Biogeochemical Cycles
Carbon Cycle

• Elemental Cycles
• H, O, and C make up more than 99% of the Earth's biomass
• all life contains H and C
• N, Ca, K, Mg, S and P are significant nutrients (elements necessary for the growth of a living organism)



• the elements cycle through organisms, they enter the atmosphere, the oceans, and even rocks. Since these chemicals cycle through both the biological and the geological world, we call the overall cycles biogeochemical cycles.

• the recycling processes form Earth's biogeochemical cycles involve chemical reactions in both living and nonliving systems

• Concepts of Biogeochemical Cycles

• reservoirs
• those parts of the cycle where the chemical is held in large quantities for long periods of time



• exchange pools
• the chemical is held for only a short time
• residence time: length of time a chemical is held in an exchange pool



• Examples
• The oceans are a reservoir for water, while a cloud is an exchange pool



• The biotic community includes all living organisms. This community may serve as an exchange pool (although for some chemicals like carbon, bound in a sequoia for a thousand years, it may seem more like a reservoir), and also serve to move chemicals from one stage of the cycle to another.



• Carbon
• carbon is found in all four spheres or reservoirs
• biosphere (organic matter)
• atmosphere (CO₂, CH₄)
• hydrosphere (H₂CO₃, HCO₃^-, CO₃⁼)
• lithosphere (CaCO₃, coal, oil, and gas)



• the key events here are the complementary reactions of respiration and photosynthesis
• respiration takes sugar and oxygen and combines them to produce carbon dioxide, water, and energy



• Photosynthesis takes carbon dioxide and water and produces sugars and oxygen



• The outputs of respiration are the inputs of photosynthesis, and the outputs of photosynthesis are the inputs of respiration. The reactions are also complementary in the way they deal with energy. Photosynthesis takes energy from the sun and stores it in the carbon-carbon bonds of carbohydrates; respiration releases that energy

• carbon comprises 50% of all living tissues



• carbon dioxide (CO2) is necessary for all plant growth



• CO₂ also helps to insulate the Earth
• the amount of CO2 in the atmosphere has varied through time



• Short-Term Cycling: Photosynthesis/Respiration
• where?
• photosynthesis turns inorganic carbon (CO₂) into organic carbon within the tissues of photoautotrophs



• 200 billion tons (26%) of CO₂ from the atmosphere during photosynthesis



• most CO₂ is returned to the atmosphere during respiration and decay of plants



• slightly more CO₂ is removed by terrestrial ecosystems than by aquatic ecosystems (why?)



• Long-Term Cycling: CaCO₃-SiO₂
• related to plate tectonics



• includes reservoirs of land, oceans, and limestone (CaCO₃)
• skeletal remains of marine organisms



• most of the carbon near the Earth's surface is in the form of these rocks



• the role of weathering of igneous rocks (CaSiO₃)



• the weathering products are transported to the oceans where they are incorporated into the skeletons of benthic organisms



• when the marine animals die, their remains settle to the seafloor, taking the carbon stored in their bodies with them



• some of the material is decomposed and released back to the water



• some of the material becomes part of the sediment on the ocean floors



• plate tectonics carries to subduction zones where it may ultimately return to atmosphere (how?)

• it all starts over again!!!

• atmospheric CO₂ through time



• Medium-Term Cycling
• involves organic matter and atmospheric oxygen
• coal, oil, gas



• starts with short-term cycling (removes CO₂ from the atmosphere and incorporates it into plants)



• the portion of CO₂ that is not returned to the atmosphere during decay is then stored in sediments
• coal
• peat -> bituminous coal -> anthracite coal



• oil and gas
• during burial of these sediments



• the oxygen that would be used to decay organic matter stays in the atmosphere



• this CO₂ has been removed from the atmosphere, but it can be returned
• when organic matter is burned it releases CO2 back to the atmosphere and oxygen is consumed (why?)



• the cycle is complete!!



• Five primary fluxes

	1. Photosynthesis
	2. Detritus decomposition
	3. Ocean cycling
	4. Fossil fuel combustion
	5. Deforestation

• 1-3 are primarily natural, 4-5 are primarily anthropogenic