A Hutton club seminar by Dr. Heather Buss from the University of Bristol.
Note: This blog has a similar premise to the rock eating fungi blog – but, this blog looks at weathering and carbon but has a stronger focus on critical weathering zones.
What was the seminar about and why is it important?
Carbon dioxide in the atmosphere and rainwater mix to form carbonic acid. This acid, when it falls to the ground aids silicate weathering of rocks. Weathering releases ions which are then transported by water to the ocean. This flux of ions from the land can result in the formation of carbonate rocks in the ocean. The more weathering on land, the more reactions that take place = faster weathering. This causes more CO₂ draw-down and carbonate rock formation.
The seminar wanted to understand the controls on chemical weathering rates as they are a major uncertainty. For example, what is the rate limiting factor of chemical weathering? There are so many factors at play and the seminar focused on unpicking the key players. Knowing the key factors that aid weathering could be useful for trying to increase carbon draw-down from the atmosphere, therefore trying to reduce the impact of global warming.
What is the critical zone?
Water is a very important part of the critical zone as it flushes away products of weathering and is able to dissolve more minerals.
What is a weathering front?
The weathering front is not uniform and can be different for different topographies, climates and lithologies etc.
Factors that affect weathering:
Water
Organisms e.g. plants, microorganisms etc
Organic acids
Trees (reduce erosion)
Topography
Lithology e.g. what minerals are present in a rock and how porous is the rock
All these factors are occurring in the critical zone simultaneously, but it is hard to know which, or all play the largest and most important role in aiding weathering.
Controls on weathering fluxes:
Depends on the reaction mechanism
We can predict the weathering rate if we know the reaction mechanism
There can be multiple paths for one reaction
The slowest part of a reaction is the rate limiting step
We want to understand the path of the reaction in the field and the rate limiting step
We can’t predict weathering for the whole globe as weathering changes significantly from front to front.
What techniques were used?
Core data:
Dr Buss looked at several weathering fronts from granitic basement rocks from different climatic regions. Granite was used because it gave the most complete cores and profiles. This was done by taking cores from weathering fronts and identifying loss of ions like calcium (which indicate weathering as weathering releases ions):
In the Czech Republic there was a 100% loss of calcium from the top 20 metres, placing the front there
In Puerto Rico, the most calcium was lost from the first 70 cm of the soil
Tropical regions have sharper weathering fronts (e.g. they occur at shallower depths)
In the USA, the average front is located at 5 metres depth
A range of climatic zones were recovered
The chemical index of alteration (this is just a calculation of aluminium to proportion of other cations to work out the degree of weathering) was used to calculate the weathering gradient of the front (this shows the steepness of the front)
Granite imaging:
Reactive transport modelling (RTM) was used to test the impact of changing parameters on a granitic bedrock-based critical zone. The zone created would be exposed to different temperatures and water conditions over a simulated 200,000-year period to see how this would affect the weathering front. The simulation tried to work out how much albite (a mineral) there was in the profile with depth.
Main Results
Core data:
Precipitation had the greatest effect on weathering in the studied profiles
As precipitation increases so too did weathering intensity
More weathering occurred with more rainfall
Sharper weathering fronts occur where there is more rainfall (e.g. more sudden, shallower fronts)
Depth of the profile decreases (shallows) with more rainfall
Cooler sites have thicker weathering fronts (are deeper)
Precipitation has a stronger effect on gradient steepness than temperature
Granite imaging:
Weathering at the surface of imaged profiles had a stronger response to water flow than to temperature
In arid environments, only shallow weathering occurred
Arid environments also had slower rates of weathering that was only fast near the profile surface
In wet environments, there was more flushing of nutrients by water which encouraged more weathering
In cooler and more humid environments there are slower weathering reactions but thicker reaction fronts
Temperature had a strong influence on weathering rates in humid and high water flow conditions, the opposite of this was true for cold temperatures
The simulations concluded that precipitation was the primary limiting factor
Biology and weathering
Biology (plants and microorganisms etc) do have major controls on weathering rates:
Respiration by microorganisms in the soil produces CO₂
Plants produce O₂
Organisms can bioturbate the critical zone
Plants can retain water
Roots can stabilise hillslopes
Biology can speed up weathering, but what is the role of biology for weathering rates if most of the weathering occurs at depth in the soil?
At 8 metres depth in the soil (e.g. of a deep weathering front):
There is less oxygen
There is more CO₂
Organisms can slow oxidation
They can speed up weathering rates with dissolution of CO₂ at depth
More organic acids produced at depth slows weathering and export production
These organic acids facilitate fast weathering at the surface and shallow depths
How did the seminar advance our knowledge?
Main conclusions:
Weathering occurs quite deep in the critical zone (on average)
Deeper weathering fronts occur in cooler environments
Shallow weathering fronts occur in arid and wet environments
Precipitation is the main primary limiting factor controlling the rate of weathering
Biology can aid weathering rates at the surface and slow weathering at depth
The seminar highlighted the importance of understanding natural weathering rates and how they can affect the carbon cycle. Precipitation was highlighted as the key influence on weathering rates and so maybe Dr. Buss will be able to narrow her study to research this key player. In time, maybe we can aid weathering rates to help reduce carbon dioxide or even just understand the natural ways that the earth combats excess carbon.
If you'd like to read more about Dr. Buss' work then please follow this link:https://scholar.google.com/citations?user=Q0Jjm7QAAAAJ&hl=en
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