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Chemical and Physical Mechanisms of Calcite Dissolution in Seawater

Citation

Naviaux, John David (2020) Chemical and Physical Mechanisms of Calcite Dissolution in Seawater. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/DA32-MY55. https://resolver.caltech.edu/CaltechTHESIS:11062019-135828667

Abstract

Calcium carbonates are among the most abundant and reactive minerals on Earth, and their dissolution/preservation in the ocean helps to regulate changes in atmospheric pCO2. The chemistry of the oceans has varied significantly over the past several billion years, and it is changing at an unprecedented rate today in response to anthropogenic burning of fossil fuels. The excess CO2 from human activities is acidifying the oceans and decreasing the saturation state (Ω = ([Ca2+][CO32-])/Ksp') of marine carbonates, increasing their propensity to dissolve. Despite its importance, the rate of carbonate dissolution in seawater is still described by a purely empirical expression, and the physical and chemical mechanisms setting the overall kinetics remain unknown. This stands in contrast to calcite dissolution in freshwater, where fully coupled surface-solution models have been identified. The lack of mechanistic understanding in seawater limits our ability to predict how carbonate dissolution kinetics, and therefore the buffering capacity of the ocean, are affected by changes in chemistry. This thesis advances our knowledge of the physical and chemical mechanisms responsible for carbonate dissolution by making new measurements in seawater both in the lab and in-situ.

I first probe the activation energy of the reaction in seawater by dissolving 13C-labeled CaCO3 across the full range of Ω at 5, 12, 21, and 37°C. I find that a surface-based framework is required to explain the strong non-linearity of the data near equilibrium. In this framework, dissolution proceeds by the retreat of pre-existing steps for 0.9<Ω<1, defect-assisted etch pit formation for 0.75<Ω<0.9, and homogenous etch pit formation for 0<Ω<0.75. I provide the first seawater estimates of kinetic coefficients (β), nucleation site densities (ns), and step edge free energies (α) for each mechanism, as well as the activation energy for detachment from steps (ϵstep) and the kinetic energy barrier to etch pit initiation (ϵinit).

Next, I use a custom designed in-situ reactor to measure calcite dissolution rates across a transect of the North Pacific. I find that the same surface mechanisms and "critical" Ωs identified in lab also govern the dissolution of calcite in the open ocean. In-situ dissolution rates are ~4x slower than in the lab, but I use a combination of chemical spike experiments and measurements in archived seawater to show that this discrepancy can be explained by the presence of dissolved organic carbon in-situ. I propose an empirical rate equation that describes all previous in-situ measurements of inorganic calcite dissolution rates.

Changes in the relation between dissolution rate and Ω can be explained by the activation of different surface processes, but the surface theory cannot account for much of the near-equilibrium dissolution behavior and temperature dependence. I therefore continue on in this thesis to combine the latest speciation models with dissolution measurements in artificial seawater of varying sulfate concentrations. I find that low sulfate solutions suppress dissolution rates by two orders of magnitude near equilibrium, while dissolution rates in the same solutions are enhanced far-from-equilibrium. Using these results, I fit a mechanistic model of dissolution that couples surface and solution processes. The model satisfies the principle of microscopic reversibility, provides an excellent estimate of calcite solubility product in seawater, and explains near equilibrium (Ω > 0.75) dissolution rates in 0, 14, and 28 mM [SO42-] seawater at 21°C. The model cannot explain dissolution rates for Ω < 0.75 when etch pits begin opening homogenously across the surface, so I suggest areas of improvement for future models.

Previous work has demonstrated that calcite dissolution rates are enhanced in the presence of the enzyme carbonic anhydrase (CA). In the final chapter of this thesis, I evaluate the mechanism of CA rate enhancement by comparing the catalytic effects of freely dissolved CA, CA immobilized within hydrogels, and CA chemically bound onto porous silica beads. At the same time, I design and test a fluidized bed reactor and demonstrate its efficacy as a carbon capture device by attaching it directly to the Caltech cogeneration power plant smokestack. I find that dissolution rates within the reactor are only enhanced when CA is freely dissolved, strongly suggesting that the catalytic mechanism is direct proton transfer from the enzyme to the calcite surface.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Calcite; carbonate; dissolution; kinetics; seawater; acidification; omega; carbon capture
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Environmental Science and Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Adkins, Jess F.
Group:Resnick Sustainability Institute
Thesis Committee:
  • Thompson, Andrew F. (chair)
  • Adkins, Jess F.
  • Menemenlis, Dimitris
  • Hoffmann, Michael R.
Defense Date:10 October 2019
Funders:
Funding AgencyGrant Number
National Science Foundation Graduate Research Fellowship1745301
Record Number:CaltechTHESIS:11062019-135828667
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:11062019-135828667
DOI:10.7907/DA32-MY55
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/J.GCA.2018.11.037DOIArticle adapted for Ch. 1
https://doi.org/10.1016/j.marchem.2019.103684DOIArticle adapted for Ch. 2
ORCID:
AuthorORCID
Naviaux, John David0000-0002-0681-3163
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11897
Collection:CaltechTHESIS
Deposited By: John Naviaux
Deposited On:16 Dec 2019 19:28
Last Modified:17 Jun 2020 20:11

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