2:45 PM - 3:00 PM
[MIS22-05] Effect of photochemical degradation on stable carbon isotope signature of dissolved oraganic matter
Keywords:marine dissolved organic matter, stable isotope, photodegradation
Dissolved organic matter (DOM) is ubiquitous in the ocean, and the majority (>95%) of DOM is biologically refractory, representing a one of the largest reduced carbon pools in the global carbon cycle. The origin of DOM in the ocean has long been assumed to be autochthonous, originally produced by phytoplankton and then altered, based on measurements of the stable carbon isotope signature and lignin phenols which are biomarkers of terrestrial vascular plants. However, it has also been reported that the δ13C value of riverine DOM, which is mostly derived from soil, increases (approaching to the δ13C values of autochthonous DOM) due to photodegradation of DOM by solar radiation. Knowledge of the mechanism of the change in the δ13C value of terrestrial DOM along with photochemical degradation is needed to quantitatively and accurately assess the origin of marine DOM.
We conducted natural sunlight irradiation experiments on several types of DOM, namely Suwannee River fulvic acid and natural organic matter, upper Mississippi River natural organic matter (from the International Humic Substances Society), solid-phase extracted riverine DOM (Ishikari and Uryu Rivers, Hokkaido, Japan), and solid-phase extracted deep ocean DOM (1000 m, Sea of Okhotsk). Natural sunlight was irradiated for up to 90 days, and dissolved organic carbon (DOC) concentrations, absorption coefficient at 350 nm (a350) and δ13C value were measured.
The initial δ13C signatures of the riverine DOM ranged from -28.4‰ to -26.7‰, which is typical value of soil-derived riverine DOM. The loss of DOC concentration and a350 with photodegradation ranged from 17.2% to 65.7% and from 82.4% to 98.9%, respectively. The δ13C values enriched from 1.5‰ to 5.3‰ with photodegradation. The DOC concentration and a350 of the deep ocean DOM also decreased by 13.3% and 83.4%, respectively, and the δ13C value also increased by 1.2‰.
The decrease in DOC concentration could be expressed as a first order reaction for all samples, regardless of source differences. The changes in δ13C were also described by the Rayleigh fractionation model for all samples. The isotopic fractionation factors (α) ranged from 0.9913 to 0.9957, which is a relatively narrow range compared to those determined for simple organic compounds, indicating that kinetic isotopic fractionation is likely to be an important mechanism for shaping the δ13C value of DOM with photodegradation. A simple estimation of changes in the δ13C value of terrestrial DOM with photodegradation indicated that the δ13C value of terrestrial DOM would be comparable to that of marine autochthonous DOM with approximately 73% of photodegradation, suggesting that the contribution of terrestrial DOC to marine DOC may be considerably greater than currently thought.
We conducted natural sunlight irradiation experiments on several types of DOM, namely Suwannee River fulvic acid and natural organic matter, upper Mississippi River natural organic matter (from the International Humic Substances Society), solid-phase extracted riverine DOM (Ishikari and Uryu Rivers, Hokkaido, Japan), and solid-phase extracted deep ocean DOM (1000 m, Sea of Okhotsk). Natural sunlight was irradiated for up to 90 days, and dissolved organic carbon (DOC) concentrations, absorption coefficient at 350 nm (a350) and δ13C value were measured.
The initial δ13C signatures of the riverine DOM ranged from -28.4‰ to -26.7‰, which is typical value of soil-derived riverine DOM. The loss of DOC concentration and a350 with photodegradation ranged from 17.2% to 65.7% and from 82.4% to 98.9%, respectively. The δ13C values enriched from 1.5‰ to 5.3‰ with photodegradation. The DOC concentration and a350 of the deep ocean DOM also decreased by 13.3% and 83.4%, respectively, and the δ13C value also increased by 1.2‰.
The decrease in DOC concentration could be expressed as a first order reaction for all samples, regardless of source differences. The changes in δ13C were also described by the Rayleigh fractionation model for all samples. The isotopic fractionation factors (α) ranged from 0.9913 to 0.9957, which is a relatively narrow range compared to those determined for simple organic compounds, indicating that kinetic isotopic fractionation is likely to be an important mechanism for shaping the δ13C value of DOM with photodegradation. A simple estimation of changes in the δ13C value of terrestrial DOM with photodegradation indicated that the δ13C value of terrestrial DOM would be comparable to that of marine autochthonous DOM with approximately 73% of photodegradation, suggesting that the contribution of terrestrial DOC to marine DOC may be considerably greater than currently thought.