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Late Holocene sediment accumulation on the northern California shelf: Oceanic, fluvial, and anthropogenic influences

Christopher K. Sommerfield^,1 and Robert A. Wheatcroft^,2

¹ University of Delaware, College of Marine Studies, 700 Pilottown Road, Lewes, Delaware 19958, USA
² Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, Oregon 97331, USA

The late Holocene sedimentary record of the northern California continental shelf archives the combined influences of fluvial and oceanic mechanisms of land to ocean sediment flux, and provides perspective on the cumulative impacts of major floods and land-use change in the drainage basin during historical time. Piston cores collected on the shelf (50–150 m water depths) off the Eel and Klamath Rivers were analyzed to identify lithologies indicative of depositional processes over the past 5 k.y., and dated using ¹³⁷Cs and ¹⁴C methods to resolve spatial patterns in sediment accumulation rates. Sediment accumulation rates averaged over the past several millennia display along- and across-shelf gradients that reflect interactions between the shelf flow field and antecedent tectonically produced topography. The highest rates (3–6 mm/yr; 0.4–0.8 g/cm²/yr) are associated with structural lows on the middle shelf, whereas the lowest rates (0.2–0.6 mm/yr; 0.03–0.05 g/cm²/yr) occur at structural highs and at the shelf edge. This accumulation pattern is spatially coincident with postglacial sediment thicknesses on the shelf, and with depocenters that form on shorter time scales in response to oceanic flood events. The sediment column exhibits an upward-fining sequence consistent with increased bypass of river silt and clay as delta plains progressively filled during the Holocene transgression. Sedimentary event beds indicative of major river floods, including two probable hyperpycnal discharges of the Eel River within the past 1 k.y., are preserved. Stratigraphic evidence of accelerated sediment delivery to the shelf during historical time, in the form of increased burial of fluvial mud relative to fine-grained sand, is superimposed on this natural record. This change is concordant with the regional history of forest clearing and related land-use practices in the watershed after the early 1800s, and appears to record the combined impacts of timber harvesting and several extreme floods after ca. A.D. 1950.

Key Words: continental shelf sedimentation • fluvial systems • northern California • late Holocene • ¹⁴ C geochronology • marine sedimentology

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