marine CaCO3 dynamics to consider the presence of several carbonate minerals simultaneously, with different compositions and structures, as they not only passively coexist but chemically interact with each other.

Model
All simulations were performed in COMSOL Multiphysics®, using the PARDISO solver and a Backward-Euler time stepping method. Eight dissolved species (H + , OH -, H2CO3 * , HCO3 -, CO3 2-, Ca 2+ , Na + , Cl -) and 2 solid species (calcite, aragonite) were included. For each dissolved 240 species, initial concentrations were determined using the PHREEQC software 50 at 25°C, for a water density of 1023.6 kg m -3 and a total alkalinity of 1950 μmol kg -1 , so that the resulting saturation state of water with respect to calcite (Ωcalcite) was about 0.64, a value typical of the deep sea (Table S1; Dunne, et al. 51 ).
245 Figure S1. Summary of the different model simulations and visualization of their meshs. The top row shows the four "static" simulations in which natural CaCO3 grains dissolve suspended in a volume of water. The middle row shows the two "sediment" simulations aiming to identify the role of aragonite dissolution at the seafloor on 250 porewater chemistry and the dissolution of surrounding calcite grains. The bottom row represents the three "shrinking" simulations in which the aragonite grain mesh was reevaluated at each time step to account for the weight loss due to dissolution. Magenta color represents calcite surfaces while bright green stands for aragonite surfaces. , where, 1 , 2 , and w are equilibrium constants of the reactions at 25°C, set to 1 = 4.5 × 10 -7 , 260 2 = 4.78 × 10 -11 54 , and w = 1 × 10 -14 . is the activity of the ℎ species, computed as the product of its concentration (ci) and total activity coefficient (γi), the latter being obtained from PHREEQC (Table S1).
Calcite and aragonite dissolution were implemented as per: where K sp calcite is the solubility constant of calcite, taken here as 10 -8.480 , and K sp aragonite is the 265 solubility constant of aragonite, set at 10 -8.336 . 54 CaCO3 reactions are not instantaneous, but instead occur with associated rates that depend on solution chemistry and on the nature of the mineral. For calcite dissolution, we use kinetics from Plummer, et al. 55 , who identified three main pathways for the dissolution of calcite: where 1 = 8.64 × 10 -5 , 2 = 4.78 × 10 -7 and 3 = 2.34 × 10 -9 are the reaction rate constants at 25°C 56 and H 2 O is set to unity. Ωcalcite is the saturation state of water with respect to calcite defined as the ratio of the ion activity product (product of Ca 2+ and CO 3 2− ) and the solubility constant of calcite. The same expression was used to compute the dissolution rate of aragonite, but Ωcalcite was replaced by Ωaragonite. This is a substantial simplification, as in reality both aragonite and calcite 275 have specific dissolution kinetics. Nevertheless, recent laboratory experiments in seawater showed that, when normalized to the mineral surface area and for similar seawater saturation states with respect to the dissolving phase, aragonite dissolves at rates similar to calcite, if not slower 24,57 . This contrasts with earlier experiments 31 reporting very fast aragonite dissolution rates, but based on synthetic rather than biogenic aragonite and overestimated estimates of aragonite solubility 6,58 .

280
Given that the dissolution rates derived from our model encompass measured dissolution rates at similar bulk seawater saturation states (Fig. S3), the simplified kinetic treatment applied here should be acceptable as a first approximation, and should be replaced by a more accurate mechanistic kinetic scheme developed for dissolution in seawater-type solutions when available.
To simulate the reactive-transport of each dissolved species in water, advection-diffusion-285 reaction equation is implemented: where, t is the time (s), ∇ is the three-dimensional space derivative operator nabla, Di is the diffusion coefficient (m 2 s -1 ) of the ℎ species, u is the prescribed water laminar velocity (m s -1 ) and Ri is the reaction input (mol m −3 s −1 ) of the ℎ species.

290
A set of CaCO3 particles was used in this study, some with shapes derived from natural grains, some more conceptual with simplified geometries; their properties are summarized in Table  S2. Planktonic foraminifera shell scans of Globoturborotalita nepenthes, a Miocene Pacific species 59 , Globorotalia menardii, a Pleistocene Carribean specimen 60 and Globigerinella adamsi, from the modern Pacific 61 , were all obtained from the Tohoku University Museum e-foram 295 database (http://webdb2.museum.tohoku.ac.jp/e-foram/). The Heliconoides inflatus pteropod shell, provided by Dr. Rosie Oakes, was obtained from a CT scan of a specimen caught at 150 mdepth in a sediment-trap located in the Cariaco Basin, in the Venezuelan shelf 62 . To make computations easier, the H. inflatus and G. adamsi e-specimens resolutions were reduced from a number of faces (triangles) of 1,969,997 and 1,107,096 for the original files, respectively, to 300 19,860 and 2,224 for the final geometry files imported in COMSOL. Volumes and surface areas for each grains were computed in MATLAB from the output .stl geometry files, using the Geometry and Mesh toolbox. Weights were computed by multiplying the volume of each grain by its density. The specific surface area (SSA) was computed as the surface area to mass ratio, see Table S2. 305

Simulations
For the purposes of the present study, nine simulations were performed in total, each with different settings (Table S3). All simulation experiments and their results are made available on Zenodo (https://zenodo.org/record/5141275). 315 First, four dissolution simulations were performed on natural CaCO3 grains kept static (i.e. in suspension) in water (Fig. S1). "Periodic" boundary conditions were applied on the external walls of water volumes, which forces concentrations on each wall to be equal to those on the opposite wall. The goal was to observe how fast each grain dissolves and what are the effects on solution chemistry within and outside the shell. 320 In order to quantify the effect of aragonite dissolution on porewater chemistry, two subsequent simulations were performed on CaCO3 grains packed in a sediment bed (Fig. S1), one with calcite grains only and another with calcite grains and one aragonite pteropod shell. For simplicity, each calcite particle in this sediment was a sphere with a 150 μm-radius, surrogate for a typical foraminifera 63 , evenly spaced so that the total porosity of this sediment is ~ 0.84, typical 325 of a deep-sea sediment 64 . This array of calcite spheres was then placed within a 3.15 × 3.15 × 3.5 mm 3 (length × width × height, Table S3, Fig. S1) water cube, in which the bottom 1.95 mms were filled up with calcite spheres, the top 1.55 mms consisted of free water, and the sediment-water interface was located between the two. A "no flux" boundary condition was implemented at the bottom, "periodic" boundaric conditions on the sides, and on the top panel solute concentrations 330 were fixed to their initial values.
Finally, three simulations were performed with a moving mesh, to estimate the grain size decrease due to dissolution for aragonite grains in three different environmental settings (in suspension, sinking, and in sediments, Fig. S1). To minimize computational costs, the dissolving aragonite grain in these simulations was a sphere. The simulation with the sinking grain was 335 performed by applying a prescribed laminar water flow velocity on the z-axis of u = 100 m d -1 , a typical sinking speed for a pteropod 39 . Bottom and top boundary conditions were set to "no flux", and boundary conditions on the sides were "periodic". In each simulation, a displacement rate, normal to the aragonite grain surface, was assigned to the aragonite reactive walls, computed as: where, wn is the displacement rate defined at aragonite surface and MV is molar volume of 340 aragonite set to 3.42 ×10 -5 m 3 mol -1 .   Figure S3. Steady-state grain surface area-normalized calcite (in blue) and aragonite (in red) dissolution rates as a function of the steady-state undersaturation state with respect to aragonite (1 −Ωaragonite) or calcite (1 −Ωcalcite). Dots represent experimental "bulk" dissolution rates from the literature 31,33 , in which the measured dissolution rates are normalized by the total area (in m 2 ) of the grains. Lines represent the dissolution rates as 365 parameterized in the present study, in which the [m -2 ] in the dissolution rate expression refers to an elementary surface area unit. Aragonite and calcite dissolution rates were recomputed using original data from Keir 31 and Walter and Morse 33 . From these two publications, all the dissolution rates from biogenic and synthetic samples were used. Units for the dissolution rates were changed to [mol m -2 s -1 ] using the specific surface areas for each grain as reported in the original publications. Dissolution rates for synthetic aragonite were not reported because 370 most of them were obtained in seawater supersaturated with respect to aragonite, thus we suspect the synthetized material was not aragonite. Saturation states were recomputed using the ionic concentration products at steady state measured by the authors, and dividing by the stoichiometric solubility constant with respect to either aragonite or calcite from Mucci 6 at the salinity and temperature of experiments, to harmonize the data. Figure S4. Depth profiles of saturation state with respect to calcite. The blue circle represents the bottom-water value, above the diffusive boundary layer. The black depth profile stands for a case without a pteropod, in which it is replaced by four calcite spheres evenly spaces one from another, while the red depth profiles represents the situation with aragonite shown on the right and in Fig. 4. The depth profile in the middle refers to the whole sediment volume 380 whereas the depth profile on the right refers to the column just above the pteropod.

Data availability
Original foraminifera CT scans used in this study are available from the Tohoku University 385 Museum e-foram database (http://webdb2.museum.tohoku.ac.jp/e-foram/). The pteropod CT scan is available on request to Dr. Rosie Oakes.

Code availability
All simulation experiments and their results are made available on Zenodo (https://zenodo.org/record/5141275).