Sedimentological and Stratagraphic Evidence for a Catastrophic Flood Along the Beaufort Margin, Arctic Ocean

Monday, 15 December 2014
Shannon Klotsko, Scripps Institution of Oceanography, La Jolla, CA, United States, Neal W Driscoll, Scripps Institution of Oceanog, La Jolla, CA, United States and Lloyd D Keigwin, WHOI, Woods Hole, MA, United States
In 2013, a cruise on the USCGC Healy mapped the Beaufort margin from Barrow, AK into the Amundsen Gulf using a towed CHIRP subbottom profiler and a hull-mounted Knudsen CHIRP subbottom profiler. Sediment cores were also acquired. The seismic data image an extensive margin-wide drift deposit, which appears to have entered the Beaufort margin from the Mackenzie River. The deposit is thickest on the eastern side of the Mackenzie Delta, near stations 5 and 9. It thins west along the Beaufort margin nearing Barrow Canyon. The deposit also thins to the east, but does not extend as far in this direction. Sediment core Healy1302-JPC15, from station 5 on the Mackenzie slope, was analyzed for grain size using a Laser Diffraction Particle Size Analyzer. These data were compared to the magnetic susceptibility data from the core. Peaks in the susceptibility correlate with increases in grain size, as well as high amplitude reflectors within the drift deposit present in the seismic data. The drift deposit appears to have a diagnostic reflection pattern observed in the seismic data with the base being characterized by a high amplitude reflector that correlates with increased IRD. Above the basal reflectors, an acoustically transparent interval is observed with thicknesses on the order of 7 m near the depocenter; this interval has diminished IRD and radiocarbon dates yield accumulation rates as high as 8 m/ky. The dominant grain size for this interval is ~5 μm. Above the transparent interval is a series of high amplitude reflectors defining the top of the drift deposit. The dominant grain size for the basal and upper high amplitude reflectors reaches ~20 μm. We postulate that the drift deposit was sourced from glacial Lake Agassiz and flowed down the Mackenzie River, entering the Arctic Ocean. It was then entrained by the Beaufort Gyre, migrating clockwise along the margin. Oxygen isotope data reflects the light δO18 signature consistent with a large glacial meltwater input. Based on radiocarbon dates from the core, the flood deposit was laid down almost instantly in geologic time, starting ~13 kya. After the flood input, sedimentation rates were significantly lower, in the range of ~9 cm/ky in the late Holocene to ~120 cm/ky in the late Pleistocene. Prior to the flood, sedimentation rates were also lower at 133 cm/kyr.