Deposition and modification of a flood layer on the northern California shelf: lessons from and about the fate of terrestrial particulate organic carbon

The 1995 hood of the Eel River in northern California provided an opportuni ty to follow the short-term history of riverine particulate matter on a con tinental shelf. Particulate organic carbon, including both vascular plant d ebris and soil carbon, was utilized as a tracer of the distribution and mod ification of the shelf flood deposit. These components were sorted during i nitial emplacement of the deposit, both in the river plume and in the benth ic boundary layer. During the 8 months following the flood, lateral transpo rt, shallow burial, and bioturbation contributed to spatial changes in the amount and character of carbon in the flood deposit. Study of the deposit s uggests that this and previous hood layers may be preserved in the shelf an d upper slope stratigraphic record as clay-rich beds with relatively high c arbon to nitrogen ratios and more negative delta(13)C values than backgroun d, non-flood sediment. Concentrations of vascular plant debris are locally characteristic of such layers, and may serve as distinctive stratigraphic m arkers. Investigation of organic carbon in the 1995 flood layer on the Eel River shelf provides insight into the fate of terrestrial particulate carbo n on continental margins in general. Flooding of the Eel River, and of simi lar river systems draining mountainous terrains, introduces large quantitie s of terrestrial carbon to the marine environment and may favor the preserv ation of both terrestrial and marine carbon. The abundant woody vascular pl ant debris discharged during floods is especially likely to survive oxidati on and sulfate reduction in the shallower zones of the seabed. This study r eveals the dominant role that physical and biological processes may play in attenuating the terrestrial organic-carbon signal on shelves over monthly to yearly time scales. (C) 1999 Elsevier Science B.V. All rights reserved.