The GEOCARB model

As stated, the exchanges of carbon between the surficial reservoir and the carbonate and organic reservoirs are used to model the carbon cycle. One such model is GEOCARB. Versions of this model have been published by Robert Berner and his colleagues in numerous papers (eg Berner and Kothavala 2001). The code for the version shown below was kindly donated by Robert Berner in October 2007.

Plants are involved in the GEOCARB models in three ways:
  1. There is a weathering effectiveness function which changes as a function of plant evolution. This function is used to scale the carbon fluxes, which means it multiplies them. It is normalised to the preindustrial present, so that today's weathering effectiveness is 1.0. Smaller values express the assumed weathering effectiveness of an entirely herbaceous or lichen-dominated flora (as in the early Palaeozoic), and forests dominated by gymnosperms. There are transition periods during which these values are linearly interpolated. These are currently 380-350 Ma (million years ago) for the advent of forests, and 130-80 Ma for the spread of angiosperms.
  2. The biological weathering feedback function expresses the extent to which CO2 values are suppressed in the atmosphere as a result of weathering (ie it is a negative feedback). In GEOCARB, the feedback due to forests is assumed to vary according to the carboxylase action of Rubisco, under the assumption that the productivity of only 35% of the world's plants are limited by atmospheric CO2. Before forests, in the early Palaeozoic, a compromise function is assumed between abiotic CO2-limited weathering and possible microbial weathering (Berner 1992). The transition period is the same as that for the advent of forests for the weathering effectiveness function.
  3. Plants also contribute material to the organic reservoir, eg coal. This is not part of my project.
Part of my project is to improve the representation of plant weathering in models such as GEOCARB. To do this, I will be using the results of experiments conducted by other members of our team and attempting to model the weathering chemistry.

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