Slope Failure Records in Gas Hydrate Bearing Regions of the Cascadia Margin

ODP Leg 204 (Hydrate Ridge region) and IODP Exp. 311 (offshore Vancouver Island) cores and site survey seismic reflection profiles reveal that seismically defined wedges observed in the slope basins adjacent to gas hydrate bearing ridges correspond to intervals of more frequent slope failure compared to the surrounding hemipelagic sediments. Both Sites 1251B, 1252A and 1325B show multiple intervals of increased slope failure that are separated by periods of slow hemipelagic sedimentation. Rather than single submarine landslide deposits, these intervals are composed of multiple sand/silt turbidites and debris flows, which are most easily observed as relative increases in magnetic susceptibility, due to the abundant detrital magnetic mineral grains found within them. Core descriptions coupled with the magnetic susceptibility data for both Leg 204 and Exp. 311 cores document apparent temporal cyclicity to these episodes of slope failure. At Sites 1251B and 1252A, which can be correlated to each other, and at Site 1325B, four major episodes of increased slope failure are observed. These data coupled with initial radiocarbon ages and diatom biostratigraphy (Watanabe, 2006 and Akiba et al., 2009) suggest that the slope failure cycles may change on glacial-interglacial timescales. Dramatic changes in sea level driven by glacial-interglacial ice sheet dynamics often regulates the frequency of slope failure near the shelf-slope break in high latitude, non-sediment starved continental margins like Cascadia. Slope failures along active continental margins can also be triggered by seismic events (Goldfinger et al., 2003); indeed, Holocene subduction zone earthquakes on the Cascadia margin are known to trigger slope failures every 500-600 years, a frequency that varies on long tectonic timescales rather than glacial-interglacial periods. These mechanisms, however, do not explain the long timescale cyclic records of slope failure in these regions. Sites 1251, 1252, and 1325, lie in portions of the margin that are generally isolated from shelf and submarine canyon sediment sources and are located near the toe of the accretionary wedge where they predominantly receive local sediment loads from the erosion of the surrounding gas hydrate bearing bathymetric highs. In this presentation, new TOC measurements, radiocarbon ages, and carbon and oxygen isotope data help characterize the stratigraphy at these sites and constrain the timing of these major slope failure episodes. The implications for the mechanisms responsible for these slope failure variations and their potential relationship to the local gas hydrate systems will also be discussed.