Watershed control on the carbon loading of marine sedimentary particles

Previous investigations of the factors governing organic carbon burial on c ontinental margins have pointed toward the important, apparently protective association of carbon with mineral particles. These studies have also reve aled dramatic transformations of carbon-particle relationships at the land- sea interface. Riverine particles in some settings lose a large portion of their loads of sorbed terrestrial carbon upon discharge to the ocean and gr adually reload to similar levels with marine carbon. The Eel River in north ern California and the adjacent continental shelf were selected as an ideal system to investigate the rates of these processes. The river is episodica lly subject to large floods, and the shelf stratigraphy preserves a record of the resultant large pulses of sediment and carbon input to the marine en vironment. Carbon isotopic, carbon to nitrogen, and carbon to surface area ratios of particles in flood deposits were expected to reflect the rapid un loading of terrestrial carbon from discharged particles, whereas nonflood s ediments that have accumulated at slower rates on the shelf were expected t o carry higher loads of marine carbon. Our results indicate, however, that particles on the Eel shelf have retained their loads of terrigenous carbon, and that a significant portion of the particle-sorbed carbon buried on the shelf is kerogen derived from the Mesozoic-Tertiary Franciscan Complex. We hypothesize that rates of uplift and mass wasting in the Eel watershed and rates of particle delivery to and burial on the continental shelf, are so rapid that kerogen is not completely oxidized and is recycled instead. The loading of carbon on clay-sized particles delivered to the shelf, moreover, is dependent on river discharge and may reflect the relative importance of different mass wasting processes during precipitation events of varying in tensity. The Eel River system is likely to be representative of other small , mountainous rivers and indicates that processes on land may play an impor tant role in governing the amount and character of carbon being buried on t he continental margins. Copyright (C) 2001 Elsevier Science Ltd.