Earth.Parts #24 - Organic carbon cycling over yearly to decadal time scales
Channel: Earth.Parts
Date: 2017-03-31

Earth's carbon cycle operates in different ways over short and long term timescales. Over timescales of years to decades the dominant global processes governing the amount of CO2 in the atmosphere are world photosynthesis by plants and algae, and respiration by consumer-organisms of organic compounds back to CO2 in air. 

Global air CO2 concentrations are broadly increasing as a result of the burning of fossil fuels by human civilization, but superimposed on that modern trend is a very old trend of yearly ups and downs in the concentration of CO2 in air. Our planet's forests are mostly in the northern hemisphere, and their springtime & summer intake of CO2 drops air concentrations by up to 5 or 6 ppm, and CO2 release during autumn & winter by decomposition of dead plant biomass (e.g. leaves) raises CO2 back up by about 5 or 6 ppm. It happens every year, and has for a geologically long time.

Global reservoir box-models are useful in figuring out how cyclic processes like the carbon cycle operate. The atmosphere contains about 760 Gigatons (Gt) of carbon (C) and every year photosynthesis draws about 60 Gt of C out of the air. About half that amount of plant biomass is directly consumed by animals and microbes, which directly respire that carbon back to air as CO2. The other half of annual world plant primary-productivity, approximately, is deposited in soil and decomposed back to air as CO2. Soils and young marine sediments are a large reservoir of C, about 1,600 Gt, and organic carbon that enters this reservoir and is not immediately decomposed back to CO2 may spend centuries or longer in that form. 

In the oceans the situation is different because there are no oceanic trees, there are no long-living giant organisms that sequester lots of C. Most primary productivity in the oceans is accomplished by algae and other forms of phytoplankton, which are single-celled or multicellular microscopic life forms. 

Primary productivity in the oceans is focused at the top, in the photic zone where light can penetrate, roughly 100 m depth. Life grows there but as individual planktonic life forms die they fall down slowly through the deeper, darker ocean depths. As they fall they are almost completely decomposed by bacteria or eaten by zooplankton - tiny floating animals. Only about 1% of biomass that sifts down through the ocean water as "marine snow", dead plankton, finds its way into longer-term storage in sediment on the seafloor. Most falling biomass is converted back to CO2 dissolved in ocean water. 

The overall process of falling dead plankton decomposing on the way down is called the Marine Biological Pump, which has the overall effect of exporting C and nutrients from the shallow ocean and delivering them by gravity & decomposition to the deep ocean. The Marine Biological Pump depletes surface ocean waters of C & nutrients, and enriches the deep ocean in dissolved CO2 and nutrients derived from planktonic carcasses.

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