Ocean Acidification Chemistry | MIT Solid-State Chem

Calcium carbonate behavior follows the reversible reaction CaCO3 + H2O + CO2 ⇌ Ca²⁺ + 2HCO3⁻, with CO2 fluctuations controlling equilibrium direction—increased CO2 drives dissolution, decreased CO2 promotes precipitation. Temperature and pressure also affect equilibrium: rising temperature and decreasing pressure favor precipitation. In oceans, calcite dissolves at ~4,000m depth (calcite compensation depth), while aragonite dissolves at shallower 1,000-2,000m depth due to aragonite's lower stability under pressure. Organisms influence precipitation through direct ion concentration in vacuoles and indirect pH elevation. Calcium carbonate exists as six polymorphs; structure depends on cation ionic radius. Seafloor spreading controls ocean Mg/Ca ratios—slow spreading decreases Mg/Ca through hydrothermal alteration, favoring high magnesium calcite and aragonite; fast spreading maintains lower Mg/Ca, favoring only low magnesium calcite. Modern oceans favor HMC and aragonite.

Calcium carbonate can exist in different crystalline forms called polymorphs, even though its chemical formula remains unchanged. The two relevant forms are calcite and aragonite. Calcite tends to be more thermodynamically stable than aragonite. Many bivalve shells contain layers of each mineral, with calcite typically forming the outer layer and aragonite forming the inner layer.

Calcite and aragonite are polymorphs of calcium carbonate (CaCO3) with identical chemical formulas but distinct crystal structures. The structural difference arises from the calcium-to-oxygen radius ratio of approximately 0.714, near the 0.732 threshold separating octahedral from cubic coordination. Calcite adopts an octahedral arrangement producing rhombohedral symmetry, while aragonite forms with cubic coordination in the orthorhombic system. A phase diagram reveals calcite's thermodynamic stability at Earth's surface conditions, yet marine organisms paradoxically form aragonite. This discrepancy stems from ocean water's magnesium-to-calcium ratio of ~5:1, exceeding the ~2:1 threshold needed to stabilize calcite, rendering aragonite metastable in marine environments.

Carbonate sediments are nearly entirely composed of calcium carbonate polymorphs—calcite and aragonite—with identical chemical formulas (CaCO3) but different crystal structures. Calcite is stable at surface ocean conditions, while aragonite is metastable and slowly recrystallizes to calcite. Magnesium substitution into calcite (exceeding ~4%) interferes with crystallization, explaining why aragonite precipitates preferentially in modern oceans with high magnesium-calcium ratios (~5:1). Aragonite incorporates strontium instead of magnesium, providing a geochemical tracer for identifying originally aragonitic crystals. Allochems (carbonate grains) derive from biological sources: green algae (aragonitic), brachiopods (low-magnesium calcitic), and foraminifera. Modern tropical carbonates feature green algae and foraminifera dominance, while Paleozoic carbonates were animal-dominated.

Carbonate sediments accumulate where waters are supersaturated with calcium carbonate and silica clastic input is minimal. The fundamental reversible reaction CaCO3 + CO2 + H2O ⇌ Ca²⁺ + 2HCO3⁻ governs dissolution-precipitation. Precipitation occurs when CO2 escapes, driven by temperature increases (promoting degassing) and pressure decreases. Salinity significantly increases carbonate solubility through foreign ion interference. Six calcium carbonate polymorphs exist: calcite (hexagonal), aragonite (orthorhombic), vaterite, monohydrate, and amorphous. Despite calcite's thermodynamic stability, aragonite dominates modern sediments because magnesium's preference for OH bonding prevents its incorporation into calcite structure. Based on Mg content, calcites are classified as low (<4%), intermediate (4-12%), and high (>12%) magnesium calcite, with more Mg causing lattice distortion and reduced stability. Relative solubility follows: low Mg calcite < aragonite ≈ high Mg calcite (12% Mg) < high Mg calcite (>12% Mg) < dolomite.

This comprehensive section covers the global carbon cycle as essential for understanding climate change. It explains CO2 sources (fossil fuels at 65-80%, land use changes) and natural sinks (oceans and land biosphere absorbing approximately half of emissions). The section introduces the Bern Simple Climate Model as a mathematical framework treating the carbon cycle as multiple reservoirs (atmosphere, biosphere, soils, surface ocean, deep ocean, sediments) with vastly different sizes and exchange rates. The model uses an impulse response function describing exponential decay through multiple timescales: fast biosphere exchange (~1.86 years), medium-term soil uptake (years to decades), slow deep ocean transport (centuries), and very long-term geochemical binding (thousands of years). The model approaches a residual of ~20-21.7% of added CO2 that remains in the atmosphere for millennia. Calibration uses 14C tracer data from nuclear bomb testing. The section contrasts this with the Sink Model (Badewannenmodell), which claims halving emissions would stabilize CO2 levels, but presents five major scientific criticisms: 20-25% of anthropogenic CO2 remains in the atmosphere for millennia; sink performance depends on emission gradients; temperatures remain elevated for thousands of years at peak levels; a fixed CO2 budget limits total allowable emissions; and zero additional emissions are required to stop warming.

Carbon flows between Earth's reservoirs including atmosphere, terrestrial biosphere, oceans, and sediments. The ocean contains 38,000 billion tons of carbon—16 times the terrestrial biosphere—making it the largest active carbon reservoir. Three factors make oceans crucial for climate regulation: their vast size, CO2 disassociation in seawater, and phytoplankton's role in keeping surface CO2 low. The ocean absorbs about one-fourth of anthropogenic CO2 emissions. Carbon exists in seawater as dissolved organic carbon (CO2, bicarbonate, carbonate), particulate inorganic carbon from calcification, and dissolved/particulate organic carbon. Three main pumps drive the marine carbon cycle: the solubility pump circulates dissolved inorganic carbon via thermohaline circulation; the biological pump sequesters carbon through phytoplankton photosynthesis with recycling by zooplankton and bacteria; the carbonate pump cycles shell-forming organisms' carbonate to the deep ocean.

The ocean is Earth's largest carbon reservoir, containing approximately 38,000 gigatons of carbon, and plays a critical role in regulating atmospheric CO2 concentrations by absorbing excess carbon from human activities; measurements show the ocean absorbs about 1.7 petagrams of carbon per year, though a significant portion of anthropogenic CO2 emissions remains unaccounted for in current carbon cycle models.

Oceans absorb approximately 30% of human-emitted carbon dioxide, making them the largest active carbon reservoir on the planet. The biological carbon pump, where phytoplankton absorb carbon and their remains descend as marine snow, removes carbon from the active cycle for millions of years. However, this process depends on ocean depth, temperature, nutrient availability, and phytoplankton community composition - all parameters changing with global warming. Ocean stratification reduces nutrient mixing, while increased temperature favors smaller species whose remains dissolve before reaching the seafloor. Climate models include oceanic parameters, but uncertainty in these parameters propagates into projections. More seafloor cartography is needed to understand sedimentation patterns and carbon accumulation.

The ocean absorbs approximately 40% of anthropogenic CO2 emissions, totaling about 140 petagrams since the industrial era, with uptake increasing by 2-3 petagrams annually. This absorption occurs primarily in the surface ocean, with limited penetration below 2000 meters. The ocean serves as a critical sink that dampens atmospheric CO2 accumulation, moderating climate change. The ocean carbon cycle comprises two components: the natural carbon cycle (operating at pre-industrial atmospheric CO2 levels of 288 ppm) and the anthropogenic perturbation from rising atmospheric CO2. The natural component includes biological pump processes and overturning circulation, creating a vertical profile where carbon concentration decreases at the surface and increases with depth. The anthropogenic component is relatively small compared to natural background. The ocean carbon sink is quantified by measuring ocean pCO2 and calculating air-sea flux using budget equations dependent on solubility, ice cover, and wind speed. However, direct pCO2 observations are extremely sparse (~1.5% of the global ocean), necessitating machine learning approaches to extrapolate from sparse observations to full coverage.

Dissolved Inorganic Carbon (DIC) and saturation state (omega) are derived by combining measured pH from profiling floats with estimated total alkalinity from global empirical algorithms (LIFER or CANCER-B). These algorithms use temperature, salinity, oxygen, location, depth, and date as inputs to estimate alkalinity with accuracy of ±6 μmol/kg globally. Using programs like CO2SYS, the combination of measured pH and estimated alkalinity enables calculation of DIC, saturation state, and pCO2. Monte Carlo uncertainty analysis shows DIC uncertainty remains relatively constant (~1 μmol/kg) across the full range of DIC concentrations (1900-2400 μmol/kg), dominated by alkalinity estimation uncertainty. Saturation state uncertainty shows a clear dependence on pH, with approximately 2% uncertainty—approaching the Global Ocean Acidification Network's climate-quality target of 1%. Top-down validation against discrete shipboard samples confirms these uncertainty estimates.

Atmospheric CO2 has increased from 280 ppm to over 410 ppm since 1760 due to anthropogenic activities, causing ocean acidification that negatively impacts marine ecosystems. When CO2 dissolves in water, it forms carbonic acid through reactions producing bicarbonate and carbonate ions. Four parameters must be measured: pCO2, pH, dissolved inorganic carbon (DIC), and total alkalinity. The DIC analyzer uses phosphoric acid to convert all carbonate and bicarbonate to CO2, extracted with carrier gas. The LI-7850 provides precision better than 0.1% (2 micromoles per liter) for seawater with DIC around 2000 micromoles per liter. The system uses a 12-port valve with one standard port and eight sample ports, requiring only one standard solution with three different volumes. The standard curve method allows using smaller volumes of the same standard in place of lower concentration standards.

The ocean contains ~37,000 petagrams of carbon—about 60 times the atmospheric reservoir—giving it significant leverage over atmospheric CO2. Less than 1% of ocean carbon is organic; the vast majority is inorganic: ~9% bicarbonate, ~1% dissolved CO2, and ~90% carbonate. Dissolved CO2 (pCO2) is the only species that exchanges directly with the atmosphere. Four parameters characterize the system: pCO2, Total Alkalinity (charge balance), DIC (total carbon), and pH. Any two can be measured, others calculated using equilibrium equations. CO2 acts as a weak acid undergoing two sequential dissociations: CO2 + H2O ⇌ H2CO3 ⇌ HCO3- + H+ ⇌ CO3^2- + H+. Electrochemical neutrality requires charge balance. At typical seawater pH (~8.1), bicarbonate dominates, followed by carbonate ion, with dissolved CO2 being least abundant.

There are commonly ten ocean acidification indicators used to monitor changes in ocean chemistry. These include measurements of total hydrogen ion content, free hydrogen ion content, pH, dissolved inorganic carbon (DIC), alkalinity, aragonite saturation state, calcite saturation state, and others. However, total alkalinity is an exception because when carbonate ions are consumed, the system releases two bicarbonate ions, which cancels out the charge change.

Scientists measure four key parameters of the ocean's inorganic carbon system to understand ocean chemistry: Dissolved Inorganic Carbon (DIC), pH, partial pressure of CO2 (or fugacity), and alkalinity. By measuring any two of these parameters, researchers can calculate the other two using known chemical relationships. Precise measurements of DIC (with accuracy of 0.01%) allow scientists to track changes in ocean carbon content over time, providing essential data for understanding how atmospheric CO2 affects ocean chemistry.

Climate change is nonlinear now, requiring a nonlinear approach to solving it, which means we need to expand beyond traditional mitigation and adaptation strategies to include carbon dioxide removal technologies like ocean alkalinity enhancement and geoengineering approaches such as stratospheric aerosol injection, while simultaneously investing in research to better understand and predict tail-end risks including glacier collapse and methane feedback loops that could accelerate warming beyond current projections.

Ocean alkalinity enhancement aims to reduce surface carbon concentration, causing the ocean to absorb more CO2 from the atmosphere. Two techniques exist: (1) Passing seawater through mechanisms that remove carbonic acid, then returning the carbon-free water to the ocean. This can be integrated with desalination processes, making it potentially cost-effective for countries needing freshwater. (2) Using crushed rock material that reacts with atmospheric CO2 to form carbonates. When distributed as fine sand on ocean surfaces, this increases the area for carbonate formation. The carbonates then sink to the ocean floor, permanently sequestering CO2. Direct Ocean Carbon Capture involves passing seawater through machines that remove CO2, but moving large quantities of seawater is energetically expensive compared to Direct Air Capture (DAC), which removes CO2 directly from the atmosphere and has advanced further in development.

Some startups are developing technology to enhance ocean alkalinity as a method of carbon removal, with one project claiming to remove up to 100 metric tons of carbon per year and committing to remove 350,000 metric tons over a decade for Microsoft. This represents an emerging geoengineering approach that involves adding substances to seawater to increase its capacity to absorb CO2 from the atmosphere.

Ocean alkalinity enhancement involves adding alkaline rocks (such as olivine) to seawater to increase the ocean's capacity to absorb CO2 from the atmosphere. As oceans become increasingly acidified due to absorbing excess atmospheric CO2, their natural ability to act as carbon sinks diminishes. By adding alkaline materials, the chemical equilibrium shifts to allow greater CO2 uptake. This approach represents one of several geoengineering strategies that aim to amplify natural Earth system processes rather than implementing purely technological solutions.

Scientists and startups are developing technologies to enhance ocean alkalinity as a method for carbon capture. The ocean acts as a giant carbon storage, absorbing CO2 from the atmosphere. When CO2 dissolves in seawater, it combines with minerals like calcium carbonate and basalt to form bicarbonate, a stable carbon form that remains trapped for thousands of years. This natural process occurs slowly, but new technologies aim to accelerate it by adding alkaline minerals (triturated or dissolved) to seawater. This could help combat ocean acidification, which occurs when excess CO2 makes seawater more acidic, preventing marine organisms from building shells and skeletons. Companies like Planetarian Canada have already removed 138 tons of carbon for clients like Shopify and Stripe, while Carbfix in the US can remove up to 100 tons annually.