Carbon model worksheet 4

A schematic of the organic and inorganic components of the modelled carbon cycle.

http://www.noc.soton.ac.uk/jmodels/images/wiki/cmodel.jpg
The aim of this carbon model worksheet is to analyse some of the properties of the carbon model.

Worksheet 4: Carbonate compensation

Carbonate compensation is the negative feedback process which regulates carbonate ion concentration [CO32-] (or, more strictly, the degree of supersaturation with respect to CaCO3, Ω) in the oceans. When [CO32-] gets too high then more CaCO3 is buried, lowering [CO32-]; conversely when [CO32-] falls too low, less CaCO3 is buried, allowing river delivery to raise [CO32-]. The aim of this practical is to allow you to carry out numerical experiments on carbonate compensation, within an ocean carbon cycle model containing a dynamic lysocline. Proceed as far as you can within the time available.

1. Try some different initial conditions (initial values of [CO32-]), to be produced as follows (keep all other variables at default values): (a) deep [DIC] = 2.20 Mol m-3; (b) deep [DIC] = 2.15 Mol m-3; (c) deep [DIC] = 2.30 Mol m-3; (d) deep [DIC] = 2.40 Mol m-3. What is the initial deep [CO32-] in each case? Does the model converge in each case? (i.e. does it homeostatically regulate [CO32-]?) Is the final value of [CO32-] always the same? [remember to reset initial conditions before making each change, so previous changes are wiped clear]
2. The mechanism for carbonate compensation is supposed to involve changes in the lysocline depth in response to changes in [CO32-]. What, if any, changes do you see immediately after (a) doubling [CO32-]; (b) halving [CO32-]? [as produced, more or less, by the DIC changes for (b) and (d) in 1. above]
3. The return to the equilibrium state is governed by the balance between river inputs and burial outputs of CaCO3 and POC. Examine the size of the CaCO3 burial flux (a) immediately after doubling of deep [CO32-]; and (b) immediately after halving of deep [CO32-]. Do they agree with the schematic overleaf?
4. In the literature (see references below) the carbonate compensation time is suggested to be somewhere between 6 and 14 thousand years, and so is relatively rapid from a geological perspective. Calculate the approximate compensation times (times to return to steady-state [CO32-]) following (a) doubling of [CO32-]; (b) halving of [CO32-]? Is it the same in response to halving as to doubling? How does the carbonate compensation time in this model compare to the literature values?
5. The Paleocene-Eocene Thermal Maximum is thought to have been caused by the injection of a massive amount of carbon (~2000 Gt) into the system, probably from methane clathrates. The volume of the ocean is 135x1016 m3 and 1 Gt C is equivalent to 8.33x1013 Moles C. What effect do you see on the lysocline depth and [CO32-] if you add all that carbon to the DIC reservoir of the deep sea? What effect does carbonate compensation have on the response of the system?
6. At the Eocene/Oligocene boundary the lysocline/CCD fell rapidly by about 1 km. One hypothesis is that this was caused by a fall in sea-level such that coral reefs that had been present for eons on the shelves were eroded following a fall in sea-level. Carry out experiments with the model initial conditions for deep ocean DIC and alkalinity to see how many Moles of CaCO3 have to be added in order to drop the CCD by 1km. [dissolution of CaCO3 adds DIC and alkalinity in a ratio of 1:2]