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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 Fall 2000, the Po River (Italy) experienced a 100-yr return period flood that resulted in a 1–25cm-thick deposit in the adjacent prodelta (10–25m water depth). In the following years, numerous post-depositional perturbations occurred including bioturbation, reworking by waves with heights exceeding 5m, as well as periods of extremely high and low sediment supply. Cores collected in the central prodelta after the Fall 2000 flood and over the following 9yr, allowed characterization of the event-strata in their initial state and documentation of their subsequent evolution. Sedimentological characteristics were investigated using X-radiographs and sediment texture analyses, whereas the composition of sedimentary organic matter (OM) was studied via bulk and biomarker analyses, including organic carbon (OC), total nitrogen (TN), carbon stable isotope composition (δ13C), lignin phenols, cutin-products, p-hydroxy benzenes, benzoic acids, dicarboxylic acids, and fatty acids. The 9-yr time-series analysis indicated that roughly the lower half of the original event bed was preserved in the sediment record. Conversely, the upper half of the deposit experienced significant alterations including bioturbation, addition of new material, as well as coarsening. Comparison of the recently deposited material with 9-yr old preserved strata represented a unique natural laboratory to investigate the diagenesis of sedimentary OM in a non-steady system. Bulk data indicated that OC and TN were degraded at similar rates (loss ∼17%) whereas biomarkers exhibited a broad spectrum of reactivities (loss from ∼6% to ∼60%) indicating selective preservation during early diagenesis. Given the relevance of episodic sedimentation in several margins, this study has demonstrated the utility of event-response and time-series sampling of the seabed for understanding the early diagenesis in non-steady conditions. [Copyright &y& Elsevier]