Introduction to climate dynamics and climate modelling

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2.3.4 Geological reservoirs

The majority of the organic carbon that is exported downward from the surface layer is remineralised in the water column. In particular, the ocean is undersaturated with respect to calcite (aragonite) below 4500m (3000m) in the Atlantic and below 800m (600m) in the Pacific. As a consequence, the long-term burial of CaCO₃ in the sediments to produce limestone mainly occurs in shallow seas (for instance in coral reefs). Averaged over the whole ocean, this long term burial corresponds to 13% of the export of CaCO₃ out of the surface layer. On short timescales, this is a small fraction of the whole carbon cycle, but it becomes a crucial component on timescales longer than a century. An even smaller percentage of the organic carbon is stored in the form of natural gas, oil and coal.

Because the sea floor spreads due to plate tectonics, sediments are transported horizontally and are eventually incorporated within the mantle through subduction along plate boundaries. At higher temperatures and pressure, limestone is transformed during subduction into calcium-silicate rocks (this is called metamorphism) by the reaction:

C a C O_{3} + S i O_{2} \to CaSi O_{3} + C O_{2}

(2.48)

The CO₂ that is released in this reaction can return to the atmosphere, in particular through volcanic eruptions.

The plate motion also allows the calcium-silicate rocks to be uplifted to the continental surface, where they are affected by physical and chemical weathering. In particular, the carbonic acid contained in rain water (in the same process as reaction 2.39) could react with the calcium-silicate rocks:

CaSi O_{3} + H_{2} C O_{3} \to C a C O_{3} + S i O_{2} + H_{2} O

(2.49)

The products of this reaction are transported by rivers to the sea where they can compensate for the net export of CaCO₃ by sedimentation. Weathering thus tends to reduce atmospheric CO₂ by taking up carbonic acid to make CaCO₃ and increasing ocean alkalinity while metamorphism and sedimentation tend to increase atmospheric CO₂. Overall sedimentation, subduction, and metamorphism and weathering form a closed loop that takes place over millions of years, and is sometimes referred to as the long term inorganic carbon cycle (Fig. 2.27).

**Figure 2.27:** Long term inorganic carbon cycle through sedimentation, subduction, and metamorphism and weathering. Figure from Skinner et al. (2004). Copyright 2004 John Wiley & Sons, Inc. Reproduced with permission.