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

Abstract

The 1995 flood of the Eel River in northern California provided an opportunity to follow the short-term history of riverine particulate matter on a continental shelf. Particulate organic carbon, including both vascular plant debris and soil carbon, was utilized as a tracer of the distribution and modification of the shelf flood deposit. These components were sorted during initial emplacement of the deposit, both in the river plume and in the benthic boundary layer. During the 8 months following the flood, lateral transport, shallow burial, and bioturbation contributed to spatial changes in the amount and character of carbon in the flood deposit. Study of the deposit suggests that this and previous flood layers may be preserved in the shelf and upper slope stratigraphic record as clay-rich beds with relatively high carbon to nitrogen ratios and more negative δ 13C values than background, non-flood sediment. Concentrations of vascular plant debris are locally characteristic of such layers, and may serve as distinctive stratigraphic markers. Investigation of organic carbon in the 1995 flood layer on the Eel River shelf provides insight into the fate of terrestrial particulate carbon on continental margins in general. Flooding of the Eel River, and of similar river systems draining mountainous terrains, introduces large quantities of terrestrial carbon to the marine environment and may favor the preservation of both terrestrial and marine carbon. The abundant woody vascular plant debris discharged during floods is especially likely to survive oxidation and sulfate reduction in the shallower zones of the seabed. This study reveals 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.