A tale of two cycles the short and longterm carbon cycles

Look in most university-level ecology textbooks and there is likely to be a diagram labelled 'The carbon cycle', and as most of these books are organized around a hierarchical series of entities, this planet scale system will probably be towards the back of the book. Although often described as the carbon cycle such diagrams are usually depicting only a small part of the planet's carbon cycle, often referred to as the 'short-term carbon cycle' by those with a more geological perspective on these things (e.g. Berner, 2004; Kump et al., 2004). These ecology texts usually either miss out the two-way fluxes of carbon between the rocks and the surficial system (i.e. oceans, atmosphere, life, and soils), or restrict it to considering the burning of fossil fuels. This is a reasonable approximation on the time scale of thousands of years; however, this is a tiny fraction of the time span of life on Earth. To put the role of life in its proper Earth Systems context we need to consider the 'long-term carbon cycle'. A generalized mass balance equation for the long-term cycle can be written as (Berner, 2004)

dMc/dt - Fwc + Fwg + Fmc + Fmg " Fbc " Fbg

See Table 8.1 for a definition of these terms and Fig. 8.2 for a graphical representation of these fluxes. The main fluxes in this equation which are described in most ecology texts are the movement of organic carbon into (Fbg) and out of (Fwg) organic sediments, along with the release of CO2 from volcanoes (Fmc). In the case of the peatland described in the introduction to this chapter the key factors were peat formation (a short-term version of Fbg) and the breakdown of peat sediments after drainage (Fwg).

Two key factors in the long term, or geological, carbon cycle are the fluxes of carbonate carbon (Fwc and Fbc), these are often intimately connected with the weathering of silicate minerals and are usually overlooked in descriptions of the carbon cycle in ecology textbooks. The main processes in the long-term carbon cycle can be summarized in two chemical equations (Berner, 1998; Watson, 1999). The first of these is the familiar equation for oxygenic photosynthesis (or respiration if read from right to left) described in the previous chapter

Table 8.1: Important variables in the long-term carbon cycle (following Berner, 2004).

Mc - Mass of carbon in surficial system

Fwc - Carbon flux from weathering of Ca and Mg carbonates

Fwg - Carbon flux from weathering sedimentary organic matter

Fmc - Degassing from volcanism, metamorphism, and diagenesis of carbonates

Fmg - Degassing from volcanism, metamorphism, and diagenesis of organic matter

Fbc - Burial flux of carbonate carbon in sediments

Fbg - Burial flux of organic carbon in sediments

Fwsi - Carbon flux from weathering Ca and Mg silicates due to transfer of atmospheric carbon to Ca and Mg carbonates

Fig. 8.2: Schematic representation of the long-term carbon cycle (after Berner 1998, 2004). See Table 8.1 for description of the various fluxes, those usually considered in ecology textbooks are encircled.

In this context it should be read as an equation for 'net photosynthesis', that is photosynthesis minus respiration, which is the amount of organic matter buried in sediments. The second key equation is that for the weathering of silicate minerals namely

where 'X' stands for either Ca or Mg. Read from left to right this equation symbolizes the uptake of CO2 from the atmosphere during the weathering of silicate minerals and its transfer to HCO3_; which is washed into the oceans where it is deposited as CaCO3 or MgCO3. Read from right to left the equation summarizes the breakdown of these carbonate minerals at depth within the Earth and the subsequent release of CO2 back to the atmosphere (Berner, 1998, 2004).

The carbon storage capacity of the sediments is far greater than that of the surficial system, as such on a time scale of millions of years the flux of carbon between these two systems must be in a steady state (Berner, 2004). So that dMJdt = 0

Even a minor departure from this would, over geological time, lead to colossal changes in the CO2 content of the atmosphere. In terms of the silicate weathering system described above it must be the case that (Berner, 2004)

wsi bc wc

In the context of this book it is obviously of interest to ask what, if anything, is the role of ecology in the long-term stability of this system?

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