SEDIMENTARY AND GEOCHEMICAL EVIDENCE FOR HYPERPYCNAL FLOW DEPOSITS ON THE EEL RIVER SLOPE, NORTHERN CALIFORNIA

Flooding by small, mountainous rivers in the Pacific Northwest delivers large amounts of sediment and particulate organic carbon (POC) to the coastal ocean, where it is subject to dispersal and burial. Determining the types and relative importance of different dispersal pathways (e.g., surface plumes vs. wave-suspended gravity flows vs. hyperpycnal flows) has important implications for the development of margin facies and the burial of organic matter. Herein, we provide a detailed look at a 2.4-meter kasten core that was collected at site O450 (40° 51.390' N, 124° 28.889' W, 458 m depth) on the upper slope offshore the Eel River during summer 2009. Digital x-radiographs revealed multiple horizons of physical stratification that ranged in thickness from several centimeters to millimeters. Gamma counting of the upper meter of the core revealed penetration of Cs-137 and excess Pb-210 to 75 and 90 cm, respectively. The excess Pb-210 profile indicated non-steady sediment accumulation, with many down-core fluctuations. Sediment accumulation rate at O450 is >1 cm/y based on the penetration of Cs-137. Analysis suggests that sediment accumulation at O450 comprises periods of hemipelagic sedimentation punctuated by rapidly deposited layers characterized by low Pb-210 activities, but high Cs-137. This covariation in activities is best explained by invoking a sediment dispersal mechanism that minimizes mixing with oceanic waters (i.e., hyperpycnal flows). Ongoing work includes organic and inorganic elemental analysis, particle size, surface-area analysis and C-14 dating. Collectively, the resulting data will allow better determination of the sediment transport mechanisms affecting site O450 on the Eel River margin.