By using floating chambers and the SrCl2 precipitation method, I collected radiocarbon isotope (C14) samples of CO2 evasion from river water surface. For the emitted CO2, the measured stable carbon isotopes ( δ13C) varied from -30.2‰ to -23.2‰, and the conventional age calculated from Δ14C values ranged from 800 to 1900 years. Could I conclude that it is the old fraction of riverine organic carbon, not the young fraction, that was degraded during fluvial delivery and emitted into the atmosphere? While numerous studies have been conducted on C14 fractionation in various biogeochemical processes, such as respiration and photosynthesis, very little has been done for the process of CO2 evasion.
Any help will be greatly appreciated. Many thanks.
You can have a look in the following studies......
https://www.nature.com/articles/nature03880
https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/18348
Hi Dr Lishan Ran,
Have you considered that the old C14 dates may have been caused by the dissolution of some limestone. In other words, the effect is coming from inorganic carbon in the water, rather than old organic carbon. Dissolved CaCO3 would also affect the C13 values. Measuring the alkalinity might help in diagnosing that.
Greeting Dr Lishan Ran
In addition to the excellent suggestions by Alastair and Mohammed above, you could also look at the oceanic CO2 literature. Radiocarbon uptake by the ocean has been used to estimate oceanic fossil fuel CO2 uptake and also gas exchange. The isotopic fractionation for 14C-CO2 (in relation to 12C-CO2) across the air-water interface is generally considered to be 2 x that of 13C-CO2 (in relation to 12C-CO2). Some articles that might be of interest:
http://onlinelibrary.wiley.com/doi/10.1029/2006GB002784/full
Article Carbon isotope fractionation during gas-water exchange and d...
You might also need to consider any fractionation that might occur during precipitation with your sample collection.
All the best with your research
Damien
Dear Dr Alastair Bain McDonald,
I have also measured the alkalinity. Because of the extensive presence of calcite with the study area, its river water alkalinity is considerably high, averaging at 5500 ueq/L. Carbonate dissolution is possibly a source of the evaded CO2. But the δ13C of the evaded CO2 varied from -30.2‰ to -23.2‰, which is more depleted than the general δ13C values of DIC derived from carbonates (i.e., -8.5‰). Thus, the contribution of carbonate dissolution to CO2 evasion is likely very small, even if the fractionation caused by gas diffusion (e.g., 4.4‰ enrichment in soil CO2 relative to soil respired CO2; Cerling et al., 1991) is considered. In contrast, the δ13C values of the evaded CO2 are close to that of C3 and C4 plants. Both C3 and C4 plants, such as soybean, millet, and maize, are planted in the study area. Actually, the 13C values of the sampled evaded CO2 presented seasonal variability. They are more depleted in spring (-30.2‰) when groundwater dominates the river flow, and are relatively enriched in summer and autumn (-24‰) when C4 plants (maize) are planted. Export and decomposition of organic matter from C4 plants may have affected the 13C signature shift (from -30.2‰ in spring to -24‰ in summer and autumn). I am not quite sure whether my interpretation is correct. Many thanks for your suggestions.
Have a nice weekend.
With best regards
Lishan
Hi Lishan,
I must first confess that I do not have a PhD, and so the title Dr is inappropriate for me. Moreover, since I am not an authority, anything I write should be checked with the scientific literature. However, I am attempting to write my first scientific paper on the carbon cycle and hence my interest in your question.
The high alkalinity you report suggest to me that there is a significant amount of inorganic carbon dissolved in your water. Its carbon will be millions of years old and so will have a d14C of zero. If you can measure the molarity of the calcium (and Mg) content of the water then you will have a value for the molarity of inorganic carbon, and you can subtract that when calculating your 14C age.
d18O fractionation due to evaporation (and condensation) is used to calculate paleo-temperatures, so I would expect that there is a d14C fractionation caused by evaporation too. But you can check that if you can measure the d14C of your water.
HTH,
Alastair.
Dear Damien and Alastair,
Many thanks for your very detailed and helpful explanations, which have greatly improved my understanding of 13C frationation during CO2 degassing across water-air interface. Isotopic carbon fractionation is a very interesting process accompanying carbon transport within a river catchment. Thanks again.
Best regards
Lishan
Dear Alastair,
Yes, the river catchment I am currently study is characterized by extensive presence of carbonates, and the DIC concentration in river water is extremely high. The measured C14 age ( 800 to 1900 years) indicates that the degassed CO2 is probably a combination of CO2 with different ages, such as recently-fixed organic matter (very young,
Hi Lishan,
Thanks for confirming my suspicions. It is always like it when I am told I am correct, which does not happen very often.
Just two points which I think you are missing. The inorganic carbon is being derived from calcium carbonate millions of years old so it will contain no 14C. Moreover, you wrote "... the 13C values of the sampled evaded CO2 presented seasonal variability" therefore the organic 14C will also vary seasonally. In other words its age is less than1 year. So what I am claiming is that from the d14C value you can calculate how much carbonate derived carbon is in your sample.
With a half life of 5700 years, 14C will have 3/4 life of 2850 years, and a 7/8 life of 1400 years which is around the average C14 age you are measuring. Therefore, one eighth of the carbon in your water is derived from carbonate!
Cheers, Alastair.