A SMALL NOTE ABOUT DISSOLVED GASES.docx.PDF

A SMALL NOTE ABOUT DISSOLVED GASES.docx.PDF

1. In **Riley's Introduction to Marine Chemistry**, the topic of **dissolved gases** is a critical aspect of
ocean chemistry, as the concentrations of various gases dissolved in seawater are fundamental to both
marine life and ocean processes. These dissolved gases play important roles in controlling the chemistry
of the ocean, as well as the biological processes that occur within it.
### Overview of Dissolved Gases
The primary gases dissolved in seawater are:
1. **Oxygen (O₂)**
Oxygen is crucial for the respiration of marine organisms. The concentration of oxygen in seawater is
influenced by factors like temperature, salinity, and biological activity (e.g., photosynthesis and
respiration). Oxygen solubility decreases as temperature increases, and it is also lower in more saline
waters.
2. **Carbon Dioxide (CO₂)**
Carbon dioxide is a key player in ocean chemistry, particularly in relation to the carbon cycle. CO₂
dissolves in water and can react with water to form carbonic acid (H₂CO₃), which dissociates into
bicarbonate (HCO₃⁻) and carbonate ions (CO₃²⁻). This process is central to ocean acidification, which is a
growing concern due to increased atmospheric CO₂ levels.
3. **Nitrogen (N₂)**
Nitrogen is the most abundant dissolved gas in the ocean, though it is relatively inert biologically
because most organisms cannot use N₂ directly. However, nitrogen in the form of nitrate (NO₃⁻) is
essential for marine plants and phytoplankton. Nitrogen is also important in the nitrogen cycle of the
ocean, as it can be fixed by certain bacteria and converted into forms that can be used by organisms.
4. **Argon (Ar)**
Argon is a noble gas and is relatively inert in the marine environment. It is present in seawater in
proportion to its atmospheric concentration, and it is often used as a tracer in physical oceanography
studies.

2. 5. **Other Gases**
- **Ozone (O₃):** Though present in very small amounts, ozone can be measured in seawater and
plays a role in photochemical processes.
- **Methane (CH₄):** Methane is produced by certain marine organisms, particularly those in anoxic
(oxygen-deprived) environments like deep-sea sediments.
- **Hydrogen Sulfide (H₂S):** This gas can accumulate in anoxic waters, particularly in deeper regions
of the ocean or in areas with high biological activity.
### Factors Affecting Dissolved Gas Concentrations
Several factors control the concentrations of dissolved gases in the ocean:
- **Temperature:** Warmer water holds less gas than cooler water. This is particularly important for
gases like oxygen and carbon dioxide.
- **Salinity:** Saltier water holds less dissolved gas than freshwater. This is because the solubility of
gases decreases as salinity increases.
- **Pressure:** The solubility of gases increases with pressure. Therefore, deeper ocean waters typically
contain higher concentrations of dissolved gases.
- **Biological Activity:** Photosynthesis and respiration by marine organisms affect gas concentrations.
For example, during the day, photosynthesizing plankton increase oxygen levels, while respiration (both
by animals and plants) consumes oxygen and produces CO₂.
- **Air-Sea Exchange:** The rate at which gases are exchanged between the atmosphere and the ocean
surface depends on factors like wind speed, wave action, and the concentration gradient of the gas.
### Chemical Reactions Involving Dissolved Gases
Dissolved gases in the ocean are involved in several important chemical processes:

3. - **Oxygen and Carbon Dioxide Cycling:** Photosynthesis by phytoplankton consumes CO₂ and releases
oxygen, while respiration by animals and other organisms consumes oxygen and releases CO₂. This cycle
is vital for maintaining life in the ocean.

- **Ocean Acidification:** Increased atmospheric CO₂ is absorbed by the ocean, leading to a decrease in
pH. This process, known as ocean acidification, has significant implications for marine life, particularly
organisms that rely on calcium carbonate (e.g., corals and shellfish).
- **Redox Reactions:** Gases like nitrogen and hydrogen sulfide are involved in redox reactions,
particularly in anoxic zones where organisms can use these gases in the absence of oxygen.
- **Nutrient Availability:** Nitrogen in the form of nitrate or ammonium is a crucial nutrient for
phytoplankton growth, and the cycling of nitrogen compounds is tightly linked to the availability of
dissolved oxygen and other gases.
REACTIVE AND NON REACTIVE GASES
1. **Reactive Gases**: These are gases that readily participate in chemical reactions. Examples include:
- **Oxygen (O₂)**: Highly reactive, especially with organic compounds and metals. It's crucial in
combustion reactions.
- **Hydrogen (H₂)**: Reacts with many elements and compounds, often exothermically.
- **Chlorine (Cl₂)**: Reacts with many metals and nonmetals to form chlorides, often quite vigorously.
- **Nitrogen oxides (NO, NO₂)**: Known for their reactivity in atmospheric chemistry and pollution
contexts.
2. **Non-Reactive (Inert) Gases**: These gases have low reactivity, particularly the noble gases.
Examples include:
- **Noble Gases (He, Ne, Ar, Kr, Xe)**: Known for their lack of reactivity due to filled electron shells,
making them stable.

4. - **Nitrogen (N₂)**: While it can react under specific conditions, it's relatively inert at room
temperature due to its strong triple bond.
.carbon dioxide system- origin.
The origin of the carbon dioxide system in seawater is a complex interplay of natural and anthropogenic processes. Here's a
breakdown of the key factors:
Natural Sources:
Volcanic Activity: Volcanic eruptions release significant amounts of CO2 into the atmosphere, which then dissolves into the
ocean.
Biological Processes: Photosynthesis by marine organisms removes CO2 from the atmosphere and incorporates it into
organic matter. Respiration by these organisms and decomposition of organic matter release CO2 back into the water.
Ocean-Atmosphere Exchange: CO2 is constantly exchanged between the atmosphere and the ocean, with the direction of
the flux depending on the partial pressure difference.
Anthropogenic Sources:
Fossil Fuel Combustion: Human activities like burning fossil fuels release large amounts of CO2 into the atmosphere, which
subsequently dissolves into the ocean.
Deforestation: Deforestation reduces the Earth's ability to absorb CO2 through photosynthesis, leading to increased
atmospheric CO2 levels.
Key Points from Riley's Introduction to Marine Chemistry:
Seawater can be considered a dilute solution of sodium bicarbonate with other acid-base species.
The total dissolved inorganic carbon (DIC) in seawater consists of bicarbonate, carbonate, and dissolved CO2.
Bicarbonate is the dominant species, followed by carbonate, and then dissolved CO2.
The equilibrium between these species buffers seawater against pH changes.
Understanding the carbon dioxide system is crucial for studying ocean acidification and its impacts on marine ecosystems.
Additional Considerations:
The distribution of the carbon dioxide system in the ocean is influenced by factors like temperature, salinity, and biological
activity.
Ocean currents play a significant role in transporting carbon dioxide around the globe.
The increasing concentration of CO2 in the atmosphere is leading to ocean acidification, which has potential consequences
for marine organisms and ecosystems.
Importance of Carbon Dioxide in Marine Chemistry
Carbon dioxide (CO2) plays a crucial role in marine chemistry and the overall global carbon cycle. Its importance stems from
several key factors:
1.Ocean Acidification:
oIncreased atmospheric CO2 levels lead to higher CO2 absorption by the ocean.
oThis absorption results in the formation of carbonic acid, lowering the ocean's pH and causing ocean acidification.
oOcean acidification has significant impacts on marine organisms, particularly those with calcium carbonate shells or
skeletons, like corals, mollusks, and some plankton.
2.Carbonate Chemistry:
oCO2 reacts with water to form carbonic acid, which then dissociates into bicarbonate and carbonate ions.
oThe equilibrium between these species influences the ocean's buffering capacity and its ability to regulate pH.
oChanges in CO2 levels can disrupt this equilibrium, affecting marine ecosystems.

5. 3.Biological Processes:
oMarine organisms, such as phytoplankton, use CO2 in photosynthesis to produce
organic matter.
oThis process removes CO2 from the atmosphere and incorporates it into the marine
food web.
oRespiration by marine organisms, including bacteria, releases CO2 back into the water.
Factors Governing the Distribution of Carbon Dioxide in
the Ocean
The distribution of CO2 in the ocean is influenced by several factors:
1.Physical Processes:
oOcean circulation patterns, such as currents and upwelling, transport CO2-rich waters
to different parts of the ocean.
oTemperature and salinity variations affect the solubility of CO2 in seawater.
oWind-driven mixing at the ocean surface facilitates the exchange of CO2 between the
atmosphere and the ocean.
2.Biological Processes:
oPhotosynthesis by phytoplankton removes CO2 from the surface waters, while
respiration by marine organisms releases CO2.
oThe vertical distribution of marine organisms influences the distribution of CO2 in the
water column.
3.Chemical Processes:
oThe equilibrium reactions between CO2, bicarbonate, and carbonate ions control the
distribution of inorganic carbon species.
oThe pH of seawater affects the speciation of these species.
4.Anthropogenic Factors:
oHuman activities, such as burning fossil fuels and deforestation, increase the
concentration of CO2 in the atmosphere, leading to higher CO2 uptake by the ocean.
Understanding the factors governing the distribution of CO2 in the ocean is essential for
assessing the impacts of climate change on marine ecosystems and for
developing strategies to mitigate these impacts.
Alkalinity, Buffer Capacity, Lysocline, and Carbonate
Compensation Depth
These are key concepts in marine chemistry, particularly relevant to
understanding the ocean's carbon cycle and its response to climate
change.
Alkalinity
Definition: Alkalinity is the measure of the capacity of water to neutralize
acids. It's essentially the sum of all bases in the water, including
bicarbonate, carbonate, and hydroxide ions.

6. Importance: Alkalinity plays a crucial role in buffering the ocean's pH,
helping to stabilize it against changes caused by the absorption of
atmospheric CO2. This buffering capacity is essential for marine
organisms, especially those with calcium carbonate shells.
Buffer Capacity
Definition: Buffer capacity refers to the resistance of a solution to changes
in pH. In the context of seawater, it's related to its alkalinity.
Importance: A higher buffer capacity means the ocean can absorb more
CO2 without significant pH changes. However, with increasing CO2
emissions, the ocean's buffer capacity is gradually diminishing, leading to
ocean acidification.
Lysocline
Definition: The lysocline is the depth in the ocean below which calcium
carbonate (CaCO3) begins to dissolve rapidly.
Importance: This depth is influenced by factors like pressure, temperature,
and the availability of carbonate ions. As you descend below the lysocline,
the increasing pressure and decreasing temperature make it harder for
CaCO3 to remain stable.
Carbonate Compensation Depth (CCD)
Definition: The CCD is the depth at which the rate of CaCO3 supply (from
the surface waters) equals the rate of dissolution.

Importance: Below the CCD, there is a net dissolution of CaCO3, and no
calcareous sediments can accumulate. The depth of the CCD varies
depending on factors like ocean circulation, biological productivity, and the
concentration of CO2 in the water.
Relationship Between These Concepts
CO2 Absorption and Alkalinity: As the ocean absorbs more CO2, it leads to
a decrease in pH and a decrease in carbonate ion concentration. This can
reduce the ocean's buffer capacity and affect the saturation state of
CaCO3, influencing the lysocline and CCD.
Ocean Acidification and CaCO3: Ocean acidification makes it more difficult
for marine organisms to build and maintain their CaCO3 shells and
skeletons. This can have significant impacts on marine ecosystems,
particularly those reliant on calcifying organisms.
By understanding these concepts, we can better appreciate the complex
interactions between the ocean's chemistry, biology, and physics, and how
they are influenced by human activities and climate change.

7. 
Ocean Acidification: A Growing Threat
Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, primarily caused by
the absorption of carbon dioxide (CO2) from the atmosphere. This phenomenon is a direct
consequence of human activities, such as the burning of fossil fuels and deforestation.  
How Does It Happen?
1.CO2 Absorption: When CO2 is absorbed by seawater, it reacts with water molecules to
form carbonic acid.  
2.Carbonic Acid Dissociation: Carbonic acid then dissociates into bicarbonate and
hydrogen ions.  
3.pH Reduction: The increase in hydrogen ions lowers the pH of the seawater, making it
more acidic.  
Impacts on Marine Ecosystems
Ocean acidification has far-reaching consequences for marine ecosystems:  
Shellfish and Coral Reefs: Many marine organisms, including shellfish, corals, and
other creatures with calcium carbonate shells or skeletons, are particularly vulnerable. As
the ocean becomes more acidic, it becomes harder for these organisms to build and
maintain their shells, leading to weakened structures and increased mortality rates.  
Food Web Disruptions: Changes in the abundance and distribution of calcifying
organisms can disrupt marine food webs, affecting fish populations and other species that
rely on them.  
Economic Impacts: Ocean acidification can have significant economic impacts on
industries such as fishing, tourism, and aquaculture, which depend on healthy marine
ecosystems.  
Mitigating Ocean Acidification
To address ocean acidification, it is crucial to reduce greenhouse gas emissions,
particularly CO2. This can be achieved through:  
Transitioning to Renewable Energy Sources: Shifting from fossil fuels to
renewable energy sources like solar, wind, and hydro power can
significantly reduce CO2 emissions.  
Improving Energy Efficiency: Implementing energy-efficient technologies
and practices can help reduce energy consumption and associated emissions.
Protecting Coastal Ecosystems: Healthy coastal ecosystems, such as
mangroves and seagrass beds, can help absorb CO2 and mitigate the effects
of ocean acidification.  

8. International Cooperation: Collaborative efforts among nations are
necessary to address the global issue of climate change and its impacts on
the ocean.  
Dissolved Oxygen: Origin and Factors Governing Distribution
Origin of Dissolved Oxygen
Dissolved oxygen (DO) is the amount of oxygen present in water.
It primarily originates from two sources:
1.Atmospheric Oxygen: Oxygen from the atmosphere dissolves into
water at the surface, especially in areas with high water
turbulence, such as rapids or breaking waves.
2.Photosynthesis: Aquatic plants, algae, and phytoplankton produce
oxygen as a byproduct of photosynthesis, releasing it into the
water.
Factors Governing the Distribution of Dissolved Oxygen
Several factors influence the distribution of dissolved oxygen in
water bodies:
1.Temperature: As water temperature increases, the solubility of
oxygen decreases. Warmer waters, therefore, tend to have lower
dissolved oxygen levels.
2.Salinity: Increased salinity reduces the solubility of oxygen in
water. Hence, saltwater bodies generally have lower DO levels
than freshwater bodies.
3.Atmospheric Pressure: Higher atmospheric pressure increases
the solubility of oxygen in water.
4.Photosynthesis: The presence of aquatic plants and algae,
especially in well-lit surface waters, increases oxygen levels
through photosynthesis.
5.Respiration: Aquatic organisms, including fish, invertebrates, and
bacteria, consume oxygen during respiration. This process
reduces the DO levels in the water.
6.Organic Matter Decomposition: The decomposition of organic
matter by bacteria consumes oxygen, leading to lower DO levels,
especially in stagnant or poorly oxygenated waters.

9. 7.Water Flow: Rapidly flowing water, like in rivers and streams, can
increase oxygen levels due to increased contact with the
atmosphere and turbulence. Stagnant water bodies, such as
ponds and lakes, tend to have lower DO levels.
Importance of Dissolved Oxygen
Dissolved oxygen is crucial for aquatic life. Fish and other aquatic
organisms rely on dissolved oxygen for respiration. Low DO levels
can lead to stress, disease, and even death in aquatic organisms.
Monitoring and maintaining adequate dissolved oxygen levels are
essential for the health of aquatic ecosystems.
Monitoring Dissolved Oxygen
Dissolved oxygen levels are measured using various techniques,
including:
Winkler Titration Method: A chemical method that involves
titrating a sample with a chemical solution to determine the
amount of oxygen present.
Electrochemical Probes: Electronic sensors that measure the
electrical current produced by a chemical reaction involving
oxygen.
By understanding the factors that influence dissolved oxygen
levels and monitoring these levels, we can take steps to protect
aquatic ecosystems and ensure the health of aquatic life.
AOU
AOU, or Apparent Oxygen Utilization, is a concept in marine chemistry that refers
to the amount of oxygen that has been consumed in a water mass since it was last
in contact with the atmosphere. It's a measure of the degree of water mass
modification due to biological and chemical processes:
Calculation: AOU is calculated by subtracting the measured oxygen
concentration from the oxygen saturation concentration at the same
temperature, salinity, and pressure.
Interpretation: A higher AOU value indicates that a water mass has
undergone more significant biological activity, such as respiration and
organic matter decomposition.

10. Applications: AOU is a valuable tool for studying ocean circulation
patterns, identifying water masses, and understanding the distribution of
nutrients and other chemical tracers.
Limitations: AOU can be influenced by factors like physical mixing and
advection, which can complicate its interpretation.
Oxygen Minimum Zones (OMZs): Formation and
Consequences
Formation of Oxygen Minimum Zones
Oxygen Minimum Zones (OMZs) are regions in the ocean where oxygen concentrations are
significantly lower than in surrounding waters. They typically form in the mid-water column,
between 200 and 1000 meters deep. Several factors contribute to their formation:  
1.Biological Oxygen Demand:
oHigh biological productivity in surface waters leads to a large amount of organic
matter sinking to the deeper layers.  
oAs this organic matter decomposes, bacteria consume oxygen, reducing its
concentration in the water.  
2.Limited Water Circulation:
oIn areas with weak water circulation, oxygen-rich water from the surface struggles
to reach the deeper layers.
 
oThis lack of oxygen replenishment exacerbates oxygen depletion.
3.Temperature and Salinity:
oWarmer water and higher salinity can reduce the solubility of oxygen in seawater,
further contributing to low oxygen levels.  
Consequences of Ocean Hypoxia
Ocean hypoxia, or oxygen depletion, can have significant ecological and biogeochemical
consequences:  
1.Impact on Marine Life:
oMany marine organisms, particularly those that are less mobile or have specific
oxygen requirements, are adversely affected by low oxygen conditions.  
oThis can lead to reduced biodiversity, altered food webs, and mass mortality
events.  
2.Nutrient Cycling:
oHypoxia can alter nutrient cycling processes, affecting the availability of essential
nutrients for marine organisms.  

11. oIt can also lead to the release of harmful substances, such as hydrogen sulfide,
which can further stress marine ecosystems.  
3.Climate Change:
oClimate change is expected to exacerbate ocean hypoxia by warming waters and
increasing stratification, which can reduce oxygen exchange between surface and
deep waters.  
oThis can lead to the expansion of OMZs and further threaten marine ecosystems.  
Mitigation and Adaptation
Addressing ocean hypoxia requires a multifaceted approach:
Reducing Greenhouse Gas Emissions: Mitigating climate change is crucial to reducing
the impact of warming waters on oxygen levels.  
Sustainable Fisheries: Sustainable fishing practices can help maintain healthy marine
ecosystems and reduce the pressure on fish populations.  
Marine Protected Areas: Establishing marine protected areas can help conserve
biodiversity and protect vulnerable marine ecosystems.  
Monitoring and Research: Continued monitoring and research are essential to
understand the dynamics of OMZs and their impacts on marine ecosystems.
Hydrogen Sulfide and Its Impact on Elemental Chemistry
Hydrogen sulfide (H S)
₂
is a colorless gas with a characteristic rotten egg smell. It
is a potent reducing agent and can significantly alter the chemistry of elements in
various environments, particularly in aquatic systems.  
Key Reactions and Impacts
1.Metal Sulfide Precipitation:
oH₂S reacts with metal ions in solution to form insoluble metal
sulfides.  
oThis process can remove metals from solution, affecting their
bioavailability and mobility.  
oExample: FeS (iron sulfide) and PbS (lead sulfide) are common
precipitates formed in anoxic environments.
2.Redox Reactions:
oH₂S acts as a strong reducing agent, leading to redox reactions with
various elements.  
oIt can reduce oxidized forms of metals, such as Fe(III) to Fe(II), and
sulfur species, such as sulfate (SO₄²⁻) to sulfide (S²⁻).

12. oThese redox reactions can influence the solubility and mobility of
metals in the environment.
3.Acidification:
oIn aqueous solutions, H₂S can dissociate into HS⁻ and H⁺ ions,
contributing to acidification.  
oAcidification can affect the solubility of minerals and the
bioavailability of metals.  
Environmental Significance
Aquatic Environments:
oIn aquatic systems, H₂S can create anoxic conditions, leading to fish
kills and other ecological problems.
oIt can also influence the geochemical cycling of elements, such as
sulfur, iron, and manganese.  
Soil Environments:
oIn soils, H₂S can affect the availability of nutrients, such as iron and
zinc, to plants.  
oIt can also contribute to soil acidification and the release of toxic
metals.
Industrial Settings:
oIn industrial processes, H₂S can corrode metal equipment and
pipelines.  
oIt is a hazardous gas and can pose health risks if inhaled.  
Mitigation Strategies
Aeration: Increasing oxygen levels in water bodies can help oxidize H₂S and
reduce its concentration.  
Chemical Treatment: Chemical treatments can be used to neutralize H₂S or
precipitate metal sulfides.  
Biological Treatment: Microbial processes can be used to oxidize H₂S,
converting it into less harmful compounds.  
Source Control: Reducing the sources of H₂S, such as industrial emissions
and wastewater discharges, is essential for long-term management.