UC San Diego

Roughly half the sediment that flows into the oceans is delivered by small rivers that drain tectonically active mountain belts. Their high sediment yield implies that a disproportionately high fraction of particulate organic carbon is also delivered to the oceans by these rivers. The fate of the coarse grained particulate carbon derived from living and soil biomass, which originates as fixed atmospheric carbon, is an important but poorly understood issue. Only by landsliding, triggered by heavy rainfall or earthquakes, will significant amounts of fresh, modern carbon be exported from mountain belts. Flood discharge from severe storms efficiently transports the organic matter from steep, mountainous catchments to adjacent oceans as turbidity currents. The resultant deposits are a potential proxy for landslide occurrence and large flood events. Rapid burial in subsiding offshore basins makes active continental margins prime settings for significant organic carbon sequestration. To demonstrate these concepts, new core data is presented, along with existing geophysical data from the offshore Eel basin, northern California. Rapid burial of terrestrial organic matter derived from coastal mountains occurs in growth synclines and slope basins that have been actively accumulating sediment for millions of years. Significant preservation of biomass-derived carbon in offshore depocenters at active margins has wide implications for longer-term carbon budgets, essentially because the living biomass represents a renewable carbon resource that can be transferred from the biosphere to the lithosphere