Abstract
Riverine organic matter consists of a labile (metabolizable) and a residual (non-metabolizable) fraction1. The labile fraction can be oxidized or 'lost' within the rivers2, their estuaries3, and in the marine environment4. Previous estimates of the river fluxes of organic carbon have not considered such potential losses, being based on measurements of the bulk carbon and nitrogen contents5. Here I report detailed chemical analyses of organic matter associated with suspended matter from several major world rivers that have allowed me to differentiate it into labile and refractory fractions. Globally, 35% (81X 1012 g Cyr−1) of it belongs to the labile fraction and may become oxidized in estuaries and in the marine environment. The rest (150 x 1012g Cyr−1) appears to be highly degraded, with the bulk entering present-day tropical and subtropical sea areas. This degraded fraction could represent a significant source of organic carbon accumulating in marine sediments.
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