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I am trying to figure out the difference between a carbon sink and a carbon reservoir.

The definition for a carbon sink from wikipedia is that “carbon sink is a natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period. ” And the definition for carbon reservoir is “ Carbon-storing natural feature (such as a forest or the land mass) that exchanges carbon with other reservoirs."

My textbook says that mature forests are carbon reservoirs but they are not carbon sinks. So what is the difference between a carbon sink and a carbon reservoir on earth?

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The difference is that a carbon sink accumulates carbon, whereas a carbon reservoir has accumulated carbon.

That is to say:

A carbon sink is an ongoing process which is increasing the amount of carbon stored in it.

Whereas although a carbon reservoir might exchange individual carbon-based molecules with other parts of the carbon cycle, as much will go out as goes in, keeping the amount of carbon in the reservoir broadly constant.

Both of these contrast with things such as coal seams and natural gas & crude oil reservoirs that humans have tapped as sources of fuel; and leakages of methane from the ground: these were carbon reservoirs until the amount of carbon in them began depleting (e.g. through mining, well-drilling, or - in the case of methane clathrate leakages - ice melt), at which point they became carbon sources.

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    $\begingroup$ Would if be fair to say that coal seams & crude oil deposits are carbon reservoirs, whereas, an active peat bog is a carbon sink? $\endgroup$ – Fred Jun 21 '15 at 12:37
  • $\begingroup$ @Fred updated to cover coal, oil & gas deposits $\endgroup$ – EnergyNumbers Jun 23 '15 at 8:06
  • $\begingroup$ Those last two paras aren't quite right. The phrase "carbon reservoir" doesn't imply anything about the change (or lack of it) with time, just that it's an identifiable part of the system that has some carbon in it. Reservoirs that are net sinks or sources over some period are still reservoirs. $\endgroup$ – Deditos Jun 23 '15 at 21:12
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A carbon reservoir is measured by the amount of carbon it contains, for example kilograms of carbon [kg C]. A carbon source or sink is measured by the amount that comes out or into the reservoir from the rest of the system per unit of time, for example kilograms per day [kg C/day]. A coal or oil deposit can be either a reservoir or source, as carbon flows out (it's a source as product is extracted) the size of the reservoir decreases.

Because carbon (or any element) is a conserved quantity, if your peat bog is growing larger, then it may be a net sink for carbon. But if is losing carbon to the atmosphere (say as methane or if it is a burning peat bog) as deposition of organic material continues, the reservoir may not be growing and the bog could be a net source of carbon to the rest of the system. (I really don't know how real peat deposits behave.)

In mathematical transport-models, you can specify sources and sinks without specifying a reservoir, but in actual systems there has to be reservoirs to provide or take-up whatever element is being transported.

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