4.1C Oceans as a carbon sink (HL) Notes

Oceans as a Carbon Sink

1. The oceans act as a global carbon sink, absorbing approximately 25–30% of anthropogenic carbon dioxide (CO₂) emitted into the atmosphere from human activities such as fossil fuel combustion, deforestation, and cement production.
2. This ability to absorb and store carbon plays a critical role in moderating atmospheric CO₂ concentrations and slowing the rate of climate change.
3. The process through which the ocean takes up carbon is called carbon sequestration, the long-term removal of CO₂ from the atmosphere and its storage in the ocean system.

Mechanisms of Carbon Absorption by Oceans

1. Physical (Solubility) Pump

1. CO₂ dissolves directly into the surface waters of the ocean, particularly in cold, high-latitude regions where gas solubility is higher.
2. The dissolved CO₂ forms a dynamic equilibrium with carbonic acid (H₂CO₃), bicarbonate ions (HCO₃⁻), and carbonate ions (CO₃²⁻).
3. Ocean currents then transport this dissolved inorganic carbon (DIC) to deeper waters, where it can remain sequestered for centuries.

$$\text{CO₂ (g)} + \text{H₂O (l)} \leftrightarrow \text{H₂CO₃ (aq)} \leftrightarrow \text{H⁺ (aq)} + \text{HCO₃⁻ (aq)} \leftrightarrow 2\text{H⁺ (aq)} + \text{CO₃²⁻ (aq)}$$

2. Biological Pump

1. Phytoplankton, microscopic photosynthetic organisms, absorb CO₂ dissolved in seawater to form organic carbon via photosynthesis.
2. This carbon is passed along the food web as zooplankton and higher consumers feed on phytoplankton.
3. When these organisms die, some organic material sinks to the deep ocean, sequestering carbon in biomass and sediments.

3. Carbonate Pump

1. Marine organisms such as corals, mollusks, and coccolithophores use CO₂ and calcium ions to form calcium carbonate (CaCO₃) for shells and skeletons.
2. When these organisms die, their remains settle on the seabed, contributing to carbonate sediment formation that can lock carbon away for millions of years.

Limits to Oceanic Carbon Uptake

1. The ocean’s ability to absorb CO₂ is not unlimited.
2. As atmospheric CO₂ concentrations rise, oceans absorb more, but chemical reactions reduce the capacity for further uptake over time.
3. A saturation point occurs when equilibrium between oceanic and atmospheric CO₂ prevents additional absorption.
4. Warming oceans also reduce CO₂ solubility, lowering their efficiency as carbon sinks.

Ocean Acidification and Long-Term Carbon Sequestration

Short-Term Sequestration: Dissolved Carbon and Acidification

1. When CO₂ dissolves in seawater, it forms carbonic acid (H₂CO₃), which dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻).
2. The increased hydrogen ion concentration lowers the ocean’s pH, a process known as ocean acidification.
3. Pre-industrial ocean pH averaged around 8.2.
4. Current measurements show 8.1 or lower, representing a 30% increase in acidity.

$$\text{Ca²⁺} + \text{CO₃²⁻} \leftrightarrow \text{CaCO₃}$$

Ecological Impacts of Acidification

1. Reduced Calcification: Acidic water lowers carbonate ion availability, hindering shell and skeleton formation.
2. Coral Reef Decline: Coral bleaching and skeletal weakening reduce biodiversity and reef resilience.
3. Food Web Disruption: Planktonic calcifiers like Coccolithophores are affected, altering food availability for higher trophic levels.
4. Ecosystem Collapse: Degradation of reefs and shellfish populations threatens fisheries and coastal protection.

Long-Term Carbon Sequestration: The Sedimentation Process

1. In the long term, carbon is stored in biological and geological forms:
   1. Biological Carbon Storage: Phytoplankton absorb CO₂ during photosynthesis. When they die, their remains sink to the seabed.
   2. Organic Carbon Burial: Some organic matter escapes decomposition and becomes buried in sediments as partially decomposed biomass.
   3. Inorganic Carbonate Formation: Marine organisms such as corals, molluscs, and foraminifera use Ca²⁺ and CO₃²⁻ to form CaCO₃ shells and skeletons.
2. Over millions of years, these sediments compact under high pressure to form limestone or fossil fuels such as oil, gas, and coal.

Transformation into Fossil Fuels

1. Over geological timescales, pressure and heat transform buried organic matter into fossil fuels such as coal, oil, and natural gas.
2. These act as long-term carbon reservoirs, keeping carbon locked away from the atmosphere.
3. However, human extraction and combustion of fossil fuels reverse this process, rapidly releasing ancient carbon back into the atmosphere.