Photochemical degradation of dissolved organic carbon and its impact on the oceanic carbon cycle

Abstract

THE processes that regulate the cycling of oceanic dissolved organic carbon (DOC), one of the largest carbon reservoirs on the Earth's surface1, are largely unknown. DOC residues in the deep sea, below 500 m, seem to be composed mainly of biologically refractory compounds2-10 such as humic substances11. The average apparent 14C age of this refractory DOC is >6,000 yr in the deep Pacific2, suggesting that its rate of turnover is slow, but the pathways and rates responsible for this apparent slow turnover are unknown. Several studies have shown that aquatic humic substances are photochemically degraded by sunlight into biologically labile and/or volatile organic compounds12-14 and carbon monoxide15,16. Here we present new data which suggest that this photochemical degradation pathway is the rate-limiting step for the removal of a large fraction of oceanic DOC. This rate will increase with increasing flux of solar ultraviolet-B radiation. We estimate the oceanic residence time of biologically refractory, photochemically reactive DOC to be 500-2,100 yr, which is less than its average apparent 14C age. The injection of 'old carbon' from sediments into the deep sea may explain this discrepancy. © 1991 Nature Publishing Group.