Page 647 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 19 ecosystem essentials 611
Carbon dioxide reservoir in the atmosphere
Carbon dioxide dissolved
in the oceans
Clearing of forest for agriculture
Agricultural burning
Volcanic activity
Refineries
Wildfires
Producers • forests
Urban combustion• plants
O 2
Power plants
production
O2 production
Fossil fuel combustion and exhaust
• transportation
Consumers
• detritus feeders, decomposers
Fossil fuel mining
Factories
Consumers • animals
▲Figure 19.5 The carbon and oxygen cycles. Carbon is fixed (orange arrows) through photosynthesis, with oxygen as a by-product. respiration by living organisms, the burning of forests and grasslands, and the combustion of fossil fuels release carbon to the atmosphere (blue arrows). These cycles are greatly influenced by human activities.
constantly cycle within each ecosystem and through the biosphere.
The most abundant natural elements in living mat- ter are hydrogen (H), oxygen (O), and carbon (C). Together, these elements make up more than 99% of Earth’s biomass; in fact, all life (being composed of organic molecules) con- tains hydrogen and carbon. In addition, nitrogen (N), cal- cium (Ca), potassium (K), magnesium (Mg), sulfur (S), and phosphorus (P) are significant nutrients, elements neces- sary for the growth of a living organism.
These key elements flow through the natural world in various chemical cycles. Oxygen, carbon, and nitrogen each have gaseous cycles, parts of which take place in the atmosphere. Other major elements, including phos- phorus, calcium, potassium, and sulfur, have sedimen- tary cycles, which principally involve mineral and solid phases. Some elements cycle through both gaseous and sedimentary stages. The recycling of gases and nutrient sedimentary materials forms Earth’s biogeochemical cycles, so called because they involve chemical reactions necessary for growth and development of living systems. The chemical elements themselves recycle over and over again in life processes.
Oxygen and Carbon Cycles We consider the oxygen and carbon cycles together because they are so closely intertwined through photosynthesis and respiration (Figure 19.5). The atmosphere is the principal reservoir of available oxygen. Larger reserves of oxygen exist in Earth’s crust, but they are unavailable, being chemically bound with other elements, especially the silicate (SiO2) and carbonate (CO3) mineral families. Unoxidized re- serves of fossil fuels and sediments also contain oxygen.
The oceans are enormous pools of carbon—about 42900 billion tonnes. However, all of this carbon is bound chemically in CO2, calcium carbonate, and other compounds. The ocean initially absorbs CO2 by means of the photosynthesis achieved by phytoplankton; it becomes part of the living organisms and through them is fixed in certain carbonate minerals, such as limestone (CaCO3). The ocean water can also ab- sorb CO2 directly from the atmosphere. In Chapter 16, we discussed ocean acidification, the lowering of ocean pH caused by excessive absorption of atmo- spheric CO2 into the water. This condition of acidity makes it harder for plankton, corals, and other organ- isms to maintain calcium carbonate skeletons.