Lecture 07 – Biosphere, Pedosphere, Carbon Cycle

Q: What effect can large volumes of glacial meltwater have on surface ocean salinity near Greenland?

They can reduce surface water salinity in the vicinity of Greenland.

Q: What is the concern about reduced surface salinity near Greenland and thermo-haline circulation?

It is unknown whether reduced salinity could slow down or stop the thermo-haline conveyor belt circulation in the next few decades.

Q: What property of glacial meltwater makes it capable of disrupting thermo-haline circulation?

Its very low TDS (total dissolved solids), meaning it is very dilute compared to seawater.

Q: What are the two most abundant elements in plant tissue by dry weight?

Carbon (45%) and oxygen (45%).

Q: What are the sources of nitrogen in plant tissue?

Rain, soil, and litter.

Q: What are the sources of potassium (K) and calcium (Ca) in plant tissue?

Minerals and litter.

Q: What is the source of phosphorus (P) in plant tissue?

Litterfall.

Q: What are the sources of magnesium (Mg) in plant tissue?

Minerals and litter.

Q: What are the sources of sulfur (S) in plant tissue?

Rain, minerals, and litter.

Q: What does photosynthesis on land produce?

Organic carbon.

Q: What determines the rate of photosynthesis on land?

Nutrient availability, specifically the limiting nutrients nitrogen (N) and phosphorus (P).

Q: Where is Net Primary Production (NPP) highest on land?

In tropical and temperate regions.

Q: What is litterfall?

The delivery of dead organic matter from trees and plants to the soil, where it is decomposed.

Q: How much carbon is returned to the atmosphere by fires, and what fraction of NPP is that?

5 × 10¹⁵ g C/yr, which is less than 10% of NPP.

Q: What is SOM?

Soil organic matter — organic material in the soil that is subject to decomposition.

Q: Why is glacial meltwater significant for ocean chemistry, and what is the concern near Greenland?

Meltwater has very low TDS, so large volumes can reduce surface ocean salinity near Greenland — potentially slowing or stopping the thermohaline conveyor belt, which could make Europe colder.

Q: What are the two most abundant elements in plant tissue, and where do they come from?

Carbon (45%, from atmospheric CO₂) and oxygen (45%, from water). Remaining dry weight: H 6%, N 1.5% (rain/soil/litter), K 1%, Ca 0.5%, P 0.2% (litter), Mg 0.2%, S 0.1%.

Q: What controls the rate of terrestrial photosynthesis?

Sunlight, water, and nutrient availability — with nitrogen (N) and phosphorus (P) as the limiting nutrients.

Q: What is global continental NPP, and where is it highest?

60 × 10¹⁵ g C/yr, highest in tropical and temperate regions. The continental biosphere stores 600 × 10¹⁵ g C total.

Q: What is litterfall, and how does it relate to NPP?

The delivery of dead organic matter from plants to the soil for decomposition. Litterfall is 55 × 10¹⁵ g C/yr (>90% of NPP); fires return another 5 × 10¹⁵ g C/yr (

Q: What is SOM and what are the two decomposition pathways?

Soil organic matter. Rapid decomposition releases CO₂, H₂O, and nutrients (N, P, K, S). Slow decomposition produces resistant organic compounds called humus.

Q: How much carbon is stored in continental soil, and how does it compare to the biosphere?

1500 × 10¹⁵ g C — 2.5 times the 600 × 10¹⁵ g C stored in the biosphere. Decomposers return 55 × 10¹⁵ g C/yr to the atmosphere.

Q: Why is the ocean nutrient-limited despite having sunlight, water, and CO₂?

Dead phytoplankton sink, removing N and P from the surface ocean. Nutrients only return via upwelling and river input, keeping surface concentrations low.

Q: What is the NPP and carbon storage of the ocean?

Surface ocean stores 1000 × 10¹⁵ g C; deep ocean stores 38,000 × 10¹⁵ g C. Sinking flux (NPP) is 7 × 10¹⁵ g C/yr; recycled production is 33 × 10¹⁵ g C/yr.

Q: How is carbon transported from continents to the ocean by rivers?

As DIC (HCO₃⁻ from chemical weathering, 0.3 × 10¹⁵ g C/yr) and DOC (from humus decomposition, 0.4 × 10¹⁵ g C/yr) — total river flux is 0.7 × 10¹⁵ g C/yr.

Q: What role do foraminifera play in the long-term carbon cycle?

They consume HCO₃⁻ and Ca to build CaCO₃ shells, which sink and form carbonate sediment (52,000,000 × 10¹⁵ g C). Subduction and volcanism eventually return this carbon to the atmosphere as CO₂ (0.15 × 10¹⁵ g C/yr).

Q: How is carbon returned from the lithosphere to the atmosphere?

Through volcanism — subducted carbonate and organic sediments release CO₂ at a rate of 0.15 × 10¹⁵ g C/yr.

Q: What form does carbon take in each major reservoir?

Atmosphere: CO₂. Vegetation and soil: organic C. Surface and deep ocean: dissolved inorganic C (HCO₃⁻). Rocks/sediments: inorganic CaCO₃ and organic C (both solid).

Q: What are the relative sizes of the major carbon reservoirs (atmosphere = 1×)?

Atmosphere 1×, vegetation 1×, soils 2.5×, surface ocean 1.6×, deep ocean 63×, rocks/sediments 100,000×.

Q: Why does carbon exchange rate differ between reservoirs?

Larger reservoirs exchange carbon more slowly. Small surface reservoirs have high fluxes and fast turnover; the deep ocean is partially isolated by the thermocline; sediments are enormous with tiny outgoing fluxes.

Q: What is the thermocline's role in the carbon cycle?

It is a zone of rapid temperature change with depth that partially isolates the deep ocean from the surface, making carbon exchange between them sluggish.

Q: How does continental NPP compare to ocean NPP?

Continental NPP (60 × 10¹⁵ g C/yr) is roughly 8–9 times greater than ocean sinking flux (7 × 10¹⁵ g C/yr).

Q: What is respiration (oxidation) in the pedosphere?

The decomposition of SOM and litter, converting reduced organic carbon back to CO₂ that is re-emitted to the atmosphere.