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Abstract: Small, mountainous river systems (SMRS) are now recognized to supply roughly half of the land-derived fine grained sediment and by inference associated bioactive constituents such as organic matter (OM), to the world oceans, yet there is an incomplete understanding of how that material is distributed on continental margins. This knowledge deficit stems, in part, from the small number of SMRS that have been studied. Herein, we present data on the grain size, OM content, and composition of surface sediments from 66 cores collected on the Umpqua River shelf and uppermost slope off the central Oregon coast. In comparison to northern California''s Eel River, the Umpqua has over an order of magnitude lower sediment supply, but a similar energetic wave climate, thus sediment and associated OM was expected to be widely dispersed across the margin. Instead, both textural and compositional data indicate the existence of a well-defined depocenter in 80m to 110m water depth that stretches along margin for roughly 40km. Centered ∼10km north of the river, the depocenter is characterized by appreciably higher mud and organic carbon contents than surrounding shelf sediments. Moreover, sedimentary OM in the depocenter, especially within its inshore half, has multiple, unequivocal indicators (e.g., carbon:nitrogen, stable carbon isotope and lignin phenol compositions) of terrestrial origin. These seemingly paradoxical results – a high dispersal capacity margin fed by a low sediment yield river, but with a clearly defined depocenter – may be explained by invoking wave-supported gravity flows, such as what has been observed offshore the Eel and other high yield systems (e.g., Waiapu, New Zealand). Testing this conjecture must await in situ observations of this low yield SMRS. [Copyright &y& Elsevier]

Abstract: In this study, organic carbon (OC), total nitrogen (TN), stable carbon isotopic (δ 13COC) and CuO reaction product compositions were used to identify the sources of organic matter (OM) and to quantify the relative importance of allochthonous and autochthonous contributions to the western Adriatic Sea, Italy. Suspended particulate material (195 samples) and surficial sediments (0–1cm, 70 samples) from shallow cross-shelf transects were collected in February and May 2003, respectively. Vertical water column profiles were acquired along the same transects. Data include depth, potential temperature, salinity, density and chlorophyll fluorimetry. Along the western Adriatic shelf in the near-shore region, the phytoplankton growth was influenced by dynamics of the buoyant plumes from the Po and Appennine rivers. A small amount of very fine terrigenous material remained suspended within the coastal current and was exported southward along the shelf to the slope. High variability in the bulk composition was detected in the Po prodelta surficial sediments, whereas the western Adriatic shelf, although a larger area, exhibited a narrower range of values. A significant decoupling was observed between suspended particles in the water column and surficial deposits. The organic material collected in the water column was compositionally heterogeneous, with contributions from marine phytoplankton, riverine–estuarine phytoplankton and soil-derived OM. Frequent physical reworking of surficial sediments likely leads to the efficient oxidation of marine OC, resulting in the observed accumulation and preservation of refractory soil-derived OC delivered by the Po and Appennine rivers. [Copyright &y& Elsevier]