Critical Zone Flashcards
The Critical Zone
An external heterogeneous layer of the ground, from the tip of the vegetation layer all the way to the groundwater layer. It is composed of rocks, soil, water, air and biota. We term it critical due to it being life-sustaining and critical to us.
What does the Critical Zone provide?
- Biological habitat and gene pool.
- Storage and transformation of water, carbon & nutrients.
- Source of raw materials.
- Platform for human activities and structures.
- Archive for physical and cultural heritage.
- Food and fibre production
Pressures on Critical Zone
There are various pressures, e.g. freshwater has been depleted by 50% and increasing land use within human development, widespread deforestation, more pastureland is pressurising the critical zone.
Earth’s Weathering Engine
Anthropogenic forcing’s are the most important contribution to Earth’s weathering engine, tectonic is a very slow contribution.
Without weathering, there is no soil formation, therefore the forcing’s influence all the services we receive from the critical zone.
Critical Zone Observatories (CZOs)
These are field research facilities to enable a multi-disciplinary approach to the observation of the Earth’s surface throughout the full extent of the critical zone. This is motivated by:
Hypothesis Testing
Process Understanding
Model Development.
Soil formation
On average, it takes 100 years for 1 cm of soil to form. The Soil chrono sequence is the timescale over which soil depth increases with time.
Weathering
This is the physical and chemical disintegration and decomposition of rocks, and the minerals within. biology can also influence weathering.
Agents of Physical Weathering: Temperature
Temperature changes ‘exfoliate’ rocks due to the differential heating between the surface and interior of the rock. Expansion and contraction of different minerals gradually weakens and fractures the rocks, especially when there is a vast temperature different between day and night. Furthermore freeze-thaw effects within rocks have the same effect, with water expanding 9% upon freezing.
Agents of Physical Weathering: Water
Water can have multiple effects on rock to weather it. flowing water carrying sediments can have an abrasive effect, creating ravines, gorges and valleys. Water also rounds particles and smooths edges.
Agents of Physical Weathering: Ice
Typically at high altitudes such as in mountains, the movement of glaciers can cause the cutting and crashing of rocks at the bed of the glacier or by causing avalanches, e.g. the Damma Glacier.
Agents of Physical Weathering: Wind
Wind is an agent of transportation for small particles such as sand, these can have an abrasive effect on rock structures, usually in arid climates. There is also a rounding effect caused by particles in wind, similar to in flowing water.
Agents of Chemical Weathering: Solution
Water is a universal solvent, and is a polar molecule. It can dissolve minerals such as halite (NaCl), this mechanism is limited in arid regions but plays a greater role in semi-arid and humid regions.
Agents of Chemical Weathering: Hydration
Water can combine with a mineral to form a new mineral (absorption), which increases the material volume and softens the material.
Agents of Chemical Weathering: Hydrolysis
One of the most important chemical processes; where the mineral structure and composition changes, e.g. Orthoclase to Kaolinite.
Agents of Chemical Weathering: Oxidation and Reduction
Iron-containing rocks can be reduced and oxidise because iron has many oxidation states, these changes are usually accompanies by a change in the colour of the rock.
Agents of Chemical Weathering: Carbonation
Reaction with Carbonic acid (H2CO3) found in rain, which can dissolve minerals such as calcium carbonate.
Agents of Biological Weathering: Root action
Root growth can apply pressure to rocks and slowly break them up, they also release chemicals into the ground which can initiate chemical weathering.
Agents of Biological Weathering: Microbial action
Microbes, e.g. fungi, can grow on rocks and enhance chemical weathering processes.
Agents of Biological Weathering: Animal action
Rodents, termites etc can mechanically break down surfaces.
Agents of Biological Weathering: Anthropogenic
Machinery and quarrying causes weathering
Soil Ecosystem Services:
- Water purification and soil containment reduction.
- Climate regulation.
- Nutrient cycling - decomposing chemicals into things which can be taken up by plants.
- Habitat for organisms.
- Food regulation.
- Source of pharmaceuticals and genetic resources.
- Foundation for human infrastructure.
- Provision of construction materials.
- Cultural heritage.
- Provision of food, fibre, and fuel.
- Carbon sequestration.
Soil Ecological functions
- Biomass production
- Protection of humans and the environment
- Gene reservoir
Soil non-Ecological functions
- Physical basis for human activities.
- Source of raw materials.
- Geogenic and cultural heritage.
Soil and the environment
- 35% of global CO2 emission come from soil and microbial respiration, and respiration of roots.
- 53% of NO2 and 33-47% of CH4 are also emitted by soil.
- Soil contains 3 times more carbon than the atmosphere and is important in local and global hydrological scale, furthermore it attenuates contaminants and provides clean water.
Soil as a Gene Reservoir
Soil contains nearly 1/4 of Earth’s biodiversity. 1g of soil contains 1 billion microorganisms, which hare important drivers of nutrient cycling and a source of antibiotics, yet we have no idea what they do or how they function as we cant culture them successfully.
Defining Soils
Soils are a natural body and a finite resource, and a product of weathering. It has physical, chemical, mineralogical and biological properties. It is used as a porous media for plant growth and provides life-supporting ecosystem services.
Soil Horizon A
At the top, made of organic material, roots, and lots of microbial activity. A.K.A Topsoil.
Soil Horizon B
Not so biological active as horizon A, due to lack of oxygen and less nutrients.
Soil Horizon C
Even less biologically active than other horizons, for the same lack of oxygen and nutrients. it is usually made up of weathered and disintegrated rocks.
Soil Horizons
Zones vary depending on the composition and location of the soil, grassland areas are more acidic with little C horizons, some mountain landscapes have no B horizon due to a lot of profile mix-up in mobile areas, preventing soil development.
How does a soil profile develop?
- Formation of parent material (e.g. weathering) - forming the C horizon.
- Soil forming processes, formation of A, B and E horizons from C.
Soil forming equation
S=f(cl,p,o,r,t) (Hans Jenny, 1941) cl = climate p = parent material o = organisms r = relief and hydrology t = time
