Carbon model worksheet 4

The aim of this carbonate compensation worksheet is to outline some numerical experiments elucidating carbonate compensation, to be carried out within the carbon cycle (which contains a dynamic lysocline).

Worksheet 4: Carbonate compensation

Carbonate compensation is the negative feedback process which regulates carbonate ion concentration [CO₃^2-] (or, more strictly, the degree of supersaturation with respect to CaCO₃, Ω) in the oceans. When [CO₃^2-] gets too high then more CaCO₃ is buried, lowering [CO₃^2-]; conversely when [CO₃^2-] falls too low, less CaCO₃ is buried, allowing river delivery to raise [CO₃^2-].

1. Try some different initial conditions (initial values of [CO₃^2-]), 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 [CO₃^2-] in each case? Does the model converge in each case? (i.e. does it homeostatically regulate [CO₃^2-]?) Is the final value of [CO₃^2-] 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 [CO₃^2-]. What, if any, changes do you see immediately after (a) doubling [CO₃^2-]; (b) halving [CO₃^2-]? [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 [CO₃^2-]; and (b) immediately after halving of deep [CO₃^2-]. How does it compare to the river flux?
4. In the scientific literature 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 [CO₃^2-]) following (a) doubling of [CO₃^2-]; (b) halving of [CO₃^2-]? 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 [CO₃^2-] 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]

Once you have completed these questions, follow this link for the answers.