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Life and Global Chemical Cycles
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Micronutrients
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Elements required in small amounts by all life
or moderate amounts by some forms of life
Macronutrients
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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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Copyright © 2014 by John Wiley & Sons, Inc.
Life and Global Chemical Cycles
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For life to persist elements must be
available at the right time, in the right
amount, and in right concentrations
relative to one another
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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
Copyright © 2014 by John Wiley & Sons, Inc.
General Aspects of Biogeochemical
Cycles
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Some chemicals cycle quickly and are
readily regenerated for biological activity
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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
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Typically lack a gas phase and are insoluble
Phosphorus is an example
Copyright © 2014 by John Wiley & Sons, Inc.
General Aspects of Biogeochemical
Cycles
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Most required nutrients are light
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Heaviest is iodine with atomic weight of 53
Since life evolved it has altered
biogeochemical cycles
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The continuation of processes that control
biogeochemical cycles is essential for
maintenance of life
Copyright © 2014 by John Wiley & Sons, Inc.
General Aspects of Biogeochemical
Cycles
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Through modern technology transfer rate
of elements into air, water, and soil have
been altered
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May benefit society, but may also pose
environmental hazard
Mankind must recognize the + and –
consequences of altering cycles
Copyright © 2014 by John Wiley & Sons, Inc.
General Aspects of Biogeochemical
Cycles
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Simplest way to view a cycle is a box-andarrow diagram
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Boxes represent places where a chemical is stored
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Donating compartment is a source
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Receiving compartment is a sink
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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
Copyright © 2014 by John Wiley & Sons, Inc.
Copyright © 2014 by John Wiley & Sons, Inc.
The Hydrologic Cycle
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The transfer of water from oceans to the
atmosphere to the land and back to the
oceans
Driven by solar energy
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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
Copyright © 2014 by John Wiley & Sons, Inc.
The Hydrologic Cycle
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Total water on earth = 1.3 billion km3
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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
Copyright © 2014 by John Wiley & Sons, Inc.
The Hydrologic Cycle
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At the regional and local level, the
fundamental unit of the landscape is the
drainage basin, a.k.a. watershed or
catchment
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The area that contributes surface runoff to a
particular stream or river
Vary greatly in size
Usually named for main stream or river
Copyright © 2014 by John Wiley & Sons, Inc.
Copyright © 2014 by John Wiley & Sons, Inc.
The Carbon Cycle
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Carbon is the element that anchors all
organic substances
Carbon has a gaseous phase
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Enters atmosphere (CO2 and CH4) through
respiration, fires and diffusion
Removed from the atmosphere by
photosynthesis
Copyright © 2014 by John Wiley & Sons, Inc.
The Carbon Cycle
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Carbon occurs in the ocean in several
forms
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Dissolved CO2, carbonate and bicarbonate
Marine organisms and their products, CaCO3
Enters the ocean by
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Simple diffusion then dissolves
Transfer from land in rivers as dissolved
carbon
Wind
Copyright © 2014 by John Wiley & Sons, Inc.
The Carbon Cycle
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Carbon enters the biota through
photosynthesis and then returned by
respiration or fire
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When organisms die, decomposition of their
remains releases carbon
If buried under certain conditions, carbon is
not released
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Transformed into fossil fuels
Copyright © 2014 by John Wiley & Sons, Inc.
Copyright © 2014 by John Wiley & Sons, Inc.
Copyright © 2014 by John Wiley & Sons, Inc.
The Carbon-Silicate Cycle
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The cycling of carbon intimately involved with
the cycling of silicon
Weak carbonic acid falls as rain and weathers
silicate rich rocks
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Releases Ca2+ and HCO3Transferred 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
Copyright © 2014 by John Wiley & Sons, Inc.
The Carbon-Silicate Cycle
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Affects the levels of CO2 and O2 in the
atmosphere
Copyright © 2014 by John Wiley & Sons, Inc.
Copyright © 2014 by John Wiley & Sons, Inc.
The Nitrogen Cycle
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N essential to life because it is necessary
for the production of proteins and DNA
Free N2 makes up 78% of atmosphere
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But most organisms can’t use it directly
Relatively unreactive element must be
converted to NO3- or NH4+
Performed by bacteria
Copyright © 2014 by John Wiley & Sons, Inc.
The Nitrogen Cycle
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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
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Some have formed symbiotic relationships in
the roots of plants or stomach in animals
Copyright © 2014 by John Wiley & Sons, Inc.
The Nitrogen Cycle
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Industrial process can now convert
molecular N into compounds usable by
plants
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Main component of N fertilizers
N in agricultural runoff potential source of
water pollution
N combines with O at high temperatures
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Oxides of N are a source of air pollution
Copyright © 2014 by John Wiley & Sons, Inc.
Copyright © 2014 by John Wiley & Sons, Inc.
The Phosphorus Cycle
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P one of the “big six” required for life
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Often a limiting factor for plant and algae
growth
Does not have a gaseous phase
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Rate of transfer slow
Copyright © 2014 by John Wiley & Sons, Inc.
The Phosphorus Cycle
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Enters biota through
uptake as phosphate
by plants, algae and
some bacteria
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Returns to soil when
plants die or is lost to
oceans via runoff
Returned to land via
ocean-feeding birds’
excrement (guano)
[Insert FIGRE 7.21a Guano Island, Peru.]
Guano deposits major
source of P for
fertilizers
Copyright © 2014 by John Wiley & Sons, Inc.
The Phosphorus Cycle
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Phosphorous mining creates
environmental problems
Overabundance of phosphorous in runoff
causes pollution problems
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Unwanted growth of photosynthetic bacteria
& algae
Oceanic dumping of organic materials high in
N & P has produced several hundred “dead
zones”
Copyright © 2014 by John Wiley & Sons, Inc.
The Phosphorus Cycle
Copyright © 2014 by John Wiley & Sons, Inc.
Copyright © 2014 by John Wiley & Sons, Inc.
Chapter Summary
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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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Copyright © 2014 by John Wiley & Sons, Inc.
Chapter Summary
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There are many uncertainties
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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
Copyright © 2014 by John Wiley & Sons, Inc.