Catalysis and chemical mechanisms of calcite dissolution in seawater

Near-equilibrium calcite dissolution in seawater contributes significantly to the regulation of atmospheric CO_2 on 1,000-y timescales. Despite many studies on far-from-equilibrium dissolution, little is known about the detailed mechanisms responsible for calcite dissolution in seawater. In this paper, we dissolve ^(13)C-labeled calcites in natural seawater. We show that the time-evolving enrichment of δ^(13)C in solution is a direct measure of both dissolution and precipitation reactions across a large range of saturation states. Secondary Ion Mass Spectrometer profiles into the ^(13)C-labeled solids confirm the presence of precipitated material even in undersaturated conditions. The close balance of precipitation and dissolution near equilibrium can alter the chemical composition of calcite deeper than one monolayer into the crystal. This balance of dissolution–precipitation shifts significantly toward a dissolution-dominated mechanism below about Ω= 0.7. Finally, we show that the enzyme carbonic anhydrase (CA) increases the dissolution rate across all saturation states, and the effect is most pronounced close to equilibrium. This finding suggests that the rate of hydration of CO_2 is a rate-limiting step for calcite dissolution in seawater. We then interpret our dissolution data in a framework that incorporates both solution chemistry and geometric constraints on the calcite solid. Near equilibrium, this framework demonstrates a lowered free energy barrier at the solid–solution interface in the presence of CA. This framework also indicates a significant change in dissolution mechanism at Ω= 0.7, which we interpret as the onset of homogeneous etch pit nucleation

A Kinetic Pressure Effect on Calcite Dissolution in Seawater

This study provides laboratory data of calcite dissolution rate as a function of seawater undersaturation state (1-Ω) under variable pressure. ^(13)C-labeled calcite was dissolved in unlabeled seawater and the evolving δ^(13)C composition of the fluid was monitored over time to evaluate the dissolution rate. Results show that dissolution rates are enhanced by a factor of 2-4 at 700 dbar compared to dissolution at the same Ω under ambient pressure (10 dbar). This dissolution rate enhancement under pressure applies over an Ω range of 0.65 to 1 between 10 dbar and 700 dbar. Above 700 dbar (up to 2500 dbar), dissolution rates become independent of pressure. The observed enhancement is well beyond the uncertainty associated with the thermodynamic properties of calcite under pressure (partial molar volume ΔV), and thus should be interpreted as a kinetic pressure effect on calcite dissolution. Dissolution at ambient pressure and higher pressures yield non-linear dissolution kinetics, the pressure effect does not significantly change the reaction order n in Rate = k(1-Ω^)n, which is shown to vary from 3.1±0.3 to 3.8±0.5 from 10 dbar to 700 dbar over Ω = 0.65 to 0.9. Furthermore, two different dissolution mechanisms are indicated by a discontinuity in the rate-undersaturation relationship, and seen at both ambient and higher pressures. The discontinuity, Ω_(critical) = 0.87±0.05 and 0.90±0.03 at 10 dbar and 1050 dbar respectively, are similar within error. The reaction order, n, at Ω > 0.9 is 0.47±0.27 and 0.46±0.15 at 10 dbar and 700 dbar respectively. This Ω_(critical) is considered to be the threshold between step retreat dissolution and defect-assisted dissolution. The kinetic enhancement of dissolution rate at higher pressures is related to a decrease in the interfacial energy barrier at dissolution sites. The impact of pressure on the calcite dissolution kinetics implies that sinking particles would dissolve at shallower depth than previously thought

A Mechanistic Study of Carbonic Anhydrase Enhanced Calcite Dissolution

Carbonic anhydrase (CA) has been shown to promote calcite dissolution (Liu, 2001, https://doi.org/10.1111/j.1755-6724.2001.tb00531.x; Subhas et al., 2017, https://doi.org/10.1073/pnas.1703604114), and understanding the catalytic mechanism will facilitate our understanding of the oceanic alkalinity cycle. We use atomic force microscopy (AFM) to directly observe calcite dissolution in CA‐bearing solution. CA is found to etch the calcite surface only when in extreme proximity (~1 nm) to the mineral. Subsequently, the CA‐induced etch pits create step edges that serve as active dissolution sites. The possible catalytic mechanism is through the adsorption of CA on the calcite surface, followed by proton transfer from the CA catalytic center to the calcite surface during CO2 hydration. This study shows that the accessibility of CA to particulate inorganic carbon (PIC) in the ocean is critical in properly estimating oceanic CaCO3 and alkalinity cycles

A Mechanistic Study of Carbonic Anhydrase Enhanced Calcite Dissolution

Carbonic anhydrase (CA) has been shown to promote calcite dissolution (Liu, 2001, https://doi.org/10.1111/j.1755-6724.2001.tb00531.x; Subhas et al., 2017, https://doi.org/10.1073/pnas.1703604114), and understanding the catalytic mechanism will facilitate our understanding of the oceanic alkalinity cycle. We use atomic force microscopy (AFM) to directly observe calcite dissolution in CA‐bearing solution. CA is found to etch the calcite surface only when in extreme proximity (~1 nm) to the mineral. Subsequently, the CA‐induced etch pits create step edges that serve as active dissolution sites. The possible catalytic mechanism is through the adsorption of CA on the calcite surface, followed by proton transfer from the CA catalytic center to the calcite surface during CO2 hydration. This study shows that the accessibility of CA to particulate inorganic carbon (PIC) in the ocean is critical in properly estimating oceanic CaCO3 and alkalinity cycles

RADIv1: a non-steady-state early diagenetic model for ocean sediments in Julia and MATLAB/GNU Octave

We introduce a time-dependent, one-dimensional model of early diagenesis that we term RADI, an acronym accounting for the main processes included in the model: chemical reactions, advection, molecular and bio-diffusion, and bio-irrigation. RADI is targeted for study of deep-sea sediments, in particular those containing calcium carbonates (CaCO3). RADI combines CaCO3 dissolution driven by organic matter degradation with a diffusive boundary layer and integrates state-of-the-art parameterizations of CaCO3 dissolution kinetics in seawater, thus serving as a link between mechanistic surface reaction modeling and global-scale biogeochemical models. RADI also includes CaCO3 precipitation, providing a continuum between CaCO3 dissolution and precipitation. RADI integrates components rather than individual chemical species for accessibility and is straightforward to compare against measurements. RADI is the first diagenetic model implemented in Julia, a high-performance programming language that is free and open source, and it is also available in MATLAB/GNU Octave. Here, we first describe the scientific background behind RADI and its implementations. Following this, we evaluate its performance in three selected locations and explore other potential applications, such as the influence of tides and seasonality on early diagenesis in the deep ocean. RADI is a powerful tool to study the time-transient and steady-state response of the sedimentary system to environmental perturbation, such as deep-sea mining, deoxygenation, or acidification events

An Atomic Force Microscopy Study of Calcite Dissolution in Seawater

We present the first examination of calcite dissolution in seawater using Atomic Force Microscopy (AFM). We quantify step retreat velocity and etch pit density to compare dissolution in seawater to low ionic strength water, and also to compare calcite dissolution under AFM conditions to those conducted in bulk solution experiments (e.g. Subhas et al., 2015, Dong et al., 2018). Bulk dissolution rates and step retreat velocities are slower at high and mid-saturation state (Ω) values and become comparable to low ionic strength water rates at low Ω. The onset of defect-assisted etch pit formation in seawater is at Ω ∼ 0.85 (defined as Ω_(critical)), higher than in low ionic strength water (Ω ∼ 0.54). There is an abrupt increase in etch pit density (from ∼10⁶ cm⁻² to ∼10⁸ cm⁻²) occurring when Ω falls below 0.7 in seawater, compared to Ω ∼ 0.1 in low ionic strength water, suggesting a transition from defect-assisted dissolution to homogeneous dissolution much closer to equilibrium in seawater. The step retreat velocity (v) does not scale linearly with undersaturation (1-Ω) across an Ω range of 0.4 to 0.9 in seawater, potentially indicating a high order correlation between kink rate and Ω for non-Kossel crystals such as calcite, or surface complexation processes during calcite dissolution in seawater

An Atomic Force Microscopy Study of Calcite Dissolution in Seawater

We present the first examination of calcite dissolution in seawater using Atomic Force Microscopy (AFM). We quantify step retreat velocity and etch pit density to compare dissolution in seawater to low ionic strength water, and also to compare calcite dissolution under AFM conditions to those conducted in bulk solution experiments (e.g. Subhas et al., 2015, Dong et al., 2018). Bulk dissolution rates and step retreat velocities are slower at high and mid-saturation state (Ω) values and become comparable to low ionic strength water rates at low Ω. The onset of defect-assisted etch pit formation in seawater is at Ω ∼ 0.85 (defined as Ω_(critical)), higher than in low ionic strength water (Ω ∼ 0.54). There is an abrupt increase in etch pit density (from ∼10⁶ cm⁻² to ∼10⁸ cm⁻²) occurring when Ω falls below 0.7 in seawater, compared to Ω ∼ 0.1 in low ionic strength water, suggesting a transition from defect-assisted dissolution to homogeneous dissolution much closer to equilibrium in seawater. The step retreat velocity (v) does not scale linearly with undersaturation (1-Ω) across an Ω range of 0.4 to 0.9 in seawater, potentially indicating a high order correlation between kink rate and Ω for non-Kossel crystals such as calcite, or surface complexation processes during calcite dissolution in seawater

On calcium-to-alkalinity anomalies in the North Pacific, Red Sea, Indian Ocean and Southern Ocean

An important factor for predicting the effect of increased CO2 on future acidification of the ocean is a proper understanding of the interactions controlling production and dissolution of calcium carbonate minerals (CaCO3). The production and dissolution of CaCO3 in the ocean can be assessed over large spatial scales by measuring seawater calcium concentrations and total alkalinity (AT), yet past studies suggest that there could be large discrepancies between calcium and AT-based balances of the CaCO3 cycle in the North Pacific and Indian Oceans. Here, we analyse water column samples collected along transects in the North Pacific, Southern Ocean, tropical Indian Ocean and Red Sea for their concentrations of calcium, nutrients, and AT. We find that there is an excess calcium over AT anomaly in the top 1000 m of the tropical Indian Ocean water-column. The source of this anomaly is the dissolution of subsurface gypsum deposits in the Red Sea. We find no evidence for calcium-over-AT anomalies in the North Pacific, in contrast to previous studies. Our results show that, in most cases, calcium and AT data agree well and can be used to reconstruct the marine CaCO3 cycle

Anaerobic diagenesis of silica and carbon in continental margin sediments : discrete zones of TCO2 production

Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B. V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 69 (2005): 4611-4629, doi:10.1016/j.gca.2005.05.011.Pore water profiles of dissolved Si, Ca2+, SO42-, CH4, and TCO2 (Dissolved Inorganic Carbon; DIC) were determined from multicores and gravity cores collected at nine sites off Southern California, the west coast of Mexico, and within the Gulf of California. These sites were located within the eastern North Pacific oxygen minimum zone at depths of 400 to 900 m and in settings where bottom water oxygen concentrations were <3 μM and sediments were laminated. Pore water profiles were defined at a resolution of millimeters (whole core squeezing), centimeters (sectioning and squeezing) and meters (gravity core sectioning and squeezing), and diffusive fluxes were calculated for different zones within the sediment column. The flux of dissolved silica across the sediment-water interface (SWI) ranged from 0.3 to 3.4 mmol Si m-2d-1, and TCO2 fluxes ranged from 0.8 to 4.6 mmol C m-2d-1. A positive correlation (r = 0.74) existed between these fluxes, yet these two constituents exhibited significantly different diagenetic behavior downcore; dissolved Si generally reached a constant concentration (between 450 and 900 μM) in the upper few cm, whereas TCO2 concentrations increased monotonically with depth. Methane was detected at micromolar levels in sediment intervals between 0 and 60 cm and at five sites, increased to millimolar levels at depths of 80 to 170 cm. At the horizon marking the appearance of millimolar levels of methane, there was a distinct change in slope of the sulfate and TCO2 gradients. A flux budget for this horizon was determined by using linear fits to pore water profiles; these budgets indicate that the upward TCO2 flux away from this horizon is 40 to 50% greater than the downward sulfate flux to this horizon. Given that the TCO2 flux to this horizon from below was quite small, this imbalance suggests that anaerobic oxidation of methane by sulfate is not the only process producing TCO2 within this horizon. A budget for TCO2 at this horizon is balanced when 40 to 80% of the sulfate flux is attributed to organic carbon remineralization. Of the DIC that diffuses across the SWI, 20 to 40% is generated by reactions occurring within or below this deep reaction horizon.Funding for the work was provided by NSF-OCE-0002250 to L. Stott, W. Berelson and R. Douglas, NSF-OCE-0129555 to W. Berelson, and NSF-OCE-9911649 to F. Sansone

Pelagic calcium carbonate production and shallow dissolution in the North Pacific Ocean

Funding was provided by NSF Grants OCE1220600 and OCE1220302 awarded to JA and WB, respectively, MINECO PID2020-113526RB-I00, the Generalitat de Catalunya MERS (#2017 SGR-1588) awarded to PZ and NERC grant NE/N011716/1 awarded to JR.Planktonic calcifying organisms play a key role in regulating ocean carbonate chemistry and atmospheric CO2. Surprisingly, references to the absolute and relative contribution of these organisms to calcium carbonate production are lacking. Here we report quantification of pelagic calcium carbonate production in the North Pacific, providing new insights on the contribution of the three main planktonic calcifying groups. Our results show that coccolithophores dominate the living calcium carbonate (CaCO3) standing stock, with coccolithophore calcite comprising ~90% of total CaCO3 production, and pteropods and foraminifera playing a secondary role. We show that pelagic CaCO3 production is higher than the sinking flux of CaCO3 at 150 and 200 m at ocean stations ALOHA and PAPA, implying that a large portion of pelagic calcium carbonate is remineralised within the photic zone; this extensive shallow dissolution explains the apparent discrepancy between previous estimates of CaCO3 production derived from satellite observations/biogeochemical modeling versus estimates from shallow sediment traps. We suggest future changes in the CaCO3 cycle and its impact on atmospheric CO2 will largely depend on how the poorly-understood processes that determine whether CaCO3 is remineralised in the photic zone or exported to depth respond to anthropogenic warming and acidification.Publisher PDFPeer reviewe