ocean chemistry 2

Lecture 12: Ocean Chemistry II

Ocean Biogeochemistry

Overview

• Focus on the interaction of ocean chemistry with biological processes, specifically concerning carbon dioxide (CO₂) in the ocean.

Carbon Dioxide (CO₂) in the Ocean

• Dissolution in Seawater:

  • CO₂ dissolves in seawater through air-sea exchange, leading to the formation of Dissolved Inorganic Carbon (DIC).

  • Removal Mechanisms:

  • CO₂ is removed from the ocean surface primarily by photosynthesis.

  • It is also produced during respiration processes.

• Profile of CO₂ Levels:

  • Inquiry into the expected CO₂ profile based on the O₂ profiles learned in previous discussions.

Chemical Dynamics of CO₂ in Seawater

Dissolution Reactions

• When CO₂ dissolves in seawater, several reactions occur:

  1. Bicarbonate Formation:
     $ext{CO}<em>2 + ext{H}</em>2 ext{O} <br>ightleftharpoons ext{H}<em>2 ext{CO}</em>3$

  2. Dissociation of Carbonic Acid:
     $ext{H}<em>2 ext{CO}</em>3 <br>ightleftharpoons ext{H}^+ + ext{HCO}_3^-$

  3. Dissociation of Bicarbonate Ion:
     $ext{HCO}<em>3^- ightleftharpoons ext{H}^+ + ext{CO}</em>3^{2-}$

• Distribution of Dissolved Inorganic Carbon (DIC):

  • DIC can be represented as:
    $ext{DIC} = [ ext{HCO}<em>3^-] + [ ext{CO}</em>3^{2-}] + [ ext{CO}_2^*]$

Impact of CO₂ on Ocean Acidity

pH Changes

• Definition of pH:

  • $ext{pH} = - ext{log}[ ext{H}^+]$

• Mechanics:

  • As the concentration of H⁺ ions increases (due to CO₂ dissolution), the ocean's pH decreases, indicating increasing acidity.

pH Range in Oceans

• Typical pH Values of Ocean Water:

  • Seawater is generally slightly alkaline:

  • Surface waters: pH ≈ 8.1

  • Deep ocean: pH ≈ 7.6 - 7.8

• pH Scale:

  • Ranges from 7.6 to 8.2, with a variation of approximately 0.3-0.4 pH units.

• Logarithmic Nature of pH:

  • A decrease of 0.3 in pH indicates about a two-fold increase in the concentration of H⁺ ions.

• Correlations with Atmospheric CO₂:

  • Increased atmospheric CO₂ levels lead to increased H⁺ concentrations in seawater.

Chemical Species and pH Dynamics

Relative Abundance

• The relative abundance of carbonic acid (H₂CO₃), bicarbonate ion (HCO₃⁻), and carbonate ion (CO₃²⁻) in seawater is dependent on the pH:

  • Higher H⁺ ion concentrations lead to fewer carbonate ions available.

Carbonate System Functionality

Buffering Capacity

• Carbonate System:

  • The carbonate system is critical for resists large changes in ocean pH, allowing for substantial CO₂ absorption without collapsing the pH.

  • Reactions involving H⁺ concentration:

  • When H⁺ is added (acidic conditions): $ext{H}^+ + ext{CO}<em>3^{2-} ightarrow ext{HCO}</em>3^-$

    • This reaction removes free H⁺ ions, slowing down the decrease in pH.

  • When H⁺ is removed (basic conditions): $ext{HCO}<em>3^- ightleftharpoons ext{H}^+ + ext{CO}</em>3^{2-}$

    • This reaction releases H⁺ ions, which slows down the increase in pH.

Le Chatelier's Principle and Implications

• Conditions of Low Carbonate Ion:

  • When CO₃²⁻ concentration is low, calcium carbonate (CaCO₃) becomes undersaturated:
    $ext{Ca}^{2+} + ext{CO}<em>3^{2-} ightleftharpoons ext{CaCO}</em>3 ext{ (solid)}$

  • Under these conditions, the equilibrium shifts leftward, which can lead to the dissolution of shells and carbonate structures.

Overview of the Carbon Cycle

• The carbon cycle visually represented includes:

  • Relationships among carbonate, bicarbonate, and carbonic acid in the ocean.

  • Interaction with atmospheric CO₂ and the oceanic components: H₂O, H⁺ ions, Ca²⁺ ions, and precipitation/dissolution processes related to calcium carbonate (CaCO₃) sediment.

Anthropogenic Effects on Ocean pH

• Impact of Increased CO₂:

  • The rise in anthropogenic CO₂ has notably reduced ocean pH.

  • Preindustrial ocean surface pH was approximately 8.2, currently observed at approximately 8.1, indicating roughly a 30% increase in H⁺ concentration.

Alkalinity Enhancement Discussion

• Carbon Dioxide Removal (CDR) Strategy:

  • A proposed method for CDR involves increasing ocean alkalinity. - Methods:

  • Addition of crushed minerals to increase alkalinity, leading to the consumption of H⁺ ions. - Resulting Equilibrium Shift:

  • Enhancing alkalinity shifts the equilibrium towards bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻), thus permitting a higher amount of atmospheric CO₂ to dissolve without causing significant pH drops.