Modern sedimentation and diagenetic processes on the Eel River shelf and slope, northern California continental margin

A suite of geochemical, radionuclide, and sedimentological methods were used to investigate modern sedimentation processes associated with river floods and to characterize early diagenetic environments of the Eel River shelf and slope, northern California continental margin. Episodic winter-season river runoff greatly increases the supply of river sediment and {dollar}sp7{dollar}Be to sediments of the margin. Because radionuclide-labeled sediment is not appreciably cycled between the seabed and water column prior to deposition, {dollar}sp7{dollar}Be is a powerful naturally occurring tracer of flood sediments and short-term sedimentation processes. The episodic input of flood sediment to the shelf yields {dollar}sp7{dollar}Be-based sediment fluxes (2.3-66.0 g/cm{dollar}sp2{dollar}/y range) that are significantly higher than {dollar}sp{lcub}210{rcub}{dollar}Pb accumulation rates (100-y average) determined for the same depositional sites. {dollar}sp{lcub}210{rcub}{dollar}Pb sediment accumulation rates decrease from maximum mid-shelf values of 0.6-1.7 g/cm{dollar}sp2{dollar}/y to values of 0.2-0.4 g/cm{dollar}sp2{dollar}/y at the shelf edge, with a spatially weighted mean of 0.5 g/cm{dollar}sp2{dollar}/y (0.4 cm/y) for the entire shelf. {dollar}sp{lcub}210{rcub}{dollar}Pb activity anomalies are observed in some cores and are associated with major Eel River floods of 1955, 1964, and 1974. Because of the coincidence of high river-flow events and southerly winds during cyclonic winter storms, net northward shelf transport allows for preferential deposition of fine-grained sediment north of the river mouth. A {dollar}sp{lcub}210{rcub}{dollar}Pb-based fine-grained sediment budget developed for the dispersal system demonstrates that a maximum of only {dollar}sim{dollar}20% (14 {dollar}times{dollar} 10{dollar}sp6{dollar} t/y) of the mean annual supply of mud from local rivers is trapped on the mid and outer shelf on a time scale of 100 y. Spatial and temporal sedimentary distributions of {dollar}sp7{dollar}Be and {dollar}sp{lcub}210{rcub}{dollar}Ph for the Eel River shelf and slope illustrate the importance of oceanic processes on the dispersal patterns and deposition rates of flood sediment on the northern California continental margin. Comparison of modern and ancient diagenetic environments of the Eel River Basin indicates that, despite the episodic depositional regime, post-depositional biogenic transport have reoxidation processes have been the primary control on solid-phase C-S-Fe signatures of sediments deposited on the margin during the Quaternary.