Dissolved organic carbon (DOC) in the ocean is one of the largest pools of reduced carbon on the earth’s surface, and the majority (>95%) of DOC is biologically refractory. The source of DOC in the ocean has long been assumed to be predominantly autochthonous, originally produced by marine phytoplankton, based on the stable carbon isotopic composition (δ¹³C) of DOC and the lignin phenol concentration of DOC, which are biomarkers of terrestrial vascular plants. However, it has also been reported that the δ¹³C values of riverine DOC, which is mostly derived from soil, increase due to photodegradation by sunlight and approach the δ¹³C values of marine autochthonous DOC. The mechanisms underlying this increase in the δ¹³C value remain poorly understood and pose an important problem for accurately evaluating the origin of marine DOC.
We conducted natural sunlight irradiation experiments on several types of DOC, namely Suwannee River fulvic acid (SRFA), Suwannee River natural organic matter (SRNOM), upper Mississippi River natural organic matter (MRNOM), and solid-phase extracted riverine DOC from the Uryu and Chubetsu Rivers in Hokkaido, Japan. These DOC extracts were irradiated by natural sunlight for up to 91 days on the rooftop of the Graduate School of Environmental Science, Hokkaido University. The concentration, absorption coefficient at 350 nm (a₃₅₀), fluorescence spectrum, and δ¹³C values of DOC were measured before and after irradiation.
The degree of decrease in the DOC concentration varied among these samples, ranging from 17.2% (Chubetsu River) to 65.7% (SRNOM). The degree of decrease in the a₃₅₀ also varied among these samples, ranging from 82.4% (Chubetsu River) to 98.9% (SRNOM). The increase in the δ¹³C value ranged from +1.51‰ (Chubetsu River) to +5.31‰ (SRNOM) at the end of the irradiation experiments. The decrease in the DOC concentration was clearly described by a first-order reaction for all samples, despite differences in the source. Changes in the δ¹³C value of DOC were explained by the Rayleigh fractionation model for all samples. These results indicate that kinetic fractionation of carbon isotopes is an important mechanism for increasing the δ¹³C values of DOC during photodegradation. The results of this study suggest that terrigenous DOC can contribute more substantially to the DOC in the ocean than was previously thought.
 

terrigenous dissolved organic matter,photodegradation,stable carbon isotope,kinetic isotopic fractionation