The Missing 13C

A couple years back we decided to try making an isotope integrated Carbon cycle model, to see if we could replicate the isotopic “excursions” evident in the geological record. This effort foundered because we couldn’t even balance the current Carbon cycle when we integrated isotopes.

The problem is that the per mil 13C PDB of atmospheric Carbon (-8), is probably the best measured and least uncertain value in the entire Carbon cycle. Yet this value is unsustainable and would drop like a rock when the best estimates of the isotopic fluxes in and out of the atmosphere are used.

Basically, the atmosphere needs more 13C. An additional input 60 GtC at +5 PDB per year to get a reasonable approximation of the -.02 PDB measured yearly decrease.

A lot of people (including me) suspect that the estimates of yearly volcanic Carbon output is low. The current estimate is .1 GtC yearly, and is ignored in the model as insignificant. This would seem an attractive place to look for the missing 13C. One might suspect that volcanic Carbon might be above the Pee Dee Belemnite (PDB) baseline. Positive numbers mean more 13C, and negative numbers less 13C than the standard.

Unfortunately, data from the Encyclopedia of Volcanoes below indicate that volcanic Carbon is predominantly negative.

Volcanic d13C credit Encyclopedia of Volcanoes.png

Adding more volcanic carbon will make the problem of stabilizing atmospheric 13C content even more difficult.

Aggregated First Year Delta 13C Per Mil Atmospheric Inputs
-0.47315 Deep Ocean
-2.08708 Mixed Lay
3.797101 Vegetation
-0.01217 Swamp
-0.00464 Plankton
-1.02431 Soil
-0.20601 Humans
-0.01025 Total (-.16 measured)

It can be seen that only the interaction with vegetation results in increased atmospheric 13C. Seemingly, this is where we must look for the missing 13C.

Either plants are absorbing more CO2 than we think, or they are respiring more than we think, or both. The run above achieves a rough balance by supposing that plants are respiring more than we think. This approach is attractive, because if plants are only absorbing more, there would be a large increase in plant biomass. Measurements indicate that plant biomass expressed as Carbon (about half of plant dry mass) is increasing about .4 GtC per year. It would take about 20 GtC/yr of increased absorption (alone) by plants to achieve a similar balance. This would result in about fifty times more biomass increase than we measure.

Any accounting for the Carbon isotope values applied to supposed fluxes indicates that current conceptions of the Carbon cycle are incorrect. The well measured isotopic value of the atmosphere would be unstable, and the measured rate of change cannot be replicated without a large additional 13C input, or a large reduction in the 12C input.

We expect the atmospheric mass as Carbon to increase yearly somewhat below the one way inputs of humans and soils, ~70 Gt. Large reductions in 12C inputs would push this mass balance in the wrong direction. The large increase in 13C from increased plant respiration proposed here puts the yearly change in bass balance in the expected range (~61 Gt).

It seems we must look to vegetation for the missing 13C


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