What happens to terrestrial organic matter in the ocean?

Each year rivers transport approximately 0.25×1015 g of dissolved (<0.5 μm) organic carbon (DOC) and 0.15×1015 g of particulate (>0.5 μm) organic carbon (POC) from continents to the ocean. Global discharge of riverine DOC is sufficient alone to sustain turnover of the entire pool of organic carbon dissolved in seawater. Similarly, the input of terrestrial POC by rivers is adequate to supply all the organic carbon buried in marine sediments. Because riverine organic matter consists of highly degraded, nitrogen-poor remains of terrestrial organisms, it might be expected to suffer minimal respiration in the ocean. One of the biggest mysteries in the global carbon cycle, therefore, is that only a small fraction of the organic matter dissolved in seawater and preserved in marine sediments appears to be land-derived. Either our global budgets and distribution estimates are greatly in error, or both dissolved and particulate organic matter of terrestrial origin suffer rapid and remarkably extensive remineralization at sea. Although many uncertainties remain, recent studies provide growing evidence for destruction of both dissolved and particulate terrestrial organic matter in the ocean. The mechanisms by which these huge masses of organic materials might be so rapidly oxidized following discharge are largely unknown. This report focuses on the transport and transformations of land-derived organic matter in the ocean, highlighting recent research on the patterns and processes involved.