Abstract

Despite existing paradigms about marine sedimentation, some submarine canyons are receiving large amounts of sediment during the present highstand of sea level. These modern examples can be used to unlock secrets of past canyon sedimentation. However, submarine canyons have complex morphology, and consequently dramatic variations in sedimentary processes and deposits can occur over a range of spatial scales as small as meters to tens of meters. Operations from surface ships usually cannot place sampling devices on the canyon seabed with this level of accuracy. The purpose of the present study was to investigate the variability of sedimentation over a range of scales, in order to delineate accurate trends along and among channels at the head of a canyon. Sampling with the ROV Ventana facilitated detailed examination of microenvironments (i.e. wall, thalweg) in Eel submarine canyon. The combination of intense sedimentation from nepheloid layers and gravity flows in a complicated morphologic system leads to clear distinctions between microenvironments, as well as some recognizable and unifying trends in sedimentation.

Inherent small-scale variability due to canyon morphology is evident in narrow and steep channels. At 1-m horizontal resolution, cores exhibit consistent sediment fabric, but laminae can differ between cores. At the 10-m horizontal scale the fabric is not consistent. Broader, more gently sloping channels reveal consistent sediment fabric at the 10-m scale. In most cases, physical stratification decreases along the thalwegs of channels as water depths increase. In contrast, channel walls generally exhibit elevated impacts of bioturbation and variable amounts of physical stratification. Radiochemical profiles (²¹⁰Pb, ¹³⁷Cs) and the predominance of physical stratification (including discrete laminae with high sand content) suggest that the northern entrants are receiving more sediment than their southern counterparts. However, ²¹⁰Pb profiles in this study demonstrate rapid accumulation of sediment everywhere in the head of Eel Canyon, with the highest rates of accumulation (> 40 mm/yr) found in the channel thalwegs. In thalwegs, evidence from sedimentary structures (e.g., erosional bases, cross-bedded sand layers) demonstrates that gravity flows occur frequently (many times each year) and have been linked by other investigations to storm impacts on the adjacent continental shelf. On decadal timescales, these processes are temporarily depositing sediment in the head of the canyon, which is removed over longer timescales—probably as larger gravity flows triggered by earthquakes. The radiochemical and sedimentological data collected at the base of entrant channels confirm that modern sediment is moving to deeper portions of the canyon.