Nutrient Cycles or Geochemical Cycles
Sunday, 11. January 2009, 19:21:18
It was my intention to write something today about nitrogen fixation in oceans and ocean nutrient cycling. As many of my readers are not earth scientists, and cycles in general are fundamental in many earth science disciplines, I shall however first write a few basic paragraphs on cycles in general. I am talking about cycles like the carbon cycle, the nitrogen cycle, the sulphur cycle and the cycles of other individual chemical elements like magnesium, cadmium, lead, mercury, zinc, cadmium, phosphorous ...
Such cycles are known as geochemical cycles, or biogeochemical cycles, or nutrient cycles. Each geochemical cycle is a model that describe the flow of a chemical element between land mass, ocean and atmosphere (and plants and animals). They are often shown as box model. Here is a general model of a geochemical cycle.
You may also see hydrosphere or water instead of ocean, and lithosphere instead of land, and air instead of atmosphere. And you can have many more boxes to give more details. The chemicals are sometimes held for long periods of time in one place. Such a place is called a reservoir. We may also talk about source, especially where the reservoir in decreasing in size, and sink, especially where the reservoir is increasing in size.
The pathways in the model above will be precipitation (rain, hail, snow), dust and other aerosols like sea spray from atmosphere to land and to ocean. Dust and degassing from land to atmosphere. Sea spray and gas evolution from ocean to atmosphere. Dissolved and suspended material transported by rivers to the ocean. Sedimentation from ocean to sediments. Uplift of sediments to land.
The vast majority of material near the Earth’s surface stays in the crust, the ocean, and the atmosphere. There appears to be little loss either to the mantle or to outer space. The material is continuously recycled. We talk about flux. The flux is the amount of material passing via a particular transport pathway in a fixed period of time - e.g. 7.4 gigatons per year. (A gigaton is 1 billion tons or 1,000,000 tons.
Geochemical cycles can also be illustrated by more descriptive diagrams, like the one I used in my post on the Oceanic Nitrogen Cycle for the nitrogen cycle:
The cow represents animals as a reservoir. This diagram is lacking in detail, and things like water and volcanoes (degassing) are missing.
Here is a more worked out model of the carbon cycle from UNEP - indeed with fluxes (called exchanges in the model) measured in gigatons.
For larger image - click here
The majority of carbon is found in rocks, either as carbonate, usually associated with calcium in limestones (as CaCO3), or as dispersed organic carbon, in sedimentary rocks, particularly shales. About three quarters of the total carbon in the outer regions of the Earth (the regions represented in geochemical cycle models) is contained in carbonates containing inorganic carbon and one quarter in rocks containing dispersed organic compounds. The combined carbon content of all other carbon reservoirs (atmosphere, plants and animals, soil, fossil fuels, dissolved compounds) comes to less than 1% of the total.
I hope that will do for an introduction.
Such cycles are known as geochemical cycles, or biogeochemical cycles, or nutrient cycles. Each geochemical cycle is a model that describe the flow of a chemical element between land mass, ocean and atmosphere (and plants and animals). They are often shown as box model. Here is a general model of a geochemical cycle.
You may also see hydrosphere or water instead of ocean, and lithosphere instead of land, and air instead of atmosphere. And you can have many more boxes to give more details. The chemicals are sometimes held for long periods of time in one place. Such a place is called a reservoir. We may also talk about source, especially where the reservoir in decreasing in size, and sink, especially where the reservoir is increasing in size.
The pathways in the model above will be precipitation (rain, hail, snow), dust and other aerosols like sea spray from atmosphere to land and to ocean. Dust and degassing from land to atmosphere. Sea spray and gas evolution from ocean to atmosphere. Dissolved and suspended material transported by rivers to the ocean. Sedimentation from ocean to sediments. Uplift of sediments to land.
The vast majority of material near the Earth’s surface stays in the crust, the ocean, and the atmosphere. There appears to be little loss either to the mantle or to outer space. The material is continuously recycled. We talk about flux. The flux is the amount of material passing via a particular transport pathway in a fixed period of time - e.g. 7.4 gigatons per year. (A gigaton is 1 billion tons or 1,000,000 tons.
Geochemical cycles can also be illustrated by more descriptive diagrams, like the one I used in my post on the Oceanic Nitrogen Cycle for the nitrogen cycle:
The cow represents animals as a reservoir. This diagram is lacking in detail, and things like water and volcanoes (degassing) are missing.
Here is a more worked out model of the carbon cycle from UNEP - indeed with fluxes (called exchanges in the model) measured in gigatons.
For larger image - click here
The majority of carbon is found in rocks, either as carbonate, usually associated with calcium in limestones (as CaCO3), or as dispersed organic carbon, in sedimentary rocks, particularly shales. About three quarters of the total carbon in the outer regions of the Earth (the regions represented in geochemical cycle models) is contained in carbonates containing inorganic carbon and one quarter in rocks containing dispersed organic compounds. The combined carbon content of all other carbon reservoirs (atmosphere, plants and animals, soil, fossil fuels, dissolved compounds) comes to less than 1% of the total.
I hope that will do for an introduction.
WIDGETIZE