Graduate Thesis Or Dissertation
 

A terrestrial organic matter depocenter on a high-energy margin adjacent to a low-sediment-yield river : the Umpqua River margin, Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/xw42nb738

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  • Small, mountainous river systems have been increasingly studied due to their importance as sources of fluvial sediment and organic matter to the coastal ocean. Thus far, studies of small, mountainous river systems have focused on rivers with very high sediment loads. The well-studied Eel River dispersal system in northern California shares many characteristics with the Umpqua River system, but delivers ten times more sediment to the coastal ocean annually. These rivers deliver sediment and terrestrial organic matter during winter floods to a narrow, high-energy coastal margin. Studies of the Eel have shown that the formation of wave-supported gravity flows allows for the accumulation of sediment in a mid-shelf depocenter, and that without this transport mechanism, Eel River sediment is dispersed widely and exported off-shelf. The Umpqua margin is expected to be highly dispersive due to the energetic coastal ocean conditions and the low sediment supply, which is thought to be insufficient to trigger wave-supported gravity flows. Two-dimensional patterns in grain size and organic matter content and character from 66 surface sediment samples were examined to determine where fluvial material is deposited on the margin. The distribution of fine-grained sediment, organic matter and lignin phenols indicate a mid-shelf depocenter located from 85 to 115 m water depth stretching more than 40 km along-shelf on the Umpqua margin, which is unexpected. Organic matter within this depocenter is characterized by elemental OC:N ratios that range from 9.9 to 13.9, δ¹³C values that range from -22.2 ‰ to -24.2 ‰ and total lignin yields that range from 2.7 to 7.5 mg/100 mg OC all of which are consistent with a terrestrial source. A mixture of terrestrial and marine organic carbon is present on the outer shelf and upper slope indicating delivery of riverine material from the Umpqua shelf and distal rivers to the south. Comparisons of sedimentation rates, organic carbon content and size of the depocenters indicate that the Eel and Umpqua depocenters may bury similar quantities of organic carbon. These results suggest that organic matter burial may occur within depocenters on other margins influenced by rivers supplying moderate or low sediment loads.
  • Keywords: Umpqua River, particulate organic carbon, continental shelves
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