Variations in downslope activity and bottom current dynamics in a land-detached submarine channel system since the Last Glacial Maximum

The Gollum Channel System is a land-detached large-scale canyon-channel system situated offshore southwest Ireland on the Northeast Atlantic margin. The system has been considered inactive with downslope gravity flows since the last glacial period, but geophysical data in some of its branches do suggest Holocene activity. Here, the hypothesis of (in)activity under Holocene highstand conditions is tested. High-resolution side-scan sonar, photography and bathymetry data were collected using an autonomous underwater vehicle (AUV) in the upper slope (350 to 1000 m water depth) section of two channels in the system, Bilbo Channel and Frodo Channel. The AUV results are quantified and validated by current meter data from a mooring station in Bilbo Channel. Additionally, two sediment cores are used to build a record of channel floor processes since the Last Glacial Maximum and detect any temporal variability in channel activity. Turbidites in the sediment cores indicate that downslope sediment transport was happening well after deglaciation of the continental shelf, revealing that the channels were active under modern climatic and oceanographic conditions in the early Holocene. Crescentic bedforms in one of the channel head gullies suggest present-day activity, but their limited spatial extent indicates present-day downslope flows are minimal in frequency and volume. As a whole, the Gollum Channel System can be considered inactive today. Rather, tidally modulated bottom currents oriented along the channel axes are the main mechanism currently transporting and redistributing sediments, forming fields of megaripples and superimposed ripples. This dynamic channel floor environment has allowed the development of cold-water coral mini-mounds since the early Holocene. This study highlights that specific environmental conditions are necessary to allow frequent downslope activity in a land-detached canyon-channel system and these may vary even in systems draining the same shelf, and that inactivity may have important consequences for deep-sea habitats, by allowing or promoting their development.