Decadal record of sediment export to the deep sea via Eel Canyon

Abstract

An active upper-canyon system, Eel Canyon, was studied to determine its role as a conduit and/or sink for terrigenous material over decadal timescales and to assess the sedimentary record preserved by transport processes. These data are used to (1) link seasonal fluctuations in sediment transport and deposition to preserved stratigraphic signatures, and (2) assess sediment storage and removal in the upper Eel Canyon (100–850 m water depth) over decadal timescales. Previous research has shown that upper thalwegs commonly experience gravity-driven flows during winter (November–March), due to increased sediment supply from Eel River flooding and intense storms that produce energetic wave/current conditions. Thick winter deposits composed of recently discharged fluvial sediment are formed in upper thalwegs, with distinct short- and long-lived radioisotopic and textural signatures (detectable 7Be and 234Th xs, lowered 210Pb activity, elevated clay content, and physical structures). Box and kasten cores were collected in the upper canyon (thalwegs and walls) to measure these signatures in recent and preserved winter deposits, and to calculate 100-yr accumulation rates. Non-bioturbated deposits (that have signatures indicative of rapid accretion by gravity-driven flows during the winter) are common in the upper canyon thalwegs. Short-lived radioisotopes ( 7Be and 234Th) show that sediment delivery to the upper thalweg varies temporally, sometimes beginning at the onset of river flooding, and at other times beginning during fall/early winter dry-storm events. In contrast, bioturbated deposits (which do not have signatures indicative of rapid deposition) are found on canyon walls. Non-bioturbated winter deposits are easily identified in the decadal record of thalwegs by decreases in 210Pb activity and increases in clay content. Stacking of multiple years of winter deposits (∼10 cm preserved per winter) results in non-steady-state 210Pb profiles and high decadal accumulation rates. However, down-core changes in 210Pb profiles show that slope failures are actively redistributing these winter deposits. Partial or total removal of multiple winter deposits appears to happen periodically (every ∼13 yr), which will inhibit preservation of the longer decadal record. 100-yr accumulation rates were calculated in the thalwegs from the resulting 210Pb profiles (i.e., the result of winter accretion and decadal removal by failures). Accumulation rates are much higher in thalwegs (1–6 cm/yr) than walls (0.1–0.8 cm/yr), which is likely the result of differing sediment delivery processes (via gravity-driven flows and nepheloid layers, respectively). At least 2.6±1.4% of the Eel River sediment budget is accumulating in the upper canyon over 100-yr timescales. However, this value greatly underestimates the total amount entering the canyon system because minimum accumulation rates were used in many areas (due to limited core length) and slope failures are moving sediment out of the budget area.