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

摘要

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 Be-7 and Th-214(xs), lowered Pb-210 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 (Be-7 and Th-234) 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.