Environmental Engineering Reference
In-Depth Information
weathering is a major control of atmospheric CO 2 on long (e.g., millions of years) time-
scales ( Berner and Kothavala 2001 ). The combination of weathering and reverse weather-
ing of alumino-silicate minerals is a major way that the silica and C cycles intersect
( Conley 2002 ). Another way that silica affects the global C balance is through its effect on
primary production in the ocean ( Dugdale and Wilkerson 1998 ).
CARBONATE PRECIPITATION
When carbonates are formed, either biotically or abiotically (the reverse of Eqs. 6.4 and
6.5 ), CO 2 is released. These reactions take place mostly in the ocean, but also in some
freshwaters. The building of coral reefs is basically this same process and is a source of
CO 2 to the water column. Over time there can be chemical changes in the cations associ-
ated with anions from a domination of Ca 11 to a mixture of Ca 11 and Mg 11 . The replace-
ment of Ca 11 by Mg 11 causes limestones to become dolomites, which have significantly
different biogeochemical properties and different uses by humans. Dolomite is harder
than calcite limestone and weathers more slowly. Hence, dolomite is preferred over lime-
stone in many construction applications.
In summary, C is found in both organic and inorganic forms and is involved in all
aspects of photosynthesis and both aerobic and anaerobic respiration. C is also controlled
by a number of abiotic reactions including chemical precipitation and weathering.
The inorganic C system is the major buffer in the sea and in most freshwaters; dissolved
CO 2 is an important acid in weathering reactions.
THE PRESENT-DAY GLOBAL CARBON CYCLE
AND THE GREENHOUSE EFFECT
You probably know quite a bit about the modern global C balance because of its con-
nection to climate change. As we pointed out in the beginning of this chapter, the global C
cycle is most often looked at from the point of view of Earth's atmosphere as the response.
Because CO 2 is a greenhouse gas (GHG), its rising concentration in the atmosphere traps
heat and leads to global warming and other interrelated climatic changes ( Houghton and
Woodwell 1989 ). Concern about GHGs has led to a great deal of research about what actu-
ally controls atmospheric CO 2 ( IPCC 2007 ). We will not discuss climate change in this
chapter but we will look in detail about the global C cycle from the perspective of atmo-
spheric CO 2 . Figure 6.2(a) shows the concentration of CO 2 measured on a high mountain
in Hawaii, Mauna Loa ( Keeling et al. 1995; Tans and Keeling 2012 ). This site provides a
reasonably accurate sample of the atmosphere of the Northern Hemisphere. Two impor-
tant patterns are apparent in the time trend: mean atmospheric CO 2 has increased by
about 30% since measurements began in 1958, and there is an annual cycle in CO 2 concen-
trations. The seasonal oscillation is caused by the seasonal variation in the balance between
photosynthesis and respiration in the Northern Hemisphere. During the growing season
the net uptake of CO 2 by photosynthesis is larger than the net release by respiration; in
the winter the reverse is true. Thus, CO 2 is higher in the winter than in the summer. This
oscillation in atmospheric CO 2 is a profound example of the influence of biology on ele-
mental cycles. The long-term increasing trend is the result of anthropogenic activities,
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