Marine Chemistry - MARS 1010

Marine Chemistry Overview

Introduction

  • Focus on

    • Dissolved Gases

    • In Situ Processes

    • Ocean Acidification

    • Carbon Pumps

Dissolved Gases in Ocean

Major Gases

  • Nitrogen (N2)

    • Relatively biologically inactive

    • Only biological process: N2-fixation

  • Oxygen (O2)

    • Very biologically active

    • Key processes: Photosynthesis and respiration

    • Low solubility in seawater

  • Carbon Dioxide (CO2)

    • Very biologically active

    • Key processes: Photosynthesis and respiration

    • Extremely soluble in seawater

Abundance of Gases

  • Most Abundant: N2, O2, CO2

  • Trace Gases:

    • Noble Gases:

    • Argon (Ar)

    • Helium (He)

    • Neon (Ne)

    • Krypton (Kr)

    • Xenon (Xe)

    • Radon (Rn)

    • Characteristics: Inert, do not react with water, non-biologically active

    • Biogenic Gases:

    • Sulfur dioxide (SO2)

    • Nitrous oxide (N2O)

    • Methane (CH4)

    • Dimethyl sulfide (DMS)

    • Produced primarily by phytoplankton near the surface

Factors Controlling Seawater Gas Concentrations

  • Seawater gas concentrations determined by several processes:

    • Atmospheric Concentrations:

    • Ocean equilibrates with the atmosphere

    • Exchange Rates with the Atmosphere:

    • Wind speed, wave action, and turbulence affect gas exchange

    • Solubility:

    • Each gas has its own solubility, influenced by:

      • Temperature

      • Salinity

      • Pressure

    • Biological Factors:

    • Processes such as photosynthesis, respiration, and decomposition

Photosynthesis and Respiration Equations

  • Photosynthesis:

    • CO<em>2+H</em>2O<br>ightarrowCH<em>2O+O</em>2CO<em>2 + H</em>2O <br>ightarrow CH<em>2O + O</em>2

    • Produces organic matter (sugar)

  • Respiration:

    • CH<em>2O+O</em>2<br>ightarrowCO<em>2+H</em>2OCH<em>2O + O</em>2 <br>ightarrow CO<em>2 + H</em>2O

    • Consumes oxygen and produces carbon dioxide

Biological Consequences of Gas Concentrations

Oxygen (O2) Dynamics

  • Biological consequences of low oxygen levels:

    • Low O2 can become limiting for marine organisms

    • Sources of O2 in ocean:

    • Atmosphere

    • Photosynthesis (only occurs at surface)

    • Losses of O2 in ocean:

    • Atmosphere (outgassing, only at surface)

    • Respiration (occurs at all depths)

Carbon Dioxide (CO2) Dynamics

  • CO2 rarely limits plant growth

  • Sources of Total Carbon Dioxide (TCO2) or Dissolved Inorganic Carbon (DIC):

    • Atmosphere

    • Respiration

    • Calcium carbonate dissolution:

    • CaCO<em>3+2H+ightarrowCa++CO</em>2+H2OCaCO<em>3 + 2H^+ ightarrow Ca^+ + CO</em>2 + H_2O

  • CO2 concentration reflects its solubility in seawater

Biological Carbon Pump

Organic Carbon Dynamics

  • Organic particles produced by biological processes

  • Mineralization (respiration):

    • CH<em>2O+O</em>2<br>ightarrowCO<em>2+H</em>2OCH<em>2O + O</em>2 <br>ightarrow CO<em>2 + H</em>2O

    • Consumes oxygen and releases carbon dioxide

  • Vertical distributions of organic matter:

    • Dead organisms sink from surface to bottom, affecting gas profiles

  • This process refers to the biological carbon pump

Photosynthesis Respiration Cycle

  • Relation of photosynthesis and respiration:

    • CO<em>2+H</em>2O<br>ightarrowCH<em>2O+O</em>2<br>ightarrowCO<em>2+H</em>2OCO<em>2 + H</em>2O <br>ightarrow CH<em>2O + O</em>2 <br>ightarrow CO<em>2 + H</em>2O

  • Analyze oxygen profiles to determine dominance of respiration vs. photosynthesis at different ocean depths (A, B, or C locations)

Depth Profiles and Gas Concentrations

Photic Zone vs. Deep Sea

  • Photic Zone:

    • Photosynthesis dominates over respiration

    • Significant atmospheric exchange

  • Mesopelagic Zone:

    • Respiration dominates over photosynthesis

  • Deep Sea:

    • Reflects initial properties from deep water formation and ongoing respiration processes

  • (extTotalCO<em>2extconcentration=CO</em>2+H<em>2CO</em>3+HCO<em>3−+CO</em>32−ext,typicallyaround2extmmol/L)( ext{Total } CO<em>2 ext{ concentration} = CO</em>2 + H<em>2CO</em>3 + HCO<em>3^- + CO</em>3^{2-} ext{ , typically around } 2 ext{ mmol/L}) is higher in deep waters

  • The variation of DIC is less compared to O2 levels at depth

Oxygen Concentration Profile

Reasons for Variation in O2 Concentration

  • High in the photic zone due to photosynthesis

  • Not maximum at the very surface due to gas exchange

  • Higher at the bottom due to cold deep water

  • Minimum levels occur in between due to respiration of organic particles

Oxygen Profile Characteristics

Vertical Profiles of Oxygen (O2)

  • Display high concentrations in the photic zone

  • A minimum in concentration due to high respiration rates in the mesopelagic zone

  • Differences in oxygen levels between Atlantic and Pacific Oceans due to varying water ages and oxygen consumption over time

    • Conveyor belt effect: More respiration in the older Pacific water results in lower deep O2 concentrations

Carbon Dioxide (CO2) Profile

Comparisons and Control

  • The CO2 profile in the Atlantic differs from the Pacific

    • More respiration has occurred in the deep Pacific water

  • Overall CO2 levels are higher in the ocean due to chemical reactions with water

    • This exposes water to be more soluble than gas alone would suggest

Buffering and pH

  • The bicarbonate system (HCO3-) helps stabilize ocean pH:

    • CO<em>2+H</em>2O<br>ightleftharpoonsH<em>2CO</em>3<br>ightleftharpoonsHCO<em>3−+H+ightleftharpoonsCO</em>32−+2H+CO<em>2 + H</em>2O <br>ightleftharpoons H<em>2CO</em>3 <br>ightleftharpoons HCO<em>3^- + H^+ ightleftharpoons CO</em>3^{2-} + 2H^+

  • Changes in atmospheric CO2 concentrations correlate with changes in ocean pH due to increased solubility

Carbonate Chemistry

Chemical Species

  • Distribution of Dissolved Inorganic Carbon (DIC)

  • Importance of buffering in seawater: prevents sudden changes in acidity or alkalinity

  • Components of DIC:

    • CO<em>2CO<em>2, H</em>2CO<em>3H</em>2CO<em>3, HCO</em>3−HCO</em>3^-, CO32−CO_3^{2-}

Calcium Carbonate and Its Role

Calcification Processes

  • Calcium carbonate (CaCO3) formation and dissolution impact carbon cycling

  • Interaction of CO2 with seawater affecting marine life

Solubility Carbon Pump

Carbon Dioxide Diffusion

  • CO2 is more soluble in cold waters

  • Deep water formation leads to high CO2 concentrations being transported downward

Quantitative Assessment

Factors Influencing Carbon Pump Efficiency

  • Sediment processes like burial are substantial for carbon removal

  • Key processes:

    • Gas exchange

    • Upwelling and mixing

  • To quantify the biological carbon pump, essential metrics to measure include:

    • Number of particles formed

    • Rate of sinking

    • Amount reaching the ocean floor

    • Quantity of material removed

Summary of Key Processes

  • Distribution of gases such as O2 and CO2 is governed by multiple physical, chemical, and biological processes

  • Sea surface exchange dynamics and deep-water consumption dynamics are critical in understanding gas concentrations

  • Biological cycling and chemistry play vital roles in total carbon dioxide dynamics within the ocean

Review Questions and Concepts

  1. What controls the distribution and amounts of gases in the ocean?

  2. Understand variations in seawater composition with depth and ocean basin.

  3. Sketch profiles of oxygen and CO2 for Atlantic vs. Pacific Oceans and explain differences.

  4. Explain how biological and solubility carbon pumps differ and their respective controls.

  5. Explore how major nutrient concentrations vary in the ocean and predictions of nutrient addition impacts.

  6. Define pH and describe examples of neutral, acidic, and alkaline solutions and how they are controlled in oceans.