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C-14 time scale and way of implementation



Dear Colleagues,

We might still consider to implement radiocarbon as an inorganic tracer using
fractionation corrected ratio as a tracer. 

Oliver Marchal has implemented radiocarbon in the 2.5-d ocean
circulation model of Stocker by 

(a) using the scale suggested by
U. Siegenthaler and as used by Toggweiler et al., 19989. In this
simulation the 'fractionation corrected C14/C12 ratio' is the only
tracer (of course beside T and S)

(b) using a ocean biogeochemical model (Marchal et al., Tellus in
press). The simulations include the following tracers: PO4, DIC,
DIC-13, DIC-14, DOC, DOC-13, DOC-14, Alk, O2. Export production of
nutrients in the form of POM and DOM is calculated by restoring to
surface phosphate. Fractionation factors for the carbon isotopes are
taken from Zhang et al.. In this simulation C-14 is carried as an
independent tracer that is part of the inorganic and organic cycles.
Big Delta C-14 units are calculated using the concentrations of DIC,
DIC-13, DIC-14 for each grid cell and applying the standard definition
of Big Delta C-14.

For both runs, the atmospheric boundary is set to 0 permil
Delta-C14. Both simulations have been ran for 15 kyr, i.e., very close
to equilibrium.

On our anonymous ftp site:

ftp 130.92.143.11
login: anonymous
pw: your e-mail
dir: pub/joos/ocmip

you will find the figures

 contour_c14.ps or contour_c14.gif	    in postscript or gif format

 timeseries_c14.ps or timeseries_c14.gif		"

 sel_series_suc14jul98.ps

Fig.  contour_c14 shows the radiocarbon fields in the Atlantic, Indian
and Pacific for the 2 simulations. Panel a) gives results for the
Toggweiler scale simulation; Panel b) for the organic model; Panel c)
shows the difference between a and b. The maximum difference is 7
permil in big Delta C-14. (I now Ernst you have published this many
years before). This is much smaller as deviations induced by other
relevant uncertainties (e.g. gas exchange rate). Thus, for the purpose
of pre-industrial C14 simulations it is not necessary to run an organic carbon cycle
model that includes C-13, DOM and POM, fractionation factors, etc.

Fig. timeseries_c14 shows the relaxation of a cell in the northern
deep pacific towards equilibrium for the two simulations when the
ocean is spun up from rest. This gives some indication on the
integration time that is necessary to reach equilibrium.

Fig sel_series_suc14jul98.ps shows the output for a different model
configuration, where vertical eddy diffusivity and thus ocean mixing
is high. Here, circulation has first been spun up from rest for 10'000
yrs, then radiocarbon has been added as a ratio at year 10'000. The
Delta C14 has been set to -150 permil in the ocean and to 0 permil
(fixed) in the atmosphere. The model has then been run for another
20'000 yrs (i.e. total time=30'000 yrs). The results suggest that one
needs to run the model about 5000 years to reach equilibrium when
ocean mixing is high i.e. modeled deep Pacific C-14 around 200 permil
instead of observed 240-260 permil.  The time to reach equilibrium is
probably longer if ocean mixing is more realistic.

Equilibrium might be achieved faster if the initial C-14 field is
prescribed according to observations. If OCMIP participants wish that the
Bern group does assess how to initialize the oceanic Delta C-14 field
'best', we can certainly perform more simulations as CPU time is not a
real issue for us.

In summary, for those OCMIP models that have difficulties to perform
long integrations, it might be better to include C-14 using the
Siegenthaler/Toggweiler scaling instead running a full carbon cycle model. One may
also recall that the coding of all the tracers, fractionation factors etc is
somewhat cumbersome and might lead to coding errors, inconsistencies
etc, especially when implementing various forms of marine biosphere models. 

We have not yet assessed the differences of the two implementations
for simulations dealing with bomb-produced C14.

Please let us know, if you wish more information about the results and
how they have been obtained.

With best regards, Fortunat
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- ----------------------------------------------------------------------------
Fortunat Joos
Physics Institute, KUP, Sidlerstr. 5, CH-3012 Bern

Phone:    ++41(0)31 631 44 61
Fax:      ++41(0)31 631 44 05
e-mail:   joos@climate.unibe.ch
Internet: http://www.climate.unibe.ch/~joos/