1. Life and Global Chemical Cycles
Micronutrients
Elements required in small amounts by all life or moderate amounts by some forms of life
Macronutrients
24 elements required by all organisms
Include the “big six,” which are the building blocks of life
Carbon, oxygen, hydrogen, nitrogen, phosphorus, sulfur
Each plays a special role in organisms
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2. Life and Global Chemical Cycles
For life to persist elements must be available at the right time, in the right amount, and in right concentrations relative to one another
Too much of some elements—can be toxic
Too little of some elements—can limit growth and development
Neutral—some elements are neutral for life
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3. General Aspects of Biogeochemical Cycles
Some chemicals cycle quickly and are readily regenerated for biological activity
They typically have a gas phase, are soluble and carried by the hydrologic cycle
Oxygen and nitrogen are examples
Other chemical elements are relatively immobile and returned by geological processes
Typically lack a gas phase and are insoluble
Phosphorus is an example
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4. General Aspects of Biogeochemical Cycles
Most required nutrients are light
Heaviest is iodine with atomic weight of 53
Since life evolved it has altered biogeochemical cycles
The continuation of processes that control biogeochemical cycles is essential for maintenance of life
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5. General Aspects of Biogeochemical Cycles
Through modern technology transfer rate of elements into air, water, and soil have been altered
May benefit society, but may also pose environmental hazard
Mankind must recognize the + and – consequences of altering cycles
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6. General Aspects of Biogeochemical Cycles
Simplest way to view a cycle is a box-and- arrow diagram
Boxes represent places where a chemical is stored
Donating compartment is a source
Receiving compartment is a sink
Amount of time an atom spends in any compartment is called its residence time
Arrows represent pathways of transfer
Flow is the amount moving from one box to another
Flux is the rate of transfer
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The Hydrologic Cycle
The transfer of water from oceans to the atmosphere to the land and back to the oceans
Driven by solar energy
Evaporation of water from oceans
Precipitation of water on land
Transpiration of water by plants
Evaporation of water from land
Runoff from streams, rivers and subsurface groundwater
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The Hydrologic Cycle
Total water on earth = 1.3 billion km3
97% in oceans
2% in glaciers and ice caps
0.76% is shallow groundwater
0.013% in lakes and rivers
0.001% in atmosphere
The rest in fresh water on land
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The Hydrologic Cycle
At the regional and local level, the fundamental unit of the landscape is the drainage basin, a.k.a. watershed or catchment
The area that contributes surface runoff to a particular stream or river
Vary greatly in size
Usually named for main stream or river
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The Carbon Cycle
Carbon is the element that anchors all organic substances
Carbon has a gaseous phase
Enters atmosphere (CO2 and CH4) through respiration, fires and diffusion
Removed from the atmosphere by photosynthesis
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The Carbon Cycle
Carbon occurs in the ocean in several forms
Dissolved CO2, carbonate and bicarbonate
Marine organisms and their products, CaCO3
Enters the ocean by
Simple diffusion then dissolves
Transfer from land in rivers as dissolved carbon
Wind
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The Carbon Cycle
Carbon enters the biota through photosynthesis and then returned by respiration or fire
When organisms die, decomposition of their remains releases carbon
If buried under certain conditions, carbon is not released
Transformed into fossil fuels
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17. The Carbon-Silicate Cycle
The cycling of carbon intimately involved with the cycling of silicon
Weak carbonic acid falls as rain and weathers silicate rich rocks
Releases Ca2+ and HCO3-
Transferred to oceans and used by marine animals to construct shells
Shells deposited on sea floor become part of sedimentary rock layer and return to surface in subduction zones
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18. The Carbon-Silicate Cycle
Affects the levels of CO2 and O2 in the atmosphere
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The Nitrogen Cycle
N essential to life because it is necessary for the production of proteins and DNA
Free N2 makes up 78% of atmosphere
But most organisms can’t use it directly
Relatively unreactive element must be converted to NO3- or NH4+
Performed by bacteria
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The Nitrogen Cycle
Nitrogen fixation—process of converting atmospheric N to NO3- or NH4+
Denitrification—process of releasing fixed N back to molecular N
Almost all organisms depend on nitrogen-converting bacteria
Some have formed symbiotic relationships in the roots of plants or stomach in animals
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The Nitrogen Cycle
Industrial process can now convert molecular N into compounds usable by plants
Main component of N fertilizers
N in agricultural runoff potential source of water pollution
N combines with O at high temperatures
Oxides of N are a source of air pollution
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The Phosphorus Cycle
P one of the “big six” required for life
Often a limiting factor for plant and algae growth
Does not have a gaseous phase
Rate of transfer slow
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The Phosphorus Cycle
Enters biota through uptake as phosphate by plants, algae and some bacteria
Returns to soil when plants die or is lost to oceans via runoff
Returned to land via ocean-feeding birds’ excrement (guano)
Guano deposits major source of P for fertilizers
[Insert FIGRE 7.21a Guano Island, Peru.]
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The Phosphorus Cycle
Phosphorous mining creates environmental problems
Overabundance of phosphorous in runoff causes pollution problems
Unwanted growth of photosynthetic bacteria & algae
Oceanic dumping of organic materials high in N & P has produced several hundred “dead zones”
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The Phosphorus Cycle
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Chapter Summary
Biogeochemical cycles tend to be complex
Earth’s biota has greatly altered the cycling of chemicals through the air, water, and soil
Continuation of these processes is essential to the long-term maintenance of life on Earth
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Chapter Summary
There are many uncertainties
In measuring the amount of a chemical in storage
In determining the rate of its transfer between reservoirs
Understanding biogeochemical cycles more completely will help us more fully understand our environment and how to properly manage its resources for now and the future
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